Climate Change Summit Leaves Hope for Limiting Global Warming

Governments left the United Nations climate summit this month with new doubts that global temperature increases can be limited to 1.5 degrees Celsius from preindustrial levels—but also with hope that a more realistic goal, 1.7 degrees, is within reach.

The target of 1.5 degrees Celsius—or 2.7 degrees Fahrenheit—has been the north star of U.N. climate negotiations since it was enshrined in the 2015 Paris accord. The deal calls for governments to cut greenhouse-gas emissions to levels that climate scientists estimate can limit warming to well below 2 degrees Celsius compared with preindustrial levels, with the ambition of keeping it below 1.5 degrees.

The earth has already warmed around 1.1 degrees since 1850, according to the U.N.’s latest climate science report. That is largely due to an increase in greenhouse gases, such as carbon dioxide and methane, caused by industrialization, the report says.

Limiting the temperature increase to 1.5 degrees requires that global greenhouse-gas emissions fall around 43% by 2030 compared with 2019, according to the U.N. That works out to cuts of an average 5% every year, starting now through 2030, said Pierre Friedlingstein, a climate scientist who coordinates the Global Carbon Project, a research consortium that provides estimates used by the U.N. The program estimates that fossil-fuel carbon-dioxide emissions alone this year are on track to rise 1% from last year. That doesn’t include emissions from deforestation or methane, another potent greenhouse gas.

At the summit in Sharm El Sheikh, Egypt, the U.S., Europe and other developed nations sought new commitments to cut emissions this decade by phasing out fossil fuels. They were rebuffed by large developing nations such as China, India and Saudi Arabia.

In the end, the two sides settled on language from an agreement reached at last year’s U.N. meeting in Glasgow that urged, but didn’t require, countries to accelerate their plans for greenhouse-gas cuts. “I said in Glasgow that the pulse of 1.5 degrees was weak,” said Alok Sharma, the U.K. climate envoy. “Unfortunately, it remains on life support. ”

Big economies, however, have made longer-term plans to cut emissions that analysts say could limit warming in line with the Paris accord. China, by far the world’s largest emitter, and India, the third-largest, last year pledged to become net-zero emitters of greenhouse gases by 2060 and 2070, respectively. If fulfilled, those commitments could limit warming to 1.8 degrees above preindustrial levels, according to the International Energy Agency, a club of large energy-consuming countries.

Before Sharm El Sheikh, the IEA lowered its estimate to 1.7 degrees, based on new emissions-cutting pledges from several countries—in particular Indonesia and Australia—and new agreements to reduce deforestation.

“That’s a journey—from well over 2 degrees to 1.8 to now 1.7—that we can be proud of, even as we recognize we are just getting started,” U.S. climate envoy John Kerry said.

The world has about 18 more years of carbon-dioxide emissions at current rates to hit 1.7 degrees, rather than nine years left for 1.5 degrees, according to the Global Carbon Project.

Getting to 1.7 would also be a lot cheaper. The IEA estimates that meeting the warmer target would require global investment in clean energy to rise from $1.3 trillion annually to $3 trillion by 2030. Hitting 1.5 degrees would require spending $4.2 trillion by 2030—effectively doubling the investment represented by the energy sector from 2% of global annual gross domestic product to 4%.

Apart from the money, there is a little-known scientific reason why 1.7 degrees is a lot easier than 1.5 degrees. Sharp cuts in fossil-fuel burning, particularly coal, also reduce sulfur dioxide and nitrogen oxides pollution. Those compounds transform into tiny droplets called aerosols that have helped cool the earth—by an estimated 0.5 degrees—by reflecting sunlight back into space.

“A lot of the reduction in CO2 is offset by the reduction in aerosols, specifically sulfur dioxides,” said Daniel Crow, climate and energy modeler at the IEA.

Climate scientists say limiting warming to 1.7 degrees would still be a significant achievement. The 1.5 degree threshold isn’t a tipping point above which the damage caused by climate change—including rising sea levels, drought or heat waves—accelerates. Rather, the damage grows steadily with each increment of warming. For example, the U.N.’s latest science reports estimates that between 70% and 90% of the world’s coral reefs are expected to die out at 1.5 degrees of warming—an estimate that grows to greater than 99% with 2 degrees of warming.

“The climate system is not like a cliff: before 1.5 everything is fine, as soon as you reach 1.5, it’s the end of the world,” said Mr. Friedlingstein. “We need to reduce emissions as fast as possible, and then see where we are.”

Even a target of 1.7 degrees poses huge challenges. The problem, officials and scientists say, is that the net-zero pledges made over the past two years are mostly vague promises that few countries have acted to enshrine in law. “Current evidence does not provide confidence that the nationally determined net-zero targets will be achieved,” the U.N. Environment Program said in a report from October.