It is time to stop quivering in our boots in pointless fear of the future and just roll up our sleeves and build it.
- Ray Pierrehumbert

Friday, August 17, 2007

Cure Worse Than Disease?

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

[1] 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?

5 comments:

Heiko said...

Following the link I get:

"If you are not a subscriber, you may purchase a single copy of this article (PDF & HTML included) for $9.00."

I think the effect of present day aerosol emissions, and what would happen, if they were reduced rapidly, needs a lot more attention.

Isn't this geoengineering already and we might be doing exactly what's considered dangerous in these "what would happen if geoengineering failed" papers, ie reduce them very quickly?

Michael Tobis said...

Thanks, Heiko. I would have thought the abstract would be available. I'll post the abstract on Monday next time I'm on campus.

Your point is well taken. I agree that in a very real sense we are geoengineering already. I just doubt that we are going to get very good at it in the foreseeable future.

Michael Tobis said...

Done Sunday; better early than never...

EliRabett said...

When I see a geoengineering proposal which has up front investment and small continuing costs, I'll bite. The ones that I have seen to date have ongoing and increasing capital costs as the level of adaptation needed increases. That is a losing proposition

Heiko said...

There's a related post on Eli's blog:

http://rabett.blogspot.com/2007/08/you-say-tomato-i-say-tomahto-brian.html#links

Maybe Eli should consider aerosol management one of several wedges, rather than an all or nothing approach, which necessarily must go hand in hand with ever rising CO2 levels. The latter is obviously not going to cut it indefinitely, for one because eventually CO2 concentrations would become actually poisonous, and secondly, because at 40,000 ppm a sudden loss of aerosol cooling would really bake the planet.

I think it's clearly much more attractive as a temporary strategy to provide time for other approaches, and in modest amounts no higher than current troposhperic aerosols, unless there's a real emergency (as in Greenland will melt in 40 years and won't grow back even with CO2 back at pre-industrial levels).

I am not sure why Eli thinks that CO2 capture from the air is speculative. Technically there's nothing speculative about it, we produce cement at very large scale, and that's basically the same thing (take CaCO3, add heat, get concentrated CO2 and CaO, dissolve the CaO in water and wait until it's reacted back to CaCO3). It's clearly an energy intensive way to get CO2 streams ready for sequestration, and today there are easier sources (eg CO2 is produced as a near pure sidestream in ammonia manufacture).

But given sufficient zero carbon energy, be it solar thermal or nuclear energy, there are no technical hurdles that need to be overcome. This is mature technology.

The issue is how to get from today's energy consumption to 3 times today's energy consumption by 2100 to meet the energy needs of future generations, and how to then add another load of carbon free energy production equivalent to today's consumption, purely to scrub CO2 out of the air. If we do that, it's not that hard to go back to pre-industrial CO2 levels (if we so desire) by 2100-2150.