NEW YORK, April 22 (UPI) -- Earth's temperatures increased more between 1971 and 2000 than during any other three-decade interval in the last 1,400 years, U.S. researchers say.
Scientists at the Earth Institute at Columbia University said new regional temperature reconstructions covering all seven continents show the current ongoing period of man-made global warming reversed a natural cooling trend that lasted several hundred years.
The reconstructed the temperature history of Earth's continents by analyzing climate data from tree rings, pollen, cave formations, ice cores, lake and ocean sediments, and historical records from around the world, a Columbia release said Monday.
On the continent of Europe, a 2003 heat wave and drought that killed an estimated 70,000 people happened during Europe's hottest summer of the last 2,000 years, the researchers said.
The reconstructions showed temperatures varied less between continents in the same hemisphere than between hemispheres, the scientists said.
The most consistent trend across all regions in the last 2,000 years was a long-term cooling, the said, likely caused by a rise in volcanic activity, decrease in solar irradiance, changes in land-surface vegetation and slow variations in Earth's orbit.
That cooling tapered off at the end of the 19th century with the onset of industrialization and an increase in industrial greenhouse gas emissions, the study suggested.
"This paper tells us what we already knew, except in a better, more comprehensive fashion," study co-author Edward Cook said.