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Science on the Risks of Climate Engineering: Optimism About a Geoengineered Easy Way Out Should Be Tempered by Examination of Currently Observed Climate Changes
Joe Romm, 31 Aug 09
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As the risks of climate change and the difficulty of effectively reducing greenhouse gas emissions become increasingly obvious, potential geoengineering solutions are widely discussed. For example, in a recent report, Blackstock et al. explore the feasibility, potential impact, and dangers of shortwave climate engineering, which aims to reduce the incoming solar radiation and thereby reduce climate warming. Proposed geoengineering solutions tend to be controversial among climate scientists and attract considerable media attention. However, by focusing on limiting warming, the debate creates a false sense of certainty and downplays the impacts of geoengineering solutions.

So begins, “Risks of Climate Engineering” (subs. req’d), an important piece in Science this month by Gabriele Hegerl and Susan Solomon. Hegerl was a coordinating lead author for the Fourth Assessment Report. Solomon is an atmospheric chemist working for NOAA and “one of the first to propose CFCs as the cause of the Antarctic ozone hole.”

Solomon was lead author of the even more important February PNAS paper, “Irreversible climate change due to carbon dioxide emissions,” which, as I noted at the time, gives the lie to the notion that it is a moral choice not to do everything humanly possible to prevent this tragedy, a lie to the notion that we can “adapt” to climate change, unless by “adapt” you mean “force the next 50 generations to endure endless misery because we were too damn greedy to give up 0.1% of our GDP each year” (see NOAA stunner: Climate change “largely irreversible for 1000 years,” with permanent Dust Bowls in Southwest and around the globe). No surprise, then, that she co-authored a paper skeptical of geoengineering.

I remain dubious of geo-engineering (see Geo-engineering remains a bad idea” and “Geo-Engineering is NOT the Answer“ and British coal industry flack pushes geo-engineering “ploy” to give politicians “viable reason to do nothing” about global warming, which includes an excellent analysis by Prof. Alan Robock). Science advisor John Holdren told me in April that he stands by his critique:

“The ‘geo-engineering’ approaches considered so far appear to be afflicted with some combination of high costs, low leverage, and a high likelihood of serious side effects.“

The new analysis by Hegerl and Solomon is sufficiently significant — Science itself featured it early in Science Express — that I’ll excerpt it below:

Discussions of “dangerous” levels of interference with the climate system often use warming as a proxy for the seriousness of greenhouse gas–induced climate change. However, climate change impacts are driven not only by temperature changes, but also by change in other aspects of the climate system, such as precipitation and climate extremes. If geoengineering studies focus too heavily on warming, critical risks associated with such possible “cures” will not be evaluated appropriately. Here, we present an example illustrative of the need for greater emphasis not only on possible benefits but also on the risks of geoengineering—in particular, the risks already suggested by observations of climate system change.

Carbon dioxide increases cause a reduction in outgoing longwave radiation, thus changing the heat balance of the planet. Several proposed geoengineering solutions aim to avoid the resulting energy imbalance that will lead to warming by reducing incoming solar radiation. This may be achieved by, for example, increasing the number of atmospheric reflecting particles in the stratosphere or by placing reflecting “mirrors” outside the atmosphere.

Kind of ironic that the two most widely discussed geo-engineering strategies are, literally, smoke and mirrors.

These measures are indeed expected to reduce the projected warming (1, 2). Blackstock et al. focus on this particular example of geoengineering, with the rationale that it may allow rapid action to be taken if a threat of catastrophic climate change emerges. Such emerging threats could, for example, be rapidly disintegrating ice sheets, or warming that is more rapid than expected (4). One of the attractions of shortwave climate engineering is the effectiveness and rapidity with which it could reduce warming, but it is also connected with considerable risks.

It is clear that reducing incoming shortwave radiation would lead to decreases in temperature. Volcanic eruptions in the 20th century led to substantial coolings that occurred within months after the eruption and lasted several years (5, 6). Strong volcanic eruptions have in the past led to anomalously cold conditions: The year without a summer (1816) noted in North America and Europe followed the eruption of Tambora in Indonesia the year before, which was the largest volcanic event observed in recent centuries (5). However, volcanic eruptions also affect precipitation (7). The 1991 eruption of Mount Pinatubo led to substantial decreases in global stream flow and to increases in the incidence of drought (see the figure) (8). An analysis of 20th-century observations indicates that volcanic eruptions caused detectable decreases in global land precipitation (9, 10). The reason is that with reduced incoming shortwave radiation and surface cooling, less energy is available for evaporation.

Greenhouse gas increases also influence precipitation, through two mechanisms: directly through reducing outgoing longwave radiation, and indirectly through warming (1113). Warming increases evaporation, thus making more water available globally for precipitation. However, because greenhouse gases reduce outgoing longwave radiation, they also reduce the effectiveness with which the atmosphere radiates out latent heat of condensation. This reduces precipitation. The net result of the two mechanisms is a relatively small increase in global precipitation in the early stages of greenhouse warming simulations (12).

That’s a nice, simple explanation of something I hadn’t seen elsewhere.

