Many of the world's nations show few signs of cutting their greenhouse gas emissions anytime soon. That's why, in recent years, more and more climate scientists have been pondering the concept of "geoengineering" as a way to slow the pace of global warming.
Pros: It's cheap, easy, and could likely avert some of the worst impacts of global warming, like sea-level rise. Cons: It could have lots of unpredictable side effects, like mucking up global rainfall patterns. It's difficult to coordinate. And geoengineering does nothing to address other severe climate impacts, like ocean acidification.
But there's one other concern we didn't really touch on in that interview: Once the world starts geoengineering, we can't really ever stop — especially if everyone keeps pumping carbon-dioxide into the atmosphere at the same time. Why? Because as soon as we quit spraying those reflective particles into the atmosphere, the Earth will heat up very, very, very rapidly. And sudden climate change is even worse than the kind we already know about.
David Appell points to a recent study in the Journal of Geophysical Research that highlights this point vividly. The authors first used 11 climate models to forecast what would happen if the world used solar geoengineering to offset a 1 percent annual rise in global carbon-dioxide emissions. The good news: Global temperatures stay pretty stable. But then they stopped the geoengineering. And that was catastrophic: Global temperatures spiked very sharply:
Once they stopped the solar geoengineering in the models, global temperatures rose by 1.5 degrees Celsius within a decade, and over 2 degrees Celsius (or 3.6 degrees Fahrenheit) in two decades. (This jibes with the results of a 2009 study on the same topic.)
Why is that so bad? It's worth noting that the speed of climate change matters almost as much as the total temperature swings. Plenty of climate policy experts think a 2-degree Celsius rise in global temperatures between now and the end of the century would prove tricky to adapt to. But a 2-degree rise in the span of just a few decades would be even harder. Many species would have difficulty moving to suitable climates. Farmers would struggle to adapt crops to the heat. Building seawalls to fend off rising oceans is much harder when you have less time to build.
For the record, the JGR researchers also found that global rainfall patterns could go haywire if solar geoengineering was stopped:
So, yes. In theory we could set up a giant hose to spray sulfate particles into the air and cool the planet. We just have to hope that nothing ever happens to the hose. Or, if we start spraying and then discover some horrid unanticipated side effects a few decades out, it will be a lot harder to stop.
This is one reason why many geo-engineering proponents, like Harvard's David Keith (who wasn't involved in this JGR study), argue that the technique would need to be done alongside reductions in greenhouse-gas emissions. "Nothing changes the fact that in the long run, the only way to manage carbon risk is to stop emitting carbon-dioxide," Keith told me in October. "But, similarly, nothing we know about cutting carbon-dioxide emissions says that's going to help us deal with the risk of CO2 that's already in the atmosphere, or deal with climate risks in the very short term." In other words, he sees geoengineering as complementary, not a substitute for cutting emissions.
(And yes, the JGR study came out a little while ago—I just missed it at the time. Thanks to David Appell for the recent pointer.)
Further reading: Should we use geoengineering to cool the Earth? An interview with David Keith. Worth noting that Keith mentions the "rapid climate change" issue in his recent book on the subject — we just didn't touch on it in the interview.