The 2010s may go down in environmental history as the decade when the fingerprints of climate change became evident in extreme weather events, from heat waves to destructive storms, and climate tipping points once thought to be far off were found to be much closer.
It was the decade when governments worldwide woke up to the risk and signed the Paris climate agreement, yet still failed to reduce the greenhouse gas emissions at the pace and scale needed. And when climate scientists, seeing the evidence before them, cast away their reluctance to publicly advocate for action.
The sum of the decade's climate science research, compiled in a series of reports by the Intergovernmental Panel on Climate Change (IPCC), suggests global warming is pushing many planetary systems toward a breakdown.
New studies showed polar ice caps melting and sea level rising much faster than just 10 years ago. Ocean researchers showed how marine heat waves kill corals and force fish to move northward, affecting food supplies for millions of people in developing countries. They tracked changes to crucial ocean currents and concluded that hurricanes will intensify faster in a warming world.
Together, the research showed how important it will be to cap the global temperature rise as far below 2 degrees Celsius—the Paris Agreement goal—as possible.
Feedback Loops on the Greenland Ice Sheet
At the start of the decade, it was unclear how fast the Greenland and West Antarctic Ice Sheets would melt. As recently as the 1990s, melting of the Greenland Ice Sheet was balanced by the buildup of new snow and ice, offering some hope that sea level rise would be slow, allowing coastal communities time to adapt.
By the end of 2019, a study published in the scientific journal Nature showed the Greenland Ice Sheet was melting seven times faster than it had been in the 1990s. That's on pace with the IPCC's worst-case climate scenario, with Greenland alone contributing 2 to 5 inches of sea level rise by 2100. Another study, looking at evidence in fossilized shells, showed temperatures are very near a threshold that will melt most of the ice sheet.
Scientists discovered feedback loops and new ways earth's systems interact to melt the ice. Global warming is expanding ice slabs beneath Greenland's snowy areas, hastening runoff and sea level rise. In Antarctica, they showed how global warming is shifting winds and pushing warmer water under floating ice shelves—both of which could contribute to rapid disintegration of ice shelves with a subsequent surge of sea level rise.
"The rate and magnitude of Greenland Ice Sheet mass loss, and of ice loss globally, has been dramatic," said Twila Moon, a climate researcher with the National Snow and Ice Data Center.
How Sea Ice Loss Influences the Atmosphere
Many studies in the second half of the decade showed how important it is to keep the global temperature rise as far below 2 degrees Celsius as possible to avoid triggering tipping points that would have cascading consequences. Arctic sea ice is one of the big concerns.
Even now, in its diminished state, the summer Arctic sea ice is a 1.6 million square-mile shield that reflects incoming solar radiation back to space. The more it melts, the more darker-colored ocean can absorb heat, speeding up the planet's overall warming.
At 2 degrees Celsius warming, Arctic Ocean sea ice will probably melt completely, said National Snow and Ice Data Center climate researcher Walt Meier. "Some ice probably will persist if warming is limited to 1.5 degrees Celsius," Meier said. He noted that research has suggested the ice could recover fairly quickly—if greenhouse gas concentrations are reduced enough to drop the temperature.
One intriguing question has been how the loss of Arctic sea ice will affect weather patterns in North America, Europe and Asia.
Melting that much of Earth's icebox could alter wind patterns that shunt weather systems around the Northern Hemisphere, scientists reasoned early in the decade. A study in 2012 suggested a mechanism: Sea ice melt alters the jet stream by reducing the temperature contrast between the Arctic and the mid-latitudes. As the jet stream weakens, it enables areas of rainy weather or hot, dry conditions to linger longer over a given area, leading to extreme rainfall or heat waves and drought.
As the decade ended, studies seemed to support that early conclusion. Research published by Michael Mann, Stefan Rahmstorf and others showed how heat waves, floods and wildfires are linked with a jet stream pattern that, in turn, is related to an over-heated Arctic. In a climate warmed by greenhouse gases, the jet stream is more likely to set up in a pattern that causes extremes to linger longer over Europe and North America.
Arctic Melting Also Effects Ocean Currents
Another major advance in climate research this decade relates to how the melting Arctic will affect a key Atlantic Ocean current that balances Earth's climate by carrying warm, salty water north in the upper layers of the ocean and colder water southward in the ocean depths.
Early in the decade, scientists were still speculating about whether such a slowdown was happening, but now, "for the first time the IPCC has confirmed that the AMOC has slowed down, as predicted by models," Rahmstorf said.
Slowing of the AMOC, the Atlantic Meridional Overturning Circulation, has been identified as one of the main tipping points in the planet's climate system. Changes in the circulation "have been responsible for some of the strongest and most rapid climate shifts during the Quaternary Period (the past 2.6 million years)," one study said.
Total collapse of the current, still seen as unlikely, would raise sea level by 30 inches around the North Atlantic, but even a slowdown will have a similar effect, to a lesser degree.
