Past exam of the short options paper shared by Parts A and B of the physics course of the University of Oxford / 2019 / S25 / 3 / a

The transient climate response to cumulative carbon emissions is $\boxed{\kappa \simeq \Delta T/\mathcal{E}}$, where $\mathcal{E}$ is cumulative emitted carbon dioxide in a stated mass unit. Here the unit is tonnes of $\mathrm{C}{\mathrm{O}}_2$, rather than tonnes of carbon. Increasing concentration weakens the incremental radiative forcing per added atmospheric tonne because forcing is approximately logarithmic. Over a substantial range this is partly offset by a weakening of carbon sinks and by the changing ratio between transient warming and forcing as ocean heat uptake evolves. These approximate compensations make warming nearly proportional to cumulative emissions; no exact cancellation follows from the logarithmic forcing law alone. The approximation is range- and pathway-dependent, and is not a theorem valid for arbitrary emissions histories.