20th Century Warming Not Unprecedented, Experts Testify

Published October 1, 2003

A central scientific issue in the debate over the sources of changes in the Earth’s climate rose to the level of a Senate hearing during the last week of July.

The hearing was called in an attempt to help determine the answers to some important questions: Was twentieth century warming unusual? Could it have been natural? Was it due to man-made greenhouse gas emissions … and how can we know that?

Those questions mask a very complex problem that is emerging as the most important issue in the global climate change debate.

The debate–and the hearing–were prompted by two academic studies that have reached different conclusions about the nature of global climate change. One study, published in March 2003, was led by Harvard astrophysicist Willie Soon. The other, published in 1998, was by University of Virginia climate statistician Michael Mann. The two studies are the key flashpoints in a larger raging debate within the field called paleoclimatology.

Paleoclimatologists study the history of the Earth’s climate and climate changes by using what are called proxy records. Since no thermometers or sensitive temperature-taking equipment existed for most of human history, paleoclimatologists use proxies, like tree rings or ice cores, to determine what the historical climate record was like.

Mann: Twentieth Century Warming Unprecedented

The fight is over the so-called “Medieval Warm Period” (MWP) and “Little Ice Age” (LIA). The MWP is thought to have occurred from roughly the year 800 to1300, and the LIA from 1300 to 1900.

The debate turns on whether the Medieval Warm Period and Little Ice Age in fact existed and, if so, if they were global phenomena. If they were, then it’s very likely warming observed in the twentieth century may be simply the climate emerging from the Little Ice Age, a perfectly natural process–not a human-induced one.

Advocates of the man-made warming theory, such as Mann and his colleagues, are climate change skeptics–that is, they believe the historical record says climate has been relatively stable compared to today, and that the twentieth century warming is anomalous, due to human influence, and deeply troubling. They contend that local periods of warming and cooling, while large, were not synchronous, so they do not represent true “climate change.” Over the past 10 years they have developed a number of statistical techniques to support their claims.

Mann’s 1998 paper was the high point in this scientific movement. Mann claimed to find no evidence of either the Little Ice Age or the Medieval Warm Period in the paleoclimate record. His sample of that record was very small, just 10 locations.

Despite that shortcoming, the Mann study was central to the 2001 Third Assessment Report of the United Nations Intergovernmental Panel on Climate Change (IPCC), when the IPCC concluded most of the late twentieth century warming was probably due to human influence. There is no overstating how influential the Mann paper has been within climate change science circles–and in the public debate beyond.

Soon: Compelling Evidence of Medieval Warmth

Soon and his colleagues published “Reconstructing Climatic and Environmental Changes of the Past 1000 Years: A Reappraisal” in March 2003 in the British journal Energy and Environment. The researchers reviewed roughly 250 paleoclimate studies, concluding the Medieval Warm Period and Little Ice Age were both real and global, albeit with significant regional variations. They wrote:

“The Medieval Warm Period of 800 to 1300 A.D. and the Little Ice Age of 1300 to 1900 A.D. were worldwide phenomena not limited to the European and North American continents. While 20th century temperatures are much higher than in the Little Ice Age period, many parts of the world show the medieval warmth to be greater than that of the 20th century.”

In other words, the warming observed during the twentieth century is not unprecedented. There have been earlier periods of warming, taking place long before the industrial revolution and thus long before humans began emitting greenhouse gases into the atmosphere.

In addition to the Soon study, there is abundant evidence from many parts of the world for the existence of both the Medieval Warm Period and Little Ice Age. The European Space Agency says the MWP and LIA were global. It also says the MWP was approximately 1º C warmer than today.

Geological Perspectives of Global Climate Change, written by geologist Lee Gerhard of the Kansas Geological Survey and others, concludes, “that the current rate of temperature increase is not unusual, despite the human-induced addition of CO2, implies that it is not possible to detect a human imprint on Earth temperatures.”

Debate Rages On

Soon and his colleagues directly address the Mann study, contending Mann’s statistical methods were flawed in such a way that they tended to hide variability, rather than reveal it. The proxy records paleoclimatologists use to recreate the climate record are considered “noisy” because the temperature signal is masked by many other variables. Mann’s method, according to Soon, tends to amplify that noise.

After the Soon study was published, Mann, along with 12 other prominent advocates of the theory of human-induced warming, published a pointed rejoinder in the July issue of the journal EOS, setting the stage for the Senate hearing.

At the hearing, and in the pages of EOS, Mann reiterated his claim that twentieth century warming is “unprecedented in the last 1000 years” and that this is the consensus view in the scientific community. The claim has often appeared in the press since the IPCC voiced it in 2001. Mann’s criticism in EOS and at the hearing prompted several misleading press accounts of the ongoing debate.

If the Medieval Warm Period and Little Ice Age, and the cycles before them, did not actually take place, then the recent warming may indeed be unprecedented. But their non-existence is by no means settled, as the Soon study makes clear. As David Legates of the University of Delaware testified before the hearing, “we chose … to … determine if the proxy records themselves indeed confirm the claim of the 1990s being the warmest decade of the last millennium. That claim is not borne out by the individual proxy records.”

And thus the debate rages on. On one side remains a prominent group of climate modelers and statisticians who fervently believe the recent observed warming is human-induced. They assert periods such as the Medieval Warm Period didn’t happen. As the EOS group puts it, “modeling and statistical studies indicate that such anomalous warmth cannot be explained by natural factors but, instead, requires significant anthropogenic (that is, ‘human’) influences during the 20th century.” To these researchers, it matters not what the proxy record says–their models cannot explain natural variability, and therefore the Medieval Warm Period and Little Ice Age could not have happened.

But much of the broader scientific community, especially many geologists and paleoclimatologists, do not accept that argument. Just because climate modelers cannot explain natural variability does not mean natural variability does not exist. They say they can see it clearly in the data.

David Wojick is an independent science journalist and policy analyst. His email address is [email protected].

For more information …

The full text of the 67-page study by Willie Soon and colleagues, Reconstructing Climatic and Environmental Changes of the Past 1000 Years: A Reappraisal, is available through PolicyBot. Point your Web browser to http://www.heartland.org, click on the PolicyBot icon, and search for document #12312.

Geological Perspectives of Global Climate Change, by Lee Gerhard et al., is available for $49 through Amazon.com. Point your Web browser to http://www.amazon.com/exec/obidos/ASIN/0891810544/theheartlandinst..