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. 

Anthropogenic Factors of Climate Change

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 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.

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.” 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.”

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.

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. 

Current CO2 data from the Global CO2 Monitoring Network are made available by the Cooperative Air Sampling Network. 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.

For at least 650,000 years and probably ever since humans walked the Earth, the carbon dioxide concentration in the atmosphere was never even close to as high as it is at present, as shown in figure 3-1. Current CO2 concentration has risen above 380 parts per million (ppm), while the preindustrial level back throughout the Holocene (the past 10,000 years) was close to 280 ppm. Similar values apply for previous interglacial periods.

We now come to the second part: the recent rise in CO2 is entirely anthropogenic. This is also undisputed. We have tracked and we know how much fossil fuel has been burned and therefore how much CO2 we have injected directly into the atmosphere. The observed increase in CO2 concentration over the past decades is equal to 57 percent of our cumulative emissions. Other parts of the climate system—the ocean and the land biosphere—have absorbed the remaining 43 percent of emissions from the atmosphere. 

For the ocean, this is documented by around 10,000 oceanographic measurements, which show that the ocean has taken up about 2 gigatons (Gt) of carbon per year, or 30 percent of anthropogenic emissions (see figure 3-2).10 This CO2 uptake of the ocean makes the sea water more acidic and threatens marine life, which in itself is sufficient reason to reduce our carbon dioxide emissions significantly, even in the absence of climate change.

The Observed Climatic Warming
It is time to turn to statement B: human activities are altering the climate. This can be broken into two parts. The first is as follows: global climate is warming. This is by now a generally undisputed point (except by novelist Michael Crichton), so we deal with it only briefly.32 The two leading compilations of data measured with thermometers are shown in figure 3-3, that of the National Aeronautics and Space Administration (NASA) and that of the British Hadley Centre for Climate Change. 

Although they differ in the details, due to the inclusion of different data sets and use of different spatial averaging and quality control procedures, they both show a consistent picture, with a global mean warming of 0.8°C since the late nineteenth century.

Temperatures over the past ten years clearly were the warmest since measured records have been available. The year 1998 sticks out well above the longterm trend due to the occurrence of a major El Niño event that year (the last El Niño so far and one of the strongest on record). 

These events are examples of the largest natural climate variations on multiyear time scales and, by releasing heat from the ocean, generally cause positive anomalies in global mean temperature. It is remarkable that the year 2005 rivaled the heat of 1998 even though no El Niño event occurred that year. (A bizarre curiosity, perhaps worth mentioning, is that several prominent “climate skeptics” recently used the extreme year 1998 to claim in the media that global warming had ended. In Lindzen’s words, “Indeed, the absence of any record breakers during the past seven years is statistical evidence that temperatures are not increasing.”)

In addition to the surface measurements, the more recent portion of the global warming trend (since 1979) is also documented by satellite data. It is not straightforward to derive a reliable surface temperature trend from satellites, as they measure radiation coming from throughout the atmosphere (not just near the surface), including the stratosphere, which has strongly cooled,34 and the records are not homogeneous due to the short life span of individual satellites, the problem of orbital decay, observations at different times of day, and drifts in instrument calibration. Current analyses of these satellite data show trends that are fully consistent with surface measurements and model simulations.

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