The young climate activists clamoring today for rapid cuts to the world's fossil fuel emissions could be well into their 30s or 40s before the impact of those changes becomes apparent, scientists said in a study published Tuesday. 

As if curbing climate change wasn't tough enough already, the new research finds that even if humans sharply reduce greenhouse gas emissions now - cutting carbon dioxide, methane and other pollutants by at least 5% or more a year - it could still take decades before it's clear those actions are beginning to slow the rate of the Earth's warming.

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."

There is some warming already in motion that hasn't materialized yet, Samset observed. What's more, the planet's temperature is influenced significantly by natural variability, so it is hard to have an immediate impact on it. Even if greenhouse gases are reduced, this variability could temporarily introduce more warming anyway.

Furthermore, greenhouse gas emissions are accompanied by polluting aerosols, some of which reflect sunlight away from the Earth and cause cooling. If emissions are reduced, the disappearance of the aerosols will actually amplify warming, at least in the short term.

The coronavirus pandemic, with its notable but fleeting dent in global greenhouse gas emissions, has demonstrated how hard it will be to repair the impacts humans are having on the planet, particularly on the ambitious time frame scientists say is necessary to avoid the most catastrophic impacts of climate change.

Emissions declined by as much as 17% in early April, compared with the previous year, but then bounced back rapidly as shuttered economies around the world began to reopen. Overall, the globe's 2020 emissions are likely to show only a single-digit drop from 2019 levels, experts project.

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% 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 Donald 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."

Ken Caldeira, a longtime researcher at the Carnegie Institution for Science who has served as a lead author on key U.N. climate reports, was not involved with Tuesday's paper but said he was not surprised by its findings.

"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% 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%.

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.

Countries around the world have agreed in principle to slash their emissions over time as part of the Paris climate accord, but it remains uncertain whether there will be political pressure to pull back if the impacts aren't immediately visible.

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.

Similarly, the Earth's warming could initially accelerate due to natural causes, even as nations scramble to cut emissions, Samset said. That could mask the impact of any policy steps for a decade or longer.

"We are in the situation that not everyone trusts us, unfortunately," Samset said. "And there will be criticism of any climate measure."

Claudia Tebaldi, a climate scientist at the University of Maryland's Joint Global Change Research Institute, came to a similar conclusion in a study in 2013. She worries about what happens if climate action doesn't immediately result in benefits.

"It is a little bit humbling because you then can imagine that, given the noise in the system, it's going to be a less immediate message for the public to assess what mitigation is doing," Tebaldi said.

Samset and other scientists argue that the absence of instant, measurable results doesn't mean that humans should forgo tackling climate change. Caldeira said his own view is that putting the world on a more sustainable path is an obligation, even if only future generations will benefit.

"If we feel a responsibility to leave the world better off for our having been here, and we want to deserve the respect of our children and their children, then we have no choice but to build a new and better energy system," he said. "The main reason we should address the climate problem is because we want to be good people. We should address the climate problem because it is the right thing to do, and not because we will personally benefit from addressing the issue."