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Climate Models: How Reliable are Their Predictions?
We often hear the assertion that our extensive use of carbon- based fuels now threatens to alter the climate of the whole world: that enhanced greenhouse warming--induced by the carbon dioxide and other gases we have added to the air--will lead to a rapid and unprecedented rise in the average temperature of the Earth within the next fifty years.
We are not accustomed to long-term forecasts of anything of such consequence. Nor can it be surprising that the initial reaction of almost anyone is to question the reliability of the prediction. For what is claimed--if indeed an accurate portrayal of the future-- seems to leave few choices: do we prepare ourselves for the impacts of lasting climate change? Should we rethink our own use of coal and oil and natural gas and gasoline, when energy use, as we all know, is very much tied to economic growth?
What must trouble many decision-makers is that the sounding of this loud environmental alarm was tripped not so much by measurements as by computer models. How certain or how controversial are these largely theoretical predictions of global warming, and on what assumptions are they based? Given the potential importance of regional climate changes for the development of national policies, and the impacts of extreme, climate-related weather events such as droughts, floods, and hurricanes on agriculture and human safety, how reliable are the projections of future change? Are the uncertainties in present climate models so great that we can ignore their predictions? What elements are the most robust? What are the prospects for substantial improvements in climate models in the near future?
These questions, so often asked, were put to a group of scientists in late 1994 in response to requests from both the White House Office of Science and Technology Policy (OSTP) and from the Government Accounting Office (GAO) which was responding, in turn, to a request from Congressmen John Dingell of Michigan. The charge to the Forum, which I chaired at the request of the U.S. Global Change Research Program, was to develop a statement on the credibility of modeled projections of climate change, to provide background to the government for considering and developing national policy options. The participants included climate modelers and other knowledgeable scientists who were chosen to bring to the Forum a wide spectrum of scientific opinion regarding the potential threat of global greenhouse warming. This review provides the author's summary of the Forum report, which is listed as a reference at the end of the article.
General circulation models
Computer-run, mathematical simulations or models of the atmosphere and ocean are the principal tool for predicting the response of the climate to increases in greenhouse gases. The most sophisticated of these, called general circulation models, or GCMs, express in mathematical form what is known of the processes that dictate the behavior of the atmosphere and the ocean. GCMs include the interaction of the atmosphere with the oceans and with the surface of the Earth, including plants and other ground cover. They allow us to test, by mathematical simulation, what should happen to climate, around the world, in response to a wide variety of changes. For example, what climatic effects would follow a major volcanic eruption, or a change in the radiation from the Sun?
The great power of mathematical models lies in their ability to simulate the behavior of systems--like the atmosphere and ocean-- that are too complex or extensive for simple, intuitive reasoning. There are limits, however, to how much complexity can be handled by the computers on which the models are run. At present, models of the global climate system cannot include physical processes whose horizontal dimensions are less than several hundred miles--a constraint that imposes simplifications on how well we can model what we know and restrictions on the level of regional detail. The key is to incorporate the best possible representation of all the important processes and feedbacks necessary to characterize the climate system, while keeping within the practical capabilities of modern computers.
Our ability to evaluate the strengths and weaknesses of climate models has grown over the last two decades. A growing number of GCMs, many with independently derived components, are available for intercomparison. We have a growing store of meteorological and oceanic observations against which model predictions can be tested. We also have information on past climate change, recorded by natural processes in rocks and sediments, that allow us to assess the ability of models to replicate the known features of climates different from that of the present day. Each of these elements is the basis for debate on the reliability of climate model projections of the future climate.
All of the GCM experiments designed to assess the impact of increases of greenhouse gases point to global warming through the coming century, with accompanying changes in rainfall and other meteorological quantities. Still, the complexity of the climate system is a tremendous obstacle to predicting future climate change. Neither climatological observations nor present climate models is sufficient to project how climate will change with certainty. A workable approach is that adopted by the Intergovernmental Panel on Climate Change (IPCC) of the World Meteorological Organization and the United Nations Environment Programme, which is based on projections of the expected growth of greenhouse gases and the combined results of many GCMs. In terms of mean global surface temperature, the consensus prediction of the IPCC is for an increase of 0.5 to 2° Centigrade (about 1 to 3.5° Fahrenheit) by the year 2050, in response to an anticipated increase of 1 percent per year in CO2. The low end is a significant change; the high end, a dramatic one. Moreover, were the amount of atmospheric carbon dioxide to double, the consensus forecast is for an eventual warming of 1.5 to 4.5°C (about 3 to 8°F.)
