In short, because of the massive amount of fossil fuels burned since the Industrial Revolution, and the complexity of the Earth’s climate, there’s no quick payoff from changing our fossil fuel habits, researchers found.
The results lend added perspective to the relatively minor drop in emissions that occurred due to worldwide shutdowns in response to the coronavirus pandemic — a drop that appears unlikely to have much effect on the planet’s overall temperature.
“This is a big ship. We’ve given a lot of speed to a big heavy system,” said Bjorn Samset, a researcher with the Center for International Climate Research in Oslo who conducted the research in Nature Communications with two colleagues, Jan Fuglestvedt and Marianne Lund. “We have never warmed the world like this before, and we have certainly never cooled it.”
Samset said the delayed benefits of climate action could complicate the push to quickly wean the world off fossil fuels, in part because politicians and policymakers might have a difficult time showing that measures to combat climate change are making a discernible difference in the short term — even if emissions cuts help stave off future warming.
“It’s one of the things that makes climate change so difficult,” he said. “You’re looking at an avoided issue in the long term. So the best thing you can hope for is to stay at the status quo.”
But the difficulty of heading off climate change has not prevented many world leaders — and growing armies of young activists — from pushing for more ambitious action. Nearly every nation in the world has agreed, at least on paper, to collectively work to slow warming. And leaders around the globe have continued to press for immediate changes, even if the payoff takes time.
“Science tells us that [on] our current path, we face at least 3 degrees Celsius of global heating by the end of the century. I will not be there, but my granddaughters will. And your grandchildren, too,” United Nations Secretary General António Guterres told a gathering of international leaders in New York in the fall. “I will not be a silent witness to the crime of dooming our present and destroying their right to a sustainable future. It is my obligation — our obligation — to do everything to stop the climate crisis before it stops us.”
A comparable reduction would need to happen year after year for rising global temperatures to level off. A United Nations report in the fall found that the world’s emissions would need to shrink by 7.6 percent each year to meet the most ambitious aims of the Paris climate agreement, which has the support of nearly every nation except the United States under President Trump.
So far, any impact of the pandemic on the globe’s temperature is unclear. This year could very well be the hottest on record. Greenhouse gases in the atmosphere continue to rise relatively rapidly. Some regions saw a noticeable decline in air pollution, but that could prove temporary.
“We have to curb our expectations a little bit,” Samset said. “Luckily, we are moving to an era where we will see cuts in emissions. But we also realize that people are expecting to see some return on these efforts. This will impact people’s livelihoods. What do we get in return? We get reduced global warming at some point, but that may not be visible for 15, 20 years — longer if we are unlucky.”
“The amount of climate change from CO2 emissions is closely related to cumulative emissions, so even a complete cessation of CO2 emissions does not cause things to cool,” Caldeira said in an email.
He pointed to well-established climate models showing the likely trajectory of the Earth’s temperature over time if humans drastically reduce the burning of fossil fuels — and if they fail to do so. In both situations, a certain amount of expected warming is already baked into the system.
“The idea that we would not be able to statistically detect the influence of policy on global mean temperatures for many decades is the sort of thing that I believe is well understood by most working climate scientists,” he said, “but not well understood by many members of the general public.”
In the new study, Samset and his colleagues used climate change models and statistical tests to determine when a clear decline in the pace of warming might be apparent after emissions are cut. For the leading greenhouse gas, carbon dioxide, they found that if emissions are cut 5 percent annually starting this year, it will probably be 2044 before any impact is statistically significant.
So far, humans have not been able to sustain anything close to an annual emissions decrease of 5 percent.
Even with an immediate halt to all carbon dioxide emissions, the study found that proof of a change would probably not emerge until 2033. The research also found that the best option for meaningful change would be tackling all drivers of global warming simultaneously. Cutting carbon dioxide, methane, nitrous oxide, black carbon and other pollutants all at once could yield results by 2040, Samset said.
Granted, metrics other than the world’s temperature — such as the globe’s annual greenhouse gas emissions, or concentrations of carbon dioxide in the atmosphere — will change faster, and might be better to focus on, he added.
In the mid-2000s, some scientists say that a climate “hiatus” occurred, in which temperatures did not appear to rise quite as rapidly as in prior decades. While the “hiatus” was at best a blip — and, some would argue, not real — it had major political impacts.
Climate change doubters immediately seized on the issue to question the severity and pace of global warming. Some scientists who do think the warming rate slowed point to natural changes in the Pacific Ocean — in other words, the natural variability of the climate, which can temporarily blunt the warming trend.