Our climate destiny is grim after 25 top climate scientists rule out best case scenarios.

I keep vigil. I wish I knew how to get people to care, but we seem determined to kill ourselves while ignoring all the flashing code red warnings. I guess it comes down to what outrages folks and it isn’t our survival as a species, obviously. Perhaps it is for the best, all of the other species have the right to exist. No need to take them all down with us.

Disturbing but not unsurprising news on the climate front as a new study (pdf) found that a narrowing range of climate sensitivity has blown us past the goal of the Paris climate accords of limiting warming to two degrees to rule out catastrophic damage to our only home. The narrowing of climate sensitivity finds a "66% chance that the sensitivity range falls between 2.6 and 3.9 C of warming (4.9 to 7 F).”

The 25 top climate scientists that worked on the study have concluded that we will have catastrophic warming. It will likely kill many of us. However, a Sapien extinction event could be averted with strong efforts to reduce our fossil fuel emissions to zero is our best defense.

Paul Voosen writes in Science:

It seems like such a simple question: How hot is Earth going to get? Yet for 40 years, climate scientists have repeated the same unsatisfying answer: If humans double atmospheric carbon dioxide (CO2) from preindustrial levels, the planet will eventually warm between 1.5°C and 4.5°C—a temperature range that encompasses everything from a merely troubling rise to a catastrophic one.

Now, in a landmark effort, a team of 25 scientists has significantly narrowed the bounds on this critical factor, known as climate sensitivity. The assessment, conducted under the World Climate Research Programme (WCRP) and publishing this week in Reviews of Geophysics, relies on three strands of evidence: trends indicated by contemporary warming, the latest understanding of the feedback effects that can slow or accelerate climate change, and lessons from ancient climates. They support a likely warming range of between 2.6°C and 3.9°C, says Steven Sherwood, one of the study’s lead authors and a climate scientist at the University of New South Wales. “This is the number that really controls how bad global warming is going to be.”

The new study is the payoff of decades of advances in climate science, says James Hansen, the famed retired NASA climate scientist who helped craft the first sensitivity range in 1979. “It is an impressive, comprehensive study, and I am not just saying that because I agree with the result. Whoever shepherded this deserves our gratitude.”

Humanity has already emitted enough CO2 to be halfway to the doubling point of 560 parts per million, and many emissions scenarios have the planet reaching that threshold by 2060. The report underscores the risks of that course: It rules out the milder levels of warming sometimes invoked by those who would avoid emissions cuts. “For folks hoping for something better, those hopes are less grounded in reality,” says David Victor, a climate policy researcher at the University of California, San Diego, who was not part of the study.

Chelsea Harvey writes in E&E News on cloud feedbacks role in the research.

In a simple sense, greenhouse gases in the atmosphere warm the Earth by trapping heat from the sun that would otherwise be radiated back out into space. But there are other factors that can affect the total amount of warming the planet experiences over time.

These include physical changes in the air that makes up the atmosphere, the amount of water vapor in the atmosphere, and the melting of snow and ice on the Earth’s surface, which can speed up the rate of climate change as they disappear.

Then there’s the question of cloud feedbacks, often cited by scientists as one of the biggest uncertainties about future climate change. Warming in the atmosphere can change the size, density and lifespan of clouds. And clouds, in turn, are capable of either worsening or lessening global warming, depending on their characteristics.

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The new report devotes a large chunk of its analysis exclusively to clouds. It examines the growing body of science on how different types of clouds respond to climate change, and how changes in these clouds may affect future climate change.

The mounting evidence suggests that clouds are unlikely to mitigate climate change on a global scale, the report concludes. On the contrary, they’re more likely to make it worse.

Plain Language Abstract from the pdf.

Earth’s global “climate sensitivity” is a fundamental quantitative measure of the susceptibility of Earth’s climate to human influence. A landmark report in 1979 concluded that it probably lies between 1.5-4.5℃ per doubling of atmospheric carbon dioxide, assuming that other influences on climate remain unchanged. In the 40 years since it has appeared difficult to reduce this uncertainty range. In this report, we thoroughly assess all lines of evidence including some new developments. We find that a large volume of consistent evidence now points to a more confident view of a climate sensitivity near the middle or upper part of this range. In particular, it now appears extremely unlikely that the climate sensitivity could be low enough to avoid substantial climate change (well in excess of 2℃ warming) under a high-emissions future scenario. We remain unable to rule out that the sensitivity could be above 4.5℃ per doubling of carbon dioxide levels, although this is not likely. Continued research is needed to further reduce the uncertainty and we identify some of the more promising possibilities in this regard.

The study appears not to take methane and other greenhouse gases, other than CO2, into the model.