Stevens (2015, hereinafter S15) used three lines of reasoning to argue that present-day effective aerosol radiative forcing
Motivated by the arguments of S15, Kretzschmar et al. (2017, hereafter K17) analyze simulations from climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5). By regressing global and hemispheric temperature trends against present-day
The idea (arising from K17’s first finding) that greater than observed Northern Hemispheric warming arising with an
S15’s arguments thus really only apply to models with
The disagreement, and this is really the heart of the matter, arises because the CSIRO-Mk3.6.0 model (and to a lesser degree some other models analyzed by K17) predict a convex1 relationship between estimates of forcing and SO2 emissions, whereas the model of S15 follows earlier studies (Boucher and Pham 2002; Carslaw et al. 2013) in assuming that this relationship is concave. If the convex relationship is correct, then this would mean that the scaling of a midcentury
The idea implicit in K17’s interpretation of the model output is that late-century (Asian dominated) emissions project more strongly onto
To test H1 we estimate the anthropogenic aerosol burden at different time periods, following methods developed in S15. That is, we calculate the difference in the reflected clear-sky shortwave irradiance, over the ocean, between a given period and a preindustrial period, here taken to be a period between 1861 and 1869 with relatively little volcanic activity. To minimize possible effects from residual volcanic aerosol, and similar to S15, we next calculate the anomaly in this quantity relative to the lowest vigintile (5%) along a line of absolute latitude. To avoid statistical outliers arising from changes in sea ice we limit our analysis to latitudes equatorward of 50°. The resultant quantity, which we denote by R, provides a simple measure of a model’s anthropogenic aerosol forcing in a given year (denoted by subscript). Indeed, despite good reason to be skeptical of the
Differences between clear-sky reflected shortwave irradiance over the ocean between 55°N and 55°S.

The most striking finding of this analysis is the strong increase in

Patterns of R for three different time periods—from top to bottom,
Citation: Journal of Climate 30, 16; 10.1175/JCLI-D-17-0034.1

Zonal anomaly in reflected clear-sky solar irradiance over the ocean from CERES. The anomaly is computed from the lowest vigintile at the absolute latitude. This quantity differs from R in that it includes the natural aerosol, which at high latitudes over the Pacific and at low latitudes over the Atlantic is dominated by mineral dust. The average of R equatorward of 55° for the globe and the Northern Hemisphere alone is indicated on the upper right of each figure panel, in units of W m−2. Original data were smoothed before contouring with two applications of a nine-point smoothing.
Citation: Journal of Climate 30, 16; 10.1175/JCLI-D-17-0034.1
Even if the CSIRO-Mk3.6.0 model is not fit for the purpose at hand, it need not call into question H2, namely that clouds over the North Pacific are exceptionally susceptible to aerosol perturbations. To test this hypothesis we use the multiple plume representation of aerosol forcing, MACv2-SP, developed by Stevens et al. (2017). This approach relaxes some of the assumptions of S15 but still allows for explicit control over the pattern of aerosol forcing and the relative contributions of

Effective aerosol radiative forcing,
Citation: Journal of Climate 30, 16; 10.1175/JCLI-D-17-0034.1
Notwithstanding the inference that shifting patterns of aerosol loading have a small influence on the net forcing, it could be argued that a more discernible effect of the pattern of emissions could arise if aerosol–cloud interactions were a substantially larger fraction of the total forcing than implied by the Stevens et al. (2017) climatology. To test this idea we artificially enhanced the potency of aerosol–cloud interactions in MACv2-SP. Instead of contributing commensurately with aerosol–radiation interactions to
We are not adverse to the idea that
We thank Stefan Kinne for valuable and critical discussions of the K17 comment and our reply. We also would like to acknowledge the collegiality of the authors of K17, and thank them for taking the time to write their comment; formal scientific exchanges are increasingly rare, a trend which belies their value. The authors acknowledge the generous and unfettered support of the Max Planck Society. Use of the supercomputer facilities at the Deutsches Klimarechenzentrum (DKRZ) is acknowledged as is funding from the FP7 project BACCHUS (No. 603445).
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By convex we mean upward concavity, so that when
K17’s