Yesterday Dan Lack of NOAA gave a talk to the NCAR media fellows about his work on pollution from shipping, and told us something I found pretty flabbergasting. Last year the International Maritime Organisation, as part of a number of measures aimed at air pollution, decided to do something about the sulphur emissions from shipping by reducing the amount of sulphur dioxide permissible from 4.5% today to 0.5% in 2020. This would have great benefits; sulphate pollution, and associated particulate matter, cause significant health problems. According to a new paper in Environmental Science and Technology by Winebrake et al, if in 2012 the world’s shipping complied with this requirement, the associated sulphate pollution would cause 46,000 premature deaths; if that shipping used today’s higher sulphur fuels the death toll would be 87,000.
However, sulphur emissions from shipping have another effect: the sulphate aerosols that form from the gas make the oceans cooler by increasing the cloud cover above them, as the image at the top of this post shows. The effect is large enough that shipping cools the planet through sulphate aerosols much more than it warms the planet through greenhouse gas emissions. In a companion paper in Environmental Science and Technology, this time with modeller Axel Lauer as first author, the same team looks at this effect. Using the same 2012 scenarios they used for the health figures the researchers find that the cooling effect using fuel like today’s, expressed in terms of radiative forcing, is about 0.57 watts per square metre. The cooling effect if everyone uses the new low sulphur fuels is 0.27 W/m². That means a difference of 0.3 W/m² — which is to say that that’s the amount of warming that switching to low-sulphur fuels would produce.
What does a radiative forcing of 0.3 W/m² mean? Here’s a chart from the IPCC showing the radiative forcings associated with all human climate-changing activities as of today. The total (with biggish error bars) is 1.6 W/m², which shows straight off that 0.3 is quite a lot. It is, for example, twice the amount of forcing as is due to N2O, 60% of the forcing due to methane, and the same as the amount due to halocarbons (HFCs). A huge amount of money is currently being spent on the HFC problem.
Put another way (and I calculated these numbers myself, so please check and correct if you have the necessary skills) 0.3 W/m² is the radiative forcing you would expect if you dumped 47.5 billion tonnes of carbon (in the form of carbon dioxide) into the atmosphere, raising the concentration of CO2 from today’s 387 parts per million to 409 parts per million. That’s well over a decade’s worth of carbon emissions and an enormous amount of warming for the IMO to have committed the world to with no-one, as far as I can see, paying very much attention. (The most obvious environmental response to the IMO changes, from the Clean Air Task Force, was to applaud the health effects of the cuts in sulphur while deploring the lack of action on greenhouse gases and not mentioning the cooling issues at all. If you accept Dan Lack’s figure of just 0.06 W/m² for the total warming from shipping, that seems an odd omission.)
Now there are obviously complexities and caveats. This is just one modelling study — but its figures for the amount of cooling due to sulphur fit with those quoted by of others, such as Dan Lack. Taken at face value it would imply both that the total cooling effect of sulphur on clouds was probably greater than the IPCC best guess, and that sulphate from shipping was responsible for a disproportionate amount of it. But the IPCC’s guess has big error bars, and you would indeed expect sulphate from ships to be peculiarly effective — it gets sprayed into places where the clouds are very susceptible to such things. (This is the effect that John Latham’s geoengineering scheme based on cloud brightening seeks to emulate). The papers compare effects for 2012 not 2020, which is when the regulations will call for al fuel to be low sulphur, but does anyone expect less shipping in 2020 than 2012?
So is this a matter of balancing 40,000 lives a year against a decade of global warming? Not necessarily. There is another sulphate reduction option: burn low-sulphur fuels when close to land, and ordinary fuels when far off. There are already some areas where ships have to use low sulphur fuels, and they could be extended to all the places where the sulphate is likely to do its greatest harm. In further scenarios the authors of the two papers looked at a world of 2012 in which ships’ sulphur was reduced to 0.5% or even 0.1% when within 200 nautical miles of land, but left unchanged in mid voyage. In terms of fatalities the 0.1% in coastal waters is slightly better than 0.5% all over the place (44,000 deaths), 0.5% in coastal waters is slightly worse. In terms of cooling these two options are lower than business as usual but higher than a global reduction to 0.5% — their forcing is 0.45-0.48 W/m².
Low-sulphur fuels in coastal areas could lessen the warming associated with a global sulphur reduction and still save as many lives — or more. They would impose other costs, though. Getting sulphur out of fuel costs money, and this might make getting down from 0.5% to 0.1% an issue. Ships would have to carry two different types of fuel, which is also problematic, though not impossible. And going low-sulphur still deprives the world of a lot of cooling, even if the regulations only apply in coastal waters. That’s largely because most shipping is coastal. (This suggests that forcing ships to take longer, less coastal routes — to put out straight to sea where possible, and spend more time further from land — might be an option. Again it has costs.)
Beyond preferring coastal controls to global controls I have no real policy case to make here. I’m aware that there is in general a trade off between air quality reasons for reducing sulphates and the possibility that their cooling effects can be climatically helpful. But the fact that this measure involves reducing sulphur emissions in places where they do no harm (the mid oceans) and where their cooling effects are greatly enhanced (by the presence of low clouds they can brighten) makes the question particularly pointed. I have no way to balance the advantages of reduced global warming against the advantages of decreased mortality. I don’t know who has. But I do think that it’s kind of extraordinary a regulatory change with this much effect on global warming could be made with so little apparent fuss.
And I also think this all makes the case for experiments with Latham-type techniques that brighten clouds to cool the seas even stronger than it already is. If, for good reason, we are actively reducing the amount of cooling provided by shipping, surely we should at least look at possible ways of putting it back?
“Mitigating the Health Impacts of Pollution from Oceangoing Shipping: An Assessment of Low-Sulfur Fuel Mandates”, Winebrake, J. J. et al, Environ. Sci. Technol., 2009, 43 (13), pp 4776–4782
“Assessment of Near-Future Policy Instruments for Oceangoing Shipping: Impact on Atmospheric Aerosol Burdens and the Earth’s Radiation Budget” Lauer, Axel et al, Environ. Sci. Technol., 2009, 43 (13), pp 5592–5598
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