November 25, 2011
IPCC 2007 Estimates Of Global Warming Challenged By Scientists
According to a new study published this week in Science, the rate of global warming from doubling of atmospheric carbon dioxide may be less than previous estimates.
The study authors said that global warming is real and that increases of atmospheric CO2 will have multiple serious impacts. However, the most Draconian projections of temperature increases from the doubling of CO2 are unlikely, which is a projection made by the Intergovernmental Panel on Climate Change (IPCC) in 2007.
"When you reconstruct sea and land surface temperatures from the peak of the last Ice Age 21,000 years ago — which is referred to as the Last Glacial Maximum — and compare it with climate model simulations of that period, you get a much different picture," he added. "If these paleoclimatic constraints apply to the future, as predicted by our model, the results imply less probability of extreme climatic change than previously thought."
Scientists have struggled trying to determine how the Earth will respond to projected increases of atmospheric carbon dioxide.
IPCC said in 2007 that the air near the surface of the Earth would warm on average by 3.2 to 8.1 degrees Fahrenheit, with a doubling of atmospheric CO2 from pre-industrial standards.
Previous studies claimed the impacts could be much more severe, with as much as an 18 degree change, although these projections come with an acknowledged low probability.
The researchers used a climate model with more data and found that there are constraints that preclude very high levels of climate sensitivity.
They compiled land and ocean surface temperature reconstructions from the Last Glacial Maximum and created a global map of those temperatures.
Atmospheric CO2 was about a third less than before the Industrial Revolution during this time, and levels of methane and nitrous oxide were much lower.
The new data changed the assessment of climate models in many ways, according to the researchers. Their reconstruction of temperatures has greater spatial converged and showed less cooling during the Ice Age than most previous studies.
High sensitivity climate models suggest that the low levels of atmospheric CO2 during the Last Glacial Maximum would result in a "runaway effect" that would have left Earth completely ice-covered.
"Clearly, that didn't happen," Schmittner said in a press release. "Though the Earth then was covered by much more ice and snow than it is today, the ice sheets didn't extend beyond latitudes of about 40 degrees, and the tropics and subtropics were largely ice-free — except at high altitudes. These high-sensitivity models overestimate cooling."
He said that uncertainty levels may be underestimated because the model simulations did not take into account uncertainties from how cloud changes reflect sunlight.
"When we first looked at the paleoclimatic data, I was struck by the small cooling of the ocean," Schmittner said in a press release. "On average, the ocean was only about two degrees (Celsius) cooler than it is today, yet the planet was completely different — huge ice sheets over North America and northern Europe, more sea ice and snow, different vegetation, lower sea levels and more dust in the air.
"It shows that even very small changes in the ocean's surface temperature can have an enormous impact elsewhere, particularly over land areas at mid- to high-latitudes," he added.
He said unabated fossil fuel use could lead to similar warming of the sea surface as reconstruction shows what happened between the Last Glacial Maximum and today.
"Hence, drastic changes over land can be expected," he said in a press release. "However, our study implies that we still have time to prevent that from happening, if we make a concerted effort to change course soon."
The research was published in the journal Science.
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