World leaders are rapidly running out of time to limit global warming to 1.5 degrees Celsius. And without dramatic action, even less ambitious goals — such as checking climate change at 2 degrees Celsius above preindustrial levels — soon could be out of reach.
That’s the sobering conclusion of the world’s top climate scientists, who today released a milestone report on the state of global warming. The analysis by the Intergovernmental Panel on Climate Change, or IPCC, wasn’t limited to temperature projections either.
It forecasts a grim future in which babies born today would grow old on a planet that’s hotter and more dangerous than the one their parents entered.
One in which rising seas and coastal floods regularly threaten half the globe. One in which fire seasons burn hotter and longer. And one that would see the world beset by extremes — from megadroughts and heat waves to moisture-laden storms that dump crippling amounts of rainfall on cities and crops.
And it leaves no doubt about who is responsible.
“It is unequivocal that human influence has warmed the atmosphere, ocean and land,” the report states. “Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred.”
Those impacts are already visible in the extreme weather events seen this summer, with some happening earlier than anticipated, scientists say. The report calls many of the changes “unprecedented” in thousands of years and notes that some of the shifts now in motion — such as sea-level rise — are irreversible.
“This report is a reality check,” Valérie Masson-Delmotte, co-chair of the IPCC’s Working Group I that produced the report, said in a statement. “We now have a much clearer picture of the past, present and future climate, which is essential for understanding where we are headed, what can be done and how we can prepare.”
Commissioned by governments for policymakers, the report was compiled by more than 230 scientists who assessed more than 14,000 studies to determine how the climate system is changing and to project future shifts.
It comes about three months before a major climate conference in Scotland where world leaders are expected to outline next steps to try and prevent the worst effects of global warming.
“The IPCC report underscores the overwhelming urgency of this moment,” said U.S. climate envoy John Kerry in a statement. “As the IPCC makes plain, the impacts of the climate crisis, from extreme heat to wildfires to intense rainfall and flooding, will only continue to intensify unless we choose another course for ourselves and generations to come. What the world requires now is real action.”
Of particular note is how the report more directly connects the dots between human-influenced climate change and increases in extreme weather events.
Much of that is due to improvements in extreme event attribution, which links human activities to more severe weather. Observational evidence of those changes and human attribution to them has strengthened since the last report in 2013, scientists said.
“A decade ago we were able to make broad statements about a warming world being one with more extreme heat events and more extreme precipitation events,” said Zeke Hausfather, a climate scientist at the Breakthrough Institute and a contributor to the report. “But we were much less able to say, ‘This particular heat wave was made 100 times more likely by historical warming.’"
The world already has warmed by more than 1 degree Celsius since preindustrial times, and the report outlines how much more severe the impacts of warming would be at 2 C and each additional fraction of warming.
Those effects could worsen natural disasters such as the ones humanity has endured this summer. That includes wildfires in Europe and the United States; floods in Germany, China and India; and heat waves across North America.
Globally, heat waves now occur five times more often and could potentially occur 14 times as often if warming reaches 2 C, said Paola Andrea Arias Gómez, an associate professor at the University of Antioquia in Colombia and an author of the report. Droughts that occurred once every 10 years now occur 70% more frequently and would arrive between two and three times as often at 2 C of warming, she added.
In addition, global surface temperatures are hitting levels not seen in roughly 125,000 years. The rate of warming since 1970 is higher than any 50-year period in the last 2,000 years, the report states.
While regional impacts will vary over time, no place on Earth will be immune, the report found.
The latest IPCC analysis includes a detailed look at how global warming is expected to affect different parts of the world. The eastern United States, for example, likely would see an increase in extreme precipitation and a rise in river and coastal flooding. Central and western North America are projected to see increases in drought and wildfire conditions.
Looking to the future
The IPCC report considered five emissions scenarios to project how the climate would respond to different levels of emissions in the future. One optimistic model looked at very low greenhouse gas emissions with CO2 declining to net zero and then net negative. A more pessimistic model looked at very high emissions in which CO2 roughly doubles.
