More big storms. More life-threatening heat waves. More flooding along the coasts. It's a familiar litany of horrors to anyone who knows about climate change.
But if any two of those events happen in rapid succession, the even more frightening result is what scientists call "compound extremes." The most recent example came in early September when Hurricane Irma gave Puerto Rico a glancing blow, followed two weeks later by the even more devastating Hurricane Maria.
Get ready for more of the same, some scientists warn.
Deep within the Climate Science Special Report released last week by 13 federal agencies is a chapter on what it calls "potential surprises" as the planet continues to warm and seas continue to rise. The chapter was cowritten by Robert Kopp, a Rutgers University professor. The report surprised many in how thorough and frank it is given the Trump administration's suspicion of climate science.
"I would say the report ended up being perhaps more extensive than the original target," said Kopp, who directs Rutgers' Institute of Earth, Ocean, and Atmospheric Sciences.
The report by the U.S. Global Change Research Program is a topic-by-topic takedown of climate denial, painstakingly outlined by the 51 scientists involved in the 18-month project.
Simultaneous heat and drought that produce wildfires, flooding associated with rain on top of snow, successive hurricanes — all of these "compound extremes" multiply the damage of single events, Kopp said. Data show that the occurrence of warm-dry and warm-wet conditions that can give rise to compound extremes has increased since the 1950s. Another way compound extremes could have a big impact would be major simultaneous droughts in different agricultural regions that could "challenge the ability of human systems" to provide food, especially in poorer regions, the report says. What's more, these compound events are harder to forecast. In fact, events could combine to create new patterns that take forecasters by surprise because they have not yet been observed in historical records, or because they haven't been predicted in simulations.
Hurricane Sandy's "compound extreme" came when rising sea levels, higher than normal ocean temperatures, and high tides combined. All those factors strengthened both the storm, and the magnitude of its storm surge.
At the same time, reduced sea ice in Greenland caused an atmospheric feature that blocked Sandy, directing it inland, making the damage even worse.
The Arctic Ocean is covered by ice in the summer and has decreased in recent decades. But if the ice is completely melted, it would mark a tipping point that could produce big changes in regional temperature and precipitation.
Further, vast amounts of carbon are locked in the Arctic permafrost and methane is locked in sediment in the continental shelves of the Arctic Ocean. Thawing can release the greenhouse gases with the combined potential to "significantly amplify both local and global warming" — even if humans stopped producing fossil fuel emissions altogether.
Scientists are watching another potential tipping point in ocean circulation that may be the reason for the "warming hole" observed in the North Atlantic. Long term, this phenomenon could accelerate global warming, while paradoxically cooling some parts of the United States.
In the Pacific, climate model experiments suggest another tipping point as warming triggers more and stronger El Niño and La Niña events, producing more extreme weather events in different parts of the country — at the same time.
Another potential tipping point: If the Amazon rain forest mostly converts to grasslands, it could lead to a huge increase in carbon dioxide, which can lead to even more warming.
"These multiple extreme events will have compound effects," Kopp says, "with the effects adding up to more than just the sum of their parts."