Geoengineering smackdown: how 5 methods might impact rising sea levels

Geoengineering smackdown: how 5 methods might impact rising sea levels

Geoengineering smackdown: how 5 methods might impact rising sea levels

By John Timmer | Last updated a day ago

The scientific community's take on geoengineering might be described as resigned—many researchers that study climate change or evaluate solutions feel that reducing carbon emissions remains the easiest and safest option. But they also seem to have concluded that our continued failure to do so means that we need to continue studying geoengineering in case we end up needing it. This week, PNAS will be releasing a paper that does just that, comparing five different methods of cooling the climate in terms of their effectiveness at limiting sea level rise.

Historically, ocean levels have tracked the global temperature. As the Earth cools, more water gets stored as ice, and the oceans themselves contract in response to decreased temperatures; both processes reverse as the planet warms. Because of the warming that's prevailed over the last century, instrument records show an increase in the ocean levels that appears to be accelerating. The last IPCC report suggested that we'd likely see between 20 and 60cm of sea level rise over the course of this century, but its authors admitted significant uncertainty in that number; the new paper cites three studies that have since suggested the number would be over a meter.

And that creates a big problem. The authors note that about 150 million people live within a meter of sea level, and the loss of land, combined with flooding of neighboring areas during storms, may cause losses that total up to 10 percent of global gross world product by the 2070s. Thus, they argue, limiting sea level rise is a pretty effective measurement by which to judge geoengineering options.

Given that the authors consider the IPCC's sea level rise estimates out of date, it's no surprise that they chose another source for their emissions scenarios.

Three of the scenarios they consider involve manipulating the Earth's carbon cycle in order to bring some of the CO₂ we've put into the atmosphere back out. They consider afforestation, the act of adding forests to the landscape where they've been lost or never existed, to be a bad solution. Even reversing all of humanity's historic deforestation would produce a drop of only 45 parts-per-million; we've already added over 100ppm. Biochar, the process of converting biological materials into solid carbon for storage, doesn't do any better, reducing the atmospheric CO₂ by only 35ppm.

The only thing that seemed to be effective on its own is biofuel production coupled with capturing and storing the CO₂ produced during combustion. Combined with biochar and afforestation, it could have a significant impact, eliminating up to 250ppm CO₂ from the atmosphere, and stabilizing the atmosphere at just above current concentrations. Sea level rise would be limited to between 20 and 40cm, most of it arising through warming that we're already committed to.

The alternative scenarios are what the authors term radiation management approaches. One option for this is space-based mirrors, which would need to block about two percent of the incoming sunlight to cancel out a doubling of CO₂. It's effective, has fewer side effects than some of the other scenarios, but comes with a very significant hitch: we'd need to place, and keep, about 20 million tons of hardware in orbit. That's unlikely to be cheap, at least within the time spans needed to block significant sea level rises.

They also evaluate a frequently referenced scenario, pumping sulfur dioxide into the stratosphere. Based on current estimates, a doubling of CO₂ would add four Watts per square meter to the climate system. Canceling it, the authors calculate, would require the equivalent of a Pinatubo-sized volcanic eruption every other year. We could do it, but it "would lead to several undesirable consequences such as disruption in precipitation patterns and stratospheric ozone, and do nothing to avert the continued absorption of CO2 by the global ocean leading to rising acidity and ecosystem damage."

As if all that weren't bad enough, they note that any system like this requires constant maintenance, and our margin for error slips as we add more CO₂ to the atmosphere. If emissions were to be offset by sulfur dioxide through to the 2070s and the program were stopped, temperature rises may reach 1.5K a decade until the atmosphere equilibrates. Sea levels would take far longer to respond, but the rise would be relatively dramatic, as well, at up to a half-meter within 25 years.

Overall, the paper does a nice job of highlighting why many people aren't so enthusiastic about geoengineering in the first place: the options are some combination of expensive, likely to produce unintended consequences, and only partially effective. Even the most innocuous among them, a combination of reforestation and biofuel production, requires a significant rethink of agriculture, forestries management, and the development of carbon capture and storage technologies (which have their own expenses and unintended consequences).

PNAS, 2010. DOI: 10.1073/pnas.1004932107 (About DOIs).