One Cheer for UNEP’s Limiting Overshoot Report

Slowly, ever so slowly, the arithmetic imperative of gigaton carbon dioxide removal is breaking through

The UN Environment Programme (UNEP) put out a fat report this week and I read it with the mix of trepidation and forlorn hope these reports always elicit. As always, I read it looking for signs that the Overton Window on climate is moving, that previously unsayable things are becoming sayable, that “respectable opinion” looks different than how it looked a year ago, or five. And, read in that spirit, I do think there’s something to cheer here.

First, and most obvious, the UN Climate Blob is finally giving up on the pretense that we can keep global warming to within 1.5 degrees Celsius above pre-industrial levels. The new buzzword is right in the title: overshoot, as in “yes, we’re going to go beyond 1.5 degrees, but then we’re going to dip back down below that target.” Eventually. Somehow.

The report itself doesn’t hedge on this anymore:

“1.5°C will be exceeded across all central estimates. Under current policies, median projected global warming reaches around 2.6°C by 2100 (range 1.9–3.6°C). Even under one of UNEP’s most optimistic scenarios, which assumes implementation of all NDCs and long-term net zero targets, central global warming estimates peak at around 1.8°C (range 1.7–2.2°C). In all cases, central estimates indicate that global warming will cross 1.5°C within the next decade.”

Acknowledging the reality of overshoot is both courageous and the very least UNEP could have done. Courageous because so many world leaders had gone so far in promising that we must never, ever go beyond 1.5 that simply deciding to stop pretending and bow to reality took courage. But it was also the very least they could have done because, for some time, every researcher who tracks average global temperature has known that we would miss the Paris target. Continuing to pretend would’ve meant abdicating from the reality-based community altogether.

Of course, having acknowledged that we’re going to overshoot, the question becomes “OK, so how are you going to come back down to 1.5 degrees eventually?” And to this question, only two answers are possible: some flavor of solar radiation management, or some form of gigaton scale carbon dioxide removal.

Obviously, a UN report like this is never going to come out in favor of solar radiation management. They can’t, too many member states would balk, too many participating scientists would walk out. The report includes the word “geoengineering” only twice, both times to say why it’s a bad idea.

That leaves gigaton scale carbon dioxide removal (CDR), which you sense the drafters embraced by process of elimination. To be sure, they embrace only the lamest forms of CDR: either tree-planting or techniques like growing a forest, burning it, capturing the carbon dioxide in the flue gas, and burying it (known as BECCS), or those absurdly impractical Direct Air Capture (DAC) machines. And even by UNEP’s own account, these approaches barely exist yet:

“Current novel CDR provides little carbon storage. Although these approaches generally provide more durable carbon storage than terrestrial biomass, they currently account for less than 0.1 per cent of conventional CDR.”

Here, UNEP falls into a trap it has set for itself.

On the one hand, they do accept the necessity of gigaton-scale CDR. On the other hand, they’re only willing to consider techniques perceived as very low risk. But those techniques are crazy expensive: with costs per ton of carbon dioxide removed in the hundreds of dollars. Worse, they have relatively high price floors: they are so land-hungry or so energy-intensive that they will probably never get much cheaper than they are now. And UNEP admits as much:

“DACCS has fewer land constraints but is highly energy-intensive and remains costly (approximately USD100–600 tCO₂⁻¹), limiting deployment where low-carbon energy is constrained…More recent studies that explicitly incorporate sustainability constraints generally limit deployment especially of BECCS to one million hectares or less and up to a few gigatons per year.”

But then, in hard-to-abate sectors like cement, steel and aviation, traditional mitigation also costs hundreds of dollars per ton of CO₂ avoided! And we’ve already seen that the world is unwilling or unable to devote resources on that scale to abate emissions when it’s that expensive to do so: that’s why we’re having this overshoot conversation in the first place.

There is simply no reason to believe that a world that has failed to mobilize $800 to prevent the emission of one ton of CO₂ from a steel mill will somehow, magically, find the will to mobilize $800 to capture a ton of CO₂ via DAC. (If anything, the opposite is true: the steel-maker has a good reason to make steel —so they can sell steel!— the DAC plant has costs but no revenues.)

What a report like Limiting Overshoot was never going to do is acknowledge something that, to me at least, is fully obvious: that there’s no point in CDR if it’s going to cost as much, ton per ton, as mitigation. That gigaton-scale CDR is only ever going to be a thing if it’s an order of magnitude cheaper than the alternative.

Look, a blue ribbon report like this can’t, obviously, say anything interesting: co-authored by dozens of researchers in universities and government labs spread over six continents, the UN machine will make sure to squeeze out any sign of originality long before a beast like this comes anywhere near publication. Many very respectable people at eminent institutions have their fingerprints on this product, none of them want to face the blowback that would come from upsetting orthodoxies. Reports like this one exist not to be provocative, but to capture the consensus position in the UN Climate Blob. By design, Limiting Overshoot was never going to be anything other than a compendium of things very cautious people can all agree on.

The takeaway is that the UN Climate Blob is moving the only way the UN knows how to move —glacially— towards some sort of accommodation with reality. UNEP has gone from pretending that mitigation alone can do the trick to pretending that CDR that’s just as expensive as mitigation can do the trick.

Is this ideal?

No!

But it is progress!

One cheer for that!