There has always been an odd tenor to discussions among climate scientists, policy wonks, and politicians, a passive-aggressive quality, and I think it can be traced to the fact that everyone involved has to dance around the obvious truth, at risk of losing their status and influence.

The obvious truth about global warming is this: barring miracles, humanity is in for some awful shit.

Here is a plotting of dozens of climate modeling scenarios out to 2100, from the IPCC:

(Global Carbon Project) So what so what so what’s the scenario?

The black line is carbon emissions to date. The red line is the status quo — a projection of where emissions will go if no new substantial policy is passed to restrain greenhouse gas emissions.

We recently passed 400 parts per million of CO2 in the atmosphere; the status quo will take us up to 1,000 ppm, raising global average temperature (from a pre-industrial baseline) between 3.2 and 5.4 degrees Celsius. That will mean, according to a 2012 World Bank report, “extreme heat-waves, declining global food stocks, loss of ecosystems and biodiversity, and life-threatening sea level rise,” the effects of which will be “tilted against many of the world’s poorest regions,” stalling or reversing decades of development work. “A 4°C warmer world can, and must be, avoided,” said the World Bank president.

But that’s where we’re headed. It will take enormous effort just to avoid that fate. Holding temperature down under 2°C — the widely agreed upon target — would require an utterly unprecedented level of global mobilization and coordination, sustained over decades. There’s no sign of that happening, or reason to think it’s plausible anytime soon. And so, awful shit it is.

Nobody wants to say that. Why not? It might seem obvious — no one wants to hear it! — but there’s a bit more to it than that. We’ll return to the question in a minute, but first let’s look at how this unsatisfying debate plays out in public.

Are scientists keeping it real?

The latest contretemps was sparked by a comment in Nature by Oliver Geden, an analyst at the German Institute for International and Security Affairs. In it, he made a simple argument. Politicians, he says, want good news. They want to hear that it is still possible to limit temperature to 2°C. Even more, they want to hear that they can do so while avoiding aggressive emission cuts in the near-term — say, until they’re out of office.

Climate scientists, Geden says, feel pressure to provide the good news. They’re worried that if they don’t, if they come off as “alarmist” or hectoring, they will simply be ignored, boxed out of the debate. And so they construct models showing that it is possible to hit the 2°C target. The message is always, “We’re running out of time; we’ve only got five or 10 years to turn things around, but we can do it if we put our minds to it.”

That was the message in 1990, in 2000, in 2010. How can we still have five or 10 years left? The answer, Geden says, is that scientists are baking increasingly unrealistic assumptions into their models.

Can we really suck a bunch of carbon out of the atmosphere?

Geden focuses on one such assumption: that substantial negative emissions will be possible in the latter half of the 21st century. We will be able to suck thousands of megatons of carbon out of the atmosphere, so humanity can go net negative by 2100, even if we emit a bunch more carbon in the short term.

The mechanism for negative emissions is supposed to be bioenergy — burning plant mass — coupled with carbon capture and sequestration. The combo is called BECCS, and in theory, it buries more CO2 than it emits.

(Sanchez et al., 2015)

If you work enough BECCS into your model, you can almost double humanity’s “carbon budget” — the amount of carbon we can still pump in the atmosphere without passing 2°C. After all, if you can suck half the carbon out, you can afford to pump twice the carbon in.

But is large-scale BECCS plausible? There’s the problem of finding a source of biomass that doesn’t compete with food crops, the harvesting of which does not spur additional emissions, and which can be found in the enormous quantities required. The IPCC scenarios that come in below 2°C require BECCS to remove between 2 and 10 gigatons of CO2 a year from the atmosphere by 2050. By way of comparison, all the world’s oceans combined absorb about 9 gigatons a year; all the world’s terrestrial carbon sinks combined absorb about 10 gigatons a year. That’s between 5 and 25 percent of total 2010 CO2 emissions.

These scenarios mean potentially doubling the capacity of terrestrial carbon sinks, capturing and burying — permanently, without leaks — gigatons of CO2 a year. How will it be monitored? What if it leaks or is breached?

