naturalSCIENCE Logo
Cover Stories
Open Forum

Global warming: It's happening

Keywords: carbon dioxide, climate change, El Niño, evaporation, floods, glaciers, global warming, precipitation, sea ice, sea level, storms.


National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307, USA,

Received December 2, 1997, published December 4, 1997

Background: Last month, a feature article (1) in Canada’s "national newspaper," the (Toronto) Globe and Mail, asserted that reports of global warming are "science fiction" invented by self-serving scientists, and that, since 1980, the Earth has cooled, not warmed. Here, at the invitation of naturalSCIENCE, Kevin Trenberth, Head of Climate Analysis at the U.S. National Center for Atmospheric Research in Boulder, Colorado, and a leading researcher in the field of climate change, responds to the claims made in the Globe and Mail article and comments on some issues that the article raises. The Globe and Mail declined to co-publish Dr. Trenberth's article.

Earth's climate is changing. Global mean temperature is rising. The year 1995 is currently the warmest on record, although 1997 looks as though it will surpass 1995, and the years 1987-1996 are the warmest ten years on record. Glaciers are melting almost everywhere. At the same time, the composition of the atmosphere is changing, with clear evidence for increases in carbon dioxide concentrations. It is now well established that these increases are caused by human activities, particularly the burning of fossil fuels and deforestation. The result is GLOBAL WARMING (Figure 1). With the meeting of nations in Kyoto in early December 1997 to debate what actions, if any, will be taken in response to it, global warming is a hot topic. The rhetoric is strong, and disinformation campaigns have been effective in confusing policy makers as well as the general public.


Figure 1. Estimated changes in annual global mean temperatures and carbon dioxide concentrations over the past 137 years relative to a 1961-90 base period. Temperature data (degrees C) go through 1996 (red line), and the carbon dioxide values are from ice cores until 1956 (blue dashed line) and from air sampling at Mauna Loa, Hawaii from 1957 through 1995 (blue solid line). carbon dioxide is measured in parts per million by volume (ppmv) relative to a mean of 333.7 ppmv. Redrawn from Trenberth (1997) (2).

The (Toronto) Globe and Mail has contributed to the confusion by publishing an article on Saturday, November 22, 1997 The day the Earth warmed up by Guy Crittenden. It was a far from unbiased view and cited only a handful of skeptics as supposed experts on the matter, rather than the recently published findings of the Intergovernmental Panel on Climate Change (IPCC). The IPCC is sponsored by the World Meteorological Organization and the United Nations Environment Program. In the IPCC 1995 Scientific Assessment, over 2000 scientists from all over the world and from all parts of the political spectrum were represented. Yet the vast majority were focussed only on making the best statements possible about the science, given our current understanding, in a very open process.

There has been a politicization of environmental science which U.S. Congressman George Brown has written about in the March 1997 issue of Environment. Somehow, what we can say about the science becomes mixed up with advocacy on what we should do about the conclusions. It should be possible to separate these two things. The first step is to make the best scientific assessment as to what can be said about the problem in question, including all the caveats and uncertainties, and then the public and politicians may debate and decide what actions to take while accounting for all world views. It is this second step that is taking place in Kyoto.

The process of building a consensus by the IPCC plays a key role in ensuring that no particular view dominates and that the scientific assessment is unbiased. This approach is in stark contrast to the biased viewpoints dominating Crittenden’s article. Why are they biased?

It is worthwhile recognizing that there is a class of people who have vested interests in the current situation. Their strategy is often to denigrate the issue or deny that there is an issue at all. Like the tobacco companies who have long denied the addictive effects of nicotine and adverse effects of smoking on lung cancer, oil-producing countries and fossil fuel companies spend huge amounts of money to publish often misleading or invalid material to deny that global warming is a problem. It is noteworthy that the only two countries who obstructed progress and continually objected to the IPCC report in the intergovernmental plenary in Madrid, Spain, in November 1995, were Saudi Arabia and Kuwait. Oil companies often publish selective and biased views in their newsletters to shareholders. Coal companies in the U.S. wage negative advertising campaigns and fund the work of skeptics.

