Global Climate Change

A STATS survey of current research

Everybody talks about the climate and, at long last, somebody is about to do something about it. But will they do the right thing? One of the most challenging scientific controversies of our time -- whether or not human activities are changing the Earth's climate -- is tied to one of the most important policy decisions: what steps should governments carry out today to curtail industrial emissions?

The stakes are enormous. If we guess wrong on one side, we could bake our own future in an oven that won't cool down. If we guess wrong on the other, we could disrupt the global economy by unnecessary regulation and taxation. So how good are our guesses? To understand this debate we need to answer some elementary questions such as what climate is, how we measure climate change, and whether scientists agree on the results.

When we want to know today's weather, we look out the window. When we want to know tomorrow's weather, we listen to a forecast. TV weathermen routinely predict the future several days in advance -- with their accuracy diminishing the farther their forecast extends.

Can we look ahead (or, for that matter, behind) a century or more with confidence? Some say yes -- they have seen the future and it percs. Others say no, we are gazing into a cloudy crystal ball whose images can only cloud our policy judgments. Thus, the climate debate has produced two hostile camps of scientists, true believers and iconoclastic naysayers, who impugn each other's motives as well as their models.

Daily weather changes by the hour and the vicinity. Global climate, however, is the average condition of temperature and precipitation that holds across day or night, summer or winter, or Arctic and Equatorial latitudes. And global climate changes through natural causes. The ice ages attest to this fact, as does the gradual warming of the atmosphere caused by the greenhouse effect, which traps heat in the atmosphere and makes life on Earth possible. But to much of the principal greenhouse gas, carbon dioxide, could produce problems or even disaster, as the greenhouse effect accelerates towards ever higher heat.

Enter the question of human beings and their economies. Modern industrial processes pump out massive quantities of carbon dioxide and other gases into the atmosphere. No one knows whether, or how much, this human-induced (or "anthropogenic") driving of the atmosphere is significant.

How can we find out? Since scientists can't look out the window a hundred years into the future, they look instead into their computer screens. Climate scientists seek to model the world's climate by replicating its features in a computer program. This is a complex challenge, since no one really knows all the factors -- such as clouds, sea currents, atmospheric particles, ozone depletion, and solar cycles -- that contribute to the global climate.

Most critically, before we can establish what role humans play, we need to accurately estimate the natural variations in climate. Only then can we identify a human "signal" against the natural background noise.

Some think they've detected the signal. As the United Nations body assigned the task of figuring out the climate future, the Intergovernmental Panel on Climate Change (IPCC) said in 1995, "the balance of evidence suggests a discernible human influence." But other climate scientists are not so sure, since the models don't accurately represent today's climate, much less the future.

Why is it so hard to get accurate temperature data?

Perhaps you've seen a chart in a news story that shows global temperatures rising throughout the 1980's. And maybe you've seen one in a different story that shows no discernible rise. Is one of these stories wrong? How is it possible that they both could be accurate?

It sounds like the simplest question in the field of climate studies: "How hot is it?" Finding an answer, however, is surprisingly complicated. First of all, why do these averages conflict? Principally because they come from two different data sets: land-based records and satellite observations.

We have a wealth of surface-based information about how hot or cold it is at any particular place on Earth. We even have a growing body of data on how hot or cold it was in the past. The only problem with the surface-based records is the locations from which they have been collected: mostly land (three quarters of the globe is covered by oceans), mostly in the Northern Hemisphere, and often near urban centers.

This last problem is the trickiest. There is a "heat island effect" near cities that must be accounted for. Notice on the local weather report how cities are warmer than surrounding rural areas. Most climatologists use an equation to factor out this urban heat, but there is considerable disagreement over what this equation should be. Global warming skeptics point out that even a slight change in the equation could account for almost all the global warming "observed" during this century.

Surface-based readings show a rise in global temperature of approximately 1 degree F this century, with a particularly warm surge in the 1980's. Satellite readings, however, don't show this at all. Why not?

Satellite readings, which show a slight cooling over the past seventeen years, have certain advantages over surface records; but they also suffer some shortcomings. They are considered to be highly accurate, and they read temperatures for the whole planet (including oceans), scanning a miles-long slice of atmosphere, rather than just the surface. Unfortunately, the satellite record only goes back to 1978.

So when you see a chart that claims to list "global temperatures," the first thing you need to know is whether they are land-based or satellite observations. This is central to the question of whether the data we plug into highly sophisticated and sensitive climate-modeling computers is accurate. Garbage in, garbage out.

