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Here's the deal.

Humanity is wife has sex with brother videoson pace to heat Earth by around 3 degrees Celsius, or 5.4 degrees Fahrenheit, above pre-Industrial Revolution temperatures by the century's end.

That level of warming would be devastating. Why? The world has warmed by a little over 1C so far. Look at what that has done to charred Australia, rapidly melting Greenland, and the relentlessly warming oceans.

This 3C train we're riding is the most plausible "business as usual" warming scenario — meaning how much warming will likely occur with stillinsufficient plans to cut our prodigious carbon emissions — say climate researchers in a new commentary published Wednesday in the journal Nature.

Importantly, this means that an even worse warming scenario — which would require burning all the coal we can possibly reach, amounting to a cataclysmic 5C of warming — should not be considered "business as usual." (The term has often been misapplied to the apocalyptic 5C warming scenario in both published research and climate chatter.)

But at 3C, we're still in hot water.

"A 3C world will have catastrophic impacts for some, the loss of islands, corals, Arctic sea ice, some land glaciers, and not to mention changes in extreme weather," said Glen Peters, the research director at the Center for International Climate and Environmental Research (CICERO) in Oslo, Norway, and an author of the commentary.

"This is a world we want to avoid," said Peters.

"Three degreesC is still bad," stressed Sarah Green, an environmental chemist at Michigan Technological University who had no role in the Naturecommentary.

"Both [3C and 5C] are pretty catastrophic and we don't want to go to either place," said Green.

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Good news or bad news?

Being on the 3C railroad is not good news.

But there is a brighter perspective here. Heading towards 3C, rather than 5C, brings civilization closer to meeting the profoundly ambitious warming targets of 1.5C or 2C agreed to by all the world's nations at the historic U.N. Paris Accords (which the Trump administration will, to the frustration of many, leave this November).

"The fact we are heading for 3C, despite virtually no climate policies, should give us some hope that we might be able to quickly move to a pathway towards 1.5 or 2C," said Peters.

"I think of the 3C story as a more optimistic one, that our chances of under 2C are better than thought," he added.

"Is it good news or bad news?" asked Green. "It could be either. We have an unknown future. It's optimistic if you think we’re going to drive the future the right way."

If we were riding a 5C train, prospects for stabilizing the climate would be truly demoralizing. "Overstating the likelihood of extreme climate impacts can make mitigation seem harder than it actually is," noted Peters and his coauthor, climate scientist Zeke Hausfather, in the commentary. "This could lead to defeatism, because the problem is perceived as being out of control and unsolvable."

Even so, no one should rest easy at night, confident that humanity won't warm beyond 3C. It's quite possible "the planet might end up much more cooked than 3C," said Jon Overpeck, a climate scientist at the University of Michigan who had no role in the commentary.

Fossil fuel companies have big plans to ramp up the extraction and sale of oil and gas in the decades ahead. "Note that the fossil fuel industry is in a fight to the death, and is willing to go to no ends to make sure the world burns as much of their product as possible," said Overpeck.

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What's more, curbing warming at even 2C is a "herculean task," according to climate research published by Peters. We might be on the 3C train, but 2C is currently far, far, far out of reach. Even if the four biggest carbon-emitters — the U.S., China, the EU, and India — succeeded in dramatically cutting emissions over the next few decades, the rest of the worldwould need to radically cut their carbon emissions to nearly zero in the next decade — and stay there.

According to the U.N., global civilization must cut carbon emissions by 7.6 percent every year for the next decade to meet 1.5 C targets. That's unprecedented. Limiting warming to 1.5C, 2C, or somewhere in between will require an unprecedented global effort to transform civilization's power, industrial, agricultural, and transportation systems.

To understand the challenge, look at the United States. The transportation sector is the leading contributor of greenhouse gas emissions in the U.S. Meanwhile, in 2018, fuel consumption reached the nation's highest point ever, according to the Federal Highway Administration.

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The end game

Though the 3C scenario is most plausible, it's not set in stone. Not nearly.

That's because — although the planet is unquestionably warming and there are already unquestionable climate impacts — there's still a range of warming possibilities around 3 C, explained Angeline Pendergrass, a climate scientist at the National Center of Atmospheric Research who had no role in the research.

Climate scientists are still understanding how Earth's profoundly complicated nexus of oceans, atmosphere, ice worlds, and land will interact in different ways as Earth warms. "How much do clouds change?" asked Pendergrass. "How much warming does that lead to?"

Earth's natural systems might not behave as the planet continues to relentlessly warm. These natural systems, like bounties of carbon released from thawing Arctic soil (permafrost) and the retreat of massive ice sheets, could "drive considerable more warming than we think," noted the University of Michigan's Overpeck.

A warming Earth may drive us well past 3C. But, as Peters shows, at least 3C is more plausible than 5C. Though, knowing what an even more catastrophic 5C scenario means is an absolutely essential scientific exercise — 5C just shouldn't be thought of as the most likely path, aka "business as usual." "For scientists, we need to look at the extremes," said Green. "But all policy doesn’t need to be based on a worst-case scenario."

In the end, whatever warming track we're on, the most important fundamental rule doesn't change. "More emissions mean more warming and fewer emissions mean less warming," said Pendergrass.

It's a physical reality we can't escape.

Carbon emissions are still on the rise. Earth's atmospheric carbon dioxide levels, a potent greenhouse gas, are now skyrocketing. CO2 levels haven't been this high in at least800,000 years — though more likely millions of years. What's more, carbon levels are now rising at rates that are unprecedented in both the geologic and historic record.

We can confidently predict that 3C will be catastrophic. But we know how to avoid this future. "The hard thing to predict is how humans will react," said Green.