A typical strategy has been to focus on a short-term satellite record of temperatures in the lower part of the atmosphere that shows a downward trend since 1979. Indeed, this was the lead-in to Crittenden's article, which begins with the blunt assertion that (on the way to the Kyoto conference on global warming) "the Earth has failed to heat up." However, because the so-called satellite record is made up of data segments from eight different satellites, it has been shown in published research (3) that the downward trend is spurious and arises from how the segments are joined up, a point ignored by the skeptics. This argument also conveniently ignores another more reliable satellite record that shows rising global temperatures for the same period. It further ignores the much longer surface record of rising temperatures and the related evidence such as the melting of glaciers almost everywhere in the world. This kind of selective use of information is designed to mislead and increase the doubts that already exist, thus discouraging action. Many other points attributed to the skeptics by Crittenden are also either wrong or, at best, misleading and out of context. Moreover, the skeptics often exaggerate the problem thereby suggesting that the changes needed would be severely disruptive to the economy, again inhibiting action.

The neglect of information does not happen when a consensus is built such as that for the IPCC. Instead all information is evaluated (including that from skeptics) and taken into account. Consensus science may not produce the best and latest result, but judgments are made as to which results are truly established.

Evidence for climate change

Global mean temperatures have risen over the past hundred years by about 0.5 degrees C (about 1 degree F). Over half of the increase has occurred in the last 30 years (Figure 1). The temperature records have been assembled from thousands of land and ocean observation sites covering a large, representative portion of the Earth's surface and carefully controlled for possible biases arising from station and instrument changes. Widespread melting of glaciers and their disappearance in many parts of the tropics (the Andes, Himalayas, New Guinea) provide further evidence and, along with thermal expansion of the oceans, have contributed to an increase in sea level over the past century of about 15 cm (6 inches). Sea ice is melting in the Arctic and Antarctic regions. Northern hemisphere spring and summer snow cover, which has been reliably monitored by satellite imagery since 1973, has decreased by 10% since 1987.

There is variability in temperatures from year to year, and also from decade to decade superposed on the longer upward trend. Presumably this variability is natural; for instance, there is a mini global increase in temperatures with El Niño, which is a natural warming of the tropical Pacific Ocean that occurs every 3 to 6 years or so. The range of natural variability in global temperature seems to be about plus or minus 0.2 degrees C, so that it is only after the late 1970s that global mean temperatures emerge from the noise of natural variability. The warming is not uniform. Some cooling has taken place in the North Atlantic and central North Pacific and is known to be a consequence of changes in the atmospheric circulation that naturally creates southerlies in some regions and northerlies in other regions, so that spatial patterns of temperature change are not and should not be expected to be uniform.

Other climate changes have also occurred in humidity, precipitation (both average values and intensity), storms and other phenomena, and these are discussed below as their relevance becomes apparent.

Human influences

By modifying the Earth's environment in various ways, human activities are changing the climate, although it is difficult to ascribe the effects with certainty. The burning of fossil fuels pollutes the atmosphere and alters the balance of radiation on Earth through both visible particulate pollution (called aerosols) and gases that change the composition of the atmosphere. The latter are referred to as greenhouse gases because they are relatively transparent to incoming solar radiation, while they absorb and reemit outgoing infrared radiation, thus creating a blanketing effect that results in warming. For example, atmospheric carbon dioxide concentrations have increased by about 30% over preindustrial values as a result of human activities (Figure 1). Emissions of carbon dioxide into the atmosphere continue to grow and concentrations increase because carbon dioxide has a long lifetime in the atmosphere. Human activities (mostly agriculture, land use changes and industry) are also causing several other greenhouse gases (methane, nitrous oxide, chlorofluorocarbons) to increase. Global warming and associated climate change is expected as a result. Increases in atmospheric aerosols may offset global warming by blocking the Sun's radiation and by increasing the brightness of clouds that also reflect radiation back to space. However, these effects are mainly localized because the lifetime of aerosols is only about a week as they are washed out of the atmosphere by rain.

