America is now under siege by climate change in ways that scientists have warned about for years. But there is a second part to their admonition: Decades of growing crisis are already locked into the global ecosystem and cannot be reversed.

This means the kinds of cascading disasters occurring today — drought in the West fueling historic wildfires that send smoke all the way to the East Coast, or parades of tropical storms lining up across the Atlantic to march destructively toward North America — are no longer features of some dystopian future. They are the here and now, worsening for the next generation and perhaps longer, depending on humanity’s willingness to take action.

"I’ve been labeled an alarmist," said Peter Kalmus, a climate scientist in Los Angeles, where he and millions of others have inhaled dangerously high levels of smoke for weeks. "And I think it’s a lot harder for people to say that I’m being alarmist now."

Last month, before the skies over San Francisco turned a surreal orange, Death Valley reached 130 degrees Fahrenheit, the highest temperature ever measured on the planet. Dozens of people have perished from the heat in Phoenix, which in July suffered its hottest month on record, only to surpass that milestone in August.

Conversations about climate change have broken into everyday life, to the top of the headlines and to center stage in the presidential campaign. The questions are profound and urgent. Can this be reversed? What can be done to minimize the looming dangers for the decades ahead? Will the destruction of recent weeks become a moment of reckoning, or just a blip in the news cycle?

The Times spoke with two dozen climate experts, including scientists, economists, sociologists and policymakers, and their answers were by turns alarming, cynical and hopeful.

"It’s as if we’ve been smoking a pack of cigarettes a day for decades" and the world is now feeling the effects, said Katharine Hayhoe, a climate scientist at Texas Tech University. But, she said, "we’re not dead yet."

Their most sobering message was that the world still hasn’t seen the worst of it. Gone is the climate of yesteryear, and there’s no going back.

The effects of climate change evident today are the results of choices that countries made decades ago to keep pumping heat-trapping greenhouse gases into the atmosphere at ever-increasing rates despite warnings from scientists about the price to be paid.

That price — more vicious heat waves, longer wildfire seasons, rising sea levels — is now irretrievably baked in. Nations, including the United States, have dithered so long in cutting emissions that progressively more global warming is assured for decades to come, even if efforts to shift away from fossil fuels were accelerated tomorrow.

‘Twice as Bad’

"What we’re seeing today, this year, is just a small harbinger of what we are likely to get," said Jonathan Overpeck, a climate scientist at the University of Michigan. Things are on track to get "twice as bad" as they are now, he said, "if not worse."

Earth has already warmed roughly 1 degree Celsius, or 1.8 degrees Fahrenheit, since the 19th century. The most optimistic proposals made by world governments to zero out emissions envision holding warming to below 2 degrees Celsius. Nations remain far from achieving those goals.

Usually, each passing year’s records are framed by the past — the hottest temperatures ever observed, the biggest wildfires in decades. However, as Cristian Proistosescu, an assistant professor at the University of Illinois, noted on Twitter, it may be time to flip that chronological framing, and consider today the new starting point.

"Don’t think of it as the warmest month of August in California in the last century," he wrote. "Think of it as one of the coolest months of August in California in the next century."

Climate change is more a slope than a cliff, experts agreed. We’re still far from any sort of "game over" moment where it’s too late to act. There remains much that can be done to limit the damage to come, to brace against the coming megafires and superstorms and save lives and hold onto a thriving civilization.

"We can certainly move in a direction that serves us a lot better," said Stephen Pyne, an environmental historian and professor emeritus at Arizona State University. "It’s not that it’s out of our control. The whole thing is in our control."

It won’t be easy, particularly if past is prologue.

Managing climate change, experts said, will require rethinking virtually every aspect of daily life: how and where homes are built, how power grids are designed, how people plan for the future with the collective good in mind. It will require an epochal shift in politics in a country that has, on the whole, ignored climate change.

One hope raised by some experts is that the current onslaught of fires and storms — the death, the destruction, the apocalyptic skies — might motivate people to unite behind calls for action. "Those orange skies — I mean, that was scary," said Kris May, a climate scientist and coastal engineer in San Francisco, referring to the midday tangerine glow over Northern California this month, a consequence of smoke from wildfires.

Yet she wondered if they would have been even more powerful had they had struck places like Washington, D.C. Perhaps there, she said, "they’d bring about more change."

When Lightning Strikes

The issue of climate change might have been back of mind for most Americans when a dramatic, rain-free lightning storm swept across Northern California in August. In a region that gets little rain in summer or early fall, the most destructive fires, like those that swept through Wine Country in 2017 and the town of Paradise in 2018, have come in October and November.

But one August night’s spectacular lightning show became the next day’s emerging disaster, as hundreds of fires were sparked, mostly in hard-to-reach terrain. Three of those blazes now rank among the four biggest California fires since record-keeping began in 1932 — part of the 3.6 million acres that have burned in the state so far.

And the traditional fire season is just beginning.

