Earth's cooling came to sudden halt in 1900, study shows

The team omitted Africa from its analysis because the proxy records are too few to be useful. Instead, the researchers looked at seven continent-scale regions in South America, the southwestern Pacific (including Australia), as well as North America, Europe, Asia, Antarctica, and the Arctic.

This approach reveals a more complex picture over various intervals than a record of global average or hemispheric temperatures would present, the researchers find.

The team also notes that their work likely understates the overall intensity of warming because the reconstruction record stops short of the record-setting warm years and the warmest decade on record that have occurred since 1990.

On millennial time scales, the cooling trend was statistically significant on all continents except North America, where the cooling was weaker, according to the study.

Over the period the study covers, the cooling was driven variously by volcanic activity, which pumps light-reflecting aerosols high into the atmosphere, subtle changes in Earth's orbit, subtle changes in the amount of light the sun was emitting at the time, and land-use changes, the researchers say. The relative influence of each factor varied, depending on location.

The Medieval Warming Period, which the team pegs at between 830 and 1110 AD, encompassed the northern hemisphere at that time, while in South America and the southwestern Pacific, a period of relatively sustained warmth ran from 1160 to 1370.

The cooling trend first appeared to take hold in the Arctic, Europe, and Asia between 1200 and 1500 AD. North America felt it too, but less intensely, the study suggests. By the late 1500s, the cooling had expanded to include Antarctica.

That came as a surprise, Kaufman says. The progression of cooling from north to south is a pattern one would expect to see from the combined effects of subtle changes in Earth's orbit and changes in its tilt, he explains. That cooling in the north came, in part, as the Arctic received less sunlight over a given surface area in the summertime. But that also should lead to more sunlight per square meter in Antarctica.

In addition, changes in the sun's output and other sources of natural "forcings" on climate appear to have been far less influential at the time in Antarctica than over the northern hemisphere's continents.

Cooling was not as pronounced there as it was at the top of the world, but the evidence that cooling happened at all represents a new puzzle for researchers to solve.

The team also notes that its reconstruction indicates that the Medieval Warming Period and later the Little Ice Age were not global events, as some have claimed, but climate excursions whose timing and intensity varied by region.

After 1900, warming hit all continents except Antarctica. The bottom of the world continued to cool. It's a trend other researchers attribute to the slow pace of temperature changes in the frigid moat of a Southern Ocean that surrounds the ice-encrusted continent. And high-altitude winds that encircle the continent have intensified, keeping cold air trapped at high southern latitudes.

Previous

1 | 2