This section of notes includes the following topics. You can click on any of the highlighted words or phrases to jump to that portion of notes.
1. What trends are atmospheric concentrations of greenhouse gases likely to follow?
2. What climatic consequences are likely?
3. What ecological effects might be anticipated from climate changes?
4. What effects might climate change have on human systems?
5. What policy steps can (or should) be taken?
General background information on the "greenhouse" effect and the gases involved, as well as a list of links to other sites that contain information about global climate change, is in another section of these notes, and you can jump to the contents for that section by clicking on "greenhouse".
TRENDS IN ATMOSPHERIC CONCENTRATIONS OF GREENHOUSE GASES?
To make predictions about changes in atmospheric concentrations of greenhouse gases, we need to know two basic things: What trends are emissions of these gases likely to follow? How will concentrations in the atmosphere change as a result of a given set of emissions?
What are the likely trends in emissions?
Trends in emissions of CO2 will depend on trends in
(1) fossil fuel combustion and
(2) land use, particularly deforestation and reforestation.
These, in turn, will be influenced by the outcome of international treaty negotiations, what happens with the rate of population growth and rates of deforestation, how quickly we develop and bring on line alternative sources of energy, and other factors,
It is likely that the global distribution of sources will change as lesser developed nations undergo development and as most of the more developed nations get serious about conservation and the development of alternative energies. It is likely that emissions from most of the more developed countries will begin to decrease soon, while those from less developed countries are likely to increase. Such changes have already been seen. For example, in 1950 the US was responsible for over 50% of global emissions of CO2, while lesser developed nations (LDC's) contributed about 7%. By 2011 the US and China were each producing nearly 20% of global emissions, and China edged "ahead" of the US. China's emissions increased by 8% in 2007 alone! (Her per capita emissions are much lower than ours, of course!)
Between 1990 and 1998, the following trends in fossil-fuel related CO2 emissions occurred:
US increase by 10.3% (1990 - 2003, the increase was about 16%)
China increase by 28% (1990 to 2003, the increase was close to 47%...........)
India increase by 55%
European Union increase by 0.7%
Japan increase by 5.6%
Russia decrease by 24% (largely because of economic difficulties)
World increase by 6.3%
Collectively, global emission of all greenhouse gases combined increased by 70% from 1970 - 2004 (IPCC 2007)….terrifying, when you consider the trajectory that we should be on if we hope to stabilize climate...
(Incidentally, 15-25% of US CO2 emissions are from transportation, and collectively the US transportation sector alone accounts for about 5% of all global CO2 emissions!! )
There is much uncertainty in trying to project what will happen with fossil fuel use and CO2 emissions in the future, of course. In part, it depends on whether signatories to the Kyoto treaty meet their commitments to decrease emissions -- and, of course, on what the US does! As noted above, emissions from LDC's are likely to increase, as they undergo continued economic development. The IPCC (Intergovernmental Panel on Climate Change), in making its predictions for future climate (see below), deals with these uncertainties by making a range of projections from continuing present rates of increase to basically holding emissions constant at year 2000 values and intermediate scenarios. None of their scenarios assume full implementation of Kyoto. (See assigned reading from IPCC's 2007 summary in course documents on the Blackboard site.)
As you can imagine, projections concerning trends in emissions of other trace gases are very difficult as well.
CH4 sources are not well understood and may be hard to control (capturing "waste" emissions, as is done at the Corvallis landfill promises some relief as do changed diets of ruminants
O3 emissions trends will depend on what happens with emissions of precursor pollutants globally, as well as what happens with tropical deforestation and burning
are on their way out already, but time lags before concentrations decrease are great. In addition, some of the main current substitutes for CFC's (such as HCFC's and HFC's) are also potent greenhouse gases
N2O emissions are very hard to control (but decreasing N fertilizer, deforestation and fossil fuel use will help)
emissions reductions will also be hard to achieve...
Predictions of climatic effects are often based largely on CO2, because its trends are better understood than are those of other radiatively active gases, recognizing that it (ignoring the influence of black soot) is only about 1/2 the story. Most of the big computer models that attempt to simulate and predict global climate ("GCM's" -- global climate models (or general circulation models) are based on "CO2-equivalents," which are the concentration of CO2 that would give the same amount of radiative forcing as the given mixture of CO2 and other greenhouse gases. Essentially, they sum the radiative forcings of all trace gases and treat the total forcing as if it comes from an "equivalent" CO2 concentration.
