East Asian air cleanup likely contributed to acceleration in global warming

Summary

RAMIP simulations and recent emissions changes in East AsiaWe first document the emissions perturbation applied in the RAMIP baseline and East Asia simulations21 (see “Methods”), and compare them to the actual emissions reductions from the same region since around 2010. Briefly, RAMIP isolates the climate effects of aerosol emissions in one region by comparing two sets of transient emission simulations; one following a global, high emissions pathway (SSP3-7.0, which assumes weak air quality policies), and one where aerosol emissions in one region (East Asia, consisting mainly of mainland China emissions) have been replaced by those from a strong air quality policy trajectory (SSP1-2.6). See “Methods,” or21, for a full description. In the present analysis, we use simulations from 8 global models, each with 10 ensemble members, for a total of 80 ensemble members. This simulation set effectively samples both model uncertainty and internal climate variability.Figure 1a shows changes in aerosol optical depth (AOD) retrieved by MODIS Terra and Aqua between the two previous decades. Consistent with previous literature, we find a dipole pattern consisting of an increase over India and a strong decrease over China following their air quality improvement initiatives. For comparison, Fig. 1b shows the pattern of AOD change between the RAMIP East Asia and baseline simulations, for the simulated period 2035–2049. Here, and elsewhere, this time period is chosen for RAMIP to sample the climate response after most of the emission reductions have occurred, while maximizing the number of years (RAMIP simulations end in 2050, so 2049 is the last year we can use while also letting the last DJF season run into the following year.) Fig. 1c, d shows the corresponding SO2 emissions and AOD change, for observations and simulations, within the box labeled East Asia in Fig. 1a (a geographical box that covers the main emission regions).Fig. 1: Observed changes in aerosol optical depth since 20...