Two Closer Looks At Geoengineering
by Bill Chameides | February 2nd, 2012
posted by Erica Rowell (Editor)
Strong volcanic eruptions inject cooling sulfur particles into the atmosphere. Can humans use a similar approach to geoengineer the climate to offset global warming? (NASA)
Two new papers on geoengineering — one gives tinkering with our Earth system a thumbs up, the other a thumbs down.
With the prospects of a global accord on reining in greenhouse gas emissions seeming ever more remote, the idea of geoengineering ourselves out of our climate-change predicament seems to be gaining more traction. (See here, here and here.)
Of the various schemes proposed so far, the one that’s probably received the most attention has been the so-called solar radiation management or SRM approach in which solar radiation impinging on the Earth is reflected back to space in order to cancel out the warming caused by greenhouse gases. In principle one can accomplish the reflection in a variety of ways — some quite futuristic (e.g., placing huge mirrors in orbit above the Earth) and some more down to earth (placing floating reflectors on the ocean). The method of choice for most SRMers has been injecting reflective sulfate particles (or sulfur containing gases that convert to particles) into the stratosphere. (See my previous post on this method.)
SRM like other geoengineering has its upsides — it can be used in an emergency situation to counteract dangerous warming and it does not require us to kick the fossil fuel habit — good news if you’re an oil glutton or an oil baron. But it raises valid and troubling questions, such as, How practical would it be to try to maintain a constant, finely-tuned burden of sulfate particles in the stratosphere, and, What about the chances of unintended consequences?
And so the scientific community has begun to carefully vet SRM as a viable option. Two recent studies provide differing perspectives.
Good News for Crops?
One of the concerns about global warming is the possibility that a changing climate may hurt worldwide food production. In a paper published last week in the journal Nature Climate Change, Julia Pongratz of the Carnegie Institution and colleagues reported on the results of climate-model simulations comparing current crop yields and production rates with future potential yields and production rates under two different scenarios. One scenario was a world with double today’s carbon dioxide (CO2) concentrations (2xCO2); the other was a world with double the current CO2 concentrations but that also had a uniform dispersal of stratospheric aerosols capable of cancelling the warming from the CO2 (2xCO2 with SRM).
If you’re concerned about food production, the results, while acknowledged as a first-order estimate, are encouraging. First of all, crops didn’t do all that bad in the authors’ 2xCO2 simulated world. While the warmer temperatures tend to suppress production, increases in the fertilizing effect of CO2 enhance production. The 2xCO2 world has a small negative impact on corn yields (roughly 3 percent) but a small positive impact on both rice (19 percent) and wheat (6 percent). But this is averaged over latitudinal bands. At the regional scale, changes in temperature and precipitation would introduce more variability such that one country’s crop may fail, meaning less food stability even though world production doesn’t change much.
In a 2XCO2-with-SRM world, crops do better. Under these conditions, where crops benefit from the CO2 fertilization effect without the punishing effects of higher temps, average yields for the three crops increase by 8 to 21 percent. Ready to celebrate and sing the praises of SRM? Hold on. There’s a wet blanket here that Pongratz and co hint at when they note that the benefits of SRM would not be felt “uniformly” and would “probably alter market shares and the ranking of top producers.” But the problem is even bigger than just the regional shifting of crop yields as the next paper highlights.
The Arctic Blues
One of the problems with SRM is that while greenhouse gases warm by changing the flow of infrared radiation through the atmosphere, the stratospheric particles cool by changing the flow of solar radiation. And these two types of radiation have very different spatial and temporal variations. Greenhouse gases warm the atmosphere in a basically continuous and spatially uniform manner. SRM only provides cooling when the sun is shining. This difference can be especially acute in the polar regions during winter — without any sunshine, SRM has no cooling effect in the polar winter.
In a paper in press at the Journal of Climate, Kelly McCusker of the University of Washington and colleagues report on a series of climate model simulations designed to elucidate how regional climate would be impacted in a world where CO2 is allowed to increase, while at the same time stratospheric aerosols were continuously injected into the stratosphere so that globally averaged temperatures did not change. The simulations are a step forward from previous work, both in the increased complexity of the model used and how the SRM is realized in the simulations.
Overall, the authors find that while SRM does reduce the impact of climate change, the climate would continue to change from greenhouse warming. In their words: “Maintaining the modern climate is not possible.” In their simulations, SRM is most effective in the tropics but least effective in the polar regions which still experience 20-50 percent of the expected temperature increase that would have occurred in a 2xCO2 world without geoengineering. The fact that the Arctic would continue to warm the most, albeit more slowly, is an important point, as it’s the very desire to stop Arctic melting that would conceivably drive us, at least in part, to consider SRM in the first place.
Another point McCusker et al make is that while using SRM to limit incoming shortwave radiation reduces temperature, it doesn’t address other impacts from a doubling of CO2, such as its impact on atmospheric and ocean circulation. These limitations mean that weather patterns (temp and precipitation) would continue to change. One further point is that SRM appears to lose its effectiveness as CO2 continues to rise. When McCusker et al ran their simulations out to a tripling and quadrupling of CO2, SRM became less effective at reducing temperature.
Two views of what a geoengineered climate might look like. Interesting? No question. Accurate? Probably not. When it comes to understanding how our tinkering with the climate would play out, I suspect that these model simulations have barely scratched the surface of the top of this iceberg.filed under: agriculture, Arctic, carbon dioxide emissions, climate change, faculty, food, global warming, science
and: climate, food security, geoengineering, greenhouse gas emissions, greenhouse gases, infrared radiation, solar radiation, solar radiation management, SRM, sulfate particles