Under every scenario, however, global surface temperatures would continue to increase until “at least” midcentury, the report states. How much so is less certain. “Global warming of 1.5C and 2C will be exceeded during the 21st century unless deep reductions in CO2 and other greenhouse gas emissions occur in the coming decades.”
When greenhouse gases are averaged over the next 20 years, the global temperature is expected to reach or exceed 1.5 C, the report states. Even then, it could take two to three decades to see global temperatures stabilize, it notes.
The scenarios span a broader range of plausible futures than the ones from the previous assessment in 2013, said Maisa Rojas Corradi, director of the Center for Climate and Resilience Research at the University of Chile.
One big difference, she added, is that none of the previous scenarios were compatible with limiting temperatures to 1.5 C because they came before the Paris Agreement set that as a target.
Human-driven greenhouse gas emissions, such as the burning of fossil fuels, have continued their ascent over the past decade, which cuts into the carbon budget the world has left.
As CO2 emissions increase, the proportion of carbon that natural sinks such as the land and ocean are able to suck from the atmosphere will grow smaller, making them less effective at mitigation.
Each of the last four decades is already “successively warmer than any decade that preceded it,” the report states. And, it notes, even if temperatures and emissions stabilize or decline, some future change is already locked in.
That’s especially true for the oceans, ice sheets and sea levels, which will continue to rise as the Earth adjusts to current levels of warming.
Global mean sea levels already have risen faster over the past century than any century preceding it for 3,000 years, the report states.
Beyond midcentury, sea-level projections will become increasingly sensitive to the emissions choices made today, said Robert Kopp, a professor in Rutgers University’s Department of Earth and Planetary Sciences and the lead author on the chapter on ice sheets and sea levels.
Under the most extreme emissions scenario, he said, scientists could not rule out rapid ice sheet loss leading to sea-level rise approaching 2 meters by the end of the century and 5 meters by 2150.
However, Kopp noted, “our emissions matter hugely for the long-term amount of sea-level rise and how quickly it comes.”
Due to rising seas, “extreme sea level events that occurred once per century in the recent past are projected to occur at least annually at more than half of all tidal locations by 2100,” the report states. Those events include storm surges and tidal floods, and they are expected to most threaten low-lying delta regions, such as New Orleans and coastal Bangladesh.
Even if global warming is limited to 1.5 C, global mean sea level will rise by 2-3 meters over the next 2,000 years, the report adds.
Richard Alley, a professor of geosciences at Penn State University who specializes in ice sheets and sea-level change and who has contributed to past IPCC reports, said they don’t have a history of being alarmist, particularly on sea-level rise. “And if they’re saying this, history would say taking it seriously is wise for society,” he said.
The IPCC report concludes that human-influenced temperature increases have driven the last half-century of ocean warming and acidification. It’s also “very likely” the main driver of global glacial retreat and the decrease in Arctic sea ice, particularly over the last decade. Partly as a result of that melting, sea-level rise has increased by an average of 3.7 millimeters a year between 2006 and 2018, the report states.
Altering the trajectory won’t be easy.
“Many changes due to past and future greenhouse gas emissions are irreversible for centuries to millennia,” the report concludes, with melting of the Greenland and Antarctic ice sheets the most difficult to reverse. In addition, it could take hundreds or thousands of years to see the full effect of the warming climate on these ice sheets.
While the report makes clear that the window to avoid catastrophic impacts is closing, it doesn’t say it would be impossible to achieve the temperature targets outlined in the Paris Agreement.
“A key message here is that it is still possible to forestall many of the most dire impacts, but it really requires unprecedented transformational change — rapid and immediate reduction of greenhouse gas emissions to net zero by 2050,” said Ko Barrett, senior adviser for climate at NOAA’s Office of Oceanic and Atmospheric Research and an IPCC co-chair.
"But the idea that there is still a pathway forward is a point that should give us some hope," Barrett added.
The report doesn’t offer prescriptions. Future IPCC reports in early 2022 will look at adaptation and vulnerability as well as mitigation. Today’s report does, however, include a lengthy FAQ for distribution to governments that explains how responses to those shifts should account for the scale and speed at which the climate is changing.
Scientists and observers said they hope the report encourages discussions about climate resilience and response.
