GUEST ESSAY The Scientific Consensus About Global Warming John Harte

Transcription

1 GUEST ESSAY The Scientific Consensus About Global Warming John Harte John Harte is Professor of Energy and Resources at the University of California, Berkeley. He is a Phi Beta Kappa distinguished Visiting Scholar, a Fellow of the American Physical Society, a Member of the California Academy of Sciences, a Guggenheim awardee, and a recipient of a Pew Scholars Prize in Conservation and the Environment. He is the author or coauthor of 7 books, including The Green Fuse and Consider a Spherical Cow, and over 100 journal articles in fields ranging from biodiversity and conservation to biogeochemistry, global warming, and sustainability. His current research focuses on three areas: ecological responses to global climate change, the relationship between ecological diversity and function, and the social role of ecological complexity. Global warming is a controversial political topic, but a global network of the roughly 2,500 of the world's leading climate experts have reached a remarkable consensus about the underlying technical issues. This essay will summarize the facts about global warming that nearly all of these climate scientists agree upon, the slightly less certain projections of potential impacts of global warming on human society, and the remedial actions that could be taken to reduce future warming. It concludes with an analysis of a collection of myths about the subject that have arisen to a great extent because of the intense political debate surrounding the issue. What do most climate scientists agree upon? First, we know that over the past 200 years humanity has altered the composition of the atmosphere. In particular, we have loaded the air with gases, such as carbon dioxide (up to 30% since 1800, largely from combustion of coal, petroleum products, and natural gas) that trap heat and thus alter our climate. The basic mechanism is simple. Light from the sun largely penetrates the atmosphere and is absorbed at the planetary surface. There, it is converted from energy in the form of light to energy in the form of heat. As the surface temperature rises because of this heat, the earth radiates more and more heat back out to space, thereby maintaining an "energy balance". But if the amount of heat absorbing gas in the atmosphere rises, then some of the escaping heat is trapped and radiated back to the planetary surface, where it causes a further rise in temperature.

2 This is called the greenhouse effect and gases like carbon dioxide are called greenhouse gases; other greenhouse gases whose concentrations have increased because of human activity are methane, nitrous oxide, and chlorofluorocarbons (which also are responsible for the ozone thinning in the stratosphere). Secondly, we know from temperature records maintained at many hundreds of sites around the world that the average global surface temperature has risen by about 0.6C o (1.1F o ) during the past 100 years. It hasn't been a steady warming but the overall trend is upward; during the period from 1940 to 1970, the temperature actually fell a little. A warming of 0.6C o (1.1F o ) is actually quite a substantial rise; the odds that this could be to due to natural variation in the strength of sunlight or to other natural causes are small. This historic warming has not been uniform; it is greater at the poles than at the equator, it is greater at night than during the day, and it is much greater low in the atmosphere than it is higher up. All three of these patterns match quite well the predictions from mathematical models used to project the future strength of the greenhouse effect, and this has given us confidence that the models are fairly reliable. They are by no means perfect, however, and much remains to be learned about the climate system. Most importantly, our understanding of clouds their formation and their influence on climate is less than adequate. That is why scientists give a range of model predictions about the future climate change. Thirdly, if current trends in fossil fuel use and other sources of greenhouse gases continue, then around during the twenty first century the rise in the average surface temperature is predicted from these models to be in the range of 2-4.5C o ( F o ). About 20,000 years ago, when the earth was in the grip of the last ice age, the average surface temperature was roughly 5.6C o (10F o ) colder than it is today. Do you see why this suggests that the projected warming could indeed have larger consequences? What are the projected, although somewhat speculative, consequences of global warming? Climatic extremes (drought, deluge, summer heat waves) are expected to occur more frequently and possibly more intense. This has obvious implications for human health and welfare, prompting some insurance companies to rethink their policies in this area of risk. Although total precipitation may increase in parts of the world, increased temperatures are likely to lead to drier soil conditions, which will stress our capacity to produce food and perhaps alter many of the earth's natural ecosystems.

