Temperatures could subsequently cool down if carbon dioxide is somehow removed from the air later in the century, the document notes. But that prospect is questionable at the massive scales that would be required, it observes.
The 31-page draft, a summary of a much-anticipated report on the 1.5 degrees Celsius target expected to be finalized in October, was published by the website ClimateHome on Tuesday, which said the document had been "publicly available on the US federal register over the past month." Last month, several news outlets including Reuters quoted from the draft but did not publish it in full.
The 1.5 C target is crucial to small island nations worried about rising seas, and other nations particularly vulnerable to warming, and was explicitly included in the Paris climate agreement as the more ambitious of two climate goals, the other being 2 degrees C (3.6 degrees Fahrenheit).
The draft document states that there is a "very high risk" of the planet warming more than 1.5 degrees above the temperature seen in the mid to late 19th century. Maintaining the planet's temperature entirely below that level throughout the present century, without even briefly exceeding it, is likely to be "already out of reach," it finds.
Jonathan Lynn, spokesman for the United Nations' Intergovernmental Panel on Climate Change, which is producing the study, cautioned that the draft is a work in progress.
"The text is highly likely to change between this draft and the final approved summary for policymakers," he said.
Duke University climate expert Drew Shindell, who is listed as one of the drafting authors of the document, also noted that the draft summary was a very early version of the full report.
"It's much rougher and much more preliminary than even the underlying document," he said.
Although worrying, the conclusion will not be surprising to those who have followed a growing body of research on just what it would take to stop warming short of 1.5 degrees Celsius. The planet has already warmed by 1 degree Celsius or more.
In some places, the report notes, the temperature increase has already exceeded 1.5 degrees Celsius. In general, warming is more intense over land than over the oceans and is already particularly intense in the Arctic.
The document finds that a warming of 2 degrees Celsius (3.6 degrees Fahrenheit) would pose substantially larger risks in many respects than 1.5 degrees C - but it also finds that some severe risks will be present at 1.5 degrees, too.
A serious risk is already emerging to highly sensitive marine ecosystems, such as coral reefs, the document states, and 1.5 C may already be too much for them. Reefs "are at risk that at 1.5 C and at 2C they will no longer be dominated by corals," the draft report notes.
The chance that Greenland or the West Antarctic ice sheet will tip toward irreversible retreat is present at both 1.5C and 2C, the study finds - but at 2C, the likelihood of commitment to major sea level rise grows larger.
What's most striking is the radical nature and rapidity of the changes that would be required to somehow preserve a world below 1.5 degrees.
The document finds that the world has only 12 to 16 years worth of greenhouse gas emissions left, from the start of 2016, if it wants a better than even chance of holding warming below 1.5 degrees.
Two of those years have already elapsed, as of this writing. A third will have nearly elapsed by the time the draft report is finalized and released in October. (In December in Poland, it will feed into a broader United Nations deliberation about the adequacy of countries current promises to cut emissions.)
And once this "carbon budget" for 1.5 degrees Celsius is used up, emissions would have to plunge to zero to preserve the 1.5 degree goal - something that would almost certainly never happen, as it would sharply impair the world economy.
Since such rapid and severe cuts aren't likely, the report notes that it's virtually unavoidable that the planet will "overshoot" 1.5 degrees Celsius. To cool the Earth down afterward and avoid staying at dangerously high temperatures for long, it would then be necessary to remove carbon dioxide from the air at a massive scale - but that, too, is highly problematic.
Carbon removal scenarios generally involve reforesting large amounts of land, or growing trees or other plants on that land and using it for energy, and storing the resulting carbon dioxide emissions underground. But "increased biomass production and use has the potential to increase pressure on land and water resources, food production, biodiversity, and to affect air quality," the draft notes. "Therefore, the scale and speed of implementation assumed in some 1.5C pathways may be challenging."
"Avoiding a 1.5C warming would be very, very difficult without a significant overshoot," said Princeton University climate scientist Michael Oppenheimer, noting that he was commenting solely on the state of the science itself, rather than the leaked document. "Such a warming would cause increased bleaching and perhaps destruction of living coral reefs at some locations although at other places, reefs would probably survive a warming closer to 2C."
"Some of the high level messages I think come as no surprise, in that we are not on track anywhere near towards 1.5 C, and getting there would require enormous changes," added Shindell, noting that he was not speaking as an author of the draft report or on behalf of the IPCC, but simply as a scientist with expertise in the matter. "That basic conclusion, I think it's OK to say that it's not a surprise to anybody. Any climate scientist would have told you that even without the report."
The document's leak has become a standard affair for major United Nations climate science reports, because they are seen by so many reviewers.
In 2013, a leaked draft of part of the Intergovernmental Panel on Climate Change's fifth assessment report helped lend credence to the questionable idea that global warming had slowed down or "paused," based on a brief passage suggesting that the rate of warming had declined somewhat between 1998 and 2012. The final draft addressed the issue with more nuance, largely undermining the notion of any significant slowdown.
The authors have until May 15 to include any new published material in the report. Still, it's unlikely to change the fundamental conclusion that there is too little time to avert 1.5 C degrees of warming - barring some massive technological intervention.
"There is . . . no documented precedent for the geographical and economic scale of the energy, land, urban and industrial transitions implicit in pathways consistent with a 1.5C warmer world," the draft report notes.
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