Even though climate scientists have not been able to identify all of the factors involved in climate regulation, or even develop trustworthy values for the ones they do know about, some eco-activists are proposing that we actively try to alter Earth's climate. Schemes to purposefully alter the environment on a global scale are called geoengineering, and it has been proposed as a way to counter act anthropogenic global warming and its side effects. The two main geoengineering options are limiting incoming solar radiation, or modifying the carbon cycle. Two articles, one in the Proceedings of the National Academy of Science and another in Nature Geoscience, report that controlling climate through geoengineering would be difficult, if not impossible, and may do more harm than good. At a time when we cannot even predict how climate will change on its own, proposals to engineer climate change are best left as thought experiments.

Geoengineering is not a new idea, it has been around in one form or another for half a century. Some want to drop iron, nitrates and phosphorous in the sea to encourage the growth of plankton colonies which can sequester oceanic CO₂. Others wish to place sunshades in space, directly between Earth and the Sun. This blog has noted some of the wackier schemes in earlier posts.

Several have won our coveted Crank of the Week award: John Latham suggested the creation of a fleet of 1500 robot sailing ships to combat global warming; Tom M. L. Wigley, Senior Scientist at the National Center for Atmospheric Research, suggested releasing sulfur dioxide (SO₂) into the stratosphere; and President Obama's science adviser John Holdren has mentioned several extreme options, including shooting pollution particles into the upper atmosphere to reflect the sun's rays and developing artificial trees to suck carbon dioxide from Earth's atmosphere.

Geoengineering schemes abound.

According to Doug Parr, chief scientist for Greenpeace UK, the scientific community is becoming so scared of our collective inability to tackle climate emissions that such outlandish schemes are being seriously considered. We here at The Resilient Earth are seldom in agreement with the neo-Luddites at Greenpeace, but we certainly do not support experimenting with Earth's climate system—our level of scientific knowledge is simply too primitive. “The scientist's focus on tinkering with our entire planetary system is not a dynamic new technological and scientific frontier, but an expression of political despair,” said Parr in an article in the UK's Guardian.

Whether the authors of the two papers presented here are in despair is debatable, but they are seriously looking into manipulating Earth's climate. Both of these recent studies address climate regulation using aerosols to reduce incoming solar radiation. The PNAS article, “Efficacy of geoengineering to limit 21st century sea-level rise,” compared reducing CO₂ emissions verses using aerosols in an attempt to limit sea-level rise. J. C. Moore, S. Jevrejeva, and A. Grinsted described their work in the article abstract:

Here we examine the impact of five geoengineering approaches on sea level; SO₂ aerosol injection into the stratosphere, mirrors in space, afforestation, biochar, and bioenergy with carbon sequestration. Sea level responds mainly at centennial time scales to temperature change, and has been largely driven by anthropogenic forcing since 1850. Making use a model of sea-level rise as a function of time-varying climate forcing factors (solar radiation, volcanism, and greenhouse gas emissions) we find that sea-level rise by 2100 will likely be 30 cm higher than 2000 levels despite all but the most aggressive geoengineering under all except the most stringent greenhouse gas emissions scenarios.

The researchers simulated the impact of both stratospheric SO₂ injection and reduction of radiation by reflecting mirrors, using measurements from the 1991 Mount Pinatubo eruption as a guide. The amount of SO₂ aerosol required to counteract the impact of a doubling of atmospheric CO₂ depends on the size of the particles and the location of injection. The models indicated that the continuous injection of SO₂ would produce larger particles than a natural volcanic eruption, which would reducing their effective cooling capability. This is because the larger particles would not stay airborne as long as finer, natural aerosol particles.

The radiative impact of the Pinatubo eruption over four years was −1.29, −1.59, or −1.89 Wm^-2, depending on which forcing reconstruction model is used. Moore et al. calculate that, to counteract a CO₂ doubling, the geoengineering effort would need to provide a constant 4 Wm^-2 reduction in radiative forcing—equivalent to a Pinatubo eruption every 18 months. Halting the particle injections would quickly reverse the cooling effect.

Results of modeling runs using three different scenarios, labeled RCP3PD, RCP 4.5, and RCP 8.5 , are shown in the figure above. Sea-level simulations (relative to mean sea level 1980 to 2000) using mean forcings from before 2000 and RCP scenarios since 2001. The past is constrained by observed global sea level, post 2010 simulation with the RCP scenarios labeling the figure with no geoengineering (black); with the scenario plus a constant −1.56 Wm^-2 (blue); the scenario plus space mirror (from 0 to −4 Wm^-2 ) linear ramp (red). Shadows represent 5–95% confidence bands in each simulation. The scenarios are described in detail by R. H. Moss et al. in "The next generation of scenarios for climate change research and assessment."

