The Climate Time Bomb in the Atmosphere

Cleaning up air pollution might increase global temperature by as much as an entire degree Celsius.

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Big container ships have traditionally run on one of the dirtiest energy sources available: bunker fuel. This is the disgusting, tarry residue left over after crude oil has been refined into gasoline, diesel, kerosene, and so on. Bunker fuel has so much sulfur, nitrogen, and other contaminants that, according to a 2009 study, a single large container ship using it produces as much asthma- and cancer-causing pollutants as 50 million cars. A separate study released this week found that ocean shipping from just 18 large U.S. retail brands produces 3.5 million metric tons of greenhouse gas emissions, as well as nitrogen oxides (NOx) equivalent to seven coal-fired power plants and other forms of air pollution.

Shipping companies were allowed to burn this stuff because of industry lobbying and lax regulation. Bunker fuel is much cheaper than any equivalent, and companies argued that because ships spend most of their time out in the open ocean, the pollution effects were relatively modest. That argument was nonsense. One study on Sweden, for instance, found new regulations on shipping emissions just in the Baltic Sea caused a substantial decline in premature deaths.

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But there is an ironic climate benefit to using bunker fuel: the “ship tracks” left by the vessels as they cross the ocean. As particulate pollution (or aerosols) trailed behind them, it catalyzed cloud formation, leaving streaks clearly visible from space. These polluted clouds, which are brighter and tend to last longer than normal ones, increase the reflectivity of the Earth, slightly decreasing global temperature.

Ship tracks are just one source of aerosols; burning coal or oil also produces them (as well as dust storms, fires, and even ocean spray). The latest Intergovernmental Panel on Climate Change report estimates that, all told, aerosols have a cooling effect on the climate on the order of 0.5–1 degree Celsius.

That suggests a potentially very alarming downside to ongoing efforts to clean up the atmosphere, including in ocean shipping. Coal is on its way out—use has plummeted in the United States by about 55 percent since 2007—and with the rise of electric cars and bikes, oil will be as well in coming decades. And while some ocean emissions continue to be high, in 2020, new international regulations sharply restricted the amount of sulfur allowed in freight ship fuel, which caused a sharp decrease in ship tracks.

Currently, the climate has warmed by about one degree Celsius, which has caused all manner of devastating problems around the world—drought, extreme weather, mass extinctions, glacier loss, and more. If aerosol emissions are mostly ended over the next decade or so and nothing is done to compensate, that potentially means another entire degree of warming taking hold rapidly. And climate impacts do not scale linearly with temperature; a second degree will be far worse than the first one.

The energy transition isn’t happening nearly fast enough to prevent another degree or so of warming just from greenhouse gases.

Kelly Wanser, head of the nonprofit SilverLining, explains this idea of “solar radiation management” on David Roberts’s Volts podcast. As a paper by the group details, aerosol effects are the least well understood aspect of what humanity has done to the climate. So in the first place, research is vitally needed to both understand the effect of aerosols and study potential ways to replace them in the best possible manner.

Any proposed scheme of aerosol replacement, and how to best implement it, is even more uncertain. Computer models are pretty good at understanding and predicting broad-scale effects from greenhouse gas emissions, but details about specific areas are much harder. That will be doubly true for geoengineering solutions, like sending planes up to spray sulfur dioxide particles into the stratosphere. (We wouldn’t want to just start using bunker fuel again, of course.) How will the aerosols spread through the atmosphere? What will be the effects on regional weather patterns? Would that vary depending on the location of the plane? Small-scale tests in the real world will be needed to answer these (and many more) questions with any confidence.

Now, this is an uncomfortable idea for an environmentalist. Spraying sulfur dioxide might harm the ozone layer, disrupt weather patterns, or have other side effects, and it wouldn’t do anything to change ocean acidification since it wouldn’t cut carbon dioxide concentrations. But one must set those potential effects against the potential effects of not doing it. Two degrees of global warming would be absolutely catastrophic.

Incidentally, while sulfur aerosols are the most commonly proposed idea in this space, it might also be possible to use ocean water instead, which would likely have many fewer side effects.

Luckily, the worst possible use of aerosols—to use them as a permanent replacement for climate policy so humanity can continue burning fossil fuels forever—is already virtually out of the question. Renewable energy is now the cheapest available and getting cheaper, and governments around the world have put enormous money and effort toward phasing out carbon energy (thanks in part to Vladimir Putin’s invasion of Ukraine, which greatly accelerated European climate efforts). The energy transition is going to happen sooner or later. It just isn’t happening nearly fast enough to prevent another degree or so of warming just from greenhouse gases. Together with another possible degree from ending aerosol emissions, there is a real risk of tripping feedback loops—like melting the Arctic ice cap, which would decrease reflectivity and boost temperature more.

In that dire circumstance, an aerosol program might be the least-worst option, as a way to buy time to carry out the transition. The World Meteorological Organization’s recent report on ozone depletion had a whole chapter about the idea, and estimated that such a “peakshaving” approach of aerosol injection for a decade or two likely wouldn’t harm the ozone layer that much, though not with much confidence.

Again, more research is needed. Unfortunately, we are currently going in the opposite direction. As the SilverLining report points out, several American satellites vital for climate research generally are reaching the end of their life spans, with no replacements proposed. (Even setting aerosols aside, this is criminal negligence.) This research would be expensive, since you’d have to actually send out planes and ships to do these test runs, and put up some more satellites. But the report estimates that $2.6 billion a year would cover the bill—a piddling amount in the context of the vast government budget.

Democrats and climate activists should be calling for this spending, and if the U.S. can’t get its act together the EU should step up to the plate. The logic here is compelling enough that even if the research isn’t done, sooner or later one of the major countries will try it in a haphazard and possibly dangerous way.

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