The temptations of deferred removals

livemint.com:

The final compromise strengthened the main goal to “well below 2°C" and advocated “pursuing efforts" towards 1.5°C. To sweeten things for the 1.5°C brigade, the agreement also asked the Intergovernmental Panel on Climate Change to go and work out how much better the lower target would be. The IPCC reported back three years later: it would be a great deal better—but also much harder.

One set of figures jumped out from its report: the four “pathways" found to offer a good chance of 1.5°C foresaw the removal of between 100bn and 1trn tonnes of carbon dioxide from the atmosphere before 2100. Those numbers saw CDR go “from a completely niche, academic, propeller-headed discussion to central policymaking," says Julio Friedmann of Carbon Direct, a carbon-management company. The trillion-tonne pathway in that 2018 report was particularly arresting.

It was the only one based on the assumption, subsequently proved correct, that carbon-dioxide emissions would not peak before the mid-2020s. According to the models used to produce such pathways, that late a peak meant there was no longer any chance of getting to net zero in time to have a 50:50 chance of staying below 1.5°C. Instead the pathway showed an “overshoot" trajectory in which, after the temperature broke the 1.5°C barrier, it was brought back down by “net-negative" emissions which significantly reduced the amount of carbon dioxide in the atmosphere.

That required heroic levels of CDR: as much as 20bn tonnes a year. The requirements for a 2°C limit are not so insanely demanding. Indeed, at the time of the Paris agreement, it was still just about possible to imagine that it might be achieved with very little CDR, should emissions start dropping immediately.