The problem of modeling

Why do we think we know so much about the global climate? What allows some scientists to make highly specific predictions about future global temperatures?

They use computers. Climate change researchers have focussed their efforts on massive computer simulations of the atmosphere, known as Global Circulation Models (GCM's). Problems arise from a number of areas: the inherent complexity of the system, limitations in current computer technology (the time taken to run some of these simulations is measured in weeks rather than microseconds), and human ignorance of the relative effects of certain key variables.

Critics point to the now-acknowledged shortcomings of models that were considered authoritative as recently as three years ago. Model-based warming projections offered as "best guesses" by the Intergovernmental panel on Climate Change (IPCC) at the 1992 Rio Conference ranged between 1.50 and 4.50 C by the year 2050. But the 1995 IPCC report, based on updated models, lowered this prediction to between 0.80 and 3.50 C by the year 2100 -- about a fourfold reduction in the annual rate of warming.

How can we be sure that these models are accurate? The best test of the predictive capability of a model is its ability to "back-predict" things that we know have already happened. By this standard, global climate models still have some way to go. For instance, even advanced models, when supplied with the relevant variables for the Pleistocene era, fail to predict glaciers -- a climatic feature that anyone living at the time would probably have found newsworthy.

The models are getting better. Computer technology is advancing, and human understanding of how the atmosphere works is nudging forward. New factors, like clouds, water vapor, aerosols, and oceans are being factored in, and these are honing the models' accuracy. Interestingly, each successive refinement and improvement to the models has moved predictions in one direction: towards ever more modest estimates of an anthropogenic effect on warming.

Why is anthropogenicity the critical question?

Some people are surprised to learn that the climate is in a constant state of flux. There was no Edenic steady state before man came along. Rather, Earth has cycled through ice ages, mini-ice ages, and epochs much hotter than even the direst warming prediction. What matters for purposes of the climate change debate is the extent to which human activity is affecting the rate of any changes in the atmospheric system.

In fact, as recently as twenty years ago, "global cooling" was considered a much greater threat to man. We are currently living in a relatively warm interlude in what has been a predominantly cold era. Based on current understanding of these cycles, we have passed the most recent warm crest, and we can expect another ice age within the next one to three thousand years. The effects of an ice age anything like the last one would be as catastrophic as they would be unstoppable, erasing all traces of human existence from large swaths of North America and Eurasia.

The greenhouse effect is a natural phenomenon, without which human life would never have evolved. We tend to think of environmental issues in terms of "pollution," but this frame of reference is less helpful in the context of carbon dioxide (CO2) and climate change. CO2 is emitted into the atmosphere by a variety of natural sources including plant decay, volcanoes and the oceans. In fact, natural sources emit far more CO2 than man does.

Then-Senator Al Gore could confidently state ten years ago that "there is no longer any disagreement in the scientific community that the greenhouse effect is real and already occurring." True -- but there most certainly is debate in the scientific community over whether human activity is influencing the greenhouse effect. Remember that 10 F rise in global temperature that the land- based record showed for this century? Most of that warming occurred prior to 1940; that is, before mankind was emitting CO2 at anything approaching today's levels.

And while almost everyone agrees that CO2 in the atmosphere is a key component of the greenhouse effect, atmospheric concentrations of CO2, which have fluctuated widely throughout Earth's history, have not always correlated with warming. 440 million years ago, during the relatively cool Ordovician age, CO2 levels were ten times what they are now. Only 5000 years ago, global temperatures were higher than today's, but CO2 levels were considerably lower.

But CO2 isn't the whole problem. Methane gas has an even stronger effect on greenhouse warming, and its concentration in the atmosphere has risen dramatically this century. Methane, however, comes from quite different sources than CO2. Large quantities of methane are released into the atmosphere by swamps (that's wetlands to you and me), termites, pigs, and cows (don't ask).

With methane, the line between anthropogenic and natural sources is not always clear. Are rice paddies "natural"? Are cows? While some methane abatement strategies are fairly obvious (plugging leaks in natural gas lines), there are already good economic reasons to do so. In fact, we are already seeing strong market-driven methane reductions in the former Soviet Union, which had been an egregious methane waster/emitter. Clearly, policies aimed at methane reduction need to be crafted differently than those aimed at CO2 reduction.

You may have noticed that the term "climate change" is largely replacing the term "global warming." This is for good reason, as researchers and policy makers are gaining a broader understanding of the various feedbacks and complex interactions that make up the atmospheric system. Again, it is important to get away from the concept of "polluting gases" that "heat up" the Earth; what we are looking for is a human influence on a complex interdependent system. When we talk about greenhouse gases, we are not necessarily talking about gases that simply produce more warmth.