HOW FAST WILL ATMOSPHERIC CONCENTRATIONS INCREASE IN RESPONSE TO INCREASING EMISSIONS?
There is considerable uncertainty here, as we'll see. However, model projections basically assume that same processes that remove these gases from the atmosphere as operate now will continue to operate. For example, they assume that the same fraction of CO2 emissions will remain in the atmosphere as is currently the case (roughly, 50%). However, we don't fully understand all of the processes that influence how much of each gas stays in the atmosphere at present, much less whether those processes will continue to operate.
FEEDBACK PROCESSES may become very important here, and are poorly understood. As CO2 emissions increase, atmospheric concentrations are likely to reflect that increase to some extent, at least initially (as it has been doing). However, at some point feedbacks may become very important, and some of these would be positive feedbacks while others would be negative feedbacks. A few examples will illustrate the point, although there are many that could be given!
1. The rate of uptake of CO2 by vegetation may increase ("CO2 fertilization"):
If photosynthesis is stimulated by increased atmospheric concentrations of CO2, then plants will tend to decrease atmospheric CO2 concentrations, pulling more out of the atmosphere. (You could model this for yourself: Make two boxes, one representing vegetation uptake of CO2 and one representing atmospheric CO2 concentrations. Put a negative sign on the arrow that represents effect of vegetation uptake on atmospheric CO2 (as uptake goes up, concentration goes down). Put a positive sign on the arrow from atmospheric concentration to plant uptake (as atmospheric concentration increases, plant uptake increases). The system would then be a negative, or stabilizing feedback.)
This feedback could be especially important if it increased storage of carbon in trees, as that is relatively long term storage. There is some (slim, in Pat's view...) evidence that this might be occurring -- some of the "missing" CO2 in the global budget may be fertilizing additional plant growth. The seasonal amplitude of atmospheric CO2 cycles, as we saw for Mauna Loa in Hawaii, has been increasing since the mid-1960's, perhaps suggesting a larger total biosphere.
However, recent evidence suggests that, while CO2 uptake by vegetation may increase, at least initially, under enriched CO2 conditions, the overall cycling rate for that carbon also increases (respiration and decomposition increase as well). That is, the carbon may simply be being passed more rapidly through the system, without then having a net dampening effect on atmospheric concentrations. Under elevated CO2, plants seem to put increased C into fast cycling parts like fine roots and leaves-and into nonstructural carbohydrates in those -- not so much into wood [relatively long term storage]. Further, they often seem to acclimate after a while in high CO2.
The rate of uptake by vegetation may decrease:
Alternatively, and probably equally likely, plants may be stressed under conditions of changed climate, particularly in areas where it becomes drier as well as warmer. In this case, the rate of uptake of CO2 by vegetation would decrease with increasing atmospheric CO2, rather than increasing. (The sign on the arrow from the vegetation uptake compartment to the atmospheric concentration compartment would remain negative (if uptake decreased, concentrations would increase) BUT the sign on the arrow running from atmospheric concentration to vegetation uptake would be negative as well, in this case (as concentration increased, uptake would decrease.) Two negative signs mean positive, or destabilizing feedback.
As an example, studies indicate that growth rates of trees in Costa Rican rain forests decrease and their release of CO2 increases as temperature increases….Such forests might not be as big a sink for future CO2 as thought (might even become a source??). For example, in the unusually hot El Nino year 1997-98, tree growth rates at LaSelva, in Costa Rica, were very low…Similar evidence comes from forests in Panama and Malaysia -- and from rice in the tropics; productivity decreases with increasing temperatures. These could be responses to warmer temperatures being associated with drying, or other associated changes, of course. Nevertheless, the findings call into question the common dogma that tropical forests are likely to be big sinks for CO2 in the future….(BioScience July/Aug 2007).
