“There is no documented historic precedent” for the sweeping change to energy, transportation and other systems required to reach 1.5 degrees Celsius, the U.N. Intergovernmental Panel on Climate Change (IPCC) wrote in a report requested as part of the 2015 Paris climate agreement.

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At the same time, however, the report is being received with hope in some quarters because it affirms that 1.5 degrees Celsius is still possible — if emissions stopped today, for instance, the planet would not reach that temperature. It is also likely to galvanize even stronger climate action by focusing on 1.5 degrees Celsius, rather than 2 degrees, as a target that the world cannot afford to miss.

“Frankly, we’ve delivered a message to the governments,” said Jim Skea, a co-chair of the IPCC panel and professor at Imperial College London, at a press event following the document’s release. “It’s now their responsibility … to decide whether they can act on it.” He added, “What we’ve done is said what the world needs to do.”

The transformation described in the document is breathtaking, and the speed of change required raises inevitable questions about its feasibility.

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Most strikingly, the document says the world’s annual carbon dioxide emissions, which amount to more than 40 billion tons per year, would have to be on an extremely steep downward path by 2030 to either hold the world entirely below 1.5 degrees Celsius, or allow only a brief “overshoot” in temperatures. As of 2018, emissions appeared to be still rising, not yet showing the clear peak that would need to occur before any decline.

Overall reductions in emissions in the next decade would probably need to be more than 1 billion tons per year, larger than the current emissions of all but a few of the very largest emitting countries. By 2050, the report calls for a total or near-total phaseout of the burning of coal.

“It’s like a deafening, piercing smoke alarm going off in the kitchen. We have to put out the fire,” said Erik Solheim, executive director of the U.N. Environment Program. He added that the need to either stop emissions entirely by 2050 or find some way to remove as much carbon dioxide from the air as humans put there “means net zero must be the new global mantra.”

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The radical transformation also would mean that, in a world projected to have more than 2 billion additional people by 2050, large swaths of land currently used to produce food would instead have to be converted to growing trees that store carbon and crops designated for energy use. The latter would be used as part of a currently nonexistent program to get power from trees or plants and then bury the resulting carbon dioxide emissions in the ground, leading to a net subtraction of the gas from the air — bioenergy with carbon capture and storage, or BECCS.

“Such large transitions pose profound challenges for sustainable management of the various demands on land for human settlements, food, livestock feed, fibre, bioenergy, carbon storage, biodiversity and other ecosystem services,” the report states.

The document in question was produced relatively rapidly for the cautious and deliberative IPCC, representing the work of nearly 100 scientists. It went through an elaborate peer-review process involving tens of thousands of comments. The final 34-page “summary for policymakers” was agreed to in a marathon session by scientists and government officials in Incheon, South Korea, over the past week.

The report says the world will need to develop large-scale “negative emissions” programs to remove significant volumes of carbon dioxide from the atmosphere. Although the basic technologies exist, they have not caught on widely, and scientists have strongly questioned whether such a program can be scaled up in the brief period available.

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The bottom line, Sunday’s report found, is that the world is woefully off target.

Current promises made by countries as part of the Paris climate agreement would lead to about 3 degrees Celsius (5.4 degrees Fahrenheit) of warming by the end of the century, and the Trump administration recently released an analysis assuming about 4 degrees Celsius (7.2 degrees Fahrenheit) by 2100 if the world takes no action.

In a statement, a U.S. State Department official expressed appreciation for all the work that went into the report but noted that “governments do not formally endorse specific findings presented by the authors.”

“From 2005 to 2017, U.S. CO2-related emissions declined by 14 percent while global energy-related CO2 emissions rose by 21 percent during the same time,” said the official. “This has been possible through the development and large-scale deployment of new, affordable, and cleaner technologies to capitalize on our energy abundance.”

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The IPCC is considered the definitive source on the state of climate science, but it also tends to be conservative in its conclusions. That’s because it is driven by a consensus-finding process, and its results are the product of not only science, but negotiation with governments over its precise language.

In Sunday’s report, the body detailed the magnitude and unprecedented nature of the changes that would be required to hold warming to 1.5 degrees Celsius, but it held back from taking a specific stand on the feasibility of meeting such an ambitious goal. (An early draft had cited a “very high risk” of warming exceeding 1.5 degrees Celsius; that language is now gone, even if the basic message is still easily inferred.)

“If you’re expecting IPCC to jump up and down and wave red flags, you’re going to be disappointed,” said Phil Duffy, president of the Woods Hole Research Center. “They’re going to do what they always do, which is to release very cautious reports in extremely dispassionate language.”

