The global effort to halt the spread of the deadly coronavirus has stalled much the global economy, setting up an unintended experiment on the impact on carbon emissions.

China’s GDP may have plummeted by 40% during the first three months of the year (on a seasonally adjusted annualized basis). The US’s GDP could drop anywhere from 30% to 50% by summer. And Britain’s economic output could shrink by 25% this quarter. The ardent hope is that economies will bounce back later in the year as the pandemic recedes, but annual output is still likely to decline in many nations. US GPD, for instance, may fall about 5%, according to one estimate.

So what’s all that going to add up to in terms of global climate emissions? Maybe about a 4% decline in 2020, according to a new estimate by CarbonBrief, based on an analysis of data sets that represent about three-quarters of worldwide emissions.

That would be a larger annual decline than any previous economic recession or war managed to bring about, writes Simon Evans, deputy editor at the site. But it still underscores just how massive and challenging a job the world faces in cutting emissions fast and deep enough to combat climate change.

To prevent 1.5 ˚C of warming, the world would need to cut emissions by 6% every year for the next decade. In other words, even after shutting down much of the economy for months, including global trade, travel, and construction, nations may still not decrease climate pollution enough this year to be on track to prevent that dangerous level of warming.

And emissions will almost certainly rebound as soon as economies get back on track, which is what happened in the wake of previous downturns. Indeed, China’s are already about within the normal range, a few months after the outbreak crested there.

CarbonBrief is quick to stress that the 4% figure is a rough estimate based on limited data, and subject to change depending on how the outbreak unfolds and how economies react in the weeks and months to come. The relationship between economic declines and emissions dips will also depend on which industries are ultimately hit hardest, Evans notes. Hospitality and entertainment, for instance, could see greater losses than the more carbon-intensive power sector.

But the finding underscores the limits of lifestyle changes and reductions in consumer demand, such as cutting down on car trips and plane travel, in ratcheting down emissions. It’s one more stark reminder that we need to fundamentally overhaul how we generate electricity, manufacture goods, produce food, and get around.