Humanity is probably in for some awful shit, or so I argued in my previous post. Specifically, it appears unlikely we'll be able to limit global average temperature rise to 2° Celsius over pre-industrial levels, the target threshold agreed to, at least notionally, by all 141 countries engaged in the Copenhagen climate accord.

It is still possible to construct scenarios with sufficient emission reductions to stop short of 2°C, with fairly small net impacts on global GDP, but they require increasingly heroic assumptions, like a globally harmonized price on carbon, 4 to 6 percent annual reductions in worldwide emissions over 50 years, and/or several gigatons' worth of negative emissions later in the century.

When the economic and technological numbers in those scenarios are translated into real-life sociopolitical action, they represent nothing short of a revolution — a collective effort on the scale of US mobilization for World War II, in every country of the world, beginning within the next decade, sustained for the remainder of the century.

Look around. There is no such grand mobilization in the offing. Progress on carbon reduction remains fitful and scattered. It is difficult to envision a sudden unanimity around carbon reduction sufficient to see global emissions peak in the next five to 10 years and decline rapidly every year thereafter.

And so the likely outcome at this point is exceeding 2°C.

My post elicited all sorts of interesting responses, over email and Twitter (see also Jonathan Koomey and Dan Lashof). A good number, however, took the form, "So what you're saying is, [thing I'm not saying]." It seems that lots of issues have gotten lumped together in discussions around the 2°C target, so I thought it would be worth a follow-up to try to tease them apart.

So what you're saying is there's no hope

No!

Remember, the main question at hand is not whether 2°C is physically possible but whether it's sociopolitically feasible, or likely. That is, to say the least, uncertain, and where's there's uncertainty there's always hope.

After all, we're talking about scenarios that model economic and technological development out to 2100. It's one thing to predict how the climate system will change over a century — climate moves slowly — but another entirely to predict human development that far ahead. Social change is non-linear; after periods of relative stasis, societies can lurch to a new equilibrium almost overnight (a pattern known as "punctuated equilibrium"). What will be technologically feasible in 2050, or 2080? What political changes will take place between now and then? We have virtually no idea. So as Koomey says, we can't know in advance whether 2°C will prove to be feasible. We can't see the future.

And in general, I agree. I have written in the past about how the integrated assessment models (IAMs) used to create these scenarios contain such a wide array of contestable assumptions, about developments in so far in the future, that it is ludicrous to interpret their results as anything but thought exercises.

There could be a fundamental shift in consciousness and behavior sometime this century. Many people responded to my post by citing the relatively big recent shifts on gay marriage and marijuana as examples.

But ... still. Those are mild, passing gusts compared with the mighty wind necessary to blow the entire world economy in a new direction. The shift required to achieve 2°C would have to be enormous — utterly without historical precedent — and it would have to get underway soon.

Hoping for a fundamental shift in human consciousness and politics in the next 10 to 15 years amounts to hoping for a miracle. That's what hoping for 2°C means — banking on a miracle.

Which is fine. It's great to have hope! Not that long ago I wrote a whole long post about hope, which came down strongly on the pro-hope side of the debate. (It was popular; you should go read it.)

But particularly when it comes to policymakers and the people advising them, we also need clear-eyed straight talk. "We don't yet know that 2°C is impossible" is very different from "We're on track." The fact is, based on what we know about carbon budgets, path dependence, and status quo bias, changing the fossil fuel energy system fast enough to stay under 2°C appears increasingly unlikely. We are rapidly losing options, and the ones that remain are unrealistic or unpleasant.

So what you're saying is we should give up

No!

I think the 2°C target has generally been a positive thing (more on that in a minute), but it has led many people to discuss the climate challenge as though it is binary — we succeed or fail, solve it or blow it, prevent it or endure it.

But that's not how climate change works. There's nothing magic about the 2°C threshold. Things won't suddenly shift from tolerable to terrible when we cross it. It's just our best guess about when the really nasty risks start to kick in. We may be, and probably are, wrong about the exact temperature thresholds that yield particular ill effects. There's tons of uncertainty around that stuff (which, as climate campaigners have struggled unsuccessfully to explain, means more risk, not less). But the rough conclusion of the IPCC is: once you clear 1°C, global impacts become net negative and get worse and more costly from there on up.

Even if we go past 2°C, we'll still want to stop before 3°C; 4°C will be worse than 3°C, 5°C worse than 4°C, and so on. It will get progressively worse until we stop pushing the system with carbon emissions.

There's no preventing climate change; it is already underway. The signal can already be detected behind the noise of typical weather variations; impacts are already being felt. The only question is, having pushed this unfathomably large system into motion, whether we can ease off pushing soon enough. Best-case scenario, the system settles into a new, hotter-but-still-tolerable equilibrium. We can't un-ring the bell, though; every bit of CO2-driven warming is, for human intents and purposes, permanent.