One of the reasons that earlier computer models consistently overstated warming projections was that they failed to account for the cooling effect of anthropogenic atmospheric pollution. Burning fossil fuels releases sulfur compounds (aerosols) into the atmosphere. These particulates create a layer of haze or smog that reflects sunlight before it reaches the Earth's surface, producing a cooling effect. In other words, many of the same processes that might lead to global warming at the same time produce its antidote. The extent of this compensatory effect is still being debated, but its inclusion in recent, more sophisticated climate models has produced results that more closely conform to the actual climate record.

Amendments to the federal Clean Air Act now mandate strong curbs on these very emissions, since they also contribute to acid rain. Ironically, then, an immediate ban on all greenhouse gas emissions in an effort to avert global warming might just lead to Another important greenhouse factor is water vapor. Increased heat evaporates more water, trapping more heat in the atmosphere. But more water vapor in the atmosphere means more clouds -- and clouds reflect sunlight. (We told you it was a complex system.)

The system may even turn out to be more complex than we need to worry about. A new line of research has yielded some startling (though still controversial) results. In 1992, two Danish researchers published a study showing a remarkable correlation between sun spot activity in this century and global temperature patterns. Solar variation (the energy emitted by the sun fluctuates over time) had already been posited as an explanation for global warming and cooling trends, but the tight fit of the sun spot observations has raised the interest of many researchers.

A current battle over whether there is a discernible anthropogenic effect on global climate is being fought over the IPCC's most recent report, "The Science of Climate Change." The IPCC report was peer reviewed by an international panel of climate experts and formally adopted by the full working group at the IPCC meeting in Madrid last November. When the report was published this summer in London, however, critics pointed to editorial changes that had been made to the "final" version previously approved by the scientists.

Here's where things turned rough. Frederick Seitz, former president of both the National Academy of Sciences and the American Physical Society, claimed that he had "never witnessed a more disturbing corruption of the peer review process than the event that led to this IPCC report." Seitz went on to charge that nearly all of the editorial changes made by the report's lead author, Benjamin Santer, "worked to remove hints of the skepticism with which many scientists regard claims that human activities are having a major impact on climate in general or global warming in particular." Others, including the independent but industry-backed Global Climate Coalition (GCC) went so far as to claim that the report had been "scientifically cleansed" in a political effort to emphasize alarm about global warming while deleting references to uncertainty.

Santer has responded that the changes he made to the original text were consistent with IPCC procedures, and that he simply clarified the text and removed redundancies. Defenders of the published version also countered that the GCC critics were funded in part by the fuel industry. Greenhouse skeptics shot back that doom-saying researchers are angling for a bigger slice of the two to three billion dollar pie the United States alone spends annually on climate studies.

Why all the heat?

Why does all this matter? If pessimistic warming scenarios are accurate, temperatures could rise quickly, turning croplands into desert, promoting the spread of infectious disease, and wiping out coastal areas and islands. If these same predictions turn out to be inaccurate, and politicians answer a false alarm, hundreds of billions of dollars stand to be sucked out of the global economy. This would mean schools that will never be built, medical breakthroughs foregone, and quality of life diminished, all for nothing.

It is difficult to say, exactly, whether human activity is having an effect on global climate. Some argue that we should apply the "precautionary principal" -- that the risk of danger is so great that we should act before knowing for certain, a sort of Pascal's wager on climate. Assistant Secretary of State Tim Wirth asserts that, "Even if the theory is wrong, we'll be doing the right thing."

Here are some things we do know: We know that the Earth has dramatic temperature shifts, and we have some idea as to what causes or at least contributes to them. We also know that just a few years ago we had no idea of some of the factors which we now consider absolutely central, and that new advances in this field are being made at an exciting pace. Our circulation models are getting better, but they are still not accurate enough to account for observed history. And we know that humans are emitting large quantities of substances whose effect on the atmosphere remains unclear.

As Penn State University professor of Geosciences Richard Alley expressed our dilemma, "We know for certain only that humans are hitting a very complex fluid-dynamic system with a big energy hammer -- with some uncertain, though potentially dramatic, future payback."



Article from July 01 1996 by Statistical Assessment Service, 2100 L St NW Suite 300 Washington DC 20037. Original link: http://www.stats.org/record.jsp?type=oped&ID=100