On our current emissions path, CO2 levels in 2100 will hit levels last seen when the Earth was 29°F (16°C) hotter

from
http://thinkprogress.org/romm/2011/01/13/207334/science-kiehl-ncar-paleoclimate-lessons-from-earths-hot-past/

By Joe Romm on Jan 13, 2011 at 5:02 pm

http://www2.ucar.edu/news/3628/earth-s-hot-past-could-be-prologue-future-climate

Earth’s hot past could be prologue to future climate
January 13, 2011

BOULDER—The magnitude of climate change during Earth’s deep past suggests that future temperatures may eventually rise far more than projected if society continues its pace of emitting greenhouse gases, a new analysis concludes. The study, by National Center for Atmospheric Research (NCAR) scientist Jeffrey Kiehl, will appear as a “Perspectives” piece in this week’s issue of the journal Science.

Building on recent research, the study examines the relationship between global temperatures and high levels of carbon dioxide in the atmosphere tens of millions of years ago. It warns that, if carbon dioxide emissions continue at their current rate through the end of this century, atmospheric concentrations of the greenhouse gas will reach levels that last existed about 30 million to 100 million years ago, when global temperatures averaged about 29 degrees Fahrenheit (16 degrees Celsius) above pre-industrial levels.

Kiehl said that global temperatures may gradually rise over the next several centuries or millennia in response to the carbon dioxide. Elevated levels of the greenhouse gas may remain in the atmosphere for tens of thousands of years, according to recent computer model studies of geochemical processes that the study cites.

The study also indicates that the planet’s climate system, over long periods of times, may be at least twice as sensitive to carbon dioxide than currently projected by computer models, which have generally focused on shorter-term warming trends. This is largely because even sophisticated computer models have not yet been able to incorporate critical processes, such as the loss of ice sheets, that take place over centuries or millennia and amplify the initial warming effects of carbon dioxide.

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