The two-degree limit isn’t mentioned in a single IPCC report. Many scientists say the benchmark underpinning talks in Paris is an arbitrary threshold based on tenuous research
The single most important benchmark underpinning this week’s talks in Paris on climate change –two degrees Celsius–has guided climate-treaty discussions for decades, but scientists are at odds on the relevance of that target.
Many researchers have argued that a rise in the planet’s average global air temperature of two degrees or more above preindustrial levels would usher in catastrophic climate change. But many others, while convinced the planet is warming, say two degrees is a somewhat arbitrary threshold based on tenuous research, and therefore an impractical spur to policy action.
“It emerged from a political agenda, not a scientific analysis,” said Mark Maslin, professor of climatology at University College London. “It’s not a sensible, rational target because the models give you a range of possibilities, not a single answer.”
Policy makers tend to assume the two-degree target expresses a solid scientific view, but it doesn’t. The exhaustive reports published by the United Nations Intergovernmental Panel on Climate Change are considered to be the most comprehensive analysis of the science of global warming. Yet the two-degree limit isn’t mentioned in a single IPCC report.
Still, many scientists back the goal because they see it as giving policy makers a clear-cut target to shoot at in the fight against global warming.
The vast majority of climatologists agree that the earth is getting warmer and that the emission of greenhouse gases by human activity is the main driver of this change. But the question of when a catastrophic tipping point might be reached is up in the air.
Some significant phenomena widely attributed to warming–such as dramatic summer melting of Arctic sea ice and glacial retreat in Greenland–are already evident today, even though the average temperature is one degree Celsius above preindustrial levels and thus some ways from the two-degree benchmark.
On the other hand, some of the more dangerous consequences of warming might not become evident until long after the two-degree line is crossed.
“The whole apocalyptic metaphor is misleading,” said Carlo Jaeger, chairman of the Global Climate Forum, a German think tank, and a professor at the University of Potsdam. “Hell is not going to break loose at two degrees–it will take hundreds of years to unfold.”
William Nordhaus, a professor of economics at Yale University, appears to have been the first to mention the two-degree figure in a paper published in 1977. But rather than making a robust scientific calculation based on the physics of climate change, his paper argued that a rise of two or more degrees would put the earth’s climate outside the observable range of temperature over the last several hundred thousand years.
Thereafter, the two-degree benchmark slowly gained traction. Other scientists argued that global air temperatures hadn’t risen beyond 1.5 or two degrees in the course of human history, and that greater temperature shifts eons earlier had triggered cataclysmic changes in sea level. They figured that a two-degree ceiling would therefore amount to a natural safety limit.
“The idea was, ‘let’s not move the human enterprise out of an evolutionary regime that we are adapted to,’ ” said Hans Joachim Schellnhuber, director of the Potsdam Institute for Climate Impact Research, who helped popularize the two-degree goal.
In 1994, at a meeting in Bonn, Dr. Schellnhuber sat down with Angela Merkel, then Germany’s minister for the environment. Since both are trained physicists, Dr. Schellnhuber offered complicated charts and figures for Ms. Merkel to peruse.
“I presented what I call ‘the tolerable windows’ approach,” recalls Dr. Schellnhuber, then the German government’s chief adviser on climate. “In terms of temperature, the tolerable window was limited to two degrees.”
Ms. Merkel backed the target. The following year, she persuaded the Council of the European Union to formally endorse it.
More scientists began to support it, too. One 2003 study concluded that beyond two degrees, “the risks increase very substantially involving potentially large extinctions or even ecosystem collapses, major increases in hunger and water shortage risks as well as socioeconomic damages, particularly in developing countries.”
In October 2014, David Victor, a professor of international relations at the University of California, San Diego, and Charles Kennel, a professor at the Scripps Institution of Oceanography in La Jolla, Calif., wrote a sharp critique of the two-degree benchmark in the journal Nature.
They argued that the yardstick was scientifically weak because it captured only a tiny portion of the planet’s climate profile. More than 93% of the extra heat, they noted, ends up in the ocean and not in the atmosphere.
For that reason, they said, policy makers should also track ocean heat content and other parameters when setting emission goals. In an article this month in Nature Climate Change, they and a third author said “a new list of planetary vital signs” is needed to take the full measure of climate change.
Prof. Victor also said the two-degree benchmark should be ditched because it is no longer an achievable target.
“The trajectory of emissions we’re on now is so steep, it’s too late,” he said. “There’s no scenario under which this could be contained below two degrees–it’s game over.”