Oregon State University -- Biology 301 -- Human Impacts on Ecosystems

Copyright 1999, Patricia S. Muir


Notes giving general background information on global climate change (the "greenhouse effect") follow. As you'll see, there are many relationships between this topic and issues that we have discussed previously, such as human population growth, agriculture, and tropospheric ozone pollution. More information on the liklihood of global climate change, feedbacks affecting it, human and ecological consequences associated with it, and policy steps that may be taken to address it, are in the sections of notes called "Probabilities, consequences and policies related to global climate change." If you want to use the study guide covering global climate change, or to obtain additional references on this topic, click on study guide .

There are many web resources that can be used to learn more about the prospect of global climate change. I list only a few here, but you will find hundreds when you search on your own, of course! I have tried to list only key sources that approach the subject from relatively unbiased perspectives. 

This one just out in February of 2014 is excellent -- a document on the science of glocal climate change, which includes great list of "frequently asked questions" with clear answers -- you can download the PDF for personal use for free. Put out by the US National Academy of Science in company with the UK analog, the Royal Society. It is entitled Climate Change: Evidence and Causes.

Oregon State University's Oregon Climate Change Research Institute provides a site that is loaded with background information, primers on weather and climate, discussions of specific aspects of climate and climate change, and rebuttals of common misunderstandings about causes and likely consequences of human-influenced climate change. It also includes a "contact us" link that you can use to communicate with one of the scientists "in the know."

The US Global Change Research Information Office (GCRIO) provides access to data and information on global climate change research, adaptation/mitigation strategies and technologies, and global change-related educational resources on behalf of the various US Federal agencies and organizations that participate in the US Global Change Research Program.

The National Center for Ecological Analysis and Synthesis, funded by the National Science Foundation, the State of California, and UC Santa Barbara provides information on a variety of topics related to global climate change (and others as well).

NASA's Goddard Space Flight Center's Distributed Active Archive Center offers NASA Mission to Planet Earth resources to study the greenhouse effect, El Nino's effect on weather and climate, the role of oceans in global climate and more. Provided are data, images, and documentation.

The Consortium for International Earth Science Information Network offers a gateway site, leading to additional sources of information pertinent to the study of global environmental change.

The National Oceanic and Atmospheric Administration maintains a National Geophysical Data Center, which leads to information on paleoclimatology, meteorology, ice and snow, and many other types of geophysical information.

Summaries of key climate change data sets, graphs, data tables, and so forth are available through "Trends Online," from the Carbon Dioxide Information Analysis Center (CDIAC) at Oak Ridge National Laboratory. This is referred to as the "single best source" of information on fossil fuels use, CO2 and temperature trends.

The Intergovernmental Panel on Climate Change (IPCC) is a consortium of over 2000 scientists and policy analysts from over 100 nations. This group, convened by the UN and the World Meteorological Organization (WMO), is charged with conducting state-of-the-science reviews of knowledge about climate change, and has produced a major report on the topic five times (1990, 1995, 2001, 2007, and 2013 -- the "Summary for Policy Makers" of the 2013 report is in your assigned reading in Course Documents). Their reports are highly anticipated and respected by the international climate change community.

An excellent site for information about renewable and conventional energy -- pros and cons, new developments, etc -- is sponsored by the U.S. Department of Energy's National Renewable Energy Laboratory (NREL).

The Union of Concerned Scientists maintains a fine informative site that includes information on many aspects of climate change, largely written with a lay (i.e., non-scientist) audience in mind.

In a rare digression from my attempts to be "strictly the facts" in my discussions with you, here is a link to a video that is very important.

To help you find what you are looking for in this topic, notes that follow are organized acording to the following outline. Click on the topic to move to that place in the notes. After you have looked at the topic, you can either continue scrolling through the subsequent topics, or return to the following outline to click on the topic that you want. You can use the box labelled "CONTENTS" to move to the master directory for the BI 301 home page. (For more information on navigating within and among these documents, click "Naviga te".)


I. What is the "greenhouse effect?"

ll. Carbon dioxide (CO2)

III. Global temperatures

IV. Other trace gases


Based on the position of Earth relative to the sun and characteristics of Earth's surface, the average temperature of Earth's surface should be -18o C (that's minus 18 o C). However, the mean temperature is closer to +15oC. Why this difference?

Earth and its atmosphere receive radiant energy from the sun. This radiation arrives in various wavelenths:

About 1/3 of this incoming radiation is reflected from Earth's surface or from clouds and particles in atmosphere, without being absorbed. The remaining 70% is absorbed by the atmosphere or by Earth's surface.

This absorbed energy is then ultimately re-emitted by the atmosphere and Earth's surface and organisms at longer, lower energy wavelengths, such as infrared wavelengths (longer wavelengths are less energetic). (Remember the law of thermodynamics that says that energy is degraded with each transfer?)

Some of the energy re-radiated from the atmosphere goes out to space, while some travels to Earth, where it is absorbed and, ultimately, radiated out again. Energy that is radiated out from Earth either:

(1) passes through the atmosphere and out to space (approximately 10% of the energy radiated from Earth), or,

(2) is absorbed by clouds, gases, and particles in the atmosphere, after which much is then returned to Earth as slightly longer wavelength (heat) radiation (approximately 90% of the energy radiated from Earth).

This energy then warms Earth. Without an atmosphere capable of trapping and re-radiating energy, the earth's surface would be below freezing (about - 18oC) rather than the current +15 o C (59 o F). Thus, Earth is warmed largely from trapping and re-radiation of heat -- infrared radiation -- by gases and particles in the atmosphere. This trapping and re-radiation of heat by gases in the atmosphere is called the "greenhouse effect."

The analogy to a greenhouse is loose, but is based on the fact that the glass in a greenhouse is transparent to incoming radiation, but blocks some of the outgoing, longer wavelength radiation, causing the greenhouse to be warmer than the outside air. The atmosphere acts similarly, in that it allows much of the incident shorter wavelength radiation in -- is relatively transparent to it -- but doesn't allow as much of the outgoing, longer wavelength radiation to pass on through. (In fact, much of the warming in a real greenhouse is simply caused by lack of mixing of air.)

The gases that are most active at trapping this radiant heat energy are referred to as "greenhouse gases" or "radiatively active gases."

Thus, Earth's temperature is not simply a function of our distance from the sun, but also of gases in our atmosphere.

The greenhouse effect, and humans' understanding of it, is not new. People have understood that Earth is warmed by greenhouse gases in the atmosphere for well over a century. For example, people have long known that water vapor is important as a greenhouse gas, and that one of the most important greenhouse gases in the atmosphere is carbon dioxide (CO2). The relationship between CO2 and global temperatures was pointed out way back in 1896 by a Swedish chemist named Arrhenius, who suggested that coal combustion in industrializing Europe might increase atmospheric concentrations of CO2 and result in global warming! (As we'll see, CO2 only comprises a bit under 0.04% of Earth's atmosphere (about 398 ppm [as of 2014]) but is very important in our energy balance.)

