Solar Warming and Our Sulfur Sunshield

Two unrelated stories concerning the science of climate change caught my attention yesterday. The first was the announcement of a new report on solar variability, published in Nature, which appeared to upend established thinking about the impact of solar cycles on the earth's climate. The other was a discussion on Shell's climate blog of the potential impact of regulations affecting the sulfur content of marine fuel oil on an effect that has been partly mitigating climate change for decades. Both are interesting in their own right, while together providing a useful reminder that climate change is much more complex than the soundbites we typically hear from the media and advocacy groups, especially after we've had a run of unusually hot or cold weather.

As a less-than-fully reformed science nerd, I loved the simple elegance of the first sentence of the abstract of the Haigh, et al paper in Nature: "The thermal structure and composition of the atmosphere is determined fundamentally by the incoming solar irradiance." Paragraphs of exposition boiled down to 16 words that neatly frame the importance of the researchers' finding that for the last several years, and contrary to what we'd have expected from being in the low part of the solar cycle, featuring few or no sunspots for several years, the earth has been receiving more energy from the sun where it really counts--in the lower part of the atmosphere, or troposphere. If their interpretation of the satellite data is correct, then it pretty well torpedoes the notion from two years ago that a weak sun was about to flip global warming into global cooling. Of course it would also defuse some of the determined attempts to attribute this year's record temperatures entirely to humanity's greenhouse gas emissions.

While this finding isn't expected to alter the decade-to-decade view of climate change, it certainly suggests that we should be paying attention to a lot more than just CO2 and its sibling GHGs over shorter intervals, and in that respect it's a nice lead-in to the discussion of atmospheric cooling due to sulfur emissions from ships. That also applies to its implication that we still have a lot more to learn about the earth's atmosphere--where climate lives--and its dynamic interaction with the solar system.

In his blog on Shell's corporate website, Shell climate advisor David Hone shared his observations from a recent meeting exploring the impact of sulfur emissions on climate change. This apparently led to discussions of sulfur-based strategies for geoengineering the climate, but even without going that far it seems clear that this issue deserves a lot more attention that it has received. I was aware that such emissions tend to offset at least part of our greenhouse gas emissions, and that previous reductions in sulfur for onshore fuels--necessary for local air quality and modern vehicle anti-pollution equipment--might have given an unintended boost to warming. However, I think this is the first time I've seen the estimated climate forcing associated with marine fuels of -0.6 W/m2, which as Mr. Hone notes is not small relative to the total greenhouse gas forcing of around 2 W/m2. This situation surely justifies a serious re-think of the International Maritime Organization's decision to slash the sulfur content of all marine fuel burned globally, particularly since it is hardly the only alternative available to address the negative effects of these emissions on most human populations. It's also a much more expensive option for shippers--and thus anyone who benefits from international trade--than confining the low-sulfur rules to coastal waters. According to the analysis cited by Mr. Hone, the latter scenario would preserve nearly 80% of our sulfur sunshade, while the global low-sulfur rule would more than halve it.

When I was involved in marine fuel supply and distribution on the West Coast early in my career, it was already clear that the emissions from burning high-sulfur bunker fuel were a major source of pollution in port cities and coastal areas, and that the importance of addressing them would grow once most onshore emission sources, from power plants, trains and other mobile sources had been dealt with. Some of the sulfur was eliminated as large marine diesel engines replaced the old steam turbines, and much of the rest was addressed with restrictions on the quality of fuel that could be burned in port and along the coast. For now, vessel owners can comply with these rules by carrying two different fuels: enough of the more expensive low-sulfur fuel for use in US and other regulated coastal waters, and the rest consisting of much cheaper high-sulfur fuel for use on the high seas. That approach, which would no longer be an option after 2020 under the IMO rules, cleans up the air where it matters most but still puts enough SO2 into the atmosphere to scatter some of the incoming solar energy and offset part of the warming from CO2.

Using one form of pollution to offset another is hardly a perfect solution, but just as many scientists and environmentalists urge caution about introducing new geoengineering measures before we understand their consequences well enough, we should think long and hard about tampering with this long-standing, if inadvertent geoengineering process until we have something better in mind to replace it, or until we no longer need it.