Start with the removal number, because it is what carries the scenario. Around 10 gigatonnes of CO2 taken out of the air each year is a quarter of the roughly 40 gigatonnes the world emits now [9][1], and it is required after residual emissions from sources like farming have already been balanced out, not instead of that [9]. Reto Knutti at ETH Zurich, who did not work on the report, puts the necessary scale-up at a factor of about 100,000 [10]. Run the factor backwards and the implied base is roughly 100,000 tonnes a year: ten gigatonnes is ten billion tonnes, divided by 100,000 [2]. What the factor does not tell you is which methods sit in that denominator, which matters because Knutti's point is that the cheap method and the scalable method are not the same one. Planting forests is cheap and runs out of land; direct air capture can scale and is, in his words, really expensive [11].
The emissions half of the pathway is no gentler. Halving global emissions by 2035 [7] means going from about 40 gigatonnes to about 20, which leaves the 10 gigatonnes of net-negative removal at half the size of what would still be there to cancel [3]. New Scientist points out that emissions are still rising worldwide [8].
What makes the return conditional rather than merely expensive is the peak. Debra Roberts of the University of KwaZulu-Natal, one of the report's authors, says a return depends on warming staying under 1.8C, because past that the irreversible changes begin to cascade and the job gets much harder [6]. Nature reads that same 1.8C differently, as the best this century can deliver on the current trajectory [2]. Konstantin Weber at ETH Zurich notes that the current-policies scenario in the latest modelling set points to about 3C by 2100 [16].
The honest description, then, is that overshoot-and-return is arithmetically available and, in Knutti's assessment, technically and financially possible but not politically plausible; his advice to policymakers is to plan adaptation for at least 2C, if not more [12][13]. James Dyke at the University of Exeter goes further, saying there are still no credible plans for large-scale carbon dioxide removal and that societies should prepare for a warmer, more dangerous world that could persist for centuries [14]. Oliver Geden, who has watched the concept move from thought experiment toward official plan B, says it may amount to a way of masking ongoing failure [17].
Inger Andersen of UNEP describes the new posture as approaching 1.5C differently, from above [3]. That framing does real work: it keeps a number nearly 200 Paris signatories adopted in 2015 [21] and that the IPCC ties to tipping-point risk growing with every tenth of a degree [22]. It also has a limit Roberts states plainly, which is that arriving back at 1.5C is not arriving back at the world that existed at 1.5C, because some of the losses in between do not reverse [20].
