Global temperatures highest in 4,000 years

Global temperatures are higher than at any time in at least 4,000 years, scientists reported Thursday, and over the coming decades are likely to surpass levels not seen on the planet since before the last ice age.

Previous research had extended back roughly 1,500 years and suggested that the rapid temperature spike of the past century, believed to be a consequence of human activity, exceeded any warming episode during those years. The new work confirms that result while suggesting that the modern warming is unique over a longer period.

Even if the temperature increase from human activity that is projected for later this century comes out on the low end of estimates, scientists said, the planet will be at least as warm as, and probably warmer than, it was at any point in the modern geological era, known as the Holocene.

That epoch began about 12,000 years ago, after changes in incoming sunshine caused vast ice sheets to melt across the Northern Hemisphere. Scientists believe the moderate climate of the Holocene set the stage for the rise of human civilization roughly 8,000 years ago and continues to sustain it by, for example, permitting a high level of food production.

In the new research, scheduled for publication Friday in the journal Science, Shaun Marcott, an earth scientist at Oregon State University, and his colleagues compiled the most meticulous reconstruction yet of global temperatures over the past 11,300 years, virtually the entire Holocene. They used indicators such as the distribution of microscopic, temperature-sensitive ocean creatures to determine past climate.

Like previous such efforts, the method gives only an approximation. Michael E. Mann, a researcher at Pennsylvania State University who is an expert in the relevant techniques but was not involved in the new research, said the authors had made conservative data choices in their analysis. “It's another important achievement and significant result as we continue to refine our knowledge and understanding of climate change,” Mann said.

Although the paper is the most complete reconstruction of global temperature, it is roughly consistent with previous work on a regional scale. It suggests that changes in the amount and distribution of incoming sunlight, caused by wobbles in the earth's orbit, contributed to a sharp temperature rise in the early Holocene.

The climate then stabilized at relatively warm temperatures about 10,000 years ago, hitting a plateau that lasted for roughly 5,000 years, the paper shows. After that, shifts of incoming sunshine prompted a long, slow cooling trend.

The cooling was interrupted, at least in the Northern Hemisphere, by a fairly brief spike during the Middle Ages, known as the Medieval Warm Period. (It was then that the Vikings settled Greenland, dying out there when the climate cooled again.) Scientists say that, if natural factors were still governing the climate, the Northern Hemisphere would probably be destined to freeze over again in several thousand years.

“We were on this downward slope, presumably going back toward another ice age,” Marcott said. Instead, scientists believe the enormous increase in greenhouse gases caused by industrialisation will almost certainly prevent that.