How Much Will the Earth Warm Up?

Green: Science

Scientists say the Earth will warm in response to increasing carbon dioxide levels in the atmosphere, but since the 1970s, they have not made much headway in narrowing down exactly how much it will warm.

Typical forecasts say that if humanity doubles the amount of carbon dioxide in the atmosphere before emissions cease, the temperature will most likely rise by about 5.4 degrees Fahrenheit, though the figure could be as low as 3.6 degrees or as high as 8.1 degrees. And scientists have not entirely ruled out far higher numbers.

Now comes a new entry in the effort to specify the value known as “climate sensitivity,” and it falls on the low side of the existing estimates. The paper, in the journal Science, calculates that a doubling of carbon dioxide will most likely lead to a warming of 4.1 degrees Fahrenheit, though the number could be as low as 3 degrees or as high as 4.7 degrees.

“Our study shows that very high climate sensitivities are virtually impossible, suggesting that we still have enough time to deal with the problem and reduce carbon emissions, which could avoid the most severe impacts,” said Andreas Schmittner, a climate scientist at Oregon State University and the study’s lead author.

Dr. Schmittner and colleagues combined current global computer forecasts with an extensive reconstruction of the Earth’s past climate to arrive at their conclusions. Their premise was that the carbon dioxide changes that accompanied the most recent ice age, which peaked about 20,000 years ago, can help determine how the planet will behave when facing CO2 fluctuations in the future.

Their reconstruction suggested that ocean temperatures varied less from today’s value than one might have thought for an ice age, an indicator of relatively low climate sensitivity. They also found a lower chance of extreme climate change – upwards of 18 degrees Fahrenheit – than some other papers have found.

Scientists not involved in the work said it was intriguing—but must be taken with caution, as one more entry in a long string of efforts to get at the sensitivity question.

Gavin Schmidt, a climate scientist at NASA, said the researchers’ data were not representative of the whole planet, covering only about 26 percent of its surface. In his view, that might have skewed the results.

Dr. Schmittner acknowledged another potential problem: that his computer analysis did not take good account of potential changes in clouds. Because clouds affect sunlight absorbed at the Earth’s surface, excluding them means “the range that we estimate for climate sensitivity may be too narrow,” he said.

Finally, there are some questions about the underlying ocean data the researchers used. Richard Alley, a geoscientist at Pennsylvania State University, said some scientists have questioned the data set’s accuracy and prefer using other ocean temperature estimates than the ones the researchers employed. This points to a flaw in the data set rather than the research design, he said in an email, and he thinks the authors might have found similar results to previous work if they had used a more “fully consistent and accurate” database.

“I would not conclude from this study that the estimates of future global warming are overblown,” Dr. Alley said. Given the vast array of data from other studies, Dr. Alley thinks that it is “much too early to assume that the climate sensitivity is low.”

David Lea, an Earth scientist at the University of California, Santa Barbara, said he found the approach promising. But he pointed out that even if the researchers’ predictions are right and the planet warms just 4.1 degrees, this would nonetheless have a big impact on the global climate.

While none of the numbers may sound large to laypeople, they are averages for the surface temperature of the entire planet, and therefore represent enormous additions of heat to the Earth. All parties agree that the warming over land would be much higher than the global average, and the warming in the Arctic would be higher still. The potential impacts include drastic sea level rise, loss of forests and wildlife, and difficulty producing food.

“Removing the outer bound of sensitivity takes away one threat, but it doesn’t change the basic problem,” Dr. Lea said. Dr. Schmidt agreed that a warming of 4.1 degrees Fahrenheit is “plenty big enough to cause all sorts of problems.”

Moreover, while climate scientists generally center their research around a potential doubling of carbon dioxide, there is no guarantee humanity will actually stop its emissions at that level, meaning the temperature increase could be higher than the forecasts.

The carbon dioxide level is up 40 percent already, emissions are rising rapidly, and global negotiations to limit them have not been very successful. Negotiators will try again starting next week in Durban, South Africa.