The number isn't perfect, but it's an important red flag.
As the United Nations climate change talks in Lima enter into their second week, one measurement that's coming up a lot is 3.6 degrees Fahrenheit.
If global temperatures rise that much, scientists believe, the earth will suffer catastrophic damage from climate change: things like crop failure, ice sheet loss, and severe weather.
Don Wuebblesis a professor of atmospheric science at the University of Illinois. He said the number (which is a conversion from two degrees Celsius) isn't perfect, but it's an important red flag signaling the consequences of sustained rises in Earth's temperature.
"There really is no magic number. It's an approximation," Wuebbles said.
"The changes we're seeing in climate right now are occurring extremely fast."
Don Wuebbles
Here's how scientists got the 3.6 degree measurement: they studied ancient core samples from ice, rocks, and deep ocean sediment. Scientists compared Earth's temperatures in the different geological eras that formed those samples, and studied how those temperatures impacted concurrent sea levels.
"Those are good hints, but most of those changes, by and large, tended to occur very slowly," Wuebbles said. "Whereas, the changes we're seeing in climate right now are occurring extremely fast."
According to the Goddard Institute for Space Studies, the earth's average global temperature has risen 1.4 degrees Fahrenheit since 1880 -- and it's increasing 0.2 degrees more every decade. To stay within that 3.6- degree threshold, Wuebbles said current emissions need to be cut 80 percent before 2050.