3. The rate of uptake (or retention) of CO2 by oceans may change:
If the oceans warm, as projected (and as is already being observed), they will begin to de-gas CO2 into the atmosphere because warmer waters can't hold as much gas as can colder waters. In this case, as CO2 increased in the atmosphere (and temperatures warmed), the oceans would amplify that increase in CO2 concentrations (rather than damping it through net uptake of CO2 as is presently the case). Since the oceans hold about 50 times more carbon than is held in the atmosphere, a change that alters their retention of carbon even slightly has potential to make a big difference!
Increasing temperatures may increase rates of decomposition (in some cases associated with melting of permafrost):
Dead organic matter in the top meter of global soils stores over twice as much carbon as is stored in the atmosphere. When this material decomposes, CO2 is given off. If temperatures increase, then decomposition may speed up (in places where it isn't too dry), and this may be a net increase in CO2 release if decomposition is stimulated more by increasing temperatures than is plant productivity.
Permafrost is widespread in Arctic and boreal regions of the N Hemisphere, where it covers about 22% of exposed land surface area (BioScience Sept '08). Some of it ("continuous permafrost") is very deep (350 - 650 m) and likely to stay frozen; some ("discontinuous permafrost") is 1 - 50 m deep and more vulnerable to thawing, and still more vulnerable to thawing is "sporadic permafrost." There is evidence that even some of the discontinuous permafrost remained frozen throughout the various interglacial periods throughout at least the early middle Pleistocene in what is now Yukon Territory (Science 19 Sept 2008), which suggests that if permafrost is more than a "few" m below the surface, it may be more stable than previously thought.
Because high latitude temperatures are likely to change faster than elsewhere - IPCC 2007 predicts as much increase as 7-8 degrees C by 2100 - large areas of permafrost are likely to be vulnerable to thawing. Permafrost is already melting in some areas, such as along the N. Slope in Alaska. There is a huge pool of stored carbon in permafrost - as peat and as organic C in soil. When formerly frozen soils melt and are very wet, evolution of both CH4 and CO2 from them increases greatly (CH4 when soils are saturated with water, and CO2 when they aren't, basically). Partially countering these increased emissions is the likely prospect that, as climate warms at these high latitudes, vegetation uptake of CO2 is likely to increase some (owing to a longer growing season and to increased nutrient availability from decomposition). The net effect of high latitude warming and permafrost melting, however, is likely to be increased emissions of both CO2 and CH4 (decomposition is likely to be stimulated more than production, basically). In fact, resultant emissions may be comparable in magnitude to those that come from land use changes elsewhere, including deforestation in the tropics!!! (BioScience Sept '08). According to the IPCC's 2007 report, the maximum area covered by seasonally frozen ground (note, this is not the same as permafrost) in the Northern Hemisphere decreased by about 7% since the year 1900.
5. Cloud and water vapor
(This one is related to feedbacks influencing global temperature, not so much gas concentrations, but I think it is important for you to hear something about it.) Recall that water vapor is an important greenhouse gas (under certain conditions). Possible feedbacks involving water vapor -- and to clouds -- are very complex and poorly understood (this is one of the major sources of uncertainty in projecting what will happen to temperatures, actually). Increased atmospheric temperatures would mean increased water holding capacity in the atmosphere. Warmer surface temperatures would also mean increased rates of evaporation. Thus, the atmosphere is likely to get "wetter." HOWEVER, whether this will be a net warmer or a net cooler is uncertain it appears that, influences of atmospheric water on temperatures will depend on its height in the atmosphere, on the thickness of clouds, on the droplet sizes that make up the clouds, and so forth. Thus, there is still much debate about whether cloud feedbacks will be positive or negative forcings on global temperatures.
BOTTOM LINE: MOST SCENARIOS PREDICT PARSIMONIOUSLY THAT ROUGHLY 1/2 OF THE NEWLY INJECTED CO2 WILL REMAIN IN THE ATMOSPHERE
We must recognize, though,that projections have wide range of uncertainty (they depend both on what happens to emissions and what happens to atmospheric storage and other sinks). IPCC's 2007 projections for concentrations of atmospheric CO2 equivalents in the year 2100 range from ~600 - 1550 ppm. Note that all of these projections are for more than a doubling over preindustrial levels, which would be ~560 ppm….(Recall that we've already had ~ 30% increase in CO2 alone since preindustrial times [~ 1860]). SO, IPCC's "best case scenario" (emissions constant at year 2000 levels) projects slightly over a doubling by 2100…..