There’s no consensus on the viability of widespread BECCS, which, after all, doesn’t exist yet. One 2014 commentary in the journal Nature Climate Change, co-bylined by 14 researchers, raised serious doubts about the feasibility of large-scale BECCS and the wisdom of betting the climate farm on it. They note that “deployment of large-scale bioenergy faces biophysical, technical and social challenges, and CCS is yet to be implemented widely,” and that “widespread deployment [of BECCS] in climate stabilization scenarios might become a dangerous distraction.”

Tips and tricks for producing optimistic model conclusions

But BECCS isn’t the only way to make models produce happier results. The scenarios that show a high likelihood of avoiding 2°C also presume policy regimes that are positively utopian: a rising price on carbon, harmonized across every country in the world; the availability, maturation, and rapid deployment of every known low-carbon technology; all bets paying off, for 50 years straight. It would be quite a run of luck.

Is it possible in models? Yes. Is it possible IRL? Climate modeler Glen Peters doesn’t think so:

There are other ways to shape model outcomes. Peters draws attention to this chart, from the IPCC AR5 report:

Row four is the total carbon budget available to humanity this century, in gigatons. As you can see, if you move right or left on the chart, relatively small changes substantially alter the carbon budget. If you tweak the scenario from having a 66 percent chance of staying under 2°C to a 33 percent chance, the carbon budget goes from 1,000 gigatons to 1,500 — 50 percent more breathing room.

If you decide 2°C is too difficult, and maybe 3°C is okay, your carbon budget goes from 1,000 gigatons to 2,400, more than doubling. That sure looks a lot easier. (Though, important note: even hitting that easier target would require substantial BECCS!)

Kevin Anderson, of the UK’s Tyndall Centre for Climate Change Research, is another frequent critic of these model assumptions. He says that models have often included unrealistically low estimates of current and future emissions growth, unrealistically early peaks in global emissions, and unequitable estimates of emission curves in developing countries (implicitly assuming stunted development).

Add to all these considerations the high rate of decline in emissions necessary after global emissions peak. It used to be that 2 percent annual global emission reductions was considered the maximum feasible (without serious economic contraction). Now models routinely show 4 or even 6 percent annual reductions, a rate of emissions decline that has never been achieved by anyone, anywhere, ever, much less consistently over 50 years.

Peters also shares this figure, from researcher Robbie Andrew:

In these scenarios, emissions never go net negative, though BECCS can get used. As you can see, for each year that emissions continue rising, the rate of decline afterward has to be steeper to stay within the budget.

Now policymakers are being told that emissions can peak in 2030 and still keep temperature rise under 2°C. To get that result in a modeling scenario, emissions have to fall 6 percent a year, even with large amounts of BECCS thrown in. To find that plausible, one has to imagine all of human society turning on a dime, beginning in 2030, deploying massive amounts of nuclear, bioenergy, wind, and solar, and doing so every year for decades.

It’s “possible,” yes, but at a certain point that term loses much meaning. Something that would require human beings to quickly and fundamentally change their collective behavior may not violate the laws of physics, but it is unlikely, given what we know about human beings, path dependence, and political dysfunction. This is what I once called the “brutal logic of climate change.”

Are scientists to blame?

The question is, who is responsible for publicizing the truth about the assumptions behind these scenarios? Is it scientists? Niklas Höhne, director of the New Climate Institute, offered a reasonable response to Geden:

“The IPCC has never advocated for any target and has not commented on the feasibility, nor has the UNEP gap report [which shows the gap between current emission commitments and what’s necessary for 2°C]. Both have shown the scenarios and the related assumptions, such as the need for net negative global emissions in some cases,” he said. “Both reports do not make a judgement on the feasibility. They leave that to the policy makers.”

I think there’s a good bit of truth in this. The integrated assessment models (IAMs) used to produce these scenarios are not meant to yield predictions, or even plausible alternatives. They show what outcomes result from a particular set of inputs; they reflect their assumptions. Theoretically policymakers ought to know this, but political misuse of modeling is as old as modeling.

