A Bigger Climate Context

Article excerpt

Byline: The Register-Guard

New research providing a longer view of the Earth's changing climate reinforces the idea that human activity is having an accelerating effect, warming the planet by 1.3 degrees Fahrenheit over the last century. The change is worse than it sounds, because the warming appears to have not just arrested but reversed a 5,000-year cooling trend.

Past attempts to plot temperatures over long periods have stretched back about 2,000 years. Many have relied on data from tree rings, which can provide accurate dates and evidence of shifts in climate but are also subject to local weather conditions.

Researchers led by geologist Shawn Marcott at Oregon State University have addressed both deficiencies. Their study, published in the journal Science, examined fossils of marine organisms taken from ocean sediment core samples. The fossils' positions in the sample strata provide a reliable dating method, and temperatures can be deduced from their chemical composition and isotopes. The group studied samples from 73 sites around the globe, smoothing any regional anomalies.

Using this method the researchers were able to construct a temperature chart stretching back 11,300 years - back to the beginning of the Holocene epoch. The Holocene began with the end of the last ice age, and coincides with the emergence of human civilization.

A long-term temperature chart is helpful, because it's known that the Earth's climate can change significantly without human agency - as the ice ages themselves show. Marcott and his partners found that changes have continued throughout the Holocene due primarily to variations in what's called "insolation" - the amount of solar energy reaching the Earth's surface.

During the first millennia of the Holocene, the northern hemisphere was relatively warm. …