“I think this pushes adaptation right to the fore, and it will also push the question of loss and damage to the fore. How do we deal with the impacts that can’t be avoided?” asked David Waskow, director of the international climate initiative at the World Resources Institute.
Rachel Cleetus, policy director for the climate and energy program at the Union of Concerned Scientists, said equity and justice also would need to be elevated since many of the communities bearing the brunt have not contributed to the problem.
“The actions that we’re taking or failing to take right now cast a very long shadow and will have very significant and serious implications for future generations,” said Cleetus. That includes the potential for widespread migration as coastlines disappear and famines become more likely because of drought and rising heat.
“Our fervent hope is that it will be received very differently from previous reports because scientists have been saying this for decades now,” she added.
“If this report doesn’t get policymakers’ attention, I just don’t know what will,” said Paul Bledsoe, a lecturer at American University’s Center for Environmental Policy.
It could provide the motivation for greater cooperation.
“The Biden-Harris administration is already proposing very aggressive action,” said Jane Lubchenco, deputy director for climate and energy at the Office of Science and Technology Policy. “We need partners in Congress and industry and in the states to help deliver on some of those.”
Methane in focus
Limiting global warming would require net-zero carbon emissions as well as “strong reductions” in other greenhouse gases, the report concludes.
One way to quickly limit warming would be to reduce methane emissions, it states. Methane emissions have rapidly increased over the past decade to reach the highest levels in at least 800,000 years. And the rate is accelerating.
Part of that could be due to feedbacks from warming, said Ilisa Ocko, a senior climate scientist at the Environmental Defense Fund. As permafrost melts, for example, it releases more methane. The spike is also due to natural gas production, one of the main contributors to methane in the atmosphere.
Methane mainly comes from agriculture, the burning of fossil fuels and waste. It lives in the atmosphere for around a dozen years but has an immense warming impact on the planet. Many scientists see a focus on reducing methane emissions as a way to stop temperatures from tipping over 1.5 C in the near term.
“It’s going to be essential over time to reduce emissions of methane in order to stabilize our climate,” said Ocko. “And if we make those cuts now, then we’ll avoid a lot more damages over the coming decades than if we didn’t.”
She co-authored a report in April that showed a rapid, full-scale effort to reduce methane pollution could slow worldwide warming by as much as 30% (Energywire, March 28). It would require actions such as switching feed for livestock to suppress methane in their guts or identifying and patching leaks in natural gas infrastructure.
Ocko said short-lived climate pollutants like methane have been undervalued in past climate reports, giving policymakers less reason to focus on the importance of methane reduction. Another reason is that the impacts of climate change weren’t yet so visible, so it made more sense to focus on longer term warming agents such as CO2.
Now perceptible climate impacts are here, “so it’s become an issue of needing to slow down warming now,” she added.
“In a situation where we have only a small window of time to tackle climate change and keep the Paris Agreement goals in sight, then obviously an agent that has a powerful but short-term warming impact becomes more important,” said Richard Black, a senior associate at the Energy & Climate Intelligence Unit.
Improvements in science
Much of what the report outlines isn’t new, say climate scientists. But it does draw on a wealth of new research, data and improvements in climate modeling that has advanced since the last report in 2013.
In addition to attribution science, the ways scientists measure global average temperature also has improved due to better coverage of the Arctic, the fastest warming region on the planet, and better measurements of ocean temperatures.
Those advances have allowed scientists to update their estimate of warming by 0.1 degree C.
Another change is climate sensitivity, what Hausfather from the Breakthrough Institute calls a “holy grail” of climate science.
It’s a measure of how much warming occurs when there is double the amount of CO2 in the atmosphere relative to preindustrial times.
The range has typically been somewhere between 1.5 C and 4.5 C for every previous IPCC report, but this one narrowed it to a likely range of 2.5 to 4 C.
With a narrower range of uncertainty, scientists can better assess future warming, Hausfather said.
That matters to help identify the type of action that’s needed.
“We can’t stop emissions tomorrow,” said Kim Cobb, director of the global change program at Georgia Tech. “What we’re doing by acting aggressively soon is making sure that these next two decades of warming may be some of our last.”