While reading the <a href="https://dl.dropboxusercontent.com/u/75831381/Stevens%20aerosol.pdf" rel="nofollow">actual Stevens paper</a>, I noticed the following, which stimulated some thoughts (below):<blockquote>To the extent that changing patterns of emissions are important for the global forcing, it would be more appropriate to express <b>F</b>aer as a function of the source strength of the different patterns of emissions, something that comprehensive models are designed to do. Two of the three models (GFDL-AM3 and GISS-E2-R) analyzed by Shindell et al. (2013) for the period between 1980 and the present day indeed show that, starting in the 1990s, a multiple (rather than single) pattern based approach might be necessary to encapsulate the global forcing, as the rise of SO2 emissions in South- and East-Asia give rise to a forcing from aerosol cloud interactions that more than offsets the reduction in forcing caused by declining North American and European emissions. The response of these two models explains the scatter in the comprehensive modelling estimates at high sulfate 159 burdens in Fig. 2 and is the basis of the claim by Shindell et al. (2013) that, despite a reduction in <b>Q</b>a<b>, F</b>aer becomes more negative over the past thirty years. However, the signal underlying this claim is very small compared to the uncertainties in the modelling, and is not robust – an equal number of studies show no change in forcing between 1980 and 2000, e.g., the blue points in Fig. 2, which are taken from Carslaw et al., as well as results from the CSIRO model, which was the third one analyzed by Shindell et al.. A more recent study even shows that there is a strong decrease in the magnitude of <b>F</b>aer over the same period (Kuhn et al. 2014).</blockquote>The reference to shifts in opposing directions of aerosol production in South-East Asia and Europe/North America reminded me of a subject that, IMO, has received far to little attention: the role of the Himalayas/Tibetan Plateau complex in driving the evolution of the climate.
For instance, increasing aerosol pollution appears to have strong interactions with convective activity associated with the summer mid/upper-level anticyclone over this area [Fadnavis <i>et al.</i> (2013)]. This means that a “<i>rise of SO2 </i>[and especially insoluble]<i> emissions in South- and East-Asia</i>” should not be seen as somehow "balancing" “<i>declining North American and European emissions</i>”.
Even the assumption that their direct radiative effects will somehow "balance" is highly questionable, considering the different trajectories and differential access to the stratosphere via the Tropical Tropopause Layer (TTL) and the Tropical Easterly Jet (TEJ) [Fueglistaler <i>et al.</i> (2004)].
Given (or, perhaps, despite) how little is known regarding cloud dynamics at all, much less the effect of aerosols of various types on the dynamics of convective systems, it seems very implausible that simple changes in load would have balancing effects, given the potentially very different convective systems they interact with. From Chen <i>et al.</i> (2012):<blockquote>The results show that (1) the dominant origin of the moisture supplied to the TP [Tibet Plateau] is a narrow tropical–subtropical band in the extended Arabian Sea covering a long distance from the Indian subcontinent to the Southern Hemisphere. [...] (3) In contrast to the moisture origin confined in the low level, the origin and fate of whole column air mass over the TP is largely controlled by a strong high-level Asian anticyclone. The results show that the TP is a crossroad of air mass where air enters mainly from the northwest and northeast and continues in two separate streams: one goes southwestwards over the Indian Ocean and the other southeastwards through western North Pacific.</blockquote>This is important for several reasons: it's quite possible that emissions from India have a different effect from those from China, given their much higher chance of influencing convection in this critical area. (Also, depending on the role of “<i>whole column air mass</i>” in carrying aerosols, there might be substantial differences among the effects of aerosols released in Southern, Northern, and Northwestern China.)
In addition, any aerosols drawn into the convective system have a much greater chance of influencing the transport of water vapor into the Stratosphere [Fueglistaler <i>et al.</i> (2004)]:<blockquote>Our analysis emphasizes the importance of particular pathways for tropical TST, with the western Pacific being the dominant source of stratospheric air in general and being the place, in particular, where ~70% of tropical TST [troposphere-to-stratosphere transport] assumes its final water mixing ratio.</blockquote>Overall, the enormous number of unknowns, both known unknowns and unknown unknowns, involved in the effects of aerosol loads from these sources render the results of <b>any</b> modelling highly questionable.
After all, “<i>Global Average Temperature</i>” is a very uninformative metric, not really related to the actual effects experienced by <b>anybody</b>. There could be any number of different result states, with different impacts on "humanity", all with the same “<i>Global Average Temperature</i>”.
<b>References:</b>
<b>Fueglistaler <i>et al.</i> (2004)</b> <a href="http://onlinelibrary.wiley.com/doi/10.1029/2003JD004069/full" rel="nofollow">Tropical troposphere-to-stratosphere transport inferred from trajectory calculations</a> by S. Fueglistaler, H. Wernli, and T. Peter <i>Journal of Geophysical Research: Atmospheres</i> Volume 109, Issue D3, 16 February 2004
<b>Fadnavis <i>et al.</i> (2013)</b> <a href="http://www.atmos-chem-phys-discuss.net/12/C13590/2013/acpd-12-C13590-2013-supplement.pdf" rel="nofollow">Transport of aerosols into the UTLS and their impact on the Asian monsoon region as seen in a global model simulation</a> by Fadnavis, S.; Semeniuk, K.; Pozzoli, L.; Schultz, M. G.; Ghude, S. D.; Das, S.; Kakatkar, R. <i>Atmospheric Chemistry and Physics,</i> Volume 13, Issue 17, 2013, pp.8771-8786
<b>Chen <i>et al.</i> (2012)</b> <a href="http://www.ualberta.ca/~eec/chen_2012.pdf" rel="nofollow">On the origin and destination of atmospheric moisture and air mass over the Tibetan Plateau</a> by Bin Chen, Xiang-De Xu, Shuai Yang, and Wei Zhang <i>Theoretical and Applied Climatology</i> December 2012, Volume 110, Issue 3, pp 423-435