Scientists are starting to clear up one of the biggest controversies in climate science – Washington Post

 In Science

This illustration obtained from NASA on Jan. 20, 2016, shows that 2015 was the warmest year since modern record-keeping began in 1880, according to a new analysis by NASAs Goddard Institute for Space Studies. (Scientific Visualization Studio/Goddard Space Flight Center via AFP-Getty Images)

How much Earth will warm in response to future greenhouse gas emissions may be one of the most fundamental questions in climate science — but it’s also one of the most difficult to answer. And it’s growing more controversial: In recent years, some scientists have suggested that our climate models may actually be predicting too much future warming, and that climate change will be less severe than the projections suggest.

But new research is helping lay these suspicions to rest. A study, out Wednesday in the journal Science Advances, joins a growing body of literature that suggests the models are on track after all. And while that may be worrisome for the planet, it’s good news for the scientists working to understand its future.

The new study addresses a basic conflict between what the models suggest about future climate change and what we can infer from historical observations alone. Some scientists have suggested that the models may be too sensitive, pointing out that the warming they predict for the future is greater than what we would expect based only on the warming patterns we’ve observed since humans began emitting greenhouse gases.

The models, for instance, suggest that if the carbon dioxide concentration in the atmosphere were to reach double its preindustrial level, the planet would warm by anywhere from about 1.5 to 4.5 degrees Celsius (2.7 to 8.1 Fahrenheit). But the warming patterns we’ve actually observed over the past 200 years or so would suggest that a doubling in carbon dioxide should only lead to about 3 degrees Celsius (5.4 Fahrenheit) of warming at the most.

The discrepancy has become a major cause for concern among climate scientists in recent years, according to the new study’s lead author Cristian Proistosescu, a research associate at the University of Washington who conducted the research while completing a PhD at Harvard. Even the United Nations’ Intergovernmental Panel on Climate Change acknowledged the issue in its most recent report, he said, stating that it could no longer provide a best estimate for the climate’s sensitivity.

“It worried a lot of us,” Proistosescu told The Washington Post. “We needed to understand why our different estimates didn’t work.”

The new study helps reconcile the models with the historical record. It suggests global warming occurs in different phases or “modes” throughout the planet, some of which happen more quickly than others. Certain slow-developing climate processes could amplify warming to a greater extent in the future, putting the models in the right after all. But these processes take time, even up to several hundred years, to really take effect — and because not enough time has passed since the Industrial Revolution for their signal to really develop, the historical record is what’s actually misleading at the moment.

This conclusion is supported by a growing body of research, which suggests that warming estimates made from the historical record alone are “potentially biased low, for reasons we are now just beginning to understand,” said Timothy Andrews, a climate scientist with the Met Office, the United Kingdom’s national weather service, in an email to The Washington Post. While Andrews was not involved with the new study, he’s one of multiple scientists whose recent research has tackled the same issue.

The new study uses a statistical method to separate “fast” and “slow” climate modes in the models. According to Proistosescu, when greenhouse gases are emitted into the atmosphere, a “fast” warming effect begins to take place almost immediately in certain parts of the planet, mainly over the land masses in the Northern Hemisphere. Indeed, these are the parts of Earth where the most rapid warming has been observed since the Industrial Revolution.

On the other hand, he said, other parts of the planet — namely, the Southern Ocean and the eastern Pacific — respond much more slowly, in part because they’re just so deep and cold to begin with. But as they absorb more heat and finally start to warm up, they may produce a variety of climate feedback effects that enhance the global warming that’s already occurring. For instance, changes in ocean temperatures can alter atmospheric patterns around the world. The models suggest that these warming ocean regions may lead to a decrease in reflective cloud cover in the future, allowing more solar radiation to make it through the atmosphere to Earth’s surface.

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