(Side note -- old versions of these notes indicated that CO2 constituted just a bit over 0.03% of Earth's atmosphere...incredible to see basic numbers like this changing over this kind of time scale...)


If the greenhouse effect is a natural phenomenon, why be concerned about it? The concern is about an acceleration of the greenhouse effect, caused by human-influenced emissions of radiatively important gases, including CO2. We often hear about it as though the greenhouse effect itself is a product of humans polluting the atmosphere, but that's not true; there was a greenhouse effect long before there were humans. The real concern is about an increase in atmospheric concentrations of radiatively active gases.

In the following sections, we'll look at several of these gases; their sources, effectiveness at trapping and re-radiating heat, and trends in atmospheric concentrations. We'll begin with CO2, and then we'll look at other trace gases, including water vapor, tropospheric ozone, methane, nitrous oxide, and halocarbons, and at the role of black soot.


Carbon dioxide (CO2 ) is one of the most important greenhouse gases (second only to water vapor), and is one of the gases of most concern. Its concentration in the atmosphere has increased greatly over recent decades, owing to human activities.

Before we discuss ways by which human activities can affect concentrations of CO2 in the atmosphere, we need to understand a bit about the natural cycling of carbon on earth and its atmosphere.

One important component of carbon cycling is the biological carbon cycle. I'm sure that you are familiar with this. It basically involves:

CO2 uptake by plants in the process of photosynthesis, with conversion of a fraction of that CO2 into organic material (sugars, initially) and

Release of CO2 by respiration of plants, animals, and microbes.

The biological carbon cycle involves a huge amount of carbon; more than 20 times the quantity that is released each year by fossil fuel combustion!

Until recently, the biological carbon cycle was usually balanced; as much was taken up by plants in photosynthesis as was given off in respiration and decomposition. That is, uptake of CO2 from the atmosphere was balanced by its release back to the atmosphere. Thus, there was no net gain or loss of atmospheric CO2, and the biomass on earth was roughly constant.

There have been times, historically, when biomass accumulated. One notable example is the Carboniferous era, when much of the biomass that later became our fossil fuels was deposited. This was a warm and damp period, during which uptake and storage of carbon from the atmosphere must have been greater than its release via decomposition and respiration. That is, there was a net increase in biomass (carbon storage) during this time. (This is, of course, what we are now rapidly returning to the atmosphere via combustion of fossil fuels....)

Currently, the biological carbon cycle in the broadest sense may not be balanced, because of human influences on it (see below for details). More CO2 may be being released via respiration, decomposition, and burning than is being taken up by plants. That is, we may be experiencing a net loss of biomass on Earth at present. However, this is not certain -- we don't know for sure to what extent deforestation is being balanced by aggrading forest area. Many scientists do believe that "anthropogenic land conversion" (basically, deforestation, and largely in the tropics) constitutes a large net input of CO2 to the atmosphere, but some suspect that aggrading forests elsewhere take up enough CO2 to balance a good bit of the CO2 given off in deforestation.

Deforestation does two things that affect CO2 concentrations in the atmosphere:

1. It removes a large sink for CO2 (a "sink" is just what it sounds like; an area or entity that removes more of something than it releases), and it
2. adds a large source of CO2 to the atmosphere (via burning after logging, or and decomposition)

Until fairly recently, most scientists thought that this land conversion would constitute a net addition of CO2 to the atmosphere, particularly because most tropical deforestation is not followed by regrowth of forest, but rather by conversion of former forest to pasture or agricultural land uses that do not take up and store as much carbon as do forests. However, some evidence now suggests that this anthropogenic land conversion may be at least partially offset by accelerated uptake (aggrading forests) in some areas of the world, particularly parts of North America and Europe (see below), as well as China, which has been conducting a vigorous reforestation and afforestation program in recent years.

Returning to our discussion of global carbon cycling in general, it is important to remember that this biological cycle is only a small component in a much larger and slower-behaving cycle in terms both of the total amount of carbon involved and time scales. The geochemical carbon cycle governs the transfer of carbon between rocks, atmoshpere, biosphere, and oceans over long (geological) time scales. (See article from Scientific American on the supplementary reading list for more information of this geochemical carbon cycle.) Briefly, the major pool of carbon on Earth is in sedimentary rocks, formed from past living creatures. Carbon is released from these rocks back into the atmosphere, but over very long time scales, through plate tectonic activity, volcanism, and so on. Thus, the geochemical cycle controls atmospheric CO2 concentrations over extremely long time windows. And, over the long term, the geochemical carbon cycle is generally balanced. As for the biological carbon cycle, there have been times of considerable fluctuation in carbon pools in the past. For example, ~100 million years ago, it is estimated that there was about 10 times more CO2 in the atmosphere than there is now, and it was much warmer during that time than it is at present. It is thought that there might have been tremendous volcanic activity going on then as the ancient continent of Pangea broke up, with resultant degassing of CO2 from geological pools into the atmosphere.

The point is; what is happening to concentrations of CO2 in today's atmosphere is likely to be just a small "blip" in the long term story of Earth. While the current blip is likely to be small, and short term, from a geological perspective, it has potential to cause serious consequences for humans and other ecosystem components, as we'll see. I simply think it useful for us to put our influences on the atmosphere into some perspective, and to be humbled a bit by the recognition that we may not be the "big guys" that we think we are!

Now let's look explicitly at the question: How do humans affect the carbon cycle?


What human activities affect atmospheric concentrations of CO2? Two kinds of activities are most important:

(1) Fossil fuel burning and cement manufacturing
(2) Changes in land use (largely, deforestation)

The CO2 concentration in the atmosphere has increased by ~ 40% since preindustrial times (~ 1860 or so). Back then, the concentration was about 280 ppm, whereas by now (2014) it is nearly 398 ppm! As mentioned above, CO2 concentrations in the atmosphere have varied widely over the geologic past, BUT past changes have been SLOW compared to the present-day rate of change.

Why this increase in concentration? Well, what are the human-influenced inputs or fluxes of carbon to the atmosphere?

(1) Burning of fossil fuels -- the biggest single human-influenced net source of CO2 to the atmosphere. This combustion oxidizes organic carbon, with carbon and oxygen combining to yield CO2. The major period of fossil fuel burning on Earth has, of course, been since the dawning of the industrial age in the mid- to late 1800's.

(2) Anthropogenic land conversion. Recall that deforestation is particularly relevant here.

I already mentioned that, historically, CO2 taken up in the biological carbon was approximately equaled by CO2 released in the biological cycle. Thus, historically:

GGP -- global gross primary production (the amount of carbon fixed by plants)
ER (global ecosystem respiration, comprised of respiration by plants (AR – for autotrophic respiration) plus respiration by all other living things on land (HR – for heterotrophic respiration))

There was no net flux of carbon to or from the atmosphere on a global basis, and there was no net change in carbon storage in terrestrial ecosystems (globally).

