Human Activity Has Likely Caused Virtually All the Warming since the Late 19th Century

The IPCC AR5 estimates that human activity is responsible for ~110% of the warming since 1951. This statement may seem far fetched to some, but a new paper from Gillett suggests that this actually may be a bit conservative. Gillett's work extends that analysis back to the latter half of the 19th century. The study concluded that "anthropogenic forcings caused 0.9 to 1.3 °C of warming in global mean near-surface air temperature in 2010–2019 relative to 1850–1900, compared with an observed warming of 1.1 °C. Greenhouse gases and aerosols contributed changes of 1.2 to 1.9 °C and −0.7 to −0.1 °C, respectively, and natural forcings contributed negligibly." In the above graph, you can see the that the impact of GHG and aerosol forcings dwarf the impact of other natural forcings. '

Observant readers may note that there are two relatively short-term temperature spikes in the instrumental record, one during the 1860s and another during the 1940s. But these warm years cooled again. So the best estimate we have now is that human activity is responsible for roughly all the warming since the late 19th century, given that the last decade is 1.1 C warmer than the 1850-1900 mean.

The paper makes good use of many of the early CMIP6 models including some of the "high sensitivity" models that have garnered so much attention in recent months. These models contain new attempts to simulate clouds, and  (as best I can tell) incorporated biases that caused these models to calculate sensitivity to be too high. But the mean of all the model runs reproduced the 20th century quite well.

Much attention has recently focused on the high climate sensitivity of some CMIP6 models40, and while we find that some of the models considered here do overestimate the response to greenhouse gases, on average the greenhouse gas response of these models matches the observations closely (the best estimate of the multimodel greenhouse gas regression coefficient in Fig. 2b is close to one).

In the graph below, you can see how closely the Model mean GMST and GSAT match with HadCRUT4.  In the AR6 report, the IPCC limited the impact of these "high sensitivity" models, and consequently the projections from AR6 were not appreciably different from AR5.

Unfortunately, the paper is behind a paywall, but you can read the abstract and see the figures below. On researchgate, you can read the first page.

Gillett, N.P., Kirchmeier-Young, M., Ribes, A. et al. Constraining human contributions to observed warming since the pre-industrial period. Nat. Clim. Chang. 11, 207–212 (2021). https://doi.org/10.1038/s41558-020-00965-9