Such changes, if realized, would represent a significant climatic change. For example, the most recent climate change of similar magnitude was the last major Ice Age that reached its peak about 18,000 years ago. The mean global temperature during that time is estimated to have been between 3 and 4° C cooler than at present. The effect of this small a change in global-mean temperature can be appreciated when we realize that during the last Ice Age, glacial ice--a mile or more deep--covered much of North America, year-round, reaching as far south as the Great Lakes and the surrounding states of present-day America. That amount of change in global-mean temperature is similar, although opposite in sign, to what is now projected due to increases in greenhouse gases. But the rate of change is not. The last Ice Age developed over thousands of years, while global greenhouse warming is projected to occur within a span of less than a century. And within the lifetime of people now living.
It is equally clear that in terms of potential impact, the difference between a 1.5° and a 4.5° C projection for future warming is very large. As a result of this uncertainty, decision- makers are confronted with a difficult question. What steps should be taken when the best indications from state-of-the-science models suggest that climate change due to human activities may be large and significant, yet the predictions are less than certain?
The scientific debate regarding these uncertainties has entered the public arena, providing considerable confusion even for those aspects of climate-model predictions that are virtually certain. The debate over how much warming--and by when, and why it hasn't yet been more clearly seen--has clouded the clearer picture that increases in carbon dioxide will increase the global-mean temperature. It has also affixed the stamp of "controversial" on almost any reference to impending global warming in the press and news media, implying, erroneously, that the general concept, and not just the details, is in serious doubt.
A method for evaluation
It is possible to get an indication of the strength of a building or other structure if we know which of its footings are solid and which are less so: in this case, to separate the aspects of predicted climate change that are virtually certain from those that are uncertain. The Forum carried out this kind of assessment of predicted global warming, to provide better illumination for policy discussions and to assist policy development.
The evaluation is divided into three parts. The first provides a basis for any discussion of climate-model predictions by identifying the foundations of the greenhouse warming theory that are most solid and robust: a series of conclusions which can be viewed as "virtually certain" based on observations, experiments, and the results of many models. The second part is a listing of specific predictions of climate models that are societally important, ranked by degree of certainty. In the last part we examine what can be done in the future to improve climate-model predictions.
Although the specific predictions of climate change are derived from models, the reasons for expecting significant global warming in the near future comes from a much deeper foundation that includes laboratory and field experiments, well-established knowledge of atmospheric behavior, and measurements that include worldwide monitoring of atmospheric conditions. Here we list seven of the principal scientific arguments for a global-warming prediction. Throughout, the stated conclusions are subject to little or no debate because of their level of certainty, and indeed, to some they may appear trivial.
First, as confirmed in laboratory experiments, certain gases that are naturally present in small amounts in the atmosphere play an active role in maintaining the Earth's temperature. They do this by absorbing energy (infrared radiation) emitted from the land, ocean, clouds, and the atmosphere itself, and then re-emitting it. The most important of these so-called greenhouse gases are, in order, water vapor, carbon dioxide, and methane, followed by nitrous oxide, ozone, and chlorofluorocarbons (or CFCs), which are manmade compounds of chlorine, fluorine and carbon.
Second, because they absorb radiated energy, increased concentrations of greenhouse gases will inevitably raise the Earth's temperature. The extent of the warming will depend on possible amplifying or damping mechanisms (feedback processes), particularly those involving water vapor and clouds, that are major players in controlling the natural greenhouse effect. Such feedbacks can change the magnitude of the warming, but there are no known cases where they bring about an opposite, cooling effect. Thus that greenhouse-gas increases will produce warming is not in question. The heart of the greenhouse debate concerns the nature and timing of temperature increase, and the associated changes and impacts of other climatic quantities, not the fact that increases in greenhouse gases will lead to a rise in global temperature.
Third, the amounts of carbon dioxide, methane, nitrous oxide and chlorofluorocarbons present in the air today are significantly higher than their "pre-industrial" levels--that is, the amount that was present, naturally, before the intensive use of energy that began with the Industrial Revolution about 200 years ago. For example, the amount of carbon dioxide that is measured in the air throughout the world today is about 30 percent greater than that found in years before about 1800, as determined from the chemical analysis of air trapped in well-dated, polar ice cores. Similar findings apply to methane (which has increased by more than 100 percent) and to other greenhouse gases, with the possible exception of water vapor. The increases can be tied directly to human activities that include fossil-fuel burning (as for heating, or in internal combustion engines), the burning of trees to clear land, and certain agricultural and industrial practices.
