2021 ended as it began: with disaster. Twelve months after an atmospheric river deluged California, triggering mudslides in burned landscapes and leaving a half-million people without power, a late-season wildfire destroyed hundreds of homes in the suburbs of Denver. In between, Americans suffered blistering heat waves, merciless droughts and monstrous hurricanes. People collapsed in farm fields and drowned in basement apartments; entire communities were obliterated by surging seas and encroaching flames.

More than 4 in 10 Americans live in a county that was struck by climate-related extreme weather last year, according to a new Washington Post analysis of federal disaster declarations, and more than 80 percent experienced a heat wave. In the country that has generated more greenhouse gases than any other nation in history, global warming is expanding its reach and exacting an escalating toll.

At least 656 people died amid the onslaught of disasters, media reports and government records show. The cost of the destruction tops $104 billion, according to the National Oceanic and Atmospheric Administration, even before officials calculate the final toll of wildfires, drought and heat waves in the West.

While the Federal Emergency Management Agency identified fewer climate-related disasters in individual counties last year, it declared eight of these emergencies statewide — the most since 1998 — encompassing 135 million people overall.

There is little doubt that the future will be worse. Steadily rising temperatures heighten the risk of wildfires, turbocharge rain storms, exacerbate flooding and intensify drought.

Yet planet-warming pollution, primarily from burning fossil fuels, surged to near-record highs last year. The Build Back Better bill, which contains the biggest clean energy investment in U.S. history, stalled in Congress. The United Nations climate conference in Glasgow, Scotland, produced pledges that put global average temperatures on track to rise about 2.5 degrees Celsius (4.5 degrees Fahrenheit) by the end of the century — a degree of warming that would transform once unthinkable disasters into near-annual occurrences.

2022 begins with two crucial questions still unanswered: Will the United States invest in ways to make extreme weather less destructive? And will the country lead the world in curbing warming before it becomes impossible for humanity to adapt?

These questions loom over Louisville, Colo., Mayor Ashley Stolzmann, who lost much of her town to wildfire on Thursday.

“When I lay awake the first night, not able to sleep from the fire, when I was evacuated from my house,” she said, “the first thing I thought of is: I need everyone to reduce their carbon emissions.”

People who never considered themselves at risk from climate change are waking up to floods and fires. (Monica Rodman/The Washington Post)

Over the course of just a few hours on Dec. 30, raging winds had propelled the flames through vegetation parched by the area’s warmest and driest summer and fall on record. The Marshall Fire burned so hot and so fast that it was able to jump a six-lane highway to engulf more than 1,000 homes in Louisville and neighboring Superior, quickly becoming the most destructive blaze in state history.

“To lose whole neighborhoods is just so very sad and devastating and unexpected,” Stolzmann said in an interview.

Fires like that are not supposed to happen in densely populated suburbs. They’re not supposed to ignite in December, long after the first snows of the winter have usually fallen.

Yet rising temperatures have intensified wildfire behavior and lengthened the season for when they can burn, scientists say. In most forest types, the proportion of fires that are “high severity” (killing the majority of vegetation) has at least doubled in recent decades. The weather necessary for fire — high temperatures, low humidity, rainless days and high winds — lasts more than a month longer than it did four decades ago.

“More people are living in more flammable landscapes,” said Chelsea Nagy, a research scientist at the University of Colorado at Boulder’s Earth Lab. “More people are going to be interfacing with disaster.”

The Post’s analysis of federal data shows that about 15 percent of Americans live in a county that experienced a declared fire disaster this year, a number that has been steadily increasing since 2018.

It is a testament to the way climate change “has loaded the weather dice against us,” climate scientist Katharine Hayhoe said.

Think of the climate as a bell curve, with temperatures distributed according to how common they ought to be. The center of the bell curve has shifted slightly, with the world just over a degree Celsius (1.8 degrees Fahrenheit) warmer than in the preindustrial era. But the area of the curve now in the “extreme” zone has increased significantly.

To assess Americans’ exposure to climate disasters, The Post considered FEMA declarations around severe storms, hurricanes, floods, wildfires and droughts — events that research shows are made more likely and more severe by rising temperatures.

For every 1 degree Celsius (1.8 degrees Fahrenheit) of warming, the atmosphere can hold 7 percent more moisture, resulting in exponentially wetter storms. When the remnants of Hurricane Ida hit the New York metro area in September, rainfall rates of three to four inches per hour overwhelmed a sewage system that was designed to handle less than half that amount. From the mountains of North Carolina to the narrows of the Grand Canyon to the streets of Bloomington, Ind., and rural parts of Tennessee, flash floods killed more than 100 Americans.

