Easing Out of the Sulfur Trap
Why We Really Need To Tackle Methane *Now*
What would happen if we stopped polluting the Earth? Like, all of a sudden?
The following graph is taken from the IPCC SR15 report, published in 2018. Focus on the yellow line, which projects average global temperatures in a scenario where all (human-caused) emissions suddenly cease:
You might notice something funny about that yellow line: at first, it goes steeply up! If we were to halt emissions, why would warming accelerate?
Not all the junk we dump into the atmosphere causes warming. Sulfur – a trace component of fossil fuels – has a substantial cooling effect: it forms aerosols which scatter sunlight back into space and influence cloud formation. According to the IPCC AR6 report, the increase in sulfur emissions since 1850-1900 is cooling the earth by almost 0.5°C. This partially masks the effects of greenhouse gases like CO₂ and methane, which are “trying to” cause even more warming than we actually see. (Volcanoes also emit sulfur, which is why major eruptions can cause cooler weather.)
Unfortunately, while CO₂ lasts for centuries and methane for about 12 years, sulfur dioxide breaks down in a matter of days. We’re balancing a long-term problem with a short-term solution.
If We Don’t Tackle Methane, Things Get Bad
Now look at the green line in the graph. This is the scenario where CO₂ and aerosol (mostly sulfur) emissions cease, but everything else stays the same. As you can see, this projection is quite scary: temperatures jump 0.3°C within about 10 years, and then continue rising1. It Would Be Bad.
This combines all the worst elements of the previous scenario. We lose the aerosol cooling immediately, without any corresponding reduction in greenhouse warming. After we end CO₂ emissions, CO₂ levels will eventually decline, but not quickly enough to balance the missing aerosols. CO₂ lasts too long.
What’s missing here is any reduction in the “supporting cast” greenhouse gasses. Manmade methane, carbon monoxide, VOCs (volatile organic compounds), and black carbon together contribute about as much warming as CO₂; but unlike CO₂, they have relatively short lifetimes. In the yellow-line scenario, these emissions also cease, past emissions quickly decay, and the temperature spike is neutralized. That’s much better than the green-line scenario.
In Realistic Scenarios, There’s No Spike At All
Sometimes someone will come across the green-line projection and write a panicky article, claiming that we’re caught in a trap where phasing out fossil fuels would make climate change worse. Fortunately, this isn’t the case, as explained in a 2019 Nature paper: Climate and air-quality benefits of a realistic phase-out of fossil fuels. The paper notes two principal factors.
First, real-world scenarios look more like the yellow line than the green line. Many “supporting-cast” emissions are associated with fossil fuel use, so as we ramp down fossil fuels, we’ll automatically emit less methane, carbon monoxide, and so forth2. This only applies to some of those emissions, so in parallel we’ll need to work urgently on methane emissions from leaking gas pipelines (reducing usage doesn’t necessarily reduce leaks), abandoned coal mines, enteric methane, and other sources. Also, there are significant error bars on these projections; by pushing hard to reduce emissions of methane et al, we can increase our safety margin against a short-term temperature spike.
Second, we’re obviously not going to switch off fossil fuels overnight. Even in aggressive keep-1.5°-alive scenarios, changes play out over decades. This spreads the impact of the aerosol loss, providing time for methane and other short-lived GHGs to decay. When we really get moving on emissions reduction, current warming trends will continue for a few years, but without any spike; then warming will slow and eventually temperatures stabilize.
Waiting Won’t Help
Phasing out fossil fuels needn’t cause a temperature spike. But even if it would, we can’t afford to wait. It would be a choice between a short-term spike, or further warming and then a spike when we finally wean ourselves. The problem only gets worse the longer we wait.
In this fashion, fossil fuels are a bit like an addictive drug. When you come off the drug, the withdrawal can be tough, and it might be better to taper off than to go cold turkey. But if you keep using, your body just gets weaker and weaker, and the withdrawal will be that much harder when you finally face up to it.
The Sulfur Transition Is Happening; We’d Best Keep Up
Sulfur emissions have all sorts of nasty effects unrelated to climate, and over the last 40 years, many countries have introduced pollution controls to limit sulfur emissions. (Unlike CO₂, sulfur mostly causes problems in the area where it’s emitted. Somehow that gets people moving faster.) In 1980, the ratio of sulfur emissions to carbon emissions was about twice as high as today. As a result, the aerosol cooling effect was larger, in relative terms, and an abrupt end to aerosols would have had more dramatic impact.
Some of the written work that projects temperature spikes is based on old data. To quote from the Nature paper, “…the world’s success in curbing SO2 emissions along with its failure to curb CO₂ emissions have led the world to a state where aerosols mask a substantially smaller portion of the effect of CO₂, greatly diminishing any ‘climate penalty’ resulting from simultaneously phasing out emissions of both pollutants.”
This very success in curbing sulfur emissions has already contributed to warming trends, and that ship continues to sail. From the Nature paper again: “…the apparent success of ongoing efforts to reduce air pollution in places such as China thus adds to the urgency to phase out fossil fuel usage.” We’re already eliminating sulfur emissions3, so if we don’t address other emissions, It Would Be Bad. We’d wind up on a trajectory even worse than the green line in the graph. So let’s get moving on phasing out the short-lived greenhouse agents: methane, carbon monoxide, VOCs, and black carbon.
Caveats: Earth’s climate is extremely complex. There is a lot of uncertainty in the models, I’m glossing over important factors such as inertia / momentum effects, and I certainly don’t fully understand the science myself. Furthermore, aerosol effects are not at all uniform across the globe, so different locations will be impacted differently. I am reasonably confident that the basic ideas in this post are correct, but don’t rely on any of the numbers.
In the previous footnote, I mention that different locations around the globe may be impacted differently. Because sulfur lasts such a short time in the atmosphere, its effects are particularly localized. Thus, some locations near major sulfur emitters may currently be experiencing a strong cooling effect, which they are going to lose. However, as countries like China and India install flue-gas scrubbers, that’s already locked in, regardless of where we go with fossil fuels. The best we can do to compensate is to reduce greenhouse emissions as quickly as possible.
It is sometimes proposed that, to cool the planet, we should deliberately pump sulfur into the atmosphere, as in Neal Stephenson’s recent novel Termination Shock. This is a complex and controversial topic, and if I ever try to address it that will be an entire post of its own. Incidentally, I enjoyed the book, but I can’t recommend it unless you’re the sort of person who just absolutely enjoys wallowing in Stephenson’s prose whether or not the story is going anywhere at the moment. (I do. I could read two entire pages about a character pouring themselves a bowl of cereal. Oh, wait, I did – that was Cryptonomicon.)