Thinking about climate crisis mitigation: time limits vs. carbon budgets

The slogan “we have 12 years to prevent a climate catastrophe” has been thrown around a lot since the IPCC released a special report in 2018, which focused on the importance of limiting global warming to 1.5 degrees Celsius. I don’t particularly like the slogan, for two reasons: one, because it implies a kind of hard, unchanging timeline, and two, because it creates the image of an apocalyptic brink, past which there is no return. Neither of these are a good way to think about the climate crisis.

Mitigating the crisis is not about how much time we have left, but how much carbon we have left. The 12-year time limit is derived from how much carbon we can put into the atmosphere before we cross 1.5 C of warming. Chapter 2, Section 2, Part 2 of the IPCC Special Report analyzes this budget, and finds that the general range of how much carbon dioxide we can emit is likely somewhere between 420 and 840 gigatonnes; the 50% confidence value of the limit is 560 GtCO2. For reference, the total amount of carbon dioxide humans have emitted so far is roughly 2200 GtCO2. Comparing this budget with the rate at which we’re emitting carbon dioxide tells us how long we have until we push warming past the 1.5 C — assuming, of course, that this rate stays the same. If fossil fuel use peaked and stabilized in 2017, at a rate of 42 GtCO2/year, then our carbon budget would be exhausted after 13-14 years, around 2030. 12 years after the report’s release in 2018 — thus, the time limit.

But since this depends on the rate of carbon emissions, the time limit will change as the emission rates change. Global fossil fuel use today is in fact growing, at a rate of 2-3%/year, which knocks a couple years off of how fast we’ll exhaust the carbon budget. One the other hand, if somehow global carbon emissions peaked this year and went on a 2% yearly decline, we would instead add two years to the time limit; a 5% yearly decline would add around nine years. Thus, there is reason to be both optimistic and pessimistic: if emissions keep rising, that 12-year time limit will shrink, but if we can peak emissions and start decreasing it, the timeline will start to slowly stretch out, giving time for even more emissions reductions.

Graph of what year the carbon budget for 1.5 C of warming gets exhausted, for different rates of carbon emission increases/decreases. If carbon emissions keep increasing by 2%/year, the budget will be exhausted in 2029; if carbon emissions are decreased by 5%/year, the budget will be exhausted in 2041.

And if we don’t meet this moving deadline…well, things will get bad. But we should be clear about when and how things will get bad; its not like we’ll suddenly be engulfed with hurricanes and mega-droughts destroying our cities and displacing billions the second the last molecule of the carbon budget is used up, whether that is in 2030 or 2040 or whenever. And its not like things will be fine and dandy until that moment, either; this much should be clear from the widespread negative impacts that 1 C of warming is already having on the world. Peaking at 1.5 C will prevent much catastrophe, but it would have been much better if we had peaked at 1 C. Likewise, peaking at 2 C will bring much worse impacts than peaking at 1.5 C (such as the destruction of the coral reefs), but will nonetheless be much better than peaking at 3 C, which in turn will be much better than peaking at 4 C, and so on. This logic holds even when accounting for feedback loops and tipping points that could push climate change beyond the scope of human mitigation, since peaking human carbon emissions earlier rather than later will (presumably) prevent more dire tipping points from being reached, and make it more likely that we can figure out some way to stabilize the climate before any extinction scenarios play out.

So people shouldn’t despair when looking at the more sensationalist headlines about “12 years to save the world”. Its not a make-or-break deadline even though the stakes are massive and planetary in scope. Whatever we can do in the crucial decade of the 2020s will make the future that much marginally better, and we shouldn’t let pessimism about whether we can have a total victory stop us from fighting like hell.

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