"The current is balanced, in part, by the slope of the sea surface as it climbs away from the East coast toward the center of the North Atlantic Ocean gyre," explained Penn State climate scientist Michael Mann. "If the current slows, that slope must decrease, which is accomplished by the sea surface rising along the coast. That can add as much as an extra foot to sea level rise over the next century over parts of the East Coast."
The slowdown will likely push tropical rainfall belts southward, disrupting agriculture and ocean ecosystems. A study in 2016 showed how global warming is shifting other currents in ways that could bring more intense storms to land areas, like the rainstorms that brought record flooding to Japan.
Marine Heat Waves Drive Ecosystem Tipping Points
Scientists have long known that the oceans are absorbing about 93 percent of the heat trapped by greenhouse gases, but it wasn't until the last few years that they showed how that's manifesting in marine heat waves that are driving some ocean ecosystems to tipping points.
The most visible might be the mass die-offs of coral reefs in 2015 and 2016 as the oceans seethed at near record warmth for months on end.
The impacts of another marine heat wave, known as the Pacific warm blob, cascaded through Pacific Ocean ecosystems in 2014 and 2015, causing widespread disruption to the food chain and promoting toxic algae blooms. Another began forming in the Pacific in 2019.
Marine heat waves don't just affect the water. The Pacific warm blob was also linked with California's extreme drought that pushed some inland forest ecosystems past the brink. The large area of stagnant heat over the ocean blocked cooler marine air from reaching the coast, enabling the heat to build up.
Since the 1920s, marine heat waves have become 34 percent more frequent, and research shows about 87 percent of them can be attributed to human-caused global warming.
Potential for West Antarctic Ice Sheet Collapse
At the start of the 2010s, scientists were just beginning to understand how vulnerable parts of Antarctica are to human-caused global warming. New studies this past decade showed that what was once considered a nearly invincible fortress of ice is crumbling at the edges.
Just a little more warming could push parts of the Antarctic ice sheets past a tipping point that would raise sea level faster and higher than anticipated by the climate assessments commonly used for coastal planning around the world.
"I'd say one of the big things this decade includes the potential collapse of West Antarctica, based on new research," said University of Colorado climate scientist Mike MacFerrin. "Just this year, we have evidence that Antarctica's recent speedup of ice loss is, in fact, human-caused."
Concerns about an abrupt collapse of parts of Antarctica's ice shelves intensified in 2016 with a study suggesting that water from surface melting could seep deep into the ice, refreeze and split off huge slabs of the shelves in a relatively short time. Radar surveys showing the thinning of the Thwaites Glacier helped trigger a massive international research effort to track the melting in that area, and scientists are also watching the Pine Island Glacier for signs of disintegration.
The worst-case tipping point scenarios shouldn't be off the table, said Jason Box, a climate scientist with the Geological Survey of Denmark and Greenland.
Box said the research showing destabilization of the West Antarctic Ice Sheet is a key climate science advance of this past decade "that should have been like a real wake-up call moment," he said.
Connecting Climate Change to Weather Extremes
Only a few years ago, scientists were reluctant to connect any specific extreme weather event to global warming.
In 2011 and 2012, Rahmstorf and other scientists triggered debates when they wrote that there was strong evidence linking specific events or an increase in their numbers to the human influence on climate. Since then, scientists who specialize in attribution research have found global warming fingerprints on nearly every heat wave they've studied.
"Looking back over the past decade, it's astonishing how firmly we've moved out to the tails of the probability distribution. Extremes are common and are commonly attributed to human activities," said Oregon State University climate researcher Philip Mote. "We're living the worst-case scenario."
One area of research during the 2010s with ominous findings was how global warming affects the intensity tropical storms, including North Atlantic hurricanes that affect the densely populated East Coast of the U.S. A 2016 study tracked a persistent shift of storm tracks toward that area.
"We've seen extreme events, with every major ocean basin having at least one big, catastrophic hurricane in the last few years," said NOAA climate scientist Deke Arndt. "We had Dorian and Michael, with rapid intensification so early in its development and close to the equator," he said. "Every basin has come up with monstrous storms."
Seeing the Risks, More Scientists Are Speaking Out
The 2010s may also be remembered as the decade when scientists cast away their reluctance to advocate for political and social changes.
The research on global warming is now so compelling that "a growing number of scientists are starting to make increasingly direct, pointed, and public statements about climate change risks to society," said Daniel Swain, a climate scientist with UCLA and the National Center for Atmospheric Research.
"My sense is that a key reason for this is the widening gap between physical reality and political reality," he said. "There is a growing sense that society is careening toward the abyss while we're collectively arguing about whether we should hit the brakes.
"Scientists who work in the climate domain are particularly, sometimes painfully, aware of the consequences that lie ahead if we continue on our current trajectory. And we're also aware that the emissions targets set forth in the Paris Agreement remain largely aspirational; we're currently still on a path to blow right past them."
Read this next from our decade in review series: Earth's Hottest Decade on Record Marked by Extreme Storms, Deadly Wildfires