Emissions Cuts Harder ... and Experts Think Climate Tipping Points More Likely

Eli is a contributing correspondent for Science magazine.

Two new studies out this week make clear the gargantuan task of cutting greenhouse gases and the risks involved.

Released today, the first bit of research asked the following question: What would be the impact on the world's climate if every country implemented its leading proposal for cutting greenhouse gases? (For the United States, that's President Barack Obama's promise of an 80% cut below 1990 carbon dioxide emissions by 2050; for Brazil, halting of deforestation by midcentury, etc.) The result, modelers found, was that the level of CO2 in the atmosphere would soar from roughly 380 ppm today to roughly 500 ppm by the year 2050 and 775 ppm by the end of the century. That would lead to a temperature rise of about 3°C (roughly 5°F) by 2100.

The second paper, published in the Proceedings of the National Academy of Sciences on Monday, polled 43 respected climate scientists on how likely five climate tipping points are to occur: the loss of polar ice sheets, leading to a rise in global sea levels; the Amazon rainforest dying; the shutdown of the Atlantic conveyor current; and the shift to a more persistent El Niño weather pattern. Scientists have long considered these events remote possibilities, with much uncertainty as to what might cause them to happen.

The bottom line: If global temperatures rise between 2°C and 4°C, the likelihood of one of these tipping points occurring, the experts say, is greater than one in six. If the temperature rises above 4°C, the probability becomes greater than 50%.

One of the co-authors of the second paper told the Copenhagen Climate Congress last week that the current trajectory of emissions trends would be like "playing Russian roulette." (The link is a video clip; go to 36:55.)

The two studies taken together suggest that the debate over emissions cuts must focus squarely on risks. Up until now, the European Union has tried to do this by making their goal to limit future temperature rise to 2° C above the preindustrial level—about 1.2° C above the current global average temperature.

It seems there's two disconnects: The first is a chasm between the difficulty of achieving that task and the proposals nations have put forward. To be sure, the first study included no cuts for China. But the cuts it did include—80% below 1990 CO2 levels for the United States and Europe, 60% below 2000 for Japan and Australia—are simply not enough, the study says. That's a hard fact to swallow since these cuts are considered very aggressive ones. In fact, they're likely to get scaled back soon during negotiations, whether in the U.S. Congress or in Copenhagen in December.

But experts say nations need to hold fast to the cuts and even propose more aggressive ones. "The negotiating position of many countries needs to change," said Beth Sawin, who led the effort and works at the Sustainability Institute in Hartland, Vermont. The analysis was published by a group of independent scientists working for nongovernmental organizations and the Massachusetts Institute of Technology and was reviewed by a team of experts.

The second disconnect is over the perception of risk. The public's view is that climate change may have some local effects here and there, but that extreme events are unlikely. The PNAS paper may have flaws—the scientists who participated may represent a "selection bias" in that they wouldn't have participated if they weren't concerned about the problem. But the authors of the paper were conservative in that they looked at the bottom range of the probability estimates that the experts gave them. For example, the one-in-six chance was the lowest probability from a broad range of guesses as to when the Greenland ice sheet would melt.

It comes down to society's level of comfort with danger. Are you comfortable with a one-in-six chance of losing Greenland? That's what the experts polled suggested the risk was. "This selection of experts shows considerable agreement that if you push the climate system too hard, there is a fairly high chance that something will fall over," says Richard Alley, a climate expert at Pennsylvania State University, University Park, who didn't participate with either effort. The climate change challenge is only getting tougher with each day.

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