3 Moreover, winter snowfall in many parts of the world is likely to be replaced by rain, resulting in a reduced snowpack and earlier snowmelt; because agriculture in much of the western United States is dependent on the snowpack for irrigation water in summer, this could add to the stress on food production. A rise in sea level is also projected, although the rate of rise is still conjectural. Most estimates suggest that the rise will be slight during the next 50 years but that within one or two hundred years it could be as much as several meters. The effects of this will be felt particularly on small island nations in the South Pacific, on large areas in places like Bangladesh and South Florida that lie near sea level, and on near shore ecosystems and shoreline construction around the world. Yet another possible consequence of global warming is the spread of tropical diseases to temperate zone nations such as the U.S. The reason is that insects that spread certain tropical diseases will be able to survive in the warmer climate of the temperate zones, where they are now virtually non existent. What remedial actions might society take to reduce the risk? The following remedial actions could be taken: 1. Promote the development of affordable and clean solar energy (including direct solar in the form of solar cells and indirect solar in the form of wind and biomass) through increased support of research and development and economic incentives. Economists sometimes refer to this as leveling the playing field because many non solar energy industries already receive large subsidies and incentives from governments. In contrast to the burning of coal, oil, and gas, solar energy results in very little greenhouse gas emissions (producing the cement for solar collector bases would release some carbon dioxide!). The cost of solar energy is dropping fast and wind energy is already competitive. With some additional nurturing solar cells could become cost competitive with conventional energy in the coming decades. 2. Promote greater efficiency in the use of energy. It was once widely believed that a nation's economy could only grow if energy consumption grew. But during the late 1970s and early 80's, energy use in the United States remained fairly constant and yet the economy grew. The reason was that people began driving more fuel efficient cars, insulating their homes better, improving the efficiency of factories, and using more efficient electric appliances. The opportunities to further reduce energy consumption without sacrificing life style are considerable.

4 3. Reduce deforestation worldwide. Deforestation is the second most important source (after fossil fuel use) of carbon dioxide to the atmosphere. The reason is that forests store a large amount of carbon in the living trees; when the trees are cut and either burned or allowed to rot, the carbon is converted to carbon dioxide. There is a growing consensus among economists and scientists that taking steps 1 and 2 above would in the long run not only reduce the threat of global warming, but would also strengthen our economy and save consumers money, reduce dependence on unstable supplies of fuel, reduce urban smog, and reduce other hazards associated with fossil fuel use, such as acid deposition and oil spills. For that reason, taking steps 1 and 2 is sometimes called a "no regrets policy"; even if global warming predictions are wrong, we wouldn't regret developing clean energy and learning to use less energy to achieve the same economic goals. What are some global warming myths? The political battle over whether to take remedial actions to reduce the threat of global warming has been heating up. An intense and well publicized attack on the current consensus of climate scientists has been underway for over a decade. Among the most pervasive myths and distortions accompanying these efforts are the following. 1. The historic global warming trend over the past 100 years is the result of an increasingly brighter sun. In fact, no mechanism is known today that could convert the observed very slight changes in solar output into a warming trend consistent with observations. 2. Satellite data disprove the hypothesis that the historic warming is due to greenhouse gases. In fact, the satellite data cover too short a period of time to disprove current climate models, but the available satellite data (mostly on upper atmosphere temperatures) are reasonably consistent with model projections. 3. Most of the warming occurred early in the century but the atmosphere has only dramatically changed more recently. In fact, the warming trend since 1970 has been even more dramatic than that early in the century, consistent with the fact that the rate of buildup of greenhouse gases is increasing. 4. We will welcome global warming because an ice age is coming. Yes, another ice age will undoubtedly come, but not in the next few hundred years; perhaps over the next several thousand years, the earth will cool, but on millennial time scales the

5 warming episode that human activity is now triggering will long have dissipated as the greenhouse gases largely disappear from the atmosphere. 5. A cooling trend during the period contradicts our climate models. In fact, the cooling trend resulted from measured increasing levels of light obscuring fine particles (from fuel burning) and dust in the atmosphere during that period; although climate modelers initially failed to include that in the analyses, when the particles are included, the model predictions are consistent with observations. The very nature of science is distorted by this debate. Scientists can never know with absolute certainty that a theory is true; all they can do is disprove a theory by making observations that contradict the theory. By arguing that uncertainties in our current state of knowledge of global warming (of which there are many) obviate the need to take action on the basis of the things we are fairly confident about, those that oppose taking action undermine the public's understanding of the scientific process. This will not well serve future generations, who increasingly will need to base sound public policy on the best available scientific information. The way we deal with the threat of climate change will be a critical test society's maturity at skillfully blending science and policy in the decades ahead.