The 20th-century climate record shows the different effects of shortwave and longwave forcing on temperature and on precipitation. Global surface temperature responds in a quite straightforward way to changes in the energy budget, irrespective of whether shortwave or longwave radiation changes are involved. Thus, temperatures in the latter part of the 20th century were dominated by anthropogenic warming (interspersed with short-term cooling after volcanic eruptions) (14). In contrast, precipitation reacts more strongly to reductions in incoming shortwave radiation, such as volcanic eruptions or shortwave climate engineering, than to reductions in outgoing longwave radiation associated with greenhouse gas forcing…. Models have been able to capture the patterns of precipitation changes with greenhouse warming (14, 15) but appear to underestimate the magnitude of precipitation changes over the 20th century in response to both shortwave and longwave forcing…. Similarly, the observed global land precipitation response to volcanic forcing over the 20th century was much stronger than that simulated by present climate models (9, 10).

Satellite data also suggest that climate models underestimate the magnitude of forced changes and of variations in precipitation extremes (16, 17). Although these data are limited (13), they all suggest that precipitation changes are being underestimated. Missing external forcings (such as by absorbing aerosols) or errors in observations could contribute to the discrepancy between observations and model simulations. However, until these discrepancies are fully resolved, models cannot reliably predict how shortwave engineering can target precipitation and temperature simultaneously (18), implying that very large risks are associated with any such geoengineering scheme.

Some models suggest a large degree of cancellation between changes in warming and in precipitation in a shortwave climate-engineered world (18). However, models have been shown to have problems simulating past precipitation variability as well as trends. Furthermore, the combination of a strong greenhouse effect with a reduction of incoming radiation could have substantial effects on regional precipitation (19), including reductions that would rival those of past major droughts (20). Geoengineered changes in the environment could thus lead not only to “winners and losers” but even to conflicts over water resources (19) and the potential for migration and instability, making shortwave climate engineering internationally very controversial.

I see the liability issue as enormous. Right now, we’re all liable for climate change, though obviously the rich countries are far more to blame. But the bulk of the liability extends back many decades and involves many hundreds of millions of people and thousands of industries. A major geo-engineering effort, however, would put all of the liability for any adverse impacts on those who undertake it. The liability could be huge but the number of parties involved might be small if it proves impossible to get a global agreement to adopt that strategy. Yet if we fail to aggressively pursue mitigation, then the very countries who would likely be the leaders on geo-engineering are going to be global pariahs for having greedily refused to spend a modest amount of money needed to reduce emissions in the first place. So if there were, say, a massive drought in India and Bangladesh in the few years after a geo-engineering effort a particulate-based effort began (the mirrors strategy seems likely to be too expensive and impractical), those behind the effort would bear tremendous responsibility.

Thus, from a purely practical perspective, a true geo-engineering strategy is going to be much tougher to pursue than is widely realized. That goes double if we don’t keep emissions near what is required to stabilize below 450 ppm.

Blackstock et al. call for a study phase, during which the possible impacts of geoengineering options could be investigated. This is clearly necessary, and optimism about a geoengineered “easy way out” should be tempered by examination of currently observed climate changes. Climate change is about much more than temperature change, and using temperature alone as a proxy for its effects represents an inappropriate risk to the health of our society and to the planet.

Hear! Hear!
  • 1. J. J. Blackstock et al., Climate Engineering Responses to Climate Emergencies (Novim, 2009), available at
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  • 4. G. A. Meehl et al., in Climate Change 2007: The Fourth Scientific Assessment, S. Solomon , Eds. (Cambridge Univ. Press, Cambridge, 2007), pp. 747–845.

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  • 10. F. H. Lambert, N. P. Gillett, D. A. Stone, C. Huntingford, Geophys. Res. Lett. 32, L18704 (2005). [CrossRef]
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  • 12. M. R. Allen, W. J. Ingram, Nature 419, 223 (2002).

  • 13. F. H. Lambert, A. R. Stine, N. Y. Krakauer, J. C. H. Chang, Eos 89, 193 (2008).
  • 14. G. C. Hegerl et al., in Climate Change 2007: The Fourth Scientific Assessment, S. Solomon , Eds. (Cambridge Univ. Press, Cambridge, 2007), pp. 663–745.

  • 15. X. Zhang et al., Nature 448, 461 (2007). [CrossRef] [Medline]
  • 16. F. J. Wentz, L. Ricciardulli, K. Hilburn, C. Mears, Science 317, 233; published online 30 May 2007.[Abstract/Free Full Text]

  • 17. R. P. Allan, B. J. Soden, Science 321, 1481; published online 7 August 2008.[Abstract/Free Full Text]
  • 18. K. Caldeira, L. Wood, Philos. Trans. R. Soc. London Ser. A 366, 4039 (2008).

  • 19. A. Robock, L. Oman, G. L. Stenchikov, J. Geophys. Res. 113, D16101 (2008). [CrossRef]

  • 20. G. T. Narisma, J. A. Foley, R. Licker, N. Ramankutty, Geophys. Res. Lett. 34, L06710 (2007). [CrossRef]

This piece originally appeared on Climate Progress

Learn more about geo-engineering possibilities and pitfalls in the WorldChanging archives:
Geoengineering and the New Climate Denialism
Geoengineering: A Worldchanging Retrospective
Why Geo-Engineering is a Debate Whose Time Has Gone
Planktos, Geo-Engineering and Politics
Directed Evolution, Natural Sequestration and Terraforming the Earth
Geoengineering Megaprojects are Bad Planetary Management

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