However, humans have recently been converting forested landscapes to grazed, cultivated, or urban landscapes. This:

(1) Removes a large sink for atmospheric carbon (because forests take up and store larger amts of carbon than do other terrestrial ecosystems), and

(2) Adds a large source for atmospheric carbon (when the trees decay or are burned, releasing carbon). (About 80% of the wood removed during tropical deforestation is destroyed (burned or decayed) or used as fuel wood, so the carbon stored in it is released rapidly as CO2, as opposed to the delayed slow release that accompanies its being used for lumber.)

A mature forest stores a large amount of carbon. When cut, it is often replaced by an ecosystem that stores less carbon, (pasture or crops) with the difference in carbon content of the ecosystems being balanced by a flux of CO2 to the atmosphere.

Thus, there is an increased flux of carbon from terrestrial ecosystems to the atmosphere, resulting from this land conversion. Estimates of the net input of CO2 to the atmosphere from ALC range from ~ 1/5 - 1/3 as much as from fossil fuel burning -- quite a range of estimates, indicating the range of unknowns. In its 2013 report, the Intergovernmental Panel on Climate Change (IPCC, a group of scientists and policy makers convened by the UN, which includes thousands of representative from around the world) estimated that ALC contributed about 10% of the anthropogenically-influenced inputs of CO2 into the atmosphere during the 2002-2011 interval, with fossil fuel combustion responsible for the remaining ~90%. Over a longer time window -- 1750 - 2011, it is estimated that about 66% of CO2 emissions were from fossil fuel combustion with 34% coming from ALC (IPCC 2013).

Most of this increased ALC flux now comes from tropical Africa and Asia, but until about 1920, North America actually provided the largest ALC flux to the atmosphere.

Why is there so much variation in estimates of the size of fluxes resulting from deforestation in the tropics? Several reasons, all relating to uncertainty in estimating the factors that contribute to the fluxes:

(1) What are actual rates of deforestation (and what counts as deforestation -- e.g., does selective logging count, or must it be clearcutting?)
(2) What is the fate of deforested land? Is it cleared permanently or temporarily, and what proportion is treated in each way?
(3) How big are the stocks of carbon stored in these forests? Estimates of the standing stock differ by a factor of two!

(4) How much CO2 does an intact forest take up and store, and how is that storage affected by age of the forest, precipitation regime, etc?

Contributions to CO2 fluxes from logging per se are generally considered to be minor or negligible. While net emissions of CO2 increase temporarily during and shortly after logging, re-growth usually compensates for that increase. Most important in terms of CO2 fluxes are actual conversions of forest lands to pasture, croplands, and degraded lands. None of these types of vegetation take up and store as much carbon as do forests.

While deforestation does certainly contribute to a CO2 flux to the atmosphere, there is recent evidence suggesting that this increased flux may be counterbalanced to an unknown extent by growth of forests (with consequent carbon uptake) in some places in the world. Places where forests may be taking up and storing enough carbon to counterbalance the increased flux from tropical deforestation include portions of Brazil, Europe, China, and North America. Biomass of forests in many of these regions has, apparently, increased over recent decades, with consequent storage of carbon.

In terms of sources and sinks, then, tropical deforestation does constitute a source of carbon to the atmosphere, but forests in other areas of the world might sinks for carbon that counter balance some of this source. (See the IPCC's report, referenced on the supplementary reading list. )

Why might biomass (and carbon storage) of some areas be increasing? There are several potential reasons, including changing land use (e.g., afforestation of areas that were formerly fields, pastures; drainage and conversion to forest of wetlands; and fire suppression, which allows woody vegetation to encroach on areas that weren't formerly occupied by woody vegetation), and possible effects of CO2 fertilization and/or nitrogen fertilization from atmospheric deposition. In addition, in some areas, humans are fertilizing forests deliberately, fostering their growth (are troubles from this fertilization likely down the road, as we are experiencing with fertilizer inputs in agricultural systems?) Currently (early 2000's), changes in land use are believed to be the dominant force behind increased carbon storage in forests, with contributions from CO2 fertilization or nitrogen fertilization believed to be relatively minor, although there is uncertainty about that too.

Soils also store much of the carbon found in the terrestrial compartment. In fact, more carbon is stored in soils (including peat) than in all of the vegetation of the world! Carbon storage in soils has also shrunk since preindustrial times. Respiration and decomposition increase with ALC, and these also increase under most kinds of agricultural systems, such that most soils now store less carbon than they did even a century ago.


Where does the added CO2 go, besides into the atmosphere? It is estimated that only about 50 % of the CO2 emissions actually stay in the atmosphere -- where do the rest go? The oceans represent a major sink for carbon. Oceans take CO2 up through chemical and biological means.

Chemically , ocean waters absorb CO2 by the formation of carbonic acid:

CO2 + H2O <-----> H2CO3

The double-headed arrow on this equation indicates that this is an equilibrium reaction. Hence, as CO2 in the atmosphere increases, more is taken up by the oceans, "pushing" the reaction towards formation of H2CO3 (carbonic acid). It is estimated that the oceans are now taking up more CO2 from the atmosphere than they have in the last 20 million years! The oceans have actually become more acidic over the past century as a consequence of this accelerated uptake of CO2, with possible consequences for much of the oceanic ecosystem (including creatures whose shells are made of calcium carbonate, which can be broken down when conditions are acidic enough. There are a couple of papers that address this concern on your supplementary reading list for this unit (and the IPCC report addresses it too).

Oceans also take up CO2 biologically, largely through photosynthesis of plankton and other algae. This "fixed" carbon is eventually removed from the water by biochemical processes (for example, the algae are eaten by shell fish, which die and sink to the ocean floor, eventually forming carbonates and entering the long term geochemical cycle that we discussed above.

The oceans hold 50-60 time more carbon in various forms than does the atmosphere, which holds it mostly as CO2. Some parts of the ocean are major sinks; such as the North Atlantic during the spring planton bloom (population explosion). On the other hand, some areas of the oceans are net sources, such as the equatorial Pacific. On balance, however, oceans are net sinks for carbon.

Can ocean uptake counter human-influenced net fluxes of CO2 into the atmsophere? The oceans are currently taking up more carbon than they are releasing, but we don't know how rapidly they can take it up in response to increases in atmospheric CO2 concentration. As we'll see, this is a big uncertain factor in attempts to model likely changes in atmospheric CO2 and climate!

There is still uncertainty in estimates of what fraction of extra CO2 emissions the oceans are really taking up, but the net influx into the oceans is probably ~ 30% of the total emissions. (Net influx means ocean uptake in excess of its giving CO2 back into the atmosphere, again by both biological and chemical means.)

So, we've got about half of emissions staying in the atmosphere (and increasing its concentration of CO2), and 15 - 30% going into the oceans.

Notice anything wrong here? Where's the rest of the CO2 going?? The remaining ~ 20% (or more) of total emissions represents the "missing carbon mystery!" What happens to this remaining ~ 20% (or more)? (See articles on your supplementary reading list for more on this mystery.)

No one really knows where that missing carbon is – perhaps component estimates are in error, perhaps it is being taken up by the aggrading forests that we discussed previously – no one knows yet!