Though the authors concluded that aerosol injection could delay sea-level rise by 40–80 years, the injection of material would need to be maintained for the indefinite future. “Aerosol injection appears to fail cost-benefit analysis,” they state. “Substituting geoengineering for greenhouse gas emission abatement or removal constitutes a conscious risk transfer to future generations.”

In “Regional climate response to solar-radiation management,” Katharine L. Ricke, M. Granger Morgan and Myles R. Allen apply similar geoengineering measures to the regulation of temperature and precipitation. They present an analysis of regional differences in a climate modified by solar-radiation management, using a large-ensemble modeling experiment that evaluated the impacts of 54 scenarios for global temperature stabilization. They describe their motivation this way:

Although using solar-radiation management (SRM) to lower the average planetary temperature is not a new idea, it has recently become the focus of greater attention. Several prominent climate scientists have raised it as a feasible, and potentially necessary, strategy for avoiding catastrophic impacts of climate change (for example, rapid sea-level rise, rapid and large increase in emission of methane from high latitudes). Research on SRM is still in its infancy, but so far modelling studies suggest that, although significant hydrological anomalies would be associated with stratospheric albedo modification, even at the regional level, such a geoengineered world bears much closer resemblance to a low-CO₂ world, than either world bears to an unmodified high-CO₂ world. Increasing planetary albedo does not mitigate impacts directly related to elevated CO₂, such as acidification of the surface ocean.

From the model results, the researchers concluded that solar-radiation management would generally lead to less extreme temperature and precipitation. All of the modeled scenarios produced stabilized five-year average global-mean surface air temperatures (SATs), at levels between approximately 14.6 and 15.7 °C. This is roughly plus or minus half a degree from the temperature at the time SRM activities are initiated—depending on the level of forcing applied. The control scenario (shown in black) resulted in an increase in global-mean SAT of approximately 2.5 °C over the course of the 80-year simulations.

But there is a component of long-wave forcing of the hydrological cycle that is independent of temperature. As a result, SRM with stratospheric aerosols cannot simultaneously compensate for the impacts of rising greenhouse gases on both temperatures and the hydrological cycle. “Although it might be possible in principle to ‘fine tune’ the hydrological response by injecting aerosols with different optical properties at different latitudes or altitudes, no proposal yet exists for how this might be implemented in practice, and some variability in response remains inevitable,” the authors state.

In short, the simulations showed that it is “not feasible” to stabilize global precipitation and temperature simultaneously if atmospheric greenhouse gas concentrations continue to rise. “Hence, it may not be possible to stabilize the climate in all regions simultaneously using solar-radiation management,” they concluded. “Regional diversity in the response to different levels of solar-radiation management could make consensus about the optimal level of geoengineering difficult, if not impossible, to achieve.”

The lesson here is clear: geoengineering is not a solution to the perceived problems of climate change. Keep in mind that both of the studies presented here are based on models that are imperfect and cannot accurately predict future climate conditions. The model results indicate ineffectiveness and possible negative impacts, but the actual results remain uncertain. Obviously, all geoengineering proposals are extremely premature, if not down right dangerous. While I have a great deal of faith in human inventiveness, any such massive effort would entail some trial and error—and any error could have devastating consequences for humanity. Nothing having to do with climate change is truly “irreversible,” short of the eventual death of our Sun, but short term effects could lead to drought and famine or to devastating floods.

Researchers, accompanied by policy experts, social scientists and journalists, are doing their bureaucratic best to develop guidelines for regulating geoengineering. Given the lawsuits by Pacific islanders—seeking reparations from developed countries because of sea level rise supposedly caused by anthropogenic global warming—just imagine the furor if geoengineers were actively messing with the climate. Every natural disaster would be blamed on geoengineering. Litigation would be rampant, all aimed squarely at the developed nations. After all, you cannot sue Mother Nature for damages.

When science cannot even predict climate change accurately, trying to change the climate on purpose is shear lunacy. Experimenting on the environment we live in is a bit like doing experimental surgery on one's self—it may work in the movies but would probably be fatal in real life. For now, geoengineering remains the domain of mad scientists and cartoon super-villains. Not wishing to go all tree-hugger here, but mankind really shouldn't be messing with Mother Nature.

Be safe, enjoy the interglacial and stay skeptical.