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34 The idea that humans can change and are in fact changing the climate of our planet has developed gradually over more than a hundred years. A fringe idea in the nineteenth and early twentieth centuries,1 it is close to a wellestablished scientific consensus at the turn of the twenty-first century.2 The history of this development is grippingly told in a small book, The Discovery of Global Warming, by science historian Spencer Weart.3 During the course of this history, the initially outlandish concept of human-caused global warming has won over practically every skeptical climatologist who has cared to look dispassionately at the evidence. But with new developments in the field almost every year—for example, the growing understanding of abrupt climate changes, the record-breaking hurricane season of 2005, or the renewed concerns about the stability of the ice sheets—the “basics” are seldom discussed any more. Few people besides climatologists themselves, even in the climate policy community, could easily recount the main cornerstones of scientific evidence on which the case for anthropogenic warming rests. The goal of this paper is to do just that: to revisit the basic evidence for anthropogenic global warming. The Meaning of “Anthropogenic Climate Change” To start, we need to clarify what we mean by “anthropogenic climate change.” It is useful to distinguish two different meanings of the term, since they are Anthropogenic Climate Change: Revisiting the Facts stefan rahmstorf 3 10865-04_CH03_rev.qxd 12/10/07 11:39 AM Page 34 often confounded. The first one, let us call it statement A, can be summed up as follows: anthropogenic emissions of greenhouse gases will lead to significant global warming. This is a statement about the future. It is reflected, for example, in the well-known range of future scenarios of the 2001 Intergovernmental Panel on Climate Change (IPCC) report, which concluded that, in the absence of effective climate policies, we must expect a warming of between 1.4 and 5.8°C (centigrade) between the years 1990 and 2100.4 The second meaning, let us call it statement B, can be phrased thus: human activities already have noticeably changed global climate. This is a statement about the past and about what we can observe now. It is reflected in the famous IPCC statement of 1996: “The balance of evidence suggests that there is a discernible human influence on global climate.”5 It is reinforced considerably in the light of new evidence in the 2001 report: “There is new and stronger evidence that most of the warming observed over the last fifty years is attributable to human activities.”6 Only statement A is relevant to policy, because no current or planned policy can affect the past. Such policies are shaped by our expectations for the future. It is important to realize that statement A is not conditional on statement B. Thus, even if too much natural variability was masking any anthropogenic trend or if the quality of the data that we have simply was not good enough to detect any human influence on climate so far, we could (and would) still come to conclusion A. Nevertheless, both statement A and statement B are supported very strongly by the available evidence. Discussions about climate change in the popular media suggest that many people are misled by fallacious logic, for example, “If the Middle Ages were warmer than temperatures today, then recent warming is perfectly natural (this questions statement B), and we do not need to worry about the effect of our emissions (this questions statement A).” Both these conclusions are, of course, non sequiturs, quite apart from the fact that their premise (warmer Middle Ages) is not supported by the data. The Carbon Dioxide Effect on Climate What evidence do we have for statement A—that anthropogenic emissions will lead to significant global warming? I break this into three parts. First, the carbon dioxide (CO2) concentration is rising. This is proven by direct measurement in the atmosphere since the 1950s, set forth as the famous Keeling curve, and it is undisputed.7 Current CO2 data from the Global CO2 Monitoring Network are made available by the Cooperative Air Sampling Network.8 Ice core data, which provide a reliable and accurate record of CO2 concentration going back hundreds of thousands of years, show further that this rise is, in fact, very unusual.9 anthropogenic climate change 35 10865-04_CH03_rev.qxd 12/10/07 11:39 AM Page 35 [3.20.238.187...

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