These uncertainties aside, it is clear that humans are putting tremendous quantities of CO2 into the atmosphere, and that fossil fuel combustion is currently the most important contributor to that flux. While this is true at present, over the period 1860-1980, the global emission of CO2 from landscape changes approximately equaled that from fossil fuel burning. At present, however, fossil fuel burning is a more important source of CO2 to the atmosphere than is anthropogenic land conversion (ALC).

What are trends in CO2 emissions from fossil fuel combustion?

Not surprisingly, emissions increased rapidly over the last century (see the graph of these emissions on the graph in Course documents). A temporary decrease (about 7%) that occurred between 1979 - 1983 (recession-related) wasn't sustained, as emissions increased again 1883-91. Between 1990 and 1991, global CO2 emissions from fossil fuel burning did decrease again (about 0.2% /yr), largely because of changes in Eastern Europe and the former USSR. Those areas of the world experienced both increasing efficiency of fossil fuel use (owing in part to decontrol of prices and the closing of inefficient, uncompetitive industries) and decreased economic activity during this time.

These decreases in E. Europe and the USSR were big enough to offset increases in emissions from lesser developed countries (LDC's) temporarily. Hoever, global emissions have basically climbed over the last century, and continue to do so now in the 21st century. Most projections about future emissions are not cheery; emissions from the US alone increased by 14.7% from 1990 - 2006 (USEPA data)!!! However, US emissions actually decreased between 2005 - 2012, by about 9%! This decrease was probably related to the economic depression, but has also been caused by a shift from use of coal to increasing use of natural gas. Whether the trend of emissions decreases from the US will continue is anyone's guess. Global emissions of CO2 increased by about 6% between 2009 - 2010, and again by about 3% in each of 2011 and 2012. A good trend, however, has been displayed by nations that are participating in the Kyoto treaty, in that emissions from those nations in 2009 were nearly 15% lower than in 1990. Emissions from LDC's are likely to increase as population and industrialization also increase in those nations. Let's hope, however, that those trends don't continue.


Not surprisingly, the US contributes almost 20% of total world CO2 emissions. China was a clear second in terms of emissions until the 21st century, but her emissions by-passed those from the US in ~ 2009 -- and China's emissions are continuing to grow rapidly (huge coal reserves in China mean a huge push to bring new coal-fired electricity generating stations on line, with associated huge emissions of CO2) . In general, emissions are increasing from the LDCs and leveling (or even decreasing) from the more developed nations.

In per capita terms, the US is still a leader... -- although, as of 2004, we ranked 9th in the world, lagging behind several mideastern nations in particular. As of 2004, per capita emissions (tons of carbon per capita) for some selected nations were:
US 5.6; Canada 5.46; Russian Federation 2.89; Germany 2.67; Japan 2.69; Mainland China 1.005; India 0.34...You get the idea....


OK, given these emission trends, what is happening to concentrations of CO2 in the atmosphere?

There is no doubt that CO2 concentration in the atmosphere has been increasing over recent decades. We know this because this gas has been measured in the atmosphere for several decades. This is a nice example of the usefulness of having long term monitoring data – if CO2 concentrations hadn't been monitored for long, we could say little about whether increased emissions really were significant enough to increase atmospheric concentrations. Unfortunately, there are many phenomena in environmental science for which such long term data are lacking, crippling our ability to make policy decisions. The case of CO2 is NOT one of these, however!

In 1958, Dr. David Keeling established a site to monitor atmospheric CO2 at Mauna Loa Observatory in Hawaii. The figure that I gave you in class, based on monitoring from this site, is by now quite famous. CO2 concentrations have increased progressively since monitoring began there. The average rate of increase since 1958 has been about 0.4%/year, which is an absolute increase of about 1.5 ppmv per year. (Recall, gas concentrations in air are often measured in "ppmv" which is parts per million by volume.) Data from other monitoring stations around the world mirror the trends shown here -- in other words, this isn't just an Hawaiian phenomenon!

The annual rate of rise in concentration over the 1995 - 2005 period was faster than anytime since instrumental monitoring began (IPCC 2007), which is sad, since it is a time during which the potential dangers of climate change became more and more apparent....

The increase since the mid-nineteenth century (or preindustrial time) has, of course, been greater than this and is over 30% (from about 280 ppmv to the current ~ 397 ppmv). Current concentrations are the highest they have been in the past 800,000 years (Science, Nov. '05)! (We'll see in a bit how we know what atmospheric CO2 concentrations were that long ago!) In fact, it is likely that atmospheric CO2 concentrations haven't been > 380 ppm at any time in the past 10 Million years (Science 24 March 06).

Note the annual ups and downs in the CO2 concentrations displayed in the Mauna Loa data? These result from the net uptake of CO2 by vegetation during the growing season and its net release during less favorable seasons. That is, it essentially represents the biota breathing. I think that it is fascinating that this seasonal signal from the biota is big enough to show clearly as annual ups and downs in atmospheric concentrations! (It reflects, largely, seasons in the northern hemisphere, which has more land mass and vegetation than does the southern hemisphere.)

Note that, because the actual concentration of CO2 in the atmosphere is quite low ( for example, 390 ppm = 0.039 %), its concentration can be increased readily by additional inputs. That is, a relatively small input in absolute terms can mean a large change in terms of percentage.

CO2 persists for quite a long time in the atmosphere; its atmospheric residence time is on the order of decades to a centuries (30 - 800 years; Catalyst fall '07). About ~ 1/2 of the emissions that we put out today will be removed from the atmosphere within 30 years, while ~ 1/5 will persist for ~ 800 years.

This increase in atmospheric CO2 is predicted to cause an increase in global temperatures and in global precipitation; how much, how fast, and how distributed globally are the important questions. We'll discuss these questions and present understanding of answers later.

There are greenhouse gases other than CO2 whose concentrations are also increasing and will discuss them in a moment.


What evidence do we have that concentrations of CO2 in the atmosphere are really related to global temperatures? Evidence comes from a variety of sources, and this evidence reflects strong relationships between gas concentrations and temperatures over a wide range of time scales.

Evidence from the ancient past links timing of plate tectonic activity (which is associated with abundant volcanic activity and degassing of CO2) with climate changes, and evidence from the more recent past links atmospheric [CO2] with climatic fluctuations during the ice ages in the Pleistocene.

How do we know what CO2 concentrations in the atmosphere were thousands of years ago? One source of information is polar ice, which forms from snowfall accumulating over centuries. Annual layers are formed in this ice, and air bubbles in the ice trap gases from the time when the ice was formed. Concentrations of gases in these ancient air bubbles can be measured.

How about ancient temperatures? The ratios of various isotopes of oxygen (16 and 18, based on the number of neutrons per atom) and of various isotopes of hydrogen in the water comprising the ice depend on the temperatures prevailing at the time of its formation, hence analysis of these isotopic ratios gives insights into temperatures.

I showed you a figure in class representing fluctuations in CO2 and temperature over the past 160,000 years. This data set comes from a 2 km long (2000 m) ice cores from Antarctica (the Vostoc ice core). This 160,000 years encompasses our current interglacial period, the last 100,000 yr ice age, a previous interglacial, and an even earlier ice age.

