ALBEDO ENHANCEMENT BY STRATOSPHERIC SULFUR INJECTIONS: A CONTRIBUTION TO RESOLVE A POLICY DILEMMA? (PDF)
An Editorial Essay
by Dr. Paul J. Crutzen
Since it is likely that the greenhouse warming is substantially negated by the cooling effect of anthropogenic aerosol in the troposphere, but possibly greater, air pollution regulations, in combination with continued growing emissions of CO2, may bring the world closer than is realized to the danger:
“Supposing, a currently envisioned low probability but high consequence outcome really started to unfold in the decades ahead (for example, 5 ◦C warming in this century) which I would characterize as having potential catastrophic implications for ecosystems . . . Under such a scenario, we would simply have to practice geo-engineering . . .”-from Schneider, 1996
There are some worrying indications of potentially large climate changes: for instance the locally drastic atmospheric warming by up to 3 W/m2 (watts per square meter, from answers.com) per decade in Alaska due to surface albedo decreases through tree and shrub expansion, the projected increase in surface temperatures by 2–3 K by the middle of this century in Africa even with the Kyoto protocol in force with great impacts on biodiversity, and potentially also the 30% slowdown in the north Atlantic overturning circulation during the past half century.
(above image of temperatures in Anchorage, Alaska from 1918-2006 A.D., from Global Warming Science)
Given the grossly disappointing international political response to the required greenhouse gas emissions, and further considering some drastic results of recent studies, research on the feasibility and environmental consequences of climate engineering of the kind presented in this paper, which might need to be deployed in future, should not be tabooed.
Actually, considering the great importance of the lower stratosphere/upper troposphere (LS/UT) for the radiation balance, chemistry, and dynamics of the atmosphere, its research should be intensified.
For instance, it is not well known how much of the large quantities of anthropogenic SO2 emitted at ground level reaches the LS/UT to produce sulfate particles, what regulates temperatures, water vapour concentrations and cirrus cloud formation in the LS/UT region, and how these factors may change in response to growing CO2 concentrations, which are already 30–40% higher than ever experienced during the past 650,000 years.
(image right from Crosslink/Aerospace Corporation Magazine, click image for detail)
Progress in the understanding of the complicated earth climate system is generally slow.
Therefore it is recommended to intensify research in order to challenge the climate
modification idea here presented, starting with model investigations and, dependent
on their outcome, followed step by step by small scale atmospheric tests.
Also, as natural sulfur injection experiments occur intermittently in the form of explosive volcanic eruptions, often at low latitudes, they provide excellent opportunities for model development and testing.
Researchers at the Lawrence Livermore Laboratory are so far the only ones who have modelled the stratospheric albedo modification scheme.
In a first study, Govindasamy and Caldeira simulated this by reducing the solar luminosity by 1.8%, to balance future climate warming by a doubling of CO2.
Although solar radiative forcing has a different physics and spatial distribution than the infrared
effects caused by CO2, the model results indicated that the global temperature response by both perturbations at the Earth’ surface and atmosphere largely cancelled out.
(image left of radiative forcing components, from wikipedia.com. click image for detail)
Although these preliminary model results would be in favor a stratospheric sulfur injection operation, the required annual S[ulfur] inputs are large, so that the possibility of adverse environmental side effects needs to be fully researched before the countermeasure to greenhouse warming is attempted.
What has to be done first, is to explore whether using a sulfur injection scheme with advanced micro-physical and radiation process descriptions will show similar model results as the simple solar luminosity adjustment scheme of Govindasamy and Caldeira.
Further studies, following those conducted by Govindasamy, should address the biological effects of the albedo modification scheme.
As already mentioned, injection of soot may be an alternative, but in need of critical analysis.
Such studies by themselves, even when the experiment is never done, will be very informative.
Among possible negative side effects, those on stratospheric ozone first spring to mind.
(image right from Pacific Marine environmental Laboratory, click image for detail)