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The most extreme climate “alarmists” in U.S. politics are not nearly alarmed enough. The chances of avoiding catastrophic global temperature rise are not nil, exactly, but they are slim-to-nil, according to a new analysis prepared for the U.K. government.

Remember, climate change is simple. We’re trying to avoid temperature rise of more than 2 degrees Celsius above pre-industrial levels, because anything over that risks severe, irreversible, and overwhelmingly negative impacts. Currently we’re around 0.8 degrees above historical levels. If current trends continue, we could hit up to 6 degrees by 2100. That would likely exceed our ability to adapt, which is a polite way of saying it would lead to massive human die-off. That, in a nutshell, is (as I like to say) the brutal logic of climate change.

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How much can we feasibly limit temperature rise at this late date? A new paper (flagged by David Atkins) tries to answer that question; it’s from a research consortium involving the U.K. Met Office, the Walker Institute, the Tyndall Centre, and the Grantham Institute. The title is, “Development of emissions pathways meeting a range of long-term temperature targets” [PDF]. Feel the excitement!

The group modeled a range of temperature targets between 1.5 and 3.0 degrees. For a given target, the group attempted to produce a systematic assessment of all the possible pathways to achieving it. Pretty ambitious, no?

Three factors most influence the shape of a pathway to a given target: the year global climate emissions peak, the rate at which they subsequently decline, and the possibility of negative emissions in the latter half of the century. (Negative emissions would be achieved by burning biofuels and sequestering the carbon emissions, or “biosequestration.”) If emissions peak later, they have to decline faster; if negative emissions are possible, there is more room for a later peak; etc.

So what did the researchers find?

First and most notably, the lowest temperature rise we can feasibly hit by 2100 is 1.6 degrees. And this is using the term “feasible” rather loosely. Hitting 1.6 degrees relies on a series of somewhat ludicrous assumptions: that global emissions peak in 2014, that they decline at a rate of 3.5 percent a year thereafter (the highest rate the researchers deem possible), and that massive biosequestration becomes available late this century. To call that “optimistic” is … charitable.

What if you think, as many do, that biosequestration will never pan out and that negative emissions are a pipe dream? (Or what if you just don’t want to bet the future of the species on it?) Well, if you take biosequestration off the table, the lowest possible temperature rise we can feasibly hit is 2 degrees. And that still requires the rather heroic assumptions that global emissions will peak by 2016 and decline at 3.5 percent. (Note: No country has ever reduced its greenhouse gas emissions by 3.5 percent a year on a sustained basis. Even 1 to 2 percent is extraordinary, with little to no precedent.)

Long story short: This new round of comprehensive modeling shows that limiting temperature rise to 2 degrees is still within the realm of the possible, but only just barely. It would require a level of immediate, global, coordinated action never before seen in human history.

Nothing approaching that level of action is on the table, in the U.S. or any other country. Just about the boldest that you hear from anyone in U.S. politics is a revenue-neutral carbon tax, but a carbon tax big enough to generate the reductions these researchers are talking about would be gargantuan, on the order of hundreds of dollars per ton, not to mention the massive tariffs that would have to be levied on carbon-intensive imports. The fact is, achieving these reductions would require rethinking and rebuilding most of the political and economic systems that govern the country. Nobody is talking about that, much less advocating for it in a serious way.