Do the finer details of climate science matter any more?

RealClimate is a long-running blog publishing, as its tagline says, “Climate science from climate scientists.” Its regular contributors are academics at the top of the field, working for NASA and the IPCC, etc, and many of their peers join the online discussion.

A recent post there by Stefan Rahmstorf, Is there really still a chance for staying below 1.5 °C global warming?,  is so relevant to our own local efforts to avert the impending climate melt-down that I wanted to share it here.

What follows is a series of excerpts from the article and comments on it, all in italics; comments are attributed to the usernames (some are real names, some not) under which they appeared on the blog. Readers can, of course, go straight to RealClimate via the link above if they want the whole story, and can visit Cicero for more about the carbon budget, too; but here’s the condensed version.

There has been a bit of excitement and confusion this week about a new paper in Nature Geoscience, claiming that we can still limit global warming to below 1.5 °C above preindustrial temperatures, whilst emitting another ~800 Gigatons of carbon dioxide. That’s much more than previously thought, so how come? And while that sounds like very welcome good news, is it true? …

Emissions budgets – a very useful concept

First of all – what the heck is an “emissions budget” for CO2? Behind this concept is the fact that the amount of global warming that is reached before temperatures stabilise depends (to good approximation) on the cumulative emissions of CO2, i.e. the grand total that humanity has emitted. That is because any additional amount of CO2 in the atmosphere will remain there for a very long time … It’s like having a limited amount of cake. If we eat it all in the morning, we won’t have any left in the afternoon. The debate about the size of the emissions budget is like a debate about how much cake we have left, and how long we can keep eating cake before it’s gone. Thus, the concept of an emissions budget is very useful to get the message across that the amount of CO2 that we can still emit in total (not per year) is limited if we want to stabilise global temperature at a given level, so any delay in reducing emissions can be detrimental – especially if we cross tipping points in the climate system, e.g trigger the complete loss of the Greenland Ice Sheet. Understanding this fact is critical, even if the exact size of the budget is not known.

But of course the question arises: how large is this budget? There is not one simple answer to this, because it depends on the choice of warming limit, on what happens with climate drivers other than CO2 (other greenhouse gases, aerosols), and (given there’s uncertainties) on the probability with which you want to stay below the chosen warming limit.

I will skip the technical detail which comprises the scientific argument here because, as we will see, it need not concern us anyway. Here’s Rahmstorf’s conclusion about the science:

In summary, both approaches used by Millar compute budgets that do not actually keep global warming to 1.5 °C.

That is, Millar was wrong and we can’t relax and keep on burning fossil fuels as we might have hoped. Now comes the key point for those of us who must argue in our own communities for action to limit climate change:

Does it all matter?

We still live in a world on a path to 3 or 4 °C global warming, waiting to finally turn the tide of rising emissions. At this point, debating whether we have 0.2 °C more or less to go until we reach 1.5 °C is an academic discussion at best, a distraction at worst. The big issue is that we need to see falling emissions globally very very soon if we even want to stay well below 2 °C. That was agreed as the weaker goal in Paris in a consensus by 195 nations. It is high time that everyone backs this up with actions, not just words.

That was the end of the blog post but many of the comments on it echo Rahmstorf’s last paragraph. Here are a few of them:

Jai Mitchell: … In the history of human existence, we do not give up when faced with a potential threat that we understand to be present. We must be made aware of our current hazard so that we can collectively press our elected leaders for a world war II scale total societal mobilization to reduce all carbon emissions …

Larry Edwards: It is worth considering that at this point our salvation lies in putting pressure on the politics rather than in increasingly nuanced climate science discussions. As poignantly noted in RealClimate in 2009 ( “Hit the Brakes Hard”, the time is late, and even more so now. With “1-in-150” of humanity affected by flooding in just one recent month (and not counting the later impact of Hurricane Maria), the matter before us really is how hard can we hit the brakes and how fast politically can we hit them. …

Mike: … I appreciate the comments above about the various tipping points that are already getting pushed. Albedo, clathrates, CO2 emissions from warmed permafrost, etc. Stefan hits the nail on the head with “does it matter?” It does not. We have a cliff ahead, we need to hit the brakes hard. The growthers all seem intent on making sure we use only enough brakes to stop with our toes over the precipice. This seems prudent to growthers. I wonder how many growthers have actually walked up to a precipice and stopped with their toes over the edge?

Jonathan Richards: … We’re in for the fight of our lives (really) and that is the message and urgency that needs to be conveyed. All this useless discussion about “budget” simply enables more of the very same destructive practices (burning carbon) as before. It also entirely misses the chaos and suffering and destruction already caused by climate change – with more to come. … We’ve already blown past the “no effect on the planetary environment or climate” as the evidence also shows. …

Polly: At this point, I think it’s more constructive to simply … work as hard as possible to get carbon emissions down. Throwing our hands up and saying it can’t be done is incredibly dangerous …

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