Fourth, it would take hundreds of years for the concentration of carbon dioxide to fall back to pre-industrial levels, even if the amount emitted were immediately and substantially reduced around the world. The reason is the slow pace of the natural processes that remove carbon dioxide from the atmosphere. Further, the projected growth in world population and energy use in the developing countries make it highly unlikely that any substantial reductions in total global carbon-dioxide emissions will take place over the next several decades. Thus the atmospheric concentration of carbon dioxide is expected to continue to rise well into the 21st century. Similar arguments apply to most other greenhouse gases.
Fifth, there are many more microscopic, airborne particles (known as aerosols) in the atmosphere than were present in pre-industrial times, concentrated in and downwind of areas of intensive human activity. Aerosols are present naturally in the atmosphere in the form of windblown dust from cultivated soils, hydrocarbons from vegetation and forests, and soot from forest and grassland fires. What has increased is the anthropogenic or human-made contribution: soot, sulfate aerosols and other particles found downwind from regions of intensive fossil-fuel combustion and biomass burning.
Sixth, laboratory and atmospheric measurements demonstrate that sulfate aerosols (containing compounds of sulfur and oxygen) that come either from volcanic eruptions or fossil-fuel combustion exert a cooling influence on the climate, by reflecting some of the incoming solar radiation back into space. The increase in airborne particles cited above could thus offset some of the warming expected from the buildup of greenhouse gases, although the magnitude and extent of aerosol cooling is not known and is difficult to quantify, in part because the regional distribution and character of past and future emissions of aerosols are poorly known.
Seventh, the globally averaged temperature at the surface of the Earth has risen about 1°F (or 0.5°C) in the last 100 years. Because of the natural variability of climate, the change cannot yet be ascribed unambiguously to the increase in greenhouse gases over the same period. Nor is the recorded temperature rise as great as that expected, based on climate-model results, from greenhouse warming, although some or all of the difference may be due to the cooling effect of aerosols, noted above, or to the action of other competing long-term effects.
These seven findings form the basis for the conclusion of a vast majority of scientists that human activities are now modifying the energy balance of the Earth system. Less certain are the magnitude and the timing of the associated climate changes, which are derived from models and which are the subject of considerable debate.
Climate Model Predictions
Predictions of future climate are imperfect because they are limited by significant uncertainties that stem from: (1) the natural variability of climate; (2) our inability to predict accurately future greenhouse-gas and aerosol emissions; (3) the potential for unpredicted or unrecognized factors, such as volcanic eruptions or new or unknown human influences, to perturb atmospheric conditions; and (4) our as-yet incomplete understanding of the total climate system. The reliability of climate-model predictions depends directly upon each of these.
With this in mind we list below, in order of certainty, the major policy-relevant predictions of present climate models.
Calculated changes in climate variables will obviously depend upon the assumptions made regarding the future concentrations of greenhouse gases in the atmosphere, which are a function of projected population growth and associated economic expansion. The modeled results that are given here assume that greenhouse- gas concentrations in the atmosphere will continue to increase in coming decades. For purposes of simplicity, the climate model used considers only carbon dioxide and assumes that it will increase 1 percent each year, which, for purposes of calculation, replicates the effect of the anticipated increases in the concentrations of all other greenhouse gases.
A Ranked List
In ranking its conclusions, the Forum adopted a system of four levels of certainty, as these terms are defined in general usage: virtually certain, very probable, probable, and uncertain.
(1) The temperature of the stratosphere--an upper region of the atmosphere that extends from about ten to fifty kilometers (six to thirty miles) above the surface of the Earth--will be significantly cooled. This cooling comes about through the combined effect of increases in carbon dioxide and the observed depletion in stratospheric ozone, and the manner in which the two gases absorb and re-emit energy. Opposite in sign to what is expected near the ground, the change had been predicted by models and has now been observed. As such, it provides potential early evidence of greenhouse warming.
(2) The surface temperature of the Earth will continue to rise through at least the middle of the 21st century. The prediction is based on (a) projected, continued increases in greenhouse-gas emissions; (b) the results from a host of model calculations; and (c) the analysis of past climates of the Earth. The best available estimate, from the international assessment by the IPCC and based on the range of available model predictions, is that the global-mean surface temperature will increase by about 0.5 to 2 °C (roughly 1 to 3.5° F) over the period from 1990 to 2050 (Fig. 1). For comparison, an increase of 0.5° C--the lower limit--is equal to the warming that has taken place in the past 100 years. Beyond the year 2050, the carbon dioxide concentration is expected to reach twice that of pre- industrial times. When that level is reached, and after the climate has reached equilibrium, the best estimate for the resulting climate change is a warming of 1.5 to 4.5°C (about 3 to 8&#