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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.

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