The Hottest Century?

“Reconstructing Climatic and Environmental Changes of the Past 1,000 Years: A Reappraisal” by Willie Soon et al., in Energy & Environment (Mar. 2003), 5 Wates Way, Brentwood Essex CM15 9TB, United Kingdom.

The world has just put a long, hot century behind it, and now the question of where the era stands in the history of the world’s climate has become an item in the debate over global warming. One influential recent study of global temperature changes over the past millennium found that, for the Northern Hemisphere at least, the 20th century was the warmest century, the 1990s the warmest decade, and 1998 the warmest year. These conclusions lend more weight to the argument that anthropogenic (human-generated) greenhouse gases have produced anomalously high temperatures. (Many other, though narrower, studies point toward this reading of climate history.) Soon, a physicist at the Harvard-Smithsonian Center for Astro­physics, in Cambridge, Massa­chusetts, and his colleagues, taking a different approach, have concluded that the 20th century was probably “not the warmest” of the millennium.

In the earlier study, Michael E. Mann, an environmental scientist at the Univer­sity of Virginia, and his colleagues attempted an ambitious mathematical reconstruction of global temperature changes over the past thousand years based on various “proxy” data, such as ice core samples. Besides selecting winners (or losers) in the “warm­est” category, they dismissed the conventional wisdom among climatologists that there were two previous periods of great divergence from the climate norm: the so-called Little Ice Age (1300–1900) and the Medieval Warm Period (800–1300). The elimination of those two epochs would cast the 20th century as even more of an anomaly.

Soon and his coauthors, taking “a non-quantitative and very ‘low-tech’” approach to the problem, examined a multitude of research results obtained from local and regional climate indicators, such as coral and tree ring growth, lake fossils, ice cores, glaciers, and seafloor sediments. The results cannot be combined into a simple hemispheric or global numerical composite, the authors say, but still are revealing. “The picture emerges from many localities” that the Little Ice Age and the Medieval Warm Period were indeed “widespread” phenomena, even if not “precisely timed or synchronous.”

As for the rising thermometer readings of the 20th century, say Soon and his colleagues, they appear in historical perspective “neither unusual nor unprecedented.” Tree ring chronologies in one study “show that the Medieval Warm Period [was] as warm as, or possibly even warmer than, the 20th century,” at least for a region of the Northern Hemisphere.

The authors agree that human activity has had a significant impact on some local environments, but just how big a role humans have played in heating the atmosphere in recent decades remains up in the air.