The Last Millennium

The time series shows the combined global land and marine surface temperature record from 1856 to 2000. Data from Jones et al., 1998; and from the Climate Research Unit at the University of East Anglia (Climate Research Unit; compilation by Phil Jones).
There was a time, not so long ago, when scientists assumed that climate is a more or less unchanging condition on the planet, perhaps punctuated by catastrophic events. (The concept of "punctuated equilibrium" in paleontology is reminiscent of this concept; see the glossary.) First Louis Agassiz's discovery of the "Great Ice Age" and subsequently the idea of multiple ice ages, in the middle of the 19th century, destroyed the idea of unchanging background conditions: climate changes all the time.

At the end of the 19th century, the first reconstructions of climate change over the last 1000 years were published by the Austrian climatologist Eduard Brückner. In recent decades, the role of climate history as a means toward understanding the Earth has received increasing attention, following the monumental work of the British meteorologist Hubert H. Lamb (1913-1997).

The chief benefit of climate history is the expansion of our horizons even beyond imagination. Without the historical perspective we are trapped within the limited wisdom provided by physics and the experience of individual observers. It is perhaps surprising that the various expert opinions in the climate-related sciences regarding the problem of man-made global warming can diverge as much as they do. But it will also surely come as a shock to realize that many of these opinions are not especially relevant to the question of how much of the warming experienced over the last several decades can be ascribed to human impact.

The reason for this is actually quite simple. Physics can tell us a lot about how the climate machine works. We are in great need of that knowledge, and we should vigorously pursue such studies. However, only climate history, that is, long-term experience, can tell us whether we are witnessing highly unusual conditions or not. In some ways, only the scientists studying long time series of climate change have anything worthwhile to say about whether the Earth is warming or not. Their data show that the Earth is warming, and it is doing so at a highly unusual rate and toward highly unusual conditions. Concerning the future, the balance between physics and history is a bit more difficult to assess. We definitely need to generate mathematical simulations of the climate machine using computer programs, and ask how these artificial climate systems would react if we were to change the trace-gas content in the atmosphere, or the amount of aerosol, or the number of hurricanes, or any number of other conditions. (After all, we cannot do these experiments any other way.)

But we also need to check somehow whether our machines perform realistically, or whether they are too constrained (from a lack of understanding) to give what we consider "reasonable" responses. To make the artificial machines into more realistic performers, they need to be constructed in such a fashion as to be able to simulate the past. In this sense, the past performance of the climate system is a guide to its future behavior.