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Some researchers, including Duffy, are skeptical of the scenarios that the IPCC presents that hold warming to 1.5 degrees Celsius, particularly the reliance on negative-emissions technologies to keep the window open.

“Even if it is technically possible, without aligning the technical, political and social aspects of feasibility, it is not going to happen,” added Glen Peters, research director of the Center for International Climate Research in Oslo. “To limit warming below 1.5 C, or 2 C for that matter, requires all countries and all sectors to act.”

Underscoring the difficulty of interpreting what’s possible, the IPCC gave two separate numbers in the report for Earth’s remaining “carbon budget,” or how much carbon dioxide humans can emit and still have a reasonable chance of remaining below 1.5 degrees Celsius. The upshot is that humans are allowed either 10 or 14 years of current emissions, and no more, for a two-thirds or better chance of avoiding 1.5 degrees Celsius.

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The already limited budget would shrink further if other greenhouse gases, such as methane, aren’t controlled or if and when Arctic permafrost becomes a major source of new emissions.

But either way — in a move that may be contested — researchers have somewhat increased the carbon budget in comparison with where the IPCC set it in 2013, giving another reason for hope.

The new approach buys some time and “resets the clock for 1.5 degrees Celsius to ‘five minutes to midnight,’ ” said Oliver Geden, head of the research division of the German Institute for International and Security Affairs.

The report is sure to be the central focus of attention this December in Poland when the next meeting of the parties to the Paris climate agreement is held, and countries begin to contemplate how they can up their ambition levels, as the agreement requires them to do over time.

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Meanwhile, the report clearly documents that a warming of 1.5 degrees Celsius would be very damaging and that 2 degrees — which used to be considered a reasonable goal — could approach intolerable in parts of the world.

“1.5 degrees is the new 2 degrees,” said Jennifer Morgan, executive director of Greenpeace International, who was in Incheon for the finalization of the report.

Specifically, the document finds that instabilities in Antarctica and Greenland, which could usher in sea-level rise measured in feet rather than inches, “could be triggered around 1.5°C to 2°C of global warming.” Moreover, the total loss of tropical coral reefs is at stake because 70 to 90 percent are expected to vanish at 1.5 degrees Celsius, the report finds. At 2 degrees, that number grows to more than 99 percent.

The report found that holding warming to 1.5 degrees Celsius could save an Alaska-size area of the Arctic from permafrost thaw, muting a feedback loop that could lead to still more global emissions. The occurrence of entirely ice-free summers in the Arctic Ocean goes from one per century to one per decade between 1.5 and 2 degrees, it found — one of many ways in which the mere half a degree has large real-world consequences.

Risks of extreme heat and weather events just rise and rise as temperatures do, meaning these would be worse worldwide the more it warms.

To avoid that, in barely more than 10 years, the world’s percentage of electricity from renewables such as solar and wind power would have to jump from the current 24 percent to something more like 50 or 60 percent. Coal and gas plants that remain in operation would need to be equipped with technologies, collectively called carbon capture and storage (CCS), that prevent them from emitting carbon dioxide into the air and instead funnel it to be buried underground. By 2050, most coal plants would shut down.

Cars and other forms of transportation, meanwhile, would need to be shifting strongly toward being electrified, powered by these same renewable energy sources. At present, transportation is far behind the power sector in the shift to low-carbon fuel sources. Right now, according to the International Energy Agency, only 4 percent of road transportation is powered by renewable fuels, and the agency has projected only a 1 percent increase by 2022.

The report’s statements on the need to jettison coal were challenged by the World Coal Association.

“While we are still reviewing the draft, the World Coal Association believes that any credible pathway to meeting the 1.5 degree scenario must focus on emissions rather than fuel,” the group’s interim chief executive, Katie Warrick, said in a statement. “That is why CCS is so vital.”

That’s an approach largely embraced by the head of the Environmental Protection Agency, which under President Trump has taken numerous steps to roll back regulations on the coal industry.

In an interview with The Post last week, the EPA’s acting administrator, Andrew Wheeler, said the United States will “continue to remain engaged in the U.N.'s effort,” despite the fact that Trump has said he intends to withdraw from the Paris climate accord as soon as legally possible.

But asked specifically about what it would take to keep the world below a dangerous level of climate change, Wheeler declined to identify a specific level. The agency’s regulatory approach is that it would allow the coal industry “to continue to innovate on clean coal technologies, and those technologies will be exported to other countries."

In the end, “one thing is for sure” in light of the IPCC report, said Niklas Hohne, a scientist who heads the New Climate Institute, in a statement.

“If we give up the goal and do not even try, we will certainly miss it a long way.”