We now have an even longer ice core record, which goes back ~ 650,000 years! This core is the "Dome C" core, and is also from Antarctica. It runs to a depth of 2 miles (miles, not kilometers!). Results on gas concentrations and temperatures from this core were published in a Nov 2005 issue of Science.

Both cores show clearly that atmospheric CO2 varied in parallel with temperatures; when temperatures were up, so was CO2. (Concentrations of two other radiatively active trace gases, methane and N2O also varied in parallel with CO2 – we'll talk about these gases later.) For example, CO2 concentrations fell in synch with temperatures at the onset of the last glacial period and then these rose together as the ice retreated about 10,000 yr ago.

Globally, temperatures were about 5 degrees C cooler during glacials than during interglacials, while concentrations of CO2 were about 25% lower during glacials than during preindustrial interglacials. (And, recall that we've now seen another ~ 30 % increase since preindustrial times...) (Note that data in the figure I gave in class are from the ice core area, so the difference in temperature between glacials and interglacials shown in the figure is greater than this global average. That is, temperature differences between glacials and interglacials are greater at high latitudes and less at equator -- as we'll see, the same pattern is being found in today's warming.)

The question raised by these data, of course, is which came first, the chicken or the egg?

That is, did temperature increases cause CO2 concentrations to increase, because of effects on biological processes and changes in ocean circulation or vice versa? Scientists still aren't sure, but it is likely that they are interactive.

Ice core data suggest that temperature changes "lead" gas changes -- that is, it seems likely that orbital forcing may initiate (trigger) the changes in climate, and then gases, changing in response to the change in temperature, amplify the temperature changes.

As we understand it today, ice ages [and coming out of them] are initiated by small changes in Earth's orbit (how circular or elliptical it is) and the tilt of its axis relative to the sun. What seem like small changes in the amount of sunlight reaching Earth at various latitudes and seasons drives these changes in temperature. These changes are enough to initiate or trigger the temperature changes, but are not enough by themselves to cause as large of temperature changes as are observed -- gas concentrations changing in response to temperature thus magnify the orbitally-induced temperature changes. It is estimated that orbital changes and changes in trace gas concentrations are each responsbile for about half of the glacial/interglacial climate changes.

For example, ocean uptake of gases such as CO2 increases as cooling occurs (remember gases have higher solubility in colder water). This pulls CO2 out of the atmosphere, and would serve to amplify the orbitally-induced cooling. As another example, the volume of oceans decreases with cooling (as more water is frozen), which results in nutrients in the water becoming more concentrated, and fostering algal blooms, which pull CO2 out of solution, again amplifying cooling. (These algal blooms also result in the production of sulfate [from DMS – dimethyl sulfide -- produced by the algae], which, as we'll see enhances cooling still further.)

However, bottom line is that it doesn't really matter which comes first; what matters is that we see that there is global climate sensitivity to radiative gases. The current best guess is that trace gas changes account for about half of the glacial/interglacial climate changes.

As Dr. James White, a geologist at UC Boulder said, "Co2 and climate are like two people handcuffed together. Where one goes, the other must follow. Leadership may change or they may march in step, but they are never free from each other."


I showed you a figure in class (figures are in Course Documents on BlackBoard) representing fluctuations in temperature and CO2 concentrations over the past 100 years. Data such as these are derived from a variety of sources, including:

· corals, which preserve the oxygen isotope ratio of the water (remember its temperature sensitivity) and can be dated

· glacial ice, whose rate of retreat can be calibrated to temperature (incidentally, all measured glaciers in various parts of the world, including those in the Cascades, are retreating)

· CO2 trapped in layered lake sediments

· carbon isotopes in tree rings, which can, of course, be dated

As illustrated on the figure I gave you in class, during the last 100 years, atmospheric CO2 increased another ~ 30% above its interglacial level, and concentrations of methane doubled again too.


Data on temperatures from a variety of recording stations and instruments give us reasonable surface air temperature data for the past 100 years or so. After applying corrections for urban heat, and other factors, we can see that recent decades have been unusually warm. (See figures from class.)

I used to say that the 1980's were the warmest decade on record (since instrumental monitoring began) but the 1990's beat the 80's and now the first decade of the 2000's beat that.

See this link for an amazing 30 second video produced by NASA that shows global temperature patterns over the past 131 years -- a picture is worth more than 1,000 words in this case!

El Nino was responsible (partly) for some of the recent extra-warm years, such as 1998, but even after correcting for the influence of these events (El Nino's) the 90's remain the warmest decade since instrumental monitoring of temperatures began.

Eleven of 12 recent years (1995 - 2006) are among the warmest since instrumental monitoring began in ~ 1850 (IPCC 2007), -- global temperatures in 2010 tied those in 2005 as the warmest on record -- and the five hottest years on record since 1880 have all occurred since 1998 (Vital Signs 06-07). The present warmth is unprecedented in at least the past 1300 years (IPCC 2007). This litany continues in the 2013 IPCC report -- it indicates that each of the last three decades has been successively warmer at Earth's surface than any preceeding decade since instrumental monitoring began in ~ 1850. In the US, 2012 was the hottest year on record -- it broke the 1998 record by a full degree, which is unprecendented; temperature records are normally broken by tenths of degrees. (Temperatures in 2012 were aggrevated by drought and by a La Nina.)

El Nino is a situation that occurs when a pool of warm water that is usually centered in the western Pacific expands to the East, usually by December. This tropical warmth displaces the jet streams that then steer unusual weather into various regions of the world, typically warming the northern US. Here's what happens:

Normally, tropical trade winds blow E to W across the equatorial Pacific, dragging water along and dumping evaporated water as monsoons over Indonesia. The surface water moves W as well, and near the equator, gets diverted poleward by Earth's rotation. The divergent flow (that is, net movement of water from E to W) causes upwelling of deeper cooler water, especially in the E Pacific (because surface water is being blown away from there) where the thermocline is shallower. As deeper cooler water comes up, it brings nutrients from beneath that feed productive food chains off Peru and Chile. During an El Nino, the trade winds weaken, warm water stays in the E Pacific, monsoons fall over the ocean instead of over SE Asia, the thermocline along the coast of Chile and Peru flattens and deepens, and marine life declines as nutrients that support it fail to be recharged by upwelling.

El Nino's used to come every 7-8 years, but during the 1990's, there were several in a row. No one is sure if global warming is related to this increased frequency and duration of El Ninos or not, but global warming effects are likely to intensify the effects of weather extremes associated with El Ninos.


Some apparent oddities show up in this relatively recent temperature record. For example, 1992 and 1993 were cooler than adjacent years (after correcting for 1992 being an El Nino year), probably because of Mt. Pinatubo's eruption in June of 1991. What should a volcano have to do with global temperatures?

A volcano injects large quantities of SO2 into the atmosphere, much of which is converted to SO4 (sulfate) in the stratosphere. Much of the sulfate is present as very small, aerosol-sized particles, which cause cooling by reflecting incoming radiation. The sulfate particles also serve as condensation nuclei for high thin clouds, which also increase the albedo (reflectance, essentially) of the stratosphere. The aerosol sizes are so small that they are more effective at reflecting shortwave solar radiation than they are at attenuating longer wavelengths of radiation emitted from Earth.

