A landmark report on climate change was released on Monday by the Intergovernmental Panel on Climate Change (IPCC), the United Nations body for assessing the science related to climate change.
The U.N. climate panel sounded a dire warning, saying the world is dangerously close to runaway warming – and that humans are "unequivocally" to blame.
Unless immediate, rapid and large-scale action is taken to reduce emissions, the report says, the average global temperature will likely cross the 1.5-degree Celsius warming threshold within the next 20 years.
Describing the report as a "code red for humanity," U.N. Secretary-General António Guterres urged an immediate end to coal energy and other high-polluting fossil fuels. "Countries should also end all new fossil fuel exploration and production, and shift fossil fuel subsidies into renewable energy."
"The alarm bells are deafening," Guterres said in a statement. "This report must sound a death knell for coal and fossil fuels, before they destroy our planet."
In its first major scientific assessment since 2014, the IPCC said that Earth's average surface temperature is projected to hit 1.5 or 1.6 degrees Celsius above preindustrial levels at around 2030, no matter what trajectory greenhouse gas emissions take in the meantime.
Already, greenhouse gas levels in the atmosphere are high enough to guarantee climate disruption for decades if not centuries, scientists warn in the report.
That's on top of the deadly heatwaves, powerful hurricanes and other weather extremes that are happening now and are likely to become more severe.
Drawing on more than 14,000 scientific studies, the report gives the most comprehensive and detailed picture yet of how climate change is altering the natural world – and what still could be ahead.
Emissions "unequivocally caused by human activities" have pushed today's average global temperature 1.1 Celsius higher than the preindustrial average – and would have pushed it 0.5 Celsius further if not for the tempering effect of pollution in the atmosphere, the report says.
That means that, as societies transition away from fossil fuels, much of the aerosols in the air would vanish – and temperatures could spike.
Scientists warn that warming more than 1.5 Celsius above the preindustrial average could trigger runaway climate change with catastrophic impacts, such as heat so intense that crops fail or people die just from being outdoors.
Every additional 0.5 Celsius of warming will also boost the intensity and frequency of heat extremes and heavy rainfall, as well as droughts in some regions.
Because temperatures fluctuate from year to year, scientists measure climate warming in terms of 20-year averages.
The 1.1 Celsius warming already recorded has been enough to unleash disastrous weather. This year, heatwaves killed hundreds in the Pacific Northwest and smashed records around the world. Wildfires fueled by heat and drought are sweeping away entire towns in the U.S. West, releasing record emissions from Siberian forests, and driving Greeks to flee their lands by ferry.
It's too late to prevent these particular changes. The best the world can do is to slow them down so that countries have more time to prepare and adapt.
"We are now committed to some aspects of climate change, some of which are irreversible for hundreds to thousands of years," said IPCC co-author Tamsin Edwards, a climate scientist at King's College London. "But the more we limit warming, the more we can avoid or slow down those changes."
'We still have choices to make'
But even to slow climate change, the report says, the world is running out of time.
If the world drastically cuts emissions in the next decade, average temperatures could still rise 1.5 Celsius by 2040 and possibly 1.6 Celsius by 2060 before stabilizing.
If the world does not cut emissions dramatically and instead continues the current trajectory, the planet could see 2.0 Celsius warming by 2060 and 2.7 Celsius by the century's end.
The Earth has not been that warm since the Pliocene Epoch roughly 3 million years ago – when the first ancestors to humans were appearing and oceans were 25 meters higher than today.
It could get even worse, if warming triggers feedback loops that release even more climate-warming carbon emissions – such as the melting of Arctic permafrost or the dieback of global forests. Under these high-emissions scenarios, Earth could broil at temperatures 4.4 Celsius above the preindustrial average by 2081-2100.
"We have already changed our planet, and some of those changes we will have to live with for centuries and millennia to come," said IPCC co-author Joeri Rogelj, a climate scientist at Imperial College London.
The question now, he said, is how many more irreversible changes we avoid: "We still have choices to make."
(With input from agencies)