Nonetheless, the heated reactions elicited by Geden’s piece do show that he’s on to something. You can see some of those reactions on BuzzFeed, ClimateWire, and Responding to Climate Change (RTCC). A few are just crazy and knee-jerk, like Bill Hare of Climate Analytics, who “lumped [Geden] in with climate skeptics and other naysayers ‘who systematically downplay the risks of climate change and argue against action to reduce emissions on spurious and ill-founded grounds.’” That is roughly the opposite of what Geden does.

Others respond by, in my view, missing the point. Stefan Rahmstorf and Michael Mann both insist that Geden is wrong, that 2°C is still physically possible.

I don’t take that as the main thrust of Geden’s argument, though. Lots of things are physically possible that nonetheless require heroic assumptions about collective human behavior (like, say, aggressive mitigation policy, in the face of powerful vested interests, harmonized across the globe, sustained for decades … and also many gigatons worth of BECCS). The question is not whether 2°C scenarios violates laws of physical science, but whether they are reasonable given what we know about human beings.

That’s not really a scientific judgment, though, is it? Geden makes the same mistake when he writes, “the climate policy mantra — that time is running out for 2°C but we can still make it if we act now — is a scientific nonsense.” No. It may be a nonsense, but it’s not a scientific nonsense. No branch of science, certainly not climatology, can tell us what the humans of 2050 are capable of. We are all, on that score, making educated guesses, and a knowledge of history, politics, and economics will be just as important to that judgment as any knowledge of the physical sciences.

Who owns the nonsense?

I imagine the scientists want to blame the policy advisors and the politicians — after all, they didn’t hide the unrealistic assumptions, they are right there in Appendix 17 for anyone interested.

And yes, theoretically, the policy advisers surrounding politicians should make clear to them exactly the assumptions required to produce the 2°C outcome. And politicians should be straight with their constituents about those assumptions.

However, as the kids say these days, politicians gonna politic. They all have enormous incentive to try to thread the needle, to accept the 2°C target on one hand while maintaining that current policy commitments are adequate, or might some day be adequate, on the other. To do that, they need evidence that success is still within reach.

There is not a politician on earth wants to tell his or her constituents, “We’ve probably already blown our chance to avoid substantial suffering, but if we work really hard and devote our lives to the cause, we can somewhat reduce the even worse suffering that awaits our grandchildren.” [crowd roars]

And Geden is right that scientists have very little incentive to tell the unpleasant truths either. They can stick to physical science and the “possibility” of 2°C for quite a bit longer, I would imagine. Geden fears that the next big thing, the next deus ex machina to save the 2°C target, is going to be solar radiation managements (aka geoengineering). If they’re told to model it, what can scientists do? They’ll model it.

The sad fact is that no one has much incentive to break the bad news (except, ahem, my colleague Brad Plumer). Humans are subject to intense status quo bias. Especially on the conservative end of the psychological spectrum — which is the direction all humans move when they feel frightened or under threat — there is a powerful craving for the message that things are, basically, okay, that the system is working like it’s supposed to, that the current state of affairs is the best available, or close enough.

To be the one insisting that, no, things are not okay, things are heading toward disaster, is uncomfortable in any social milieu — especially since, in most people’s experience, those wailing about the end of the world are always wrong and frequently crazy.

(Shutterstock) Phoenix, 2060.

Who wants to put on the posterboards, go out to the street corner, and rant?

Yet here we are. The fact is, on our current trajectory, in the absence of substantial new climate policy, we are heading for up to 4°C and maybe higher by the end of the century. That will be, on any clear reading of the available evidence, catastrophic. We are headed for disaster — slowly, yes, but surely.

Even as many climate experts are now arguing that 2°C is an inadequate target, that it already represents unacceptable harms, we are facing a situation in which limiting temperature even to 3°C requires heroic policy and technology changes.

And yet … the world doesn’t appear to be ending; there’s no big, visible threat. Climate change moves so slowly that its pace is evident primarily through graphs and statistics. It rarely rises above the background noise.

So people want to hear that there’s hope of 2°C. Politicians want to say that there’s hope of 2°C. When asked, modelers are still able to produce scenarios that show 2°C. And nobody wants to be the one to pee in the punch bowl.