What if, instead of reducing the greenhouse gas concentrations that hold excess heat in our atmosphere, we injected something in the atmosphere to reflect sunlight back into space? That's the idea behind sulfate geo-engineering. As Bill wrote in his post "Can we engineer our way out?", there are a plethora of problems with geo-engineering, but scientists still study it as an option of last resort.

The idea of injecting sulfates into the atmosphere is based on the observation that large volcanic eruptions can cause short-term global cooling. But in addition to the usual problems with geo-engineering (for example, it does nothing to stop ocean acidification from excess CO₂), scientists have found a new one. Sulfate geo-engineering could endanger food and water supplies for billions of people in Africa and Asia, according to a recent paper in the Journal of Geophysical Research [PDF].

A Study with Some Scary Findings

Continuous sulfate injections into the stratosphere might cool the Earth, but does it matter where you do it? And what else might it do? To address these questions, the researchers ran a global climate model under four scenarios:

1. Business-as-usual with no geo-engineering
2. Low-level sulfate injections in the Arctic
3. Medium-level sulfate injections in the tropics
4. High-level sulfate injections in the tropics

In all the geo-engineering scenarios, sulfate was injected continuously for 20 years, and then abruptly turned off. The medium-level is roughly equivalent to a Pinatubo eruption every four years, the high-level every two years.

Compared to business-as-usual, the geo-engineering strategies slowed or even reversed global warming and the loss of Arctic summer sea ice. So far so good. But scientists found serious drawbacks when they looked at (1) regional effects and (2) the consequences of suddenly ending sulfate injections.

One of the most prominent dangers was the effect on summer monsoons in Africa and Asia, on which billions of people rely for food and water supplies. Both monsoon systems decreased markedly when sulfate was injected into the atmosphere - regardless of how much or where.

And what about when sulfate injections stopped, as might happen if a real-world geo-engineering strategy encountered technical difficulties or lost political support? The resulting warming rebound and sea ice loss was "more rapid … than has occurred in the past century or than is projected with business as usual." Since the rate of warming can be as damaging as temperature alone, warming rebound could have tremendous environmental and social consequences.

Best to Tackle the Root of the Problem

This paper adds to the growing list of geo-engineering risks (for example, see 20 reasons why geoengineering may be a bad idea [PDF]). As RealClimate scientists put it in their coverage of the latest paper, geo-engineering is like methadone: "an emergency treatment to substitute one addiction (carbon emissions) with another." What we really need to do is tackle the root of the problem and decrease greenhouse gas emissions.

This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense Fund.