10 Degrees Hotter by 2100? Odds Are Good, Unless We Act

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The future impacts of global warming may be twice as worse as we thought just a few years ago.

If current emissions trends continue, there's a very high probability that by century's end, the Earth's median surface temperature may increase 9.3 degrees F (5.2 degrees C) over average temperatures between 1981-2000, according to a team of MIT researchers .

In the Arctic, where climate changes are amplified, temperatures could rise as much as a median 20° F -- at which point the death knell for the Arctic ice cap and the Greenland ice sheet will have long sounded.

This is an update to a 2003 study made using the MIT Integrated Global System Model, which predicted an increase of 4.3 degrees F (2.4 degrees C). Initially released in February, yesterday the new research was published in the American Meteorological Society's Journal of Climate. While scientists will no doubt continue to discuss the particulars of how the data was analyzed, publication in a peer-reviewed journal lends a lot of weight to the findings, and to the team's conclusions, as reported by Reuters:

These projections indicate that "without rapid and massive action," this dramatic warming will take place this century, the statement said.

The outcome looks much worse if nothing is done to combat climate change, compared to earlier projections. But there is less change if strong policies are put in place now to cut greenhouse gas emissions.

Without action, said study co-author Ronald Prinn, "there is significantly more risk than we previously estimated. This increases the urgency for significant policy action."

This time around, the MIT Joint Program on the Science and Policy of Global Change used updated economic data, improved economic modeling, and data involving "carbon-nitrogen interaction in the terrestrial ecosystem" when they re-ran and analyzed their model. They project an astonishing median greenhouse gas concentration of 866 ppm by century's end. That's over twice the current level of around 389 ppm which is itself considered much too high by many climate experts.

Why the big difference? As the researchers explain on their web site (hat tip to Joe Romm for the find),

There is no single revision that is responsible for this change. In our more recent global model simulatations, the ocean heat-uptake is slower than previously estimated, the ocean uptake of carbon is weaker, feedbacks from the land system as temperature rises are stronger, cumulative emissions of greenhouse gases over the century are higher, and offsetting cooling from aerosol emissions is lower. No one of these effects is very strong on its own, and even adding each separately together would not fully explain the higher temperatures. Rather than interacting additively, these different affects appear to interact multiplicatively, with feedbacks among the contributing factors, leading to the surprisingly large increase in the chance of much higher temperatures.

What's the alternative? Well, MIT's team estimates that if:

• The Kyoto Protocol is implemented in 2010 by all countries that agreed to caps in the original protocol.
• The world then achieves the emissions limits [of 675 ppm CO2 equivalent, or 4.2 trillion metric tons globally] estimated by the MIT [hypothetical policy scenario] to meet the Level 2 stabilization scenario described by the U.S. Climate Change Science Program

...then, the odds that the median global surface temperature will increase by more than 6 degrees F go down significantly, and the probability that the temperature increase will be under 5 degrees F goes up to 80%.

What's all this stuff about probabilities and likelihoods and percentages?  Well, the MIT researchers acknowledge the extent of the unknowns in climate scenarios -- such as whether we hit a climate tipping point that causes an extreme condition sends everything to hell in a handbasket faster than can be currently anticipated.  Say, that melting permafrost sends such massive new quantities of greenhouse gases into the atmosphere that the climate's changes become unstoppable.

They've developed a range of probabilities based on different reactions by us -- no political policy changes, some political policy changes.  They call it the "Greenhouse Gamble."

Study co-author Ronald Prinn, the co-director of the Joint Program and director of MIT's Center for Global Change Science, says in an MIT statement,

Because vehicles last for years, and buildings and powerplants last for decades, it is essential to start making major changes through adoption of significant national and international policies as soon as possible... "The least-cost option to lower the risk is to start now and steadily transform the global energy system over the coming decades to low or zero greenhouse gas-emitting technologies."

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Image: Policy and No Policy Roulette Wheels, via MIT Joint Program on the Science and Policy of Global Change