The Intergovernmental Panel on Climate Change doesn’t hedge in its latest assessment of the state of our planet. “It is unequivocal that human influence has warmed the atmosphere, ocean and land,” the authors write. “Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred.”
Climate Change 2021: the Physical Science Basis, part of the IPCC’s sixth assessment cycle, provides a thorough update to the state of climate science since the IPCC’s fifth assessment in 2014. The assessment builds on the latest data to provide a snapshot of how much humans have shifted the climate since industrialization and what’s in store in the future. The new report has 234 authors, references more than 14,000 scientific papers, and was subjected to review from numerous climate experts and government officials. “The most important takeaway is that climate change is now, certain, and it’s here,” says Diana Bernstein, a climate scientist at the University of Southern Mississippi. “If we don’t address it immediately … it’s going to make everybody’s lives miserable.
Setting a two million year record in atmospheric carbon levels
Since the Industrial Revolution, human-generated emissions of greenhouse gases, primarily carbon dioxide, have caused the surface temperature of the globe to warm by 1.1ºC. Just considering temperatures over land (which is less than a third of Earth’s surface), the average global warming is even more—about 1.6ºC. Each of the last four decades has been warmer than any decade preceding it dating back to 1850.
Carbon dioxide is now concentrated at 410 parts per million in the atmosphere, a level that hasn’t been so high since two million years ago. It is true that the planet has always had warmer and cooler fluctuations in climate, but those shifts occurred very slowly over many thousands of years—glacial and interglacial cycles. The pace of warming today, on the other hand, is without parallel in the history of our species. What does that mean for us? As Laura Gallardo, coauthor of the report and climate scientist at the University of Chile, puts it: “The planet is going to survive … as long as the sun is more or less alive,” she says. “However, this short time framework in which things are changing, makes it extremely difficult for us to adapt and change in time.”
The window for action is nearly closed on keeping the Paris Agreement
Even if we manage to immediately cease burning fossil fuels (which is unlikely), the long-lived nature of carbon dioxide in the atmosphere means we will continue to warm the planet for a while. Global temperatures will creep upward until at least midcentury, the assessment found. That makes keeping warming within international targets like 1.5 or 2ºC challenging. “Global warming of 1.5°C and 2°C will be exceeded during the 21st century unless deep reductions in CO2 and other greenhouse gas emissions occur in the coming decades,” write the IPCC authors.
Humanity’s best hope is to cut emissions to net zero (with any human-caused emissions offset by some form of carbon removal) and ideally go negative. Still, even under the IPCC’s “very low emissions” modelled trajectory, it’s estimated that temperatures will crest at 1.6ºC of warming by midcentury before dipping back to 1.4ºC by 2100. With just a temporary overshoot of 1.5ºC, we can avoid crossing many dangerous climate tipping points.
In developing their projections, IPCC scientists narrowed their range of “climate sensitivity,” which is how much the planet is expected to warm if the carbon dioxide concentration in the atmosphere is doubled from pre industrial levels to 560 parts per million. The likely range of warming from a doubling of CO2 is now 2.5ºC to 4ºC, with a “best estimate” of 3ºC, narrowing from 1.5 to 4.5 °C—a range that hasn’t been updated since the 1970s (and the earliest estimate of climate sensitivity, 5ºC, was published in 1896).
Certain changes are irreversible
The extra CO2 we’ve added so far is responsible for much more than a bump in global temps. Here are a few major findings from the IPCC report:
- Almost all the world’s glaciers are retreating, and ice loss is causing polar regions to warm extra fast due to the loss of heat-reflecting frozen cover.
- Heat waves are hotter and more frequent, and the hottest days in a decade are 1.2ºC hotter than they were between 1850 and1900.
- Ocean heat waves have doubled in frequency since the 1950s, and CO2 emissions are also driving acidification and a decline in oxygen in marine waters. Global mean sea level has risen by 20 centimeters since 1901 and will continue to rise up to about a meter by 2100.
- Precipitation has intensified, with heavy downpours becoming more frequent. The proportion of major hurricanes has increased and the latitude of their peak intensity has shifted northward.
- Even with heavier rains, we still have more drought—once-in-a-decade droughts happen twice as often now. With increased temperatures, water evaporates faster and plants also use more of it.
In this year’s report, you can now also see how climate change will affect your region. The latest IPCC assessment had an increased focus on regional outlooks for climate change, including the incidence of extreme events. “Eight years ago we weren’t completely sure whether some of the extreme events we’re seeing were due to human-caused climate change,” said co-author Jessica Tierney, geoscientist with the University of Arizona, in a statement. “Now we’re pretty sure that that is the case. So some examples might be the extreme heat waves that we’re seeing around North America right now. The IPCC, this new report, says that those heat waves were unlikely to occur without human climate change.”
If we manage to stop polluting the atmosphere with greenhouse gases, temperatures will eventually stabilize—though we will continue warming for at least a couple decades. But there are some changes to the climate that are more permanent. For hundreds or even thousands of years to come, changes to the ocean, ice sheets, and sea level won’t be restored. The ocean will continue to absorb heat, leading to continued ice melting and sea level rise. Even if humans totally reverse course and move to negative emissions, the seas will remain elevated for perhaps thousands of years, rendering many coastal zones unsafe or impossible to live in. “Climate is not a linear thing,” says Gallardo.
Every bit of emissions avoided counts
It’s understandable to feel deflated by the news. With even 1.5ºC of warming, which is all but inevitable now, droughts, floods, tropical storms, wildfires, and heat waves will intensify and impact millions of people. But things can also get a lot worse—the frequency and magnitude of these extreme events grows even with a 0.5ºC rise, the assessment says. “Limiting warming to the 1.5 degree C target of the Paris Agreement would require immediate, rapid, and large-scale reduction in greenhouse gas emissions,” Mathew Barlow, a coauthor and climate scientist at the University of Massachusetts Lowell said in a statement. “However, regardless of any specific temperature target, every degree matters: Reducing emissions will reduce impacts.”
The bottom line: We’ll very likely reach or exceed 1.5ºC in the next 20 years, and immediate and expansive action is needed to limit the effects of climate change. That means a rapid shift to carbon-free energy, widespread electrification, and redesigning both cities and farms. For the United States, a big step could be coming this fall, says Bernstein: passing a clean energy standard and clean energy tax incentives through an upcoming infrastructure bill via a process known as budget reconciliation.
For climate change, the takeaway is to go big, or lose the only home we have. The planet will persist without us, but if humanity wants to stay, it needs to fight.