The fires, along with others in places including Colorado, Oregon and Washington, destroyed entire towns and sent smoke tens of thousands of feet high. San Francisco, Portland and Seattle have suffered some of the unhealthiest air quality on the planet, beating cities such as Beijing and New Delhi for the title. Smoke spread all the way across the continent, with particles coloring sunsets on the East Coast.

There was no place to escape. Evidence of global warming — which, scientists said, helps drive a rise in wildfire activity by creating hotter and drier conditions — was hanging visibly in the air.

For a long time, "there was so much focus on how climate change would affect the most vulnerable, like low-lying island nations or coral reefs — things that don’t dramatically affect the economic powerhouses of the world," said Katharine Mach, an associate professor at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science. "There’s often been this arrogant assumption that wealth provides protection."

Recent events, she said, are a vivid reminder that "we’re all in this together."

That notion raises a counterintuitive bit of hope: The more people who are affected, particularly the affluent and influential, the more seriously the issue gets addressed.

First, experts broadly agreed, if we want to stop the planet from relentlessly heating up forever, humanity will quickly need to eliminate its emissions of planet-warming greenhouse gases. That means cleaning up every coal plant in China, every steel mill in Europe, every car and truck in the United States.

It’s a staggering task. It means reorienting a global economy that depends on fossil fuels. So far, the world has made only halting progress.

But experts also made a point they say is often underappreciated: Even if we start radically slashing emissions today, it could be decades before those changes start to appreciably slow the rate at which Earth is warming. In the meantime, we’ll have to deal with effects that continue to worsen.

"In terms of being reversible, I can only think of things in sci-fi films — Superman trying to spin the earth in the other direction so Lois Lane doesn’t die," said Juan Declet-Barreto, a social scientist at the Union of Concerned Scientists. "Seriously, it is not reversible."

Again and again, climate scientists have shown that our choices now range from merely awful to incomprehensibly horrible.

If we cut emissions rapidly, about one-seventh of the world’s population will suffer severe heat waves every few years. Failure to do so doubles or triples that number. If we act now, sea levels could rise another 1 to 2 feet this century. If we don’t, Antarctica’s ice sheets could destabilize irreversibly and ocean levels could keep rising at an inexorable pace for centuries, making coastal civilization all but unmanageable.

The best hope is to slow the pace of warming enough to maintain some control for humanity.

"In our research, we’ve found that most systems can cope with a 1.5-degree or two-degree world, although it will be very costly and extremely difficult to adapt," said Dr. Hayhoe of Texas Tech University. "But in a four-degree world, in many cases, the system just doesn’t work anymore."

So, even as nations cut emissions, they will need to accelerate efforts to adapt to the climate change they can no longer avoid. "We need to figure out how to put ourselves less in harm’s way," said Gernot Wagner, a climate economist at New York University.

Humans are remarkably resilient. Civilizations thrive in climates as different as Saudi Arabia and Alaska.

When disaster strikes, we’ve demonstrated an ability to unite and respond. In 1970 and 1991, two major tropical cyclones hit Bangladesh, killing a half-million people. The country then built an extensive network of early-warning systems and shelters, and strengthened building codes. When another major cyclone struck in 2019, just five people died.

"The human capacity for adaptation is extraordinary — not unlimited, but extraordinary," said Greg Garrard, professor of environmental humanities at the University of British Columbia. He added, "I’m much more concerned for the future of the nonhuman than I am for the future of humans, precisely because we’re just very, very good at adaptation."

But as the case in Bangladesh illustrates, adaptation is usually a reactive measure, not a preventive one. Adapting to climate change means envisioning bigger disasters to come — again, flipping the framing away from history and into the future.

If You Can’t See It, Is It Real?

"Humans have difficulty imagining things that we haven’t experienced yet," said Alice Hill, a senior fellow at the Council on Foreign Relations who oversaw resilience planning on the National Security Council during the Obama administration.

"After every major catastrophe, whether it’s Pearl Harbor or 9/11, people always look back and say it was a failure of imagination. That also applies to climate change," she said. "It’s hard to visualize the entire West Coast aflame until you actually see it. And if we can’t see it, we tend to discount the risk."

There are concrete steps that can be taken today. Consider wildfires. After a deadly spate of Western blazes in 1910, the United States government scaled up its firefighting force, committing to extinguish wildfires wherever they occurred. For decades, that worked, giving Americans confidence that they could move into forested areas and remain safe.

But that policy led to a buildup of dense vegetation in the nation’s forests, which, when combined with a warmer and drier climate means that those forests are increasingly primed to burn bigger and hotter, overwhelming the nation’s firefighting capacity.

Going forward, experts said, the country will have to shift its mentality and learn to live with fire. States and communities will need to impose tougher regulations on homes built in fire-prone areas. Federal agencies will have to focus on managing forests better, selectively thinning some areas and even preventively setting controlled fires in others to burn off excess vegetation that can fuel runaway blazes.