Increases in greenhouse gases in the atmosphere produce global warming through an increase in heating at the Earth’s surface. This not only increases surface temperatures but also evaporates surface moisture. In fact, most of the heating at the surface goes into evaporating moisture which enhances the hydrological cycle. The analogy here is with the human body: if the heat is turned up, we sweat. The same is true for planet Earth.

Increases in moisture content (i.e., humidity) of the lower atmosphere (which are observed to be happening in many places) are a consequence of warming. The additional atmospheric moisture provides a resource for all precipitating weather systems, whether thunderstorms or extratropical rain or snow storms, because all of these systems feed upon the available moisture within their reach. This means enhanced rainfall or snowfall events, thus increasing risk of flooding, which is a pattern observed to be happening in many parts of the world. In particular in the United States and southern Canada, atmospheric moisture content is observed to have trended upward by about 10% over the past 20 years or so and records show that heavy rainfall and snowfall events are increasing at the expense of more moderate falls.

Although an increase of a few percent may not seem like much, it can be the straw that breaks the camel's back--or just what is needed to broach a dyke, as was the case in Grand Forks, North Dakota, in April 1997, when melting snow caused extensive flooding in the Red River basin (the river peaked at 54 feet (16.5 m) above flood stage and the levies and dyke system broke at 51.5 feet (15.69 m) above flood stage). Increased evaporation also leads to the expectation of enhanced droughts (earlier onset, longer lasting, greater intensity) and greater wilting of vegetation. It also means that average precipitation should increase as a whole, and this is observed mainly over land in mid to high latitudes.

Other climate phenomena are exhibiting unusual behavior, for example, El Niño, which disrupts weather patterns around the globe causing floods and droughts. Since the late 1970s there are signs that El Niño is becoming more frequent compared with the previous hundred years, but the associated changes in precipitation in the tropics dominate and complicate the record there. A new major El Niño is currently underway and is expected to continue developing throughout the winter. Is this behavioral change related to global warming? It could be, but we cannot yet be sure. Nevertheless, because El Niño brings droughts to Australia, Indonesia, parts of southern Africa, Southeast Asia, northeast Brazil and Columbia, and floods to the west coast of South America and some other places, these naturally occurring floods and droughts are apt to be worse with global warming effects superposed.

Causes of change

It is one thing to identify changes in climate that deviate from past patterns. But it is much more difficult to demonstrate that such changes are the result of human activity. A parallel here is trying to link lung cancer to smoking. There are always some people who smoke who do not get lung cancer, and some who get lung cancer who do not smoke. Yet the evidence is compelling that there is a link. Still there are always people who do not want to believe and justify their beliefs by feeding on the legitimate uncertainties that exist.

After carefully examining all of the available evidence, the Scientific Assessment Report in 1995 of the IPCC concluded that "the balance of evidence suggests a discernible human influence on global climate." The evidence examined included all the observations of changes, including paleoclimatic indicators from the distant past, and the patterns of changes, such as how temperatures are changing with altitude and geographically. Climate models also played a role by estimating what changes should have occurred given observed changes in atmospheric composition, the Sun and other effects (such as volcanic eruptions) over the past century, and by helping to assess the extent of natural variability. Thus far, the human-induced effects are relatively small compared with the huge day-to-day variations of weather. In addition, natural variability occurs on seasonal-to-interannual and decadal timescales and contributes to the climate record for the planet Earth. As a result, any anthropogenic signal in the climate record is hard to detect.

But the projections made with climate models are clear. Estimates are made of future emissions of carbon dioxide and are translated into expected concentrations of gases. The models indicate that carbon dioxide concentrations will continue to increase unless emissions are substantially reduced below today's values. It is estimated, for instance, that carbon dioxide concentrations will probably increase to 700 parts per million by volume (ppmv) by the year 2100 (compared with 360 ppmv in 1996 and 280 ppmv 200 years ago). The best estimates are that global mean temperatures will continue to increase, by about 1.0 to 3.5 degrees C (2 to 6 degrees F) by the year 2100 and sea level will increase by another 15 to 95 cm (6 to 37 inches). Nevertheless, because the extremes undergo the biggest percentage change, the extremes are the main way that we will notice climate change: the very hot or the very humid days, the heavy rains, the droughts, the fewer very cold days, and so on.

