Humans are on track to release so much greenhouse gas in less than three years that breaching a key threshold for limiting global warming will be nearly unavoidable.
A new study predicts that by early 2028 there will be enough of the heat-trapping carbon dioxide gas in the atmosphere to create a 50-50 chance or greater that the world will be locked in to 1.5 degrees Celsius of long-term warming since preindustrial times.
That level of gas accumulation, which comes from the burning of fuels like petrol, oil and coal, is sooner than the same group of 60 international scientists calculated in a study last year.
"Things aren't just getting worse. They're getting worse faster," said study co-author Zeke Hausfather of the tech firm Stripe and the climate monitoring group Berkeley Earth. "We're actively moving in the wrong direction in a critical period of time that we would need to meet our most ambitious climate goals. Some reports, there's a silver lining. I don't think there really is one in this one."
That 1.5 degree goal, first set in the 2015 Paris agreement, has been a cornerstone of international efforts to curb worsening climate change.
Scientists say crossing that limit would mean worse heat waves and droughts, bigger storms and a rise in sea-levels that could imperil small island nations.
In Thursday's Indicators of Global Climate Change report, researchers calculated that society can spew only 130 billion metric tons of carbon dioxide before the 1.5 limit becomes technically inevitable. The world is producing 442 billion metric tons a year, so that inevitability should hit around February 2028, the scientists wrote.
The report, published in the journal Earth System Science Data, shows that the rate of human-caused warming per decade has increased to nearly 0.27 degrees Celsius per decade, Hausfather said.
And the imbalance between the heat Earth absorbs from the sun and the amount it radiates out to space, a key climate change signal, is accelerating, the report said.
"It's quite a depressing picture unfortunately, where if you look across the indicators, we find that records are really being broken everywhere," said lead author Piers Forster, director of the Priestley Centre for Climate Futures at the University of Leeds in England. "I can't conceive of a situation where we can really avoid passing 1.5 degrees of very long-term temperature change."
Reduced particle pollution, which includes soot and smog, is another factor because those particles had a cooling effect that masked even more warming from appearing, scientists said.
That all shows up in Earth's energy imbalance, which is now 25 per cent higher than it was just a decade or so ago, Forster said.
Earth's energy imbalance "is the most important measure of the amount of heat being trapped in the system," Hausfather said.
Earth keeps absorbing more and more heat than it releases. "It is very clearly accelerating. It's worrisome," he said.
That 1.5 is "a clear limit, a political limit for which countries have decided that beyond which the impact of climate change would be unacceptable to their societies," said study co-author Joeri Rogelj, a climate scientist at Imperial College London.
He added that efforts to curb emissions and the impacts of climate change must continue even if the 1.5 degree threshold is exceeded.