Climate modelers predicted that the eruption of Mt. Pinatubo would decrease global mean temperatures by about 0.5 degrees C -- enough to temporarily mask warming -- and this did occur between the summers of 1991 and 1992 (briefly impeded by an El Nino early in 1992). Most of the particles resulting from the eruption then precipitated out of the atmosphere, so by 1993, global temperatures were back on an upswing. Climate modelers were pleased that global temperatures responded as predicted to the eruption of this volcano, as the correspondence between model predictions and actual events serves to validate the models; they accurately predicted these climate consequences.

The warming over recent decades is uneven over the globe. It tends to be greatest over high and mid- latitude continental areas (40 - 70 degrees N). For example, summer temperatures in N Siberia have recently been hotter than any time in the past millenium. The Arctic region has warmed at more than twice the global rate in the last 50 years.

In 2013, the IPCC's summary indicated that the global average temperature has inceased 0.85 degrees C (~ 1.5 degrees F) over the 1880 - 2012 interval, an increase that is unprecedented during the last 1000 years. Remember that these data reflect global means; warming has been more in some places and less in others (e.g., average Arctic temperatures have increased at about twice the global average rate over the past century [IPCC 2007, 2013]).

The linear warming trend over the past 50 years is nearly twice that for the past 100 years overall -- that is, warming seems to be accelerating (IPCC 2007).

Even if one ignores the instrumental record, the evidence for warming over the past 60 years is unequivocal. We have observed decreases in cover by alpine glaciers, increased rates of permafrost melting, decreases in Arctic sea ice thickness, later freezing and earlier thawing of ice on lakes and rivers, and increased calving of Antarctic ice sheets. (We'll talk in more detail about all of these later.)


This is, of course, the "$64,000 question!" It is critical to know whether this is really unnatural warming. That is, are we outside of the normal range of variability in temperatures? In the summer of 1988, NASA scientist James Hansen testified to the US congress that "greenhouse warming is here." At that time, he was attacked by many others as being premature in his claim. We still don't know for sure whether the warming trend is unnatural. There is much natural variability in the climate system, probably related to changes in ocean circulation, perhaps to variations in the sun's intensity, volcanic activity, etc.

To determine whether warming is unusual and potentially attributable to human activity, what do we need to know?

(1) What natural climate variability is like over relevant time scales.

(2) Whether human influences on radiative gases could be enough to cause the observed warming. To address this, scientists use large computer models that we'll say more about later.

(3) What is the "signal to noise ratio." That is, how large is the trend compared to the background variability in temperatures. A variety of factors influence global temperatures -- some reviewed above, some related to solar activity, and many other forces. To assess this, scientists run computations that include all of the known natural factors that affect temperatures, and then test whether the ratio exceeds some statistical detection threshold, using a variety of statistical techniques ("fingerprint methods.") That is, how much unexplained variation (or trend) is in the temperature record, after accounting for known natural forces -- and then, of course, question 2 is addressed to answer whether human influences could account for the residual variation in temperatures.

In the past, I included lots of detailed notes on each of these items here, but as the consensus about human influence on global temperatures has strengthened, I've deleted those. Please see me if you are interested, though, as I have the notes and related articles. See also some of the web sites that I give links to at the beginning of this section of notes (e.g., the Union of Concerned Scientists site and the OSU Global Environmental Change Organization sites are particularly helpful for these matters).

There is, by now, almost no doubt about the fact that global temperatures are rising in large measure because of an accumulation of greenhouse gases into the atmosphere -- an accumulation that is caused by human activity.

One can trace the increasing certainty about this assertion by tracing language used in successive IPCC reports. Recall that the Intergovernmental Panel on Climate Change [IPCC] is a consortium of over 2500 scientists from nations all over the world, which is convened periodically by the UN and the World Meteorological Organization. The scientists review all of the relevant published literature on topics related to climate, and then publish, about every 5 years, a major synthesis report. That is, they do not carry out original research themselves, but work with existing, peer-reviewed literature.

In their 1990 report, they stated, "...observed warming is broadly consistent with predictions of climate models, but it is also of the same magnitude as natural climate variability....the observed increase could be largely due to this natural variability..."

In their 1995 report, they concluded: "The balance of evidence suggests a discernible human influence on global climate."

Their 2001 report strengthened this language, making it clear that little doubt remained about the fact that the current warming has been influenced by human-related emissions of greenhouse gases. They wrote, "There is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities."

In 2007, they wrote, "Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations." (Note -- they define "very likely" as having probability > 0.9, and "anthropogenic greenhouse gas" excludes water vapor.)

In2013, their report swtated, "It is extremely likely [probability 95 - 100%] that human influence has been the dominant cause of the observed warming since the early 20th century."

Most nations that attended the Kyoto conference on global climate in December of 1998 took the position that humans are already influencing the climate – and that that influence is almost certain to increase in the future unless the international community takes MAJOR steps to decrease this influence -- that is, to lower emissions of greenhouse gases.

(We'll say more about it later, but the purpose of the Kyoto conference was that parties to the 1992 Rio framework convention on climate change, which was ratified by the US and more than 120 other nations, would meet again to put "teeth" into the Framework Convention. The Framework simply called for stabilization of greenhouse gas concentrations at levels that will protect human interests and nature, but it did not target particular concentrations, nor was it binding. At Kyoto, the parties to the convention met to try to prepare a clear protocol for implementing the convention (that is, move beyond a general agreement to stabilize, to agreement about what levels to stabilize at and a timeframe.) There have been additional meetings since then, and, as of 2008, the US has continued to refuse to ratify the treaty.).

As you can see, in each of its five year reports, the IPCC increased its certainty that humans are affecting the global climate. Its earlier caution about separating natural climate variability from human influence came about because of some of the following "anomalies" in the temperature record and other concerns:

(1) We would expect a steady warming from the fairly steady increase in greenhouse gases, but have seen instead (please refer to the figure from class that shows recent links between CO2 and temperature):

(a) A period of rapid warming until the end of World War II (this is now is largely believed to have resulted from natural causes, including higher solar output and many years with low amounts of volcanic activity overlaid on a gradually increasing load of greenhouse gases in the atmosphere.

(b) slight cooling through the mid 1970's (maybe related to a lot of volcanic activity then, especially in the Northern Hemisphere, with volcanic aerosols [and human emissions, such as sulfate]). Others say that Earth was in a natural cooling time, and that the cooling during this time would have been much greater without counteracting greenhouse gases.

(c) A second period of rapid warming since the mid-1970's, which, in the opinion of most scientists, can't be accounted for soley by natural phenomena.

(2) Computer models suggested that there should have been warming, given the increases in atmospheric concentrations of CO2 and other radiatively active trace gases. However, early models predicted more warming than we have had, given the increase in radiatively active gases.