Does climate change matter?

So what does all this mean? What, if anything, should be done? Why should we care? Clearly, addressing these questions involves much more than scientific judgments but relates to value systems and considerations such as to what kind of stewards we are for the planet Earth and what kind of environment we wish to leave to future generations.

It is important to realize that we have set in motion on planet Earth an experiment, which we cannot turn off should we not like what we discover to be the immediate or eventual consequences. This is because of the long lifetimes of carbon dioxide (centuries) and other greenhouse gases in the atmosphere and because of the thermal inertia of the oceans. The oceans overturn very slowly and take hundreds of years to adjust fully to changes, so that manifestations of changes that have already occurred are not yet fully seen.

It is clear that effects of global warming are fairly small at present, but they are unmistakably emerging and having impacts. The insidious thing about global warming is that human-induced changes are always in one direction, and thus they accumulate with time. Moreover, the changes will continue long into the future even if we want them to stop and even in the unlikely event that we abruptly reduce carbon dioxide emissions. Although some warming may be beneficial, for example by lengthening the growing season in places like Canada, other projected changes, such as rising sea levels and more extreme droughts and floods, are more likely to have adverse effects. The problem is that just as the climate has changed to a point where you may like it, it continues to change and in ways not fully predictable.

The process of change itself is also very disruptive. It is disruptive to the natural environment and ecological systems that over the past 10,000 years have not experienced rates of change as large as those that are projected. It is also disruptive to human systems, including agriculture, water resources, fisheries, and energy use, because suddenly we find that the recent past weather is no longer a useful guide to the future. For instance, if the return period of a particular severe storm changes from once per hundred years to once in fifty years, design criteria for dams, levies, buildings and so on become obsolete. Thus, climate change disrupts planning. Our understanding of the climate system is such that unanticipated surprises producing disruptive impacts in some areas are highly probable. It is for these reasons that the projected rate of climate change is so alarming to many scientists.

I have found it useful to recognize that there are several world views that help to characterize the issues. At perhaps one extreme is the environmentalist who believes that we should conserve the environment and who therefore has a political agenda that calls for actions to mitigate and abate the increases in greenhouse gases, for instance with policies or incentives designed to limit emissions into the atmosphere. At another extreme are those who think that change is inevitable, but that technology will enable us to adapt to climate change as it happens. Of course most people fall somewhere in between. For example, one growing approach is to recognize limits to growth and subscribe to sustainable development that places a premium on use of renewable resources. Also, it is essential to recognize the class of people with vested interests. These are all legitimate viewpoints and must be accommodated in any actions contemplated. The question should be not what to do, but how to do it so that employment is maintained, economies are strengthened, and resources conserved.


(1) Crittenden, Guy. 1997. The day the Earth warmed up. (Toronto) Globe and Mail November 22 p D1.

(2) Trenberth, Kevin E. 1997. The use and abuse of climate models. Nature 386: 131-133.

(3) Hurrell, J.W. and K.E. Trenberth. 1997. Spurious trends in satellite MSU temperatures from merging different satellite records. Nature 386:164-167.

1 Dr. Kevin E. Trenberth is Head of the Climate Analysis Section at the U.S. National Center for Atmospheric Research, which is sponsored by the National Science Foundation. He was a convening lead author of the 1995 IPCC Scientific Assessment of Climate Change and he is Co-chair of the Scientific Steering Group for the World Climate Research Programme's Climate Variability and Predictability (CLIVAR) program. He is a fellow of the American Meteorological Society and American Association for Advancement of Science, and an honorary fellow of the Royal Society of New Zealand.

Your comment on this article is invited and should be addressed to: For further information on submitting a contribution to naturalSCIENCE, please see the Author Guide

Cover Stories
Open Forum
What's New
Copyright © 1998 Kevin E. Trenberth