When a major science journal publishes a major climate article led by our top climate scientist–and is wise enough to make it an open access article — you MUST read it. I reviewed an early draft of “Global Temperature Change,” by James Hansen et al in the current Proceedings of the National Academy of Sciences and consider it one of the most important articles published this year — or any year — worth reading a few times.

The article is already making headlines for showing that by 2050 the Earth will be warmer than it has been for a million years, thanks to human emissions of greenhouse gases. If we take strong actions to limit further emissions starting today — what the authors call the AS [alternative scenario] — we can limit total warming from preindustrial levels to about 2°C, but even that risks sea level rise of one meter per century.

Failing to act, the authors warn, ensures far higher temperature rise, “which could yield sea level rise of several meters per century with eventual rise of tens of meters, enough to transform global coastlines.” That is “the disastrous BAU [business as usual]” case.

Equally worrisome is that “species loss under BAU has the potential to be truly disastrous, conceivably with a majority of today’s plants and animals headed toward extermination.”

The paper notes that the alternative scenario limits the severe but difficult-to-quantify risks of melting tundra and other serious carbon cycle feedbacks that Climate Progress has discussed. Yet even one more decade of inaction is likely to render that scenario “infeasible.”

The time to act is now.

Two other highlights are worth noting. First, the article sets the record straight on recent warming trends and how climate models have accurately predicted them. NASA’s press release points out, “the Earth is now reaching and passing through the warmest levels seen in the last 12,000 years. And the warming is accelerating:

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Australian climate scientist Barry Pittock gave a terrific and terrifying talk at the 20th Anniversary of the Climate Insitute last week. He made the case that the U.N.’s Intergovernmental Panel on Climate Change (IPCC), the key international process for determining the “consensus” view on climate, is systematically underestimating the future impacts of climate change. Since Pittock was a major contributor to the IPCC’s Third Assessment Report (2001) and since their Fourth Assessment is due out next year, we should pay attention to what he says.

You can see all of Pittock’s 10 reasons online in the abstract for his talk. Let me pull out four of the underestimations:

1. “The climate sensitivity, or global warming after a doubling of the pre-industrial carbon dioxide concentration, is probably in the range of 2º–6°C rather than the 2001 IPCC estimate of 1.5º–4.5ºC. This suggests a more than 50% chance of that global warming by 2100 will be 3ºC or more, a level that many consider dangerous.”

3. “Permafrost melting is widespread,” which “leads to emissions of carbon dioxide and methane,” a dangerous vicious climate cycle that CP has written about.

7. & 8. “Rapid changes in Antarctica” and “Rapid melting and faster outlet glaciers in Greenland,” which combine to threaten far faster and greater sea level rise than climate models have been predicting.

I found his talk very compelling as it matched what I’ve been hearing from a number of climate scientists I interviewed for my book, including James Hansen. Pittock concludes:

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