However, as understanding of factors regulating climate has improved, so have models, and so has the correspondence between model predictions and reality. In particular, modelers : (a) Realized that early models underestimated the ability of oceans to take up atmospheric warming (that is, they credited the oceans with more thermal inertia than they have). (b) Recognized that early models didn't account well for the involvement of other pollutants (such as SO4 aerosols), which slow warming basically by reflecting incident radiation away from Earth and by modifying the shortwave-reflective properties of clouds. For example, SO4 in clouds increases their albedo (provides a greater abundance of very small droplets high in the atmosphere, which can mean reduced warming). Once these (and other) effects were included in the models, predictions of temperature change attributable to greenhouse gases match much more closely the observed changes

Thus, the general consensus in the scientific community has changed since 1990 to the view that warming is already being detected, that human activities are important drivers of it, and that it is sure to increase in the future as emissions continue to increase.

As another example, the American Geophysical Union (comprised of over 35,000 international earth and planetary scientists) issued a statement in 1999: "There is no known geologic precedent for the transfer of carbon from the Earth's crust to the atmoshpere in amounts comparable to fossil fuel burning without simultaneous change in the climate system." "There is a compelling basis for legitimate public concern." and "The present level of scientific uncertainty does not justify inaction in the mitigation of human-induced climate change."

It is easy to see why some people have a hard time accepting that we are in a human-induced warming period. Natural causes of climate variability can, in some years and places, mask the underlying trend -- so, for example, if there is an unusually cool winter in some part of the US, those who would like to argue against human-caused climate change tend to say, "See, we're not warming up! Temperatures this week broke records for cold!" The fallacy results from confusing changes over small time scales with longer term trends -- AND failure to recognize that natural causes of climate variability are always at work. However, such natural sources of variability can mask the underlying trend for only a short time -- a temporary mask. (See Science 2 May 08 and 5 Jan. 07 for useful related articles.)

Even for those who are reluctant to agree that we are already experiencing the predicted warming, there is little doubt that eventually warming would occur. The open questions are how much and how fast, and how drastically emissions of radiatively active trace gases will need to be reduced to prevent further change.

I will mention briefly a concern about what might happen to temperatures at least in some parts of the Northern Hemisphere that has emerged fairly recently (early 2000's, basically). This is that global warming, once in place, could trigger a sudden change (a shut down, basically) in an ocean current that warms Northern latitudes (thermohaline circulation, also called meridional overturning circulation). The North Atlantic Current, a branch of the Gulf Stream, carries warm equatorial waters north in the Atlantic, which warms the nearby land masses (particularly Northern Europe). As the water moves north, it cools and becomes saltier (from evaporation), becomes denser, sinks, and returns south. The concern is that, if the circulation stopped, we could see sudden and fairly dramatic cooling of Northern Europe and the NE US and Canada, as this ocean "conveyor belt" that warms these areas stopped. This has apparently happened in the past in response to dramatic climate changes. From the past, there are records of as much as 10 degree C temperature swings in just a few years in regions, such as Greenland, that are affected by this ocean current. This shut down has apparently been caused by pulses of fresh water coming into the N Atlantic from melting glaciers and ice caps, and from the increased precipitation associated with the previous warming. (These waters are less dense than salt water, and are also warmer than the cooled ocean water, so could prevent the sinking of water that fosters movement of the "conveyor belt.") Whether this kind of abrupt change may be on the horizon is still being debated -- and the phenomenon was popularized -- and sensationalized -- in the movie, "Day After Tomorrow." Certainly there are pulses of fresh water entering the N Atlantic from melting ice! . In any case, phenomena such this serve to remind us that the global climate system is probably full of surprises! (In their 2007 and 2013 reports, the IPCC concluded that it is likely that some slowing of this circulation is likely but that it is very unlikely that a large change will occur -- and further, that any cooling induced by its slowing would be more than offset by warming caused by the increased atmospheric burden of greenhouse gases.) There is an excellent review article on this topic on the supplementary reading list (see Seager 2006).


I mentioned above that other trace gases (in addition to CO2) are important in causing the greenhouse effect.

Their importance should not be underestimated: other trace gases collectively contribute almost as much warming as does CO2! (This excludes water vapor, which, by itself, contributes more warming potential than is contributed by the sum of CO2 plus O3, CH4, N2O, and halocarbons.) Furthermore, the concentration of many of them has been increasing as rapidly or more rapidly than that of CO2 recently (which has been increasing at about 0.4%/yr).

The radiative forcing of a gas depends on its ability to absorb infra red radiation ("heat" radiation), which is determined in part by "undamental spectroscopic properties of the molecule. In addition, the contribution of a given gas depends on what wavelength is absorbs in -- to be really effective, it must absorb in a wavelength where the atmosphere is not already strongly absorbing. The contribution of a given gas also depends, of course, on its atmospheric concentrations, which in turn depends on emissions of that gas and its atmospheric residence time. CO2 currently dominates radiative forcing largely because the increase in its concentration has been so large (Science 30 March 07).

Water as a Trace Greenhouse Gas

WATER VAPOR is an important trace gas, which all by itself probably contributes on the order of 60% of the global warming potential contributed by all gases (Science 5 Dec. 03). It absorbs radiation of about the same wave length as CO2. (When I say above that other trace gases contribute almost as much warming potential as does CO2, I am excluding water vapor.)

It is not clear if human activities are having any net global effect -- yet -- on the concentration of water vapor in the atmosphere, hence controls on water vapor aren't being discussed at present in negotiations about controlling emissions of greenhouse gases. However, if we have significant warming, will have more evaporation and hence more water vapor in the atmosphere. Whether this will amplify or dampen warming is unclear, as the effects of water vapor in the atmosphere depend on the droplet sizes and their height in the atmosphere. The IPCC's 2013 report, however, indicates that water vapor is likely to have a net warming effect.

Tropospheric Ozone as a Trace Greenhouse Gas

TROPOSPHERIC OZONE is another greenhouse gas. Of course, human activities are affecting its concentration in some areas of the world, as we have already discussed. Increases in its concentration may contribute to warming. This is particularly so since one mole of ozone contributes 2000 times as much warming potential as one mole of CO2. This is because it absorbs radiation of different wavelengths than CO2; essentially it "seals in a crack" around the window of wavelengths absorbed by CO2. (Remember, of course, that we need ozone in the stratosphere -- there it protects us against uv-b radiation, and doesn't act as a greenhouse gas!)

In contrast to most other greenhouse gases, however, ozone has a short (60 hr) atmospheric residence time, which means that it doesn't accumulate in the atmosphere to the same extent that they do -- and also means that its concentration in the atmosphere will resond rapidly to changes in its production!

It is difficult to assess its concentration globally, and it is probably variable hemisphere to hemisphere. Best estimates are that radiative forcing from it is about 15% of that contributed by the longer-lived radiative gases.

Methane as a Trace Greenhouse Gas


Methane (CH4) is another very important greenhouse gas. While it is present in lower concentrations in the atmosphere than CO2, it is very effective at causing warming because absorbs radiation of a different wavelength than CO2. Like ozone, it fills in a crack around the CO2 absorption window. Methane is about 25 - 30 times more effective at causing warming than is CO2, mole for mole, largely for this reason. Methane currently contributes a bit over half as much warming as CO2 does (IPCC 2013; see also Science 30 March 07 for a higher estimate, which takes into account the fact that methane contributes to production of CO2, O3 and water vapor in the atmosphere, which add to its total radiative forcing...).