Of course, people, including policy makers, are pretty uncomfortable with such levels of uncertainty! In addition, contrarians try to suggest that uncertainty means that the scientific consensus is probably "wrong" about human influences on climate. We must remember that science is essentially always uncertain! Very few things can be stated with 100 % certainty - that's why we rely on statistical statements - p-values, probabilities of error. IPCC is always careful to provide bounds on its levels of certainty for various projections. [A note about skeptics versus contrararians taken from James Hansen [WorldWatch Nov/dec 2006] -- a skeptic objectively weighs all evidence and comes to a conclusion based on the balance of that evidence (all good scientists are skeptics!); a contrarian addresses questions as do attorneys, whose job is to defend a client rather than to seek the truth; such a person presents only evidence that supports their desired conclusion. This is not a scientific approach to deriving understanding!.]
WHAT EFFECT WILL A DOUBLING OF ATMOSPHERIC CO2-EQUIVALENTS HAVE ON CLIMATE?
This is, of course, the $64,000 question -- and I hope that the previous discussion of feedbacks makes it clear why it is hard to answer, never mind uncertainties about what will happen with emissions!
No clear information is available from historical (and prehistorical) records. We do know that CO2 concentrations nearly doubled between 160,000 yrs ago and now, but concentrations were lower then and it isn't clear that there will be a linear relationship. We also know that, at the peak of the last ice age (18,000 yr ago), temperatures were about 5 C cooler than at present (globally averaged) and that CO2 was about half what it is now ([CO2]atm was 180-200 ppm; pre-industrial was about 280 ppm; now over 382 ppm). Thus, we may be looking at a globally averaged temperature change of a similar magnitude, but in the opposite direction.
HOW DO WE KNOW WHAT TO PREDICT FOR GLOBAL CLIMATES?
We use ice core records, of course, but what other tools are at our disposal? Obviously, we can't do lab experiments with climate -- the system is too big and complex. (You could argue that we are running a big experiment right now putting gases into the atmosphere and we'll see what happens. Not a very good experiment, however -- no replication, no controls....)
What scientists do instead is rely on mathematical climate models called global climate models (or general circulation models) ("GCMs") (or CGCMs for coupled general circulation models). Models are a third approach to achieving scientific understanding, in addition to observational and experimental approaches, which we've discussed in the past. Models, of course, depend on those two approaches, as the data put into them are derived from those approaches. We run out the models - as we do in economics or other areas of science or ecology - to make predictions.
Several of these mega-models exist at various laboratories and universities in the world. These models have components representing the ocean, atmosphere, and land, and include equations that represent what we know about the behavior of the present climate system, interactions between the atmosphere and the oceans, and chemical reactions that occur in the atmosphere. They run at very large spatial scales; they are coarse-grained and not good for predictions in small areas. In model runs, the modeler specifies concentrations of greenhouse gases and runs it. Simulating global climate for one year takes several hours on the fastest super computers.
How are the models validated? That is, how do people know that they are generating reasonable predictions? They are generally verified by comparing model output of climate for current gas concentrations, and models then can be refined as needed. Alternatively, the modeler puts in past concentrations of CO2 and checks model output against what we know of past climates. The performance of the models has improved greatly in the last couple of decades, although none of them are "perfect" yet.
WHAT DO THE MODELS PREDICT FOR FUTURE CLIMATE?
While the models disagree in details, they are in general agreement:
The IPCC makes projections of global surface air temperatures for the year 2100 based on review of model output for all major models using various emissions and atmospheric concentration scenarios, ranging from optimistic (e.g., reductions in emissions) to pessimistic (continued rapid growth in emissions). In their 2007 report, the most likely range of temperature increases, over this century (comparing ~ 2100 [2090 - 2099] to ~2000 [1980 - 1999]) is predicted to be 1.8 - 4.0 degrees C, with a possible range between 1.1 and 6.4 degrees C (~ 2 - 11.5 degrees F), depending on assumptions. If temperatures do increase in this range by 2100, that rate of global warming would be, as far as we know, unprecedented over the last 10,000 years.
