The Earth's only been this warm for about a quarter of the time over the last 11,300 years, a new reconstruction of the planet's temperature history shows.
Using data from 73 sites around the world, a team from Harvard and Oregon State University has looked all the way back to the last Ice Age, putting today's climate into a broader context.
"We already knew that on a global scale, Earth is warmer today than it was over much of the past 2,000 years," says OSU post-doctoral researcher Shaun Marcott. "Now we know that it is warmer than most of the past 11,300 years. This is of particular interest because the Holocene spans the entire period of human civilization."
Over the past 5,000 years, says the team, Earth on average cooled about 1.3 degrees Fahrenheit - until the past 100 years, when it warmed ̴ 1.3 degrees. The largest changes were in the northern hemisphere, where there are more land masses and greater human populations.
Climate models project that global temperature will rise another 2.0 to 11.5 degrees Farenheit by the end of this century, largely dependent on the magnitude of carbon emissions.
"What is most troubling is that this warming will be significantly greater than at any time during the past 11,300 years," says OSU paleoclimatologist Peter Clark.
One natural factor affecting global temperatures over the past 11,300 years is gradual change in the distribution of solar insolation associated with Earth's position relative to the sun.
"During the warmest period of the Holocene, the Earth was positioned such that Northern Hemisphere summers warmed more," says Marcott. "As the Earth's orientation changed, Northern Hemisphere summers became cooler, and we should now be near the bottom of this long-term cooling trend - but obviously, we are not."
The research team relied mainly on fossils from ocean sediment cores and terrestrial archives to reconstruct the temperature history.
"The Earth's climate is complex and responds to multiple forcings, including CO2 and solar insolation," says Marcott.
"Both of those changed very slowly over the past 11,000 years. But in the last 100 years, the increase in CO2 through increased emissions from human activities has been significant. It is the only variable that can best explain the rapid increase in global temperatures."