When it comes to hacking climate change, the solutions tend to match the scale of the problem. Geo-engineers have proposed pumping tons of sulphur into the atmosphere (to deflect sunlight), fertilizing the ocean with millions of tons of lime (to improve its CO2 absorbing capacity), and planting trees across 2.5 billion acres of desert. Faced with planetary crisis, there's no space for thinking small. The question, however, is whether any these methods would actually work, and what side-effects there might be along the way. If the cure is worse than the disease, it might be better to focus on more conventional responses.
A new study from Germany runs the rule over five possible solutions. Using a computer model to simulate many of the Earth's systems (oceans, vegetation, atmosphere), researchers at the Geomar Helmholtz Center for Ocean Research, in Kiel, compared the positive and negative effects on the environment. All the solutions had been modeled individually before, but never using the same computer and emissions scenarios.
The result? Even the most workable ideas for climate engineering don't offer much in the way of upside, but provide plenty of downside, the researchers say. In some cases, they may even exacerbate climate disorder. "Our simulations suggest that the potential for these types of climate engineering to make up for failed mitigation may be very limited," the paper, which is published in Nature Communications, says.
Below, lead author David Keller, walks us through the five methods his team studied, and outlines what the possible consequences of each approach.
The technical term for planting lots of trees to combat climate change is "afforestation." But whatever you call it, it doesn't seem to be very feasible as a large-scale measure. The research tested the idea of seeding vegetation all over the Sahara desert, in North Africa, and the Australian Outback, and the effect was minimal. "Even if you could plant trees in this whole area, they will take carbon up, but they won't take enough to have a large impact on temperature," Keller says. The trees would absorb some carbon, but also reduce the reflectiveness of the surface (the albedo effect) keeping more heat at land level.
Artificial ocean upwelling, which was tested in a real-life experiment off Hawaii not long ago, is when you propel cold, nutrient-rich water from the bottom of the ocean to the top. The idea is to spur growth of plankton that suck in carbon and then sequester it below the surface when they die. The model showed it could reduce ocean temperatures, but that it would be dangerous if you stopped pumping. "If you stop it, the water gets warmer than if you did nothing at all, and it gets warmer very quickly," Keller says. "It's one of the methods that when started it, you wouldn't be able to stop, unless you dealt with the CO2 in the atmosphere." Cooling the ocean surface might also reduce evaporation and thus affect rainfall, he adds.
By increasing the ocean's alkalinity, you can increase its capacity to absorb CO2 (and reverse the problem of acidification at the same time). The problem is you need a lot of lime to make a difference. "If you took every ship we had in world and loaded it up with this powder and continuously dumped it in the ocean, it would be effective in reducing carbon," Keller says, "but not enough to deal with all the CO2 that's being emitted. You would really have to increase the shipping capacity and you would have to increase [limestone] mining. It would be like grinding up the Alps and turning it into quicklime."
Another idea to stimulate plankton growth is to fertilize an ocean that's limited in iron. The researchers focused on a scenario in the Southern Ocean, but found, while it could be logistically feasible, it wouldn't do much for CO2 absorption. "Even if we fertilize the ocean completely, which is probably possible with a big effort, we just can't get enough plankton growth because it's cold and dark most of the year, and you don't get much carbon take-up that way," Keller says.
Solar radiation management (SRM), where you use mirrors or sulphur aerosols to block sunlight, is widely seen as the most feasible method among geo-engineers. Helping its cause is that we know aerosols work: when there's a big volcanic eruption, and tons of ash go into the atmosphere, temperatures tend to drop. The problem, says Keller, is that it's only a "Band-Aid." If you have to stop spraying sulphur, the problem would return. "You get really rapid warming because the CO2 is still there," he says. Moreover, SRM could also significantly affect rainfall patterns, perhaps "as much as climate change itself."
There are other reasons to be skeptical about geo-engineering apart from the unpredictable processes these experiments might unleash. For one, governments would probably have to agree at a global level on any large-scale actions—something that is hard to see, as geo-engineering, like climate change, could benefit some countries and imperil others.
Keller isn't completely dismissive. He thinks geo-engineering like SRM could be useful in some circumstances, in conjunction with reducing emissions. But he thinks the latter is much more important. "I think if someone tries these methods, we need to know what could happen—that these are the side-effects," he says. "Personally, though, I would prefer to see emissions reduced than doing geo-engineering."
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