Temperature increases are predicted to be greater, in general, at high latitudes. (Exceptions include Antarctica and areas of the N Atlantic where deep ocean mixing is likely to moderate changes.) The average temperatures in high latitude regions have already increased at several times the global mean increase, which you'll recall has been about 0.76 degrees C in the past 100 years or so. More dramatic increases are particularly likely for northern high latitudes, such as Northern Canada, AD, and Siberia, which have already warmed 0.6 C (1.1 F) in the last 40 yrs, while smaller changes are expected towards the equator. Mid-continental regions will probably warm more than coastal areas and warming is likely to be greater over land than over the oceans (due to thermal inertia of the water).
At our latitude here in Oregon, more increases are predicted for winter than for summer. The average increase for Oregon by ~2100 is predicted to be about 4 C [or roughly 8 degrees F] -- averaged across all seasons.
Such changes would diminish the temperature differential between the climate of equatorial regions and polar regions, and would also change the temperature differentials over land versus over the oceans, with possible effects on global patterns of air and water circulation, which are largely driven by temperature differences between these regions. Consequences of disrupted global circulation patterns are hard to anticipate because controls over and effects of these global circulation patterns are not well understood.
Such destabilization of global circulation patterns is likely to have climatic consequences itself. There is some evidence for destabilization already - IPCC 2007 says that data already indicate:
Note - it is important that we not take one year's extreme in the opposite direction from these (say, occurrence of an unusual cold snap) as evidence against climate change. Many factors, both natural and unnatural, influence climate, and "weather" in a given place and time is not "climate."
Now, a 1.1 - 6.4 degree C increase over the next century doesn't sound like much, does it? Don't be fooled by apparently little numbers! Such a rapid increase in global mean temperatures would be unprecedented in human history. The change would be comparable to the 5 degree C warming between the peak of the last ice age 18,000 yr ago and today, but would happen about 10 times faster. I think you have a concept of how different climate and ecosystems were then (at the peak of the last ice age) from now; much of North America was covered with ice, for example, and biogeographic zones were shifted way to the South of where they are now.
The rate of change is very important. Back then, global mean temperatures increased at a rate of about 2 C/1000 yrs. (Some places, such as Greenland, experienced periods of more rapid change, though.) We are now talking about warming by more than 2 degrees C in a century or less. Projected temperatures for 2100 would make Earth then the warmest it has been for the past 2 mill yrs. This rate of change in climate is very important when considering the capacity of ecological (and human) systems to adapt or move to new places.
There is concern that surprises may lurk in the climate system -- things we don't understand that may suddenly flip up to faster change (just like we didn't understand how the presence of surfaces in the stratosphere would speed up destruction of ozone there). "Surprises" can arise from nonlinear behavior of the climate system -- when rapidly forced, nonlinear systems are prone to unexpected behavior!
For example, the movie, "Day After Tomorrow" was about one of these potential surprises. While exaggerated, it contained a grain of potential truth, as follows: The "Atlantic Conveyor Belt" (a branch of the Gulf Stream -- also referred to as Thermohaline Circulation or Meridional Overturning Circulation) brings warm water north from the Caribbean to the North Atlantic (along eastern North America) and out toward N Europe before it cools, gets denser, (and saltier, from evaporation) sinks, and turns back southward. (There isn't an analog in the N Pacific.) Its sinking can be slowed - or even stopped - by changes in temperature or salt content of N Atlantic waters; warmer = less dense; fresher = less dense; lower density = don't sink as much or at all. SO, for example, if there's a big pulse of fresh water dumped into the N Atlantic, as from melting ice, or if the N Atlantic warms a lot from global warming, it could slow or stop the conveyor. The consequence would be that warm water (and warmed air) wouldn't come N as much from equatorial regions, and temperatures around the N Atlantic could get abruptly cooler. (See American Scientist July/Aug 2006 for an excellent treatment of this phenomenon.)
There is evidence from ocean sediments that this has happened in the past - associated with the melting of Glacial Lake Agassiz and Greenland ice for example. However, the movie and the media have probably exaggerated the climatic consequences of a slowing or shutting down of this large conveyor belt. Northern Europe wouldn't be plunged into an ice age, as it is also kept relatively warm (compared to eastern North America - compare England's winters to New England's) by the fact that prevailing westerly winds bring ocean warmth to the continent (because of the ocean's thermal inertia they retain > summer heat than does land). This climatic influence is also apparent in the north Pacific basin - we're warmer here in winter than is Eastern Russia! That is, it is likely that climatic consequences of a shut down would be dramatic but not catastrophic.
