A Very Different Earth™: Geoengineering No Simple Solution

Can we use geoengineering to avoid the worst effects of climate change caused by increasing atmospheric carbon dioxide? Don't count on it, say two recent scientific articles.

Sea Level Rise Can't Be Halted By Geoengineering

Researchers from China/Finland, the UK and Denmark modeled the effect of various geoengineering ideas on sea level rise. Their paper "Efficacy of geoengineering to limit 21st century sea-level rise" appears in the Proceedings of the National Academy of Sciences (abstract and access here). (They deserve thanks for making their paper open access.)

They find "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." This means that even if we spend trillions on vast projects, and even if those projects work as planned, unless we also adopt tough measures to cut emissions sea level still goes up a foot.

And they say "Loss of low-lying land, combined with asset exposure to urban flooding due to the combined effects of climate change (sea-level rise and increased storminess), may reach 10% of projected global gross world product (GWP) in the 2070’s."

There are two things to consider when digesting that statement:

The authors are only talking about a meter of sea level rise in this century, at the lower end of current projections, and
The current recession cut the global economy by less than 2% from what it might have been without the recession. Imagine 10%.

They modeled several proposed projects:

Giant mirrors in space--These reduce climate forcing from the Sun's radiation, but leave greenhouse gases in the atmosphere where they continue to have their other negative effects such as ocean acidification. Also this project would be enormously expensive.

Who could think that launching 20 million tonnes of mirrors into space could be less of a blow to the economy than cutting greenhouse gas emissions by reducing coal use?
For comparison, the International Space Station weighs only 370,000 tonnes, and it has taken a decade to get it up there. It is the most expensive object ever constructed. It has been estimated to cost €100 billion over its 30-year life. Could taxpayers afford €5 trillion?

Aggressive afforestation, replanting cut forests and planting trees to create new forests, might be able to take some CO₂ out of the atmosphere, reducing that climate forcing. But CO₂ could only be reduced by about 45ppm. Humans have put more than a hundred ppm of CO₂ into the atmosphere over the past century or so, and are adding about 2ppm per year. So models still showed significant sea level rise.
Increasing biochar levels in the soil would reduce the CO₂ levels by even less--about 35ppm.
Aggressive conversion from petroleum and coal to biofuels with capture and storage of the resulting CO₂ from their combustion could reduce atmospheric CO₂ 180ppm by 2100.
A combination of afforestation, biochar sequestration and biofuels with carbon capture and storage could take 250ppm of CO₂ out of the atmosphere and might keep sea level rise to "only" 20 to 40 cm.
Injecting SO₂ into the atmosphere, equivalent to a major volcanic eruption every year and a half, could reduce global temperature rise a degree or so, but would leave the CO₂ in the atmosphere. Such a project might reduce sea level rise by 20cm or so. And if the program were ever ended warming would bounce back in a short time, with accompanying sea level rise.

And see the other findings below for more problems with SO₂ injection.

In the end they conclude "Substituting geoengineering for greenhouse gas emission abatement or removal constitutes a conscious risk transfer to future generations."

Geoengineering Helps Some But Hurts Others

Researchers from Carnegie Mellon and Oxford studied the regional impacts of various geoengineering concepts. Their work is reported in Nature Geoscience. (Abstract only here. Unfortunately these researchers did not make their findings open access.)

They modeled solar-radiation management by adding reflecting aerosols to the stratosphere. Previous modeling studies suggested that such an approach could stabilize global temperatures and reduce global precipitation. Their findings:

Our results confirm that solar-radiation management would generally lead to less extreme temperature and precipitation anomalies, compared with unmitigated greenhouse gas emissions. However, they also illustrate that it is physically not feasible to stabilize global precipitation and temperature simultaneously as long as atmospheric greenhouse gas concentrations continue to rise. Over time, simulated temperature and precipitation in large regions such as China and India vary significantly with different trajectories for solar-radiation management, and they diverge from historical baselines in different directions. Hence, it may not be possible to stabilize the climate in all regions simultaneously using solar-radiation management. 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.

So shooting millions of tonnes of SO2 into the stratosphere can cool the globe, but its effects are uneven. An article in The Economist provides more details:

A particularly salient example of this comes from Asia. There were a number of geoengineering scenarios in which the climate in both India and China in the 2020s looked quite like that of the 1990s, though in all geoengineering scenarios using this particular model India gets a bit wetter than it was before and in most of them China gets a bit drier. Go out to the 2070s, though, and the geoengineering scenarios strong enough to keep China’s temperature 1990s-ish cool India below its baseline temperature, while weaker scenarios that keep India’s temperature at the levels of the 1990s see China heat up.
That said, in both cases all the geoengineering options gave results for both temperature and precipitation closer to 1990s levels than the models projected for a world without geoengineering. In this sense both countries were "winners"—but maximising the benefits for one would still come at the expense of the other. The same lesson seems to apply quite generally across the world. Few if any regions stand out as certain losers from geoengineering if you accept that it is worth trading off a large change in temperature for a smaller drop in precipitation (in terms of change on the levels seen before the geoengineering). But different levels of geoengineering seem optimal for different regions.

So the question would be, who gets to decide what geoengineering to do? Presumably those who pay for it will control which projects get done. And naturally they will undertake projects that benefit themselves, even if they cause harm elsewhere in the world. This would be geopolitically very unfriendly, sort of like declaring war.

If these results hold I doubt we will see much geoengineering until and unless climate change gets so disruptive that the community of nations falls apart and its every one for itself.

A Scientific American blog post also covered the sea level rise story.