The planet's temperature has climbed to levels not seen in thousands of years, warming that has begun to affect plants and animals, researchers report in Tuesday's issue of Proceedings of the National Academy of Sciences.
The Earth has been warming at a rate of 0.36 degree Fahrenheit per decade for the last 30 years, according to the research team led by James Hansen of NASA's Goddard Institute for Space Studies in New York.
That brings the overall temperature to the warmest in the current interglacial period, which began about 12,000 years ago.
Hansen, who first warned of the danger of climate change decades ago, said that human-made greenhouse gases have become the dominant climate change factor.
The study said the recent warming has brought global temperature to a level within about one degree Celsius - 1.8 degree Fahrenheit - of the maximum temperature of the past million years.
"If further global warming reaches 2 or 3 degrees Celsius, we will likely see changes that make Earth a different planet than the one we know. The last time it was that warm was in the middle Pliocene, about 3 million years ago, when sea level was estimated to have been about 25 meters (80 feet) higher than today," Hansen said.
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