IPCC 2007 finds it "very unlikely" that we'll see an abrupt and large transition in this circulation during the 21st century, but lpredicts that it is likely to slow somewhat. Unlikely to have major climatic consequences, though,at least in part because temperatures in the Atlantic region are likely to increase anyway, because of global warming. .
Other things likely to change include:
Models aren't as well validated for precipitation as for temperatures, nor is output from various models as convergent in what is predicted for a given region as they are for temperature predictions. However, models do predict that we will have a wetter world, globally, largely because of increased rates of evaporation.
While mean precipitation will increase globally during the 21st century, patterns will vary geographically -- increases are expected over higher latitudes, but decreases are anticipated for some subtropical regions (particularly those that are already relatively dry). Mid-continents (such as the US Midwestern breadbasket region) are predicted to receive less precipitation in summers than they do at present. Despite globally increased precipitation, we should remember that evaporation and transpiration will increase too, owing to warmer temperatures. (These increases in evaporation and transpiration, of course, are what fuel the global increase in precipitation that is anticipated.)
Precipitation has increased by about 10% on average across the US since ~ 1910 (also in Russia, China, Australia, Japan) - particularly in winter , and the proportion coming in very heavy events has also increased (the latter is true over most land areas, according to IPCC 2007).
For Oregon, models generally predict that our winters will be warmer and wetter (sorry!) and summers will be both warmer and effectively drier. (I say "effectively" because there might be slightly more rain in summer than there is now, but the warmer temperatures will cause higher rates of evaoptranspiration, potentially offsetting that increase.) Snowpack is expected to decrease by as much as 60% by ~ 2050 under IPCC 2007 "moderate" emission scenario projections (more winter precipitation will come as rain rather than as snow, and snow levels should increase by about 1000' by ~ 2050 because of warming in winters). This is likely to lead to disruptions in water supply here, as described under consequences for humans in another section. (For OR, summer stream flows are likely to decrease 20 - 50% under the precipitation and snow regime described earlier in this paragraph.) These changes may also tend to increase fire frequencies in our area -- and cause more winter flooding.
The climate in southwestern North America is likely to become >> arid than it is today (Science 25 May 2007).
Frequency and severity of storms and unusual weather events:
Influenced in part by a changing and destabilized relationship between oceans, lands, and atmosphere (changing temperature differentials), the frequency of estreme weather events is likely to increase (signs of instability in the climate system). For example, we may see more years of back-to-back droughts, increases in frequency and severity of hurricanes and storms, and so forth.
There is evidence already of an enhanced hydrological cycle (1999). Precipitation has increased by about 10% across the contiguous US since 1910, mainly in winter, and, globally, the proportion of precipitation coming in very heavy events has increased as well. In 1995 it was announced that, for the US, the Greenhouse Climate Response Index has been high since the 1970's (from a report by National Climatic Data Center scientists) and that this index is high globally as well. (The index combines information on above normal temps, rain intensity, heavy storms, drought and so forth. It was high briefly in the 1930's and in the drought period of the 1950's, but has had a sustained high since the late 70's.) These scientists estimate that there is a 95% probability that we are seeing abnormal weather variability for the lower 48 states.
IPCC's 2007 report notes that the following anomalies have been observed in recent years:
Sea level rise:
A global temperature rise of a few degrees between 1990 and 2100 is likely to cause sea levels to rise by 0.18 - 0.59 meters (0.59 m = ~ 2 feet) (IPCC's 2007 report).
Sea levels have risen an average of 17 cm (range 10-25 cm) globally over the past 100 yrs (after correcting for changes in vertical land movements), averaging 3 mm per year between 1993 and 2003, and averaging 1.8 mm / yr between 1961 and 2003 (i.e. the rate of rise seems to be accelerating).
In fact, a paper in Science that appeared just after IPCC released its 2007 report (Science 19 Jan 2007; Rahmstorf) projected that sea level rises would be more like 0.5 - 1.4 m by 2100. This prediction was semi-empirical, based on projecting the observed rate of rise in the 20th century into the future with its range of potential warming. And, as we'll see, below, sea level rises could be even more than this, depending on what happens with ice….
