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ments (initiated in 1958) with estimates of global fossil fuel combustion and then makes crude estimates of future concentrations. It then combines concentration estimates with early radioactive connective models to estimate temperature change, and compares estimated changes to observed changes with consideration given to intrinsic climate variability. Finally, it speculates about possible impacts beyond temperature such as the CO2 fertilization of plant growth.

The National Academy of Sciences 1977 report on Energy and Climate serves as another demonstration of the consistency in our understanding of the climatic implications of transferring large quantities of carbon from fossil reservoirs to the atmosphere. When compared to recent Intergovernmental Panel on Climate Change (IPCC) reports, these older studies show that although there is a wealth of new science, what is really new is the growing will toward real action—and sadly an unambiguous trend to toward increasing in the length and opacity of climate assessment reports.

Are we seeing an anthropogenic climate signal yet? What signal will we see if we double or triple CO2 concentrations? These are the two crucial scientific questions. Although they are often deliberately blurred in public debate, they are sharply distinct. On the first, my answer is we cannot yet claim to have seen an unambiguous anthropogenic signal. Although we can measure the signal (the climate record) reasonably well, the crux of this so-called detection and attribution problem is our uncertainty about the climate's unforced variability over decade-to-century time scales—the climate's noise spectrum—and our uncertainty about the magnitude of natural forcing such as solar variability. Without robust understanding of the noise we can't calculate the signal-to-noise ratio very confidently and thus cannot yet answer the detection question unequivocally.

It is the second question, however, that ought to matter for policy: If we double or triple CO2, will we see a big signal? Here the answer is an unequivocal yes. With the exception of a few outspoken individuals, there is essentially no serious scientific dissent on this question.

When you move beyond the climate science to consider the impacts of climate change, the answer bifurcates again. If you ask, “will there be substantial impacts on many lightly managed ecosystems and on some human populations?” the answer is unequivocally yes. Examples include the ecosystems of the high Arctic and the inhabitants of low-lying islands.

If, however, you want to know about the net economic impacts of climate change, the answer is much less certain. Most economic modeling studies suggest that if we do nothing to abate climate change, we will suffer a loss of a few percent of global gross domestic product (GDP) by 2100 and that modest efforts at abatement are capable of reducing this loss (e.g., the benefits of abatement can outweigh the costs). Modeling our economic future on 100-year time scales is, however, an uncertain and untested art; both the costs of mitigation and the estimates of impact are highly uncertain.

When judged in gross economic terms, climate change will not be catastrophic and is in fact (arguably) minor compared to other economic uncertainties such as the rate of technological change or the evolution of the inequality in distribution of the world's wealth. Despite some overheated rhetoric, our civilization will not collapse if we fail to act aggressively to counter climate change. Humans are very robust. Nevertheless, climate change poses very serious environmental problems. If we double or triple carbon dioxide—as many business-as-usual projections suggest we will within this century—we will see substantial climate change and very substantial changes to many natural ecosystems. Many of us value these ecosystems and value the rights of human communities that will be most affected. We have shown by other actions that we are willing to pay a price to protect such values. I hope and expect that we will take strong action to stabilize CO2 concentrations. The history of pollution control technologies gives reason to hope that the price of achieving deep cuts in CO2 emissions will be lower than we predict. The topic of this meeting gives us an explicit basis for that hope.



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