The atmospheric concentration of methane has been increasing for about the past 300 years, and over geological time its concentration has tended to vary in parallel with CO2's (and hence with temperatures). Its recent increase began before the recent rapid increase in CO2, beginning about 300 years ago, as compared to about 100 years ago for CO2. Its concentration has increased more than 100% in the last 100 years (that is, has more than doubled -- actually increased ~ 145% since preindustrial times). In contrast, concentrations of CO2 "only" increased a bit more than 40% since preindustrial times. The average rate of increase in methane over the 1984-1994 decade was about 0.6% per year or 10 ppbv, but its rate of increase slowed beginning in the mid-to late 1990's for reasons that are not understood (Science 30 March 07). Rates of increase in atmospheric concentrations of methane have, however, increased rapidly in recent years.

As is true for CO2, atmospheric concentrations of methane are now higher than they have been in the last 800,000 years (IPCC 2007). Atmospheric concentrations of methane in 2008 were about 1,800 ppb compared to a range of 400 - 700 ppb over that long time period.

Sources of methane:

About 80% of atmospheric methane has biological sources. Note, however, that biological doesn't necessarily mean "natural," as humans have affected many of the biological sources. In fact, anthropogenically-related sources contribute about twice as much as natural sources (340 vs 160 tg/yr). Methane is produced by (listed in order from largest to smallest):

Note that all of these sources are linked to the rising human population and to agriculture! How?

Methane has direct warming effects on its own, and it also contributes to the production of CO2, ozone, and water vapor in the atmosphere, which contribute about as much warming as the methane itself.

Methane has approximately a 10 year atmospheric residence time, which is shorter than that of CO2 (which is about 30 - 800 years) or halocarbons, which are also about 100 years (see below).

Nitrous Oxide as a Trace Greenhouse Gas


Nitrous oxide has mostly "natural" sources, which contribute about twice the anthropogenic sources. It is produced by bacterial action as part of the nitrogen cycle. It is produced by aerobic nitrification, in which NH4 is oxidized to NO2, releasing N2O along the way, and also by anaerobic denitrification, in which NO3 and NO2 are reduced to molecular nitrogen, and by other bacterial transformations. (Note that this is not one of the "NOx" compounds that we discussed relative to tropospheric O3 production.) IPCC [2007] estimated that natural sources are about twice as large as anthropogenic sources.

However, anthropogenically related emissions are increasing, and, as for methane, many are connected to human population and agriculture:

Its concentration in the atmosphere has increased about 12% since preindustrial times (275 up to 312 ppbv).

Halocarbons as Trace Greenhouse Gases

HALOCARBONS (chlorofluorocarbons and HCFC's -- and we'll lump in other heavily fluorinated gases too, such as the CFC substitutes, HFC's , and also SF6 and perfluorocarbons, such as CF4, derived from smelting and electronics industries).

The only source of the CFC's, HFC's and HCFC's is anthropogenic, as they are not naturally occurring. We discussed these when we talked about stratospheric ozone depletion. Recal: these are synthetic chemicals. They are halogenated carbon compounds, such as CFC11 (CFCl3 or Freon) They all contain carbon and halogens, such as Cl (chlorine), F (fluorine), or Br (bromine), and, in the case of the HCFC's, they also contain H (hydrogen). They were until recently, used in refrigeration, aerosols, for puffing foams, as solvents for cleaning in the electronics industry, and in automobile air conditioners.

In addition to their effects on stratospheric ozone, these are important greenhouse gases. They are tremendously effective at producing warming because, even though they are present in low concentrations in the atmosphere, they absorb heat radiation of different wavelength than CO2. They are 12,000 - 15,000 times more effective at causing warming than is CO2, mole for mole.

Their concentrations in the atmosphere have been monitored since the late 1970's and they increased steadily and rapidly over most of that time; at rates of 3-5% per year.

However, both production and emissions of the CFC's fell precipitously from 1989 on, as result of international treaties intended to halt destruction of stratospheric ozone,and now their concentrations in the atmosphere are actually beginning to decline as well. The decline in atmospheric concentrations lagged greatly behind the decline in emissions, because these compounds are very long-lived (atmospheric residence times on the order of 75 - 120 years).

Replacements for CFC's (largely hydrochlorofluorocarbons (HCFC's) and hydrofluorocarbons – (HFC's)) are also greenhouse gases, but are expected to make a relatively small contribution to the global warming potential contributed by other greenhouse gases. Current models suggest that warming due to all halocarbons (CHC's , halons, and their replacements) will be at most 4-10% of the total expected greenhouse warming by 2100 -- and, gladly, as we saw earlier, they are on their way out!


More attention is now being given to the warming potential contributed by "black soot," (also referred to as "black carbon"), which is produced from biomass burning, coal burning, and combustion of diesel fuel. These small particles, in contrast to SO4 (sulfate) aerosols, which reflect incoming radiation and so act as cooling agents, absorb incoming sunlight and warm up, radiating that heat to Earth. They also absorb outgoing light reflected from Earth's surfaces and clouds and reradiate some of that energy as warmth. A couple of papers from the year 2008 express concern that the IPCC, in its 2007 report, "badly underestimated" the warming potential of this black carbon (Science March '08, reporting on a March '08 paper from Nature Geoscience). These claim that it may, in fact, be the second most important atmospheric warming agent, behind only CO2 (again, ignoring water vapor...).

Not only does black soot have these warming effects in the atmosphere, but it also settles on snow and ice and darkens them. This reduces their albedo (reflectance), so warms them -- and accelerates the melting of ice.

Black soot lasts for only about 1 week in the atmosphere, so it should be quickly responsive to efforts to reduce its emissions.

(While you might think that black soot would cool the atmosphere by blocking incoming radiation -- like a shade -- but it blocks that radiation by absorbing it and then radiates that as heat, so is in fact warming...)


· There are other greenhouse gases besides CO2,

· Collectively they are adding almost as much warming as CO2 (more, collectively, if estimates above about black soot are correct!)

· All are increasing under human influence

· Many are involved in more than one environmental problem (for example, tropospheric ozone causes problems in its own right and also contributes to excess warming; CFC's, deplete stratospheric ozone and also contribute to warming

Models of effects of gases on climate must take all of these gases into account. This is a great challenge, as they have complex atmospheric chemistry, it is challenging to predict trends in their production, and there are complex feedbacks and interactions among them. Policy decisions must also take this variety of gases into account. Thus, it is a mistake to think that the prospect of global climate change is reducible to CO2 alone.

To move to notes on predictions for climate in the future, click "predictions;" to move to implications for ecological systems, click "ecology;" and to move to implications for humans and policy steps that are being taken, click "policy."

This page is maintained by Patricia S. Muir at Oregon State University. Page last updated PARTIALLY!! March 2014.