When temperatures increased 5 degrees C (globally averaged) after the peak of the last ice age 18,000 yr ago, sea levels rose 100-130 m (the huge ice cap that had covered much of N. America melted). Nothing of that scale is predicted this time around the current distribution of ice is such that much will not melt, unlike then when there was much ice in marginal areas (like N America).
Why is a rise in sea level predicted to occur?
(1) Thermal expansion of oceans is likely to be the most important cause (oceans in some parts of the world have already warmed). Remember, warmer water occupies more volume than an equivalent mass of cooler water. Substantial warming of many ocean waters has been reported for depths between the surface and 3000 m. IPCC 2007 estimates that the oceans have been absorbing > 80% of the new heat that's been added to the climate system - buffering rises in air temps, of course. IPCC estimates thta thermal expansion has been responsbile for a bit over half of the observed rises in sea level.
(2) Melting of mountain glaciers and ice caps is another factor contributing to the anticipated sea level rise. Glaciers across the world are retreating, including those in the Cascades Range, Peru, and Mt. Kilamanjaro. Two thirds of Glacier National Park's ice fields have vanished since 1850, and if present retreat rates continue, there will be none left there within 30 - 40 years. Seven of the largest of Mt Hood's 11 glaciers have shrunk by an average of 30% since 1900 (COS 2007). (Again, it isn't completely clear if there is a "human fingerprint" behind this melting or not, but it seems increasingly likely that there is.)
GREENLAND AND ANTARCTICA:
IPCC 2007 - Melting of Greenland and Antarctic ice sheets combined is estimated to contribute about 0.42 mm per yr of the total 3.1 mm per yr of sea level rise observed between 1993 - 2003. Melting of overland ice makes the biggest contribution to sea level rise since floating ice already displaces its weight in sea water.
So far air temperatures over Antarctica are still below freezing, by and large, but warmer water coming in under ice melts it from beneath - the same melting from beneath also occurs in Greenland and in the Arctic. In some cases, the melting thins ice or its attachments so that they break off and then float and melt Rising sea levels themselves contribute to further sea level rises because more water can gets under ice and melt it from beneath. Further, ice sheet demise allows ice streams to flow out into the ocean instead of being caught by ice shelves. Are also increasingly seeing melting from the top as air temperatures warm - for Antarctica, so far this is true only for the western peninsula that sticks out towards S America (summer temps there can be > 0 C for longer than in the past; there has been about a 2.5 C increase in temperatures there since the mid-1940's). Several ice collapse events have happened there -e.g., in March 2008 a one hundred sixty sq mile chunk of the Wilkins ice shelf broke off - and that wasn't the largest calving in recent years!
IF the entire W Antarctic ice sheet melted, we'd see about a 6 - 7 m rise in mean sea levels - but it appears that it didn't do so in the last interglacial - much of the sea level rise then came from Greenland ice melt. The W Antarctic ice sheet is, however, unlikely tototally melt; it is mostly stable and likely to remain in place for hundreds to thousands of years.
IPCC 2007 projects that sea ice will diminish in both the Arctic and Antartica under all scenarios. Predictions are that the Antarctic ice sheet will not melt from the top and that it may gain mass due to increased snow fall, but that net loss could occur if "dynamical ice discharge" dominates the ice sheet's mass balance; For Greenland, ice mass losses are likely to increase with temperature more rapidly than gains will happen due to increased precipitation. (More snow is likely to fall in warmer winters -- and because of an enhanced hydrological cycle overall.) To date, there haven't been statistically significant average trends in Antarctic sea ice extent, consistent with lack of warming in atmospheric temps over most of that continent, but there have been localized changes (IPCC 2007)
Greenland's ice has lost meters of thickness in many places over the past few decades, and its mass has diminished. Greenland's ice sheets are moving faster because of melt-induced water lubrication (water flows down cracks in the ice sheets and lubricates them from below) BUT the outlet glaciers haven't sped up all that much (in 2007, it seemed like people thought they had sped up more - time will tell!) . Surface meltwater lubrication is likely to have a substantive but not catastrophic effect on Greenland's ice - this is a dynamic field of investigation and little understood. (Science 9 May 08 and 18 Apr 08.)
