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IPCC RCP6.0 projections for temperature rise
This projection shows how a rise of more than 2 degrees Celsius is expected to affect North America, Greenland and the Arctic. (NASA Graphic)

A statistical analysis led by researchers at the University of Washington sees almost no chance that the world’s nations will be able to keep the rise in global temperatures below 2 degrees Celsius (3.6 degrees Fahrenheit) over the course of the 21st century, as promised in last year’s Paris climate accord.

“Our analysis shows that the goal of 2 degrees is very much a best-case scenario,” lead author Adrian Raftery, a UW professor of statistics and sociology, said today in a news release. “It is achievable, but only with major, sustained effort on all fronts over the next 80 years.”

The analysis, published in Nature Climate Change, is consistent with the mainstream view held by climate scientists and policymakers.

Adrian Raftery
Adrian Raftery is a professor of statistics and sociology at the University of Washington. (UW Photo)

In discussions about the future effects of climate change, the 2-degree mark has been called a “speed limit” that, if broken, would significantly heighten humanity’s peril. But even as the goal was being set, experts voiced worries that it would be very hard to stay below the speed limit by 2100.

The analysis conducted by Raftery and his colleagues focuses in on the main reason for the anticipated rise: It won’t be primarily because of the world’s growing population or greater affluence, but because of the heightened carbon emissions that are associated with increased economic activity.

Changing that part of the equation could make a difference in the expectations for future climate change, although there’s a lot of uncertainty about how big the difference might be.

“We weren’t able to figure out an emissions level that would guarantee hitting the 2 [degrees] C target,” Raftery told GeekWire in an email.

Much is at stake: The Paris pact calls for nations to hold the rise in global mean temperatures, as compared with a 1986-2005 reference period, to “well below” 2 degrees C, with an aspirational target of 1.5 degrees.

“Countries argued for the 1.5 C target because of the severe impacts on their livelihoods that would result from exceeding that threshold,” said study co-author Dargan Frierson, a UW atmospheric scientist.

“Indeed, damages from heat extremes, drought, extreme weather and sea level rise will be much more severe if 2 C or higher temperature rise is allowed,” Frierson added. “Our results show that an abrupt change of course is needed to achieve these goals.”

The study worked with U.N. projections for population growth, as well as statistical models for the anticipated rise of gross domestic product per capita and the levels of industrial carbon emissions associated with GDP.

Last month, the United Nations issued an updated projection suggesting that global population was likely to rise from its current level of 7.6 billion to 11.2 billion by 2100. However, the researchers said that would not be a significant driver for increased carbon emissions, because most of the population growth would be in sub-Saharan Africa, which uses little fossil fuel.

Higher GDP is associated with higher rates of carbon emissions, but only up to a point. Once a peak level is reached, the emission rate per unit of purchasing power declines, the researchers said.

Raftery and his colleagues said their purely statistical analysis went beyond the climate scenarios sketched out by the Intergovernmental Panel on Climate Change.

“Our model is not a ‘business as usual’ scenario, but rather is based on data which already show the effect of emission mitigation policies,” they wrote.

The data on population and economics were combined with the IPCC’s climate models, and then run through thousands of simulations. To validate their computer models, the researchers checked how well the readings for 2010 could be projected using data from time frames that started in 1950 and ended no later than 2000.

At the end of the exercise, the team came up with a spectrum of statistical outcomes ranging from not great to pretty bad. The simulations pointed to a median value of 3.2 degrees C (5.76 degrees F) for the temperature rise by 2100, and a 90 percent chance that the figure would fall somewhere between 2 and 4.9 degrees C (3.6 to 8.8 degrees F).

There was only a 5 percent chance that global mean temperatures would rise by 2 degrees C or less, and a 1 percent chance that the rise would be no more than 1.5 degrees C.

Scientists say a temperature rise of 3 degrees C would spark a significant drop in global food production and a significant rise in climate refugees. But for now, that grim vision is a projection rather than a reality – analogous to the vision provided by the Ghost of Christmas Yet to Come in Charles Dickens’ “A Christmas Carol.”

“Our forecasting model does not explicitly incorporate future legislation that could change future emissions,” the researchers write. “It is based on past emissions, which implicitly account for accumulating legislation and regulation over the past 30 years since climate change became a global issue.”

They note that many of the world’s economies are improving their record on carbon emissions, and that decreasing prices for solar and wind power could accelerate the shift to renewable energy sources.

The current rate of improvement is factored into the researchers’ model, but if there’s a sudden speed-up in carbon efficiency, that would lead to a more optimistic projection.

“The reverse is also possible due to decreases in fossil fuel prices, which have dropped in recent years,” the researchers write. And although the paper doesn’t mention it, the Trump administration has signaled that it would pull out of the Paris climate pact and reverse U.S. policies aimed at cutting back on carbon emissions.

In addition to Raftery and Frierson, the authors of the Nature Climate Change paper, “Less Than 2-Degree Warming by 2100 Unlikely,” include Alec Zimmer, Richard Startz and Peiran Liu.

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