Earth’s climate is warming at a faster rate than previously thought, and with greater and more widespread consequences, according to a landmark report by the world’s top climate scientists.
The window for decisive action to avoid the worst consequences is still open, the report concludes, but just barely, as the planet approaches the watershed mark of 1.5 degrees Celsius above preindustrial temperatures a decade earlier than expected.
In a summer marked by catastrophic wildfires, a deadly heat dome, and unprecedented flooding, this report, issued by the United Nations’ Intergovernmental Panel on Climate Change, shows how humanity’s burning of fossil fuels is driving changes to the planet’s climate unseen for thousands — if not hundreds of thousands — of years.
The report provides the scientific explanation for the “devastating set of changes that are already part of our lived experience,” said Peter Frumhoff, the director of science and policy at the Union of Concerned Scientists and a lead author of a past IPCC report. “As a human, it’s impossible to read this and not find that it’s just heartbreaking — I don’t know what other word to ascribe. We are wreaking such havoc on the planet.”
The report represents the work of 234 authors from 66 countries, plus an additional 517 contributing authors. It updates a similar UN report from 2013, and is the first of four releases that together will make up the panel’s Sixth Assessment Report.
The UN panel’s reports are considered the world’s most authoritative and comprehensive assessments of the impact and progression of Earth’s warming climate. But because they must be agreed upon by so many scientists and policymakers from around the world, they tend to present a conservative take on what the latest science says about the extreme nature of the crisis, experts say.
“But here, even though it is likely conservative, the reality this report presents is alarming,” said Sergey Paltsev, deputy director of the MIT Joint Program on the Science and Policy of Global Change and a lead author of a past IPCC report.
Climate change has now progressed to a point that scientists contributing to the UN report were able to study the impacts of warming on actual events rather than relying solely on theoretical models. That has enabled scientists to more accurately pinpoint the role of climate in events like severe weather and heat waves like those experienced across much of the United States this summer. It has also allowed them to pinpoint the share of warming due to human activity.
“They have found the enemy, and it’s us,” said Paltsev. “It’s humans who are doing it, so we better fix it.”
The report found that the world has already warmed roughly 1.1 degrees C over preindustrial times, a level frighteningly close to the 1.5 degree goal set forth by the Paris Agreement, the international climate accord endorsed by 197 countries. That amount of warming is considered a point after which the consequences of climate change become increasingly dire. According to the report, the planet will hit 1.5 degrees in the next 20 years, a decade earlier than previously thought.
The report notes that even if all emissions ceased today, the effects of global warming will continue, because the carbon already in the atmosphere will take thousands of years to dissipate. That means centuries of sea level rise, increased coastal flooding, and a reshaping of coastlines around the world. Extreme heat waves are now happening five times as often as they did in preindustrial times, and if warming reaches 2 degrees C, that is expected to jump to 14 times as often, according to the report.
For the first time, the UN panel broke out the regional impacts of warming. On the East Coast of the United States, the authors projected an increase in average annual rainfall, as well as an increase in rainfall from extreme events. Massachusetts residents need to look no further than this summer — when July smashed records with 10.3 inches of rain, six inches above normal — to get a glimpse of what that may look like.
While the report lays out a stark future, it also provides a clear vision of how the choices we make now will have drastic repercussions later.
It describes five possible futures, ranging from one in which the world takes decisive action to stop using fossil fuels to one in which emissions are allowed to increase. Thanks to the emissions the world has already released into the atmosphere, all five of the scenarios are expected to have roughly 1.5 degrees C of warming by 2040.
By the time the mid-century rolls around, according to the report, our choices will begin to reap their consequences. Under the scenario of aggressive action, the mid-century — from 2041 to 2060 — would see just modest warming before temperatures decline slightly and then stabilize at 1.5 degrees C.
But the higher-end emissions scenarios would see temperatures rising as high as 3 degrees C of warming by the middle of the century, en route to what experts estimate would be 4.4 degrees C of warming by 2100.
“We know that there’s no going back from some changes in the climate system, however some of these changes could be slowed and others could be stopped by limiting warming,” said Ko Barrett, a senior adviser for climate at the National Oceanographic and Atmospheric Administration and vice chair of the UN panel.
Sea level rise is already locked in at an expected six to 12 inches on average globally by 2050, according to Robert Kopp, a climate and sea level scientist at Rutgers University and a lead author of the report. But the pace of what happens after that will be determined by the choices we make now.
If the planet continues roughly on the trajectory it is currently on, reaching 3 degrees C or 4 degrees C above preindustrial times by the end of the century, we are most likely looking at a bit over two feet of average global sea level rise by 2100.
At 2 degrees C of warming, however, that’s more like 1.5 feet of sea level rise. That extra half foot of sea level rise will be marked by increased flooding and inundation of coastal cities and low-lying islands. Beyond 2100, the seas will likely continue to rise, but a slower pace will give nations longer to adapt, Kopp said.
The path the world is currently on, based on the pledges made so far under the Paris Climate Accord, is just above the report’s middle scenario that will have warming of 2.7 degrees C by the end of the century.
Even achieving that will require that countries not just pledge to cut emissions, but that they actually formalize legislation that will require them to do so. In the United States, as well as many countries, that work remains to be done.
“Urgent climate action was needed decades ago and now we’re almost out of time and facing dire consequences,” said Peter de Menocal, the president and director of the Woods Hole Oceanographic Institution. “There is hope. All these impacts, and thus human suffering, scales directly with the amount of CO2 in the atmosphere. If we act now we can avoid the worst outcomes.”
Sabrina Shankman can be reached at firstname.lastname@example.org. Follow her on Twitter @shankman.