Sorry, then, but no one's allowed to give up — our children and grandchildren will still be fighting this battle. Even if it does become finally, physically impossible to hit 2°C, so what? Carbon emissions still need to be driven to zero as quickly as possible to avoid even higher temperatures. The fight remains the same, no matter the temperature outcome.

So what you're saying is we should abandon the 2°C target

No!

Well, this one's a bit more complicated. The 2°C target has been called upon to carry quite a bit of weight in climate discussions — perhaps too much.

When it comes to something as complex as climate change, there's a huge need for simple, accessible metrics that can ground policy comparisons and orient everyone in the same direction. The 2°C target has served that purpose well, and insofar as it still represents anything like "safety" (and there's plenty of debate on that point), then yes, it is worth preserving. As much as anything, it is a statement of values, or at least aspirations.

A shared target allows us to add up the various commitments made at international climate talks and determine whether they are adequate, whether they represent our purported values. If they do not, that fact deserves to be known to the public. Without a clear metric, there's no way to make that judgment; it's tennis with no net.

A separate but related question is whether the 2°C target is an effective mechanism for driving policy. On that score, the record is mixed, to say the least. There's an argument to be made — political scientist David Victor and others have made it — that the obsessive focus of international climate talks on long-term targets has mainly served to elicit a lot of hand-waving and lowest-common-denominator pledges.

Politicians have very little control over exact emission levels in their countries decades hence, which are driven as much by demographics and global trade as by policy. Lots of politicians, especially in developing countries, lack access to the institutions and policy tools they need to lower carbon substantially. And for almost all politicians, economic growth remains the prime objective, no matter what they say about other targets.

Given those limitations, pledges about far-future emissions can't help but be cautious and hollow. It may prove healthy for UN talks to take the turn they have, to what might be characterized as normal politics, i.e., short- and medium-term policy commitments based on national interests and circumstances. They don't yet add up to a 2°C scenario, and might never, but talks are more likely to elicit action if the bar to entry is lower, if there are clear initial steps available.

The 2°C limit can be a useful marker, a statement of common purpose, without being the primary policy driver. For policy, there are more useful, less abstract, less distant goals, like the percentage of clean electricity in the power system, the rate of deforestation, or the level of investment in clean energy RD&D and infrastructure. These are things policymakers have more control over and thus more confidence promising.

So what you're saying is scientists are big fat liars

No!

Neither I nor Oliver Geden (in the commentary I referenced) accused scientists of faking anything, despite what a few Twitter trolls concluded. Climate modelers don't have to make anything up or lie about anything to make 2°C scenarios work. They just have to tweak various assumptions about when global emissions will peak, how fast they will fall, and what kinds of clean (or negative-emissions) energy will be commercially available. There's nothing nefarious about that; those are all contestable assumptions with a wide range of plausible inputs.

Rather, the idea is simply that the assumptions required to generate 2°C scenarios are becoming more and more implausible, and that's not being communicated to the public, or even necessarily to policymakers. All most people seem to hear is, "2°C is possible!" Someone needs to communicate to policymakers and the public that success, as it's currently defined, relies on miracles nowhere yet in evidence — and the task becomes more difficult with each passing year of inaction.

Geden thinks the climate modelers should communicate that. I'm not so sure. I mean, poor climate scientists. You never hear, say, microbiologists being called upon not just to do science, but to be articulate and effective spokespeople for their science, to guide politicians toward good policies regarding their science, or to spark a massive social revolution based on their science. Mostly they just do science. Yet climate scientists are expected to shoulder every load.

The policy and political communities bear some responsibility in this. The assumptions and limitations of models are clearly communicated in scientific reports (if perhaps not in the press releases accompanying them), and there's nothing stopping policymakers from approaching model results with the appropriate sobriety, or communicating them clearly to the public. Scientists can't do everything.

What I'm really saying

At this point, it looks like we're going to pass 2°C this century. And that's going to mean considerable suffering for lots of people (especially the world's poorest) and species.

Right now we are mitigating for 4°C and adapting for 2°C; we need to do the opposite. If we're going to hit 3°C or 4°C this century, we need to start making the investments necessary to ameliorate the effects (a process somewhat deceptively known as "adaptation"). This is especially true in poorer countries already ill-prepared for natural disasters, food and water shortages, and resource conflicts. Policymakers will find that whatever was saved by skimping on mitigation is absorbed many times over by coping with the impacts.

Still, 2°C or no 2°C, the task ahead remains the same: to get to zero carbon, or as close as possible, as soon as possible; to prepare for a hotter, more volatile future; to protect the most vulnerable, who did the least to cause the problem; and, along the way, to tell the truth about how we're doing.