The answer is almost everything, according to the Intergovernmental Panel on Climate Change (IPCC). The impact of human-induced warming is worse than previously feared, the report released Monday says, and only drastic, coordinated action will keep the damage short of catastrophe.

To this point, climate change has been a slow-motion calamity whose impacts, month to month and year to year, have been hard to perceive. Unfortunately, according to the report, that is about to change.

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The burning of fossil fuels on an industrial scale has raised global temperatures by about 1.8 degrees Fahrenheit. That may not sound like much, but look at the consequences we’re already seeing: Stronger, slower, wetter tropical storms. Unprecedented heat waves. Devastating floods. Dying coral reefs. A never-before-seen summer shipping lane across the Arctic Ocean.

Meanwhile, humankind continues to pump heat-trapping carbon dioxide into the atmosphere at a tragically self-destructive rate. The IPCC calculates that a further temperature rise of 1.5 degrees Celsius — almost inevitable, given our dependence on coal, oil and gas — would be challenging but manageable. An increase of about 2 degrees, however, would be disastrous.

What’s the difference? With a 1.5-degree rise, about 14 percent of the world’s population would be vulnerable to severe and deadly heat waves every five years; with a 2-degree rise, that figure jumps to 37 percent. With a 1.5-degree rise, an additional 350 million city dwellers worldwide will face water shortages; with a 2-degree rise, 411 million people will suffer such drought. With a 1.5-degree rise, coral reefs will experience “very frequent mass mortalities”; with a 2-degree rise, coral reefs will “mostly disappear.”

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Small differences can have huge impacts. Under the 1.5-degree scenario, up to 69 million people will be newly exposed to flooding. Under the 2-degree scenario — which the report estimates would boost sea-level rise by as much as 36 inches — the number rises to 80 million.

Please don’t dismiss all of this as just another boring compendium of carefully hedged facts and figures. I have followed the IPCC’s research since covering the first Earth Summit in Rio de Janeiro in 1992. The new report strikes a different tone that combines weary fatalism with hair-on-fire alarm. In dry, just-the-facts language, it predicts declining fisheries, failing crops, more widespread risk from tropical diseases such as malaria, economic dislocation in the most-affected countries — and, by logical extension, greater political instability.

All of these impacts are bad with 1.5 more degrees of temperature rise. With 2 degrees they are much, much worse.

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The obvious solution is to dramatically reduce carbon emissions. The IPCC says emissions need to decline by at least 40 percent by 2030 and to reach net zero by 2050, if we are to hold warming to 1.5 degrees. Yet last year, according to the International Energy Agency, global emissions hit an all-time high.

Since 2016, representatives of 195 nations — including all the big emitters — signed on to the landmark Paris agreement calling for systematic emissions reductions beginning in 2020. But Trump, who has ignorantly called climate change a “hoax,” decided to withdraw the United States from the pact. Even worse, Trump is aggressively trying to increase reliance on coal, which contributes a disproportionate amount of carbon dioxide emissions compared with other fossil fuels.

U.S. carbon emissions actually fell slightly in 2017, because of the expansion of the renewable energy sector. But Trump administration policies are designed to reverse that trend; and if they fail to do so, it will be because the rest of the world is already moving toward clean energy — a huge economic shift that threatens to leave the United States behind.

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When you read the IPCC report, you see that what the world really needs is visionary leadership. As the world’s greatest economic power and its second-largest carbon emitter, the United States is uniquely capable of shepherding a global transition to renewable energy. Instead, the Trump administration rejects the science of climate change and actively favors dirty energy sources over clean ones.

Humanity has no time for such foolishness. “I’m the president of the United States. I’m not the president of the globe,” Trump thundered at a recent rally. On what planet does he think this nation resides?