"There’s a lot we can do," said Jennifer Balch, a wildfire expert at the University of Colorado at Boulder. "We’ve just been stuck in an emergency response rather than thinking and looking ahead."

Whether Americans can adopt that mentality remains an open question.

"We’ve often heard the argument that it will be too expensive to cut emissions and it will just be easier to adapt," said Noah Diffenbaugh, a climate scientist at Stanford University. But we’ve now had decades of warnings, he said, "and we’re not even adapted to the present climate."

Cascading Disasters

Adaptation can quickly become bogged down in a tangle of competing motivations and unintended consequences. Proposals for stricter building codes or higher insurance premiums face opposition from builders and voters alike.

And there’s the moral hazard problem, which is when people are shielded from the costs of their decisions and thus make bad ones. For instance, local communities reap increased property taxes from allowing buildings to rise in disaster-prone areas, but they don’t pick up most of the tab for disaster recovery — the federal government does.

Another challenge to adaptation is that, as climate change intensifies, it increases the risk of "compound hazards," when numerous disasters strike simultaneously, as well as the risk that one disaster cascades into another.

In late 2017, large wildfires scorched Santa Barbara, Calif., burning away vegetation that stabilized hillside soils. Heavy rainfall followed a month later. The result: devastating mudflows that killed 23 and injured 163.

In Houston in 2017, Hurricane Harvey shut down gasoline refineries, strained hospitals and spread toxic substances and pathogens as floodwaters swamped the city. And when the Camp Fire destroyed Paradise, Calif., in 2018, nearly 20,000 displaced people arrived in nearby Chico, which suddenly found its sewage system pushed to the limits.

"It’s really challenging to predict exactly where and how all of those cascading risks will unfold," said Amir Aghakouchak, a climate scientist at the University of California, Irvine, who studies compound hazards.

Experts also noted that climate change is an accelerant of inequality. Those most affected, globally and in the United States, tend to be the most vulnerable populations. Many are also among the people at highest risk for Covid-19.

As thousands have fled fires in recent weeks, farmworkers have continued to pick ripe crops, sometimes in evacuation zones. "They already live in a state of crisis that has been magnified, compounded by the pandemic," said Dr. Declet-Barreto of the Union of Concerned Scientists.

One concern is that adaptability will not be a collective effort. Wealthier people may find ways to protect themselves, while others are left fending for themselves. Even after American disasters, for example, relief is often off-limits to undocumented residents, experts said.

"Here in South Florida, people are building these amazing homes that float on the water. They can withstand Category 4 hurricanes, but they cost $6 million," said Dr. Mach of the University of Miami. "So how do we manage these risks so that it’s not just people with resources who stay safe?"

A Lifetime of Clues

For well over a century, science has provided us with powerful clues that this was coming.

As early as the 1850s, researchers realized that greenhouse gases like carbon dioxide could trap heat on Earth. This came at the dawn of the Industrial Age, which brought fossil-fuel-burning factories that ultimately not only filled people’s lives with modern conveniences, but also filled the sky with the carbon dioxide now warming the world.

By the 1990s, scientists had a deep understanding of the future risks of a warming world. By the 2010s, researchers could show how the extreme heat waves, droughts and floods now unfolding were influenced by climate change.

Technology offered solutions as well, whether solar power or electric cars. Yet governments have been slow to rein in reliance on fossil fuels.

"I feel like the climate scientists have kind of done our job," said Dr. Kalmus, the Los Angeles-based scientist. "We’ve laid it out pretty clearly, but nobody’s doing anything. So now it’s kind of up to the social scientists."

Will the recent spate of disasters be enough to shock voters and politicians into action?

"We have a lot of evidence that that doesn’t happen," said Dr. Garrard of the University of British Columbia.

One 2017 study found that people who experience extreme weather are more likely to support climate adaptation measures than before. But the effect diminished over time. It may be that people mentally adjust to unusual weather patterns, updating their perception of what they consider normal.

All of it can feel overwhelming, particularly for people wanting to make a difference. Susan Cutter, who directs the Hazards and Vulnerability Research Institute at the University of South Carolina, noted that climate change’s biggest problem may be the sense that it is beyond our control. The planet is burning, so does it really matter if I turn off the light?

"There’s too much complexity and, frankly, too much that needs to be changed, that we’re flitting from one concern to another," she said.

Even so, some important steps are being taken. Cities like Montecito, Calif., and Austin, Texas, have pursued difficult measures to protect against future wildfires. Britain, the birthplace of the Industrial Revolution, now goes coal-free for months at a time, having rapidly shifted to cleaner forms of electricity.

And if optimism springs from knowledge, the good news is that scientific research lays out what to do. It’s not a mystery, nor is it beyond the bounds of human ability.

"What’s beautiful about the human species is that we have the free will to decide our own fate," said Ilona Otto, a climate scientist at the Wegener Center for Climate and Global Change. "We have the agency to take courageous decisions and do what’s needed," she said. "If we choose."