Conversely, the relationship between air temperature and humidity means that warmer conditions make the atmosphere “thirstier.” Water quickly evaporates from vegetation and soil, intensifying drought and making forests more likely to burn. On the Klamath River, water shortages sickened fish and emptied wells. In California, the Dixie Fire burned an area larger than New York City, Los Angeles, Chicago and Dallas combined. More than 90 percent of the United States west of the Rockies was in drought last year.

Meanwhile, melting polar ice caps have raised global sea levels an average of eight or nine inches since 1880, heightening the risk of coastal floods. The heat absorbed by the oceans lends fuel to hurricanes, enabling mere tropical depressions to rapidly intensify into devastating Category 4 and 5 storms. Not a single structure was left undamaged after Hurricane Ida made landfall near Grand Isle, La., in September. Surging waves ripped apart houses and obliterated levees; howling winds left the barrier island community buried under three feet of sand.

The Post also analyzed heat wave data from roughly 7,500 NOAA temperature monitors across the nation, finding that 80 percent of Americans live in a county that experienced at least one day of abnormally high temperatures last year.

Although extreme heat is not officially considered a disaster, it is one of the most fatal forms of severe weather. Officials say that at least 227 people were killed by the record-shattering heat dome that struck the Pacific Northwest in late June — a figure that is almost certainly an undercount.

The science of attribution allows researchers to link these crises to human-caused warming. Using sophisticated computer models, they compare real-world disasters to historical phenomena and simulations of how those events might have unfolded in a preindustrial world.

Two of last year’s biggest weather crises cannot be definitively tied to climate change: the February “deep freeze” that left millions without power from Texas to Ohio, and the December tornado outbreak that killed dozens of people across Kentucky. Combined, these disasters affected counties that are home to roughly 15 percent of the U.S. population.

Even though climate change will make frigid weather less likely, some scientists have suggested that warming in the Arctic can lead to fluctuations in the polar vortex, allowing tongues of cold air to lash out into lower latitudes. Other studies indicate that warmer winter temperatures provided fuel for the late-season thunderstorms that gave rise to December’s twisters.

But Michael Wehner, a climate scientist at Lawrence Berkeley National Laboratory, said more attribution research on both types of these disasters is needed to determine whether climate change increased the likelihood of these events. For that reason, The Post did not include the deep freeze and tornadoes in its climate disaster analysis.

On the other hand, some recent events were made so extreme by rising temperatures that “they break statistical models,” Wehner said. The Pacific Northwest heat dome, which scientists say was “virtually impossible” without climate change, was one such event. Last week’s Marshall Fire is likely to be another.

“It’s such an outlier, we don’t have anything to compare it to. How do you put statistics on that?” Wehner said.

This is what makes climate change so dangerous. In a steadily warming world, disasters can happen in places, at times and with intensities never seen before. They overwhelm infrastructure that wasn’t built to deal with them. They catch communities unprepared.

Yet people can change even faster than the climate, Wehner said, giving the United States a chance to adapt.

Climate was not the only factor contributing to last week’s conflagration in the Denver suburbs. The initial spark almost certainly came from a human source. The 100 mph winds made the blaze far more difficult to fight. Ongoing development in fire-prone areas adds to the devastation when the inevitable occurs.

Many of these factors are within people’s control, Nagy said. Public awareness campaigns around issues such as extinguishing campfires and cigarette butts, as well as industrial safety measures like burying power lines, can cut the number of “human ignitions,” which are behind the vast majority of fires that burn in the United States. Building codes and zoning regulations can be updated to make structures less flammable and prevent people from moving into areas that are impossible to defend.

Saving lives also requires investing in social infrastructure, said Patricia Romero-Lankao, a sociologist at the National Renewable Energy Laboratory. It means strengthening health-care access, so people are less vulnerable to the effects of smoke from fires and mold from floods. It means improving government outreach to non-English speakers and other marginalized groups. Cultivating connections between neighbors, so that people know who needs help during an evacuation and whom to check on when the mercury spikes to dangerous highs, can protect the most vulnerable.

“We need to build safety nets, a sense of belonging, a sense of shared space to deal with this,” she said. “It’s not only technical solutions.”

Louisville, where hundreds of homes were lost last week, has already adopted some of these measures, Stolzmann said. Flameproof building materials and carbon-free appliances are required in new construction. The community’s scattered residents are connected by text chains, which they use to check on one another’s houses, recommend contractors, offer reassurance.

Still, there are limits on people’s capacity to adapt, Wehner said, since disasters will become exponentially worse for every incremental increase in temperature.

“When we look back at the world that was and compare it to our 1 degree Celsius warmer world, the change is a lot,” Wehner said. “But when you compare that to 1.5 degrees, it’s a larger change. And then compare it to 2 to 3 degrees” — where scientists say the world is headed — “it’s kind of off the charts.”