Changes in Arctic ice have been particularly dramatic (although changes in sea ice there don't contribute much, if any, to sea level rise, since floating ice was displacing water anyway). Melting of the ice CAP there would make a contribution…So far, there has been rapid loss of summer sea ice there - with consequences for native peoples, polar bears, and exploration for oil reserves!
In the mid-1980's >1/2 of the Arctic ocean was covered in yr-round ice; today (2008) perennial ice occurs on < 30% of the ocean (NASA satellite data ). There has been a 40 - 50% decrease in sea ice thickness in the Arctic Ocean since the 1950's. Over the period 1978 - 2007, perennial Arctic sea ice cover extent in summer decreased by an average of 7.4% per decade. In summer 2007, for the first time on record the whole NW Passage from the Atlantic to the Pacific was ice free. Russian, US, Norwegian, and Canadian energy companies are rushing to lay claims for drilling - we are likely to see expanded drilling and development there...(WorldWatch July/Aug 2008).
If present melting trends continue there, the entire Arctic could be ice free during mid summers (Some of IPCC 2007's projections reflect this possibility by 2100 or sooner) -- perhaps as soon as 2030 [Time Oct. 15, 07]. While shipping agencies may like the idea of a short cut across the top of the world (and the Russians did send two ships through en route to the Bahamas in 1999!), the implications for Arctic people and wildlife, who depend on ice for calving sites, for feeding near ice margins, and so on are not as cheery. In addition, the major influx of freshwater that this melting would allow into oceans could affect the patterns in ocean currents, as described above. .
The transition in Arctic ice is likely to speed as the ice thins and becomes more vulnerable. The decrease in Arctic sea ice extent is caused by a combination of thermodynamics (changes in temperature and radiative conditions - albedo) and dynamics - processes involving changes in ice circulation in response to changes in winds and ocean currents (Science 10 March 2007) .
These changes in sea level would be likely to cause trouble for both natural and human systems. For example, the Everglades ecosystem and Miami Florida would be inundated by rising sea waters, and, in fact, 20% of the world's human population lives on lands likely to be dramatically changed by rising waters. A 1 foot rise in sea level would lead to the loss of 20-40% of US wetlands (many of which are coastal), damaging fisheries and wildlife habitat.
ROLE OF CAUTION IN SCIENCE:
IPCC tries to be very cautious in its statements about what has been observed to-date and what is likely for the future. It works solely from review of published literature. Caution is typical of scientists. BUT, in some cases, caution may not be wisest if conservative projections lead to complacency. For example, relative to sea level rise, IPCC's projections for the future didn't include quantified contributions from melting of glaciers and ice sheets because they believed that the science of ice dynamics wasn't understood well enough to justify inclusion - there was no basis in published literature for making projections into future. It did state, though, as a footnote, that the rises could be greater than their published projections if major changes in those ice masses occur.
As mentioned earlier, a paper published subsequent to the release of their report projected faster increases in sea level - and a paper even later (Science 5 Sept 2008) predicted that, while increases of more than 2 m are physically untenable, increases up to 2 m could happen if all realistic forces worked in that direction (e.g., melt rates and so on), and that 0.8 m could readily be reached. This paper, then, predicts 0.8 - 2.0 m of rise by 2100….in contrast with IPCC's relatively conservative 0.18 - 0.59 m prediction.
SO, what is the responsibility of scientists in terms of communicating to the public and to policy makers? What responsibility do they have to let people know about the risks of the most dangerous climate change? How warn of the relatively extreme possibilities while retaining credibility? IPCC tries to tread a careful line -- it brackets projections as "very likely" "likely" and so on, with a range of probabilities associated with each projection bracket. However, for something like the prospect of much greater sea level rise, they don't give a numeric projection to because of lack of concrete information - is that a service or a disservice?
To move to the next section of these notes, which deals with ecological and agricultural consequences of climate changes, click ecological effects here. To move to notes on implications for humans and policy steps that are being taken, click "policy." To jump back to the master Table of contents for these pages, click Contents here, and for reminders on how to navigate within and among these pages, click Navigate.
Page maintained by Patricia Muir at Oregon State University. Last updated -- very partially -- March 3, 2012; prior update = Nov. 4, 2008.