Trenberth and Dai in GRL argue that injection of aerosol into the upper atmosphere reduces the vigor of the hydrological cycle, and thus is not a good compensation for greenhouse gas forcing. Even the abstract is behind the firewall! (That seems a bit counterproductive on any model of scientific publishing.) Here is the abstract:
Effects of Mount Pinatubo volcanic eruption on the hydrological cycle as an analog of geoengineering
Kevin E. Trenberth and Aiguo Dai
National Center for Atmospheric Research, Boulder, Colorado, USA
 The problem of global warming arises from the buildup of greenhouse gases such as carbon dioxide from burning of fossil fuels and other human activities that change the composition of the atmosphere and alter outgoing longwave radiation (OLR). One geoengineering solution being proposed is to reduce the incoming sunshine by emulating a volcanic eruption. In between the incoming solar radiation and the OLR is the entire weather and climate system and the hydrological cycle. The precipitation and streamflow records from 1950 to 2004 are examined for the effects of volcanic eruptions from El Chichón in March 1982 and Pinatubo in June 1991, taking into account changes from El Niño-Southern Oscillation. Following the eruption of Mount Pinatubo in June 1991 there was a substantial decrease in precipitation over land and a record decrease in runoff and river discharge into the ocean from October 1991–September 1992. The results suggest that major adverse effects, including drought, could arise from geoengineering solutions.
Received 27 April 2007; revised 4 June 2007; accepted 26 June 2007; published 1 August 2007.
Keywords: Pinatubo, hydrological cycle, geoengineering.
Greenhouse forcing enhances the hydrological cycle, but in general not enough to compensate for evaporation, leading to much headscratching among the general public about how increased flooding and increased drought could both be valid predictions.
I think their overall conclusion, that we should not rely on geoengineering to extract us from our predicament, is true enough for another reason I have rarely seen cited; if we don’t have the political structures to limit climate disruptions it is hard to see how the decisions to control any geoengineering effort can be put in place.
This isn’t to say that T & D are wrong, of course.
If they are right it raises some interesting questions, in the context of this summer of astonishing flooding here in Texas and neighboring states, in the UK, in China, in Korea. To what extent do existing anthropogenic aerosol emissions already suppress the otherwise anthropogenically enhanced hydrological cycle?