Burning fossil fuels spews carbon dioxide into the air, which warms the climate through the greenhouse effect (as if you didn’t know that). But burning fossil fuels also spews sulfur dioxide into the air, and sulfur dioxide forms aerosols that can deflect the sun’s rays and thus cool the climate. It has thus been argued that phasing out fossil fuels would have the undesirable effect of accelerating the warming of the planet in the near term, since we’d be getting rid of the cooling aerosols at the same time.

This very argument was made by countries with serious air pollution issues, and it indicated to Intergovernmental Panel on Climate Change policymakers that the countries were struggling to figure out how, and how much, to limit emissions.

But climate scientists Drew Shindell and Christopher Smith have now re-analyzed the modeling data and concluded that there is no way we could halt emissions quickly enough for the aerosols' "climate penalty" to be meaningful. "Even the most aggressive plausible transition to a clean-energy society," they write, "provides benefits for climate change mitigation."

Shindell and Smith looked at the 42 ways that IPCC policymakers suggested keeping global temperature rise to 1.5ºC above pre-Industrial levels. (We're currently at about 1ºC.) The most ambitious phase-out of fossil fuels is limited by things like technology development, financial concerns, and the amount of social and behavioral change that might be necessary. Despite these limits, it should be possible to have a median decrease of 60% in all fossil fuel usage by 2050 and a drop of 90% in the use of fossil fuels to generate electricity. That will put us on a path to drop all fossil fuel use 85% in 2100.

Under this scenario, models indicate that there is no near-term spike in global warming caused by a drop in aerosols. Why, then, are people under the misapprehension that removing aerosols along with greenhouse gases will produce a climate penalty?

We need asteroids

Well, there could be a climate penalty if carbon emissions were to cease instantaneously. That's because the carbon dioxide stays in the atmosphere for centuries (as we're seeing) while the cooling sulfur dioxide only sticks around for days to weeks. But an instantaneous abatement of fossil fuel burning is essentially impossible without multiple asteroid strikes.

"In reality, it takes a substantial amount of time to transform energy, transportation, and industrial systems under least-cost pathways," Shindell and Smith note. Since the climate will continue to warm from past and persisting carbon emissions, any warming caused by the reduced sulfur will be masked in a more gradual, realistic phase-out of fossil fuels.

Older studies that warned of the climate penalty also probably overestimated the mitigating effects of sulfur dioxide because clean air laws have since halved the amount of sulfur dioxide that is being emitted for each unit of carbon dioxide. Those cuts have come because aerosolized SO 2 doesn't only deflect the Sun’s warming rays; it is also a toxic pollutant that, when inhaled, causes millions of early deaths.

Over the past 40 years, many countries have already curbed emissions of the cooling sulfur dioxide by about 20%, despite the fact that carbon emissions have gone up in that span. Which means that, to an extent, we've already lived through much of the warming pulse that eliminating sulfur emissions would have caused. We just didn't notice since it was swallowed up by the slow but steady warming that increasing carbon emissions has wrought.

Nature, 2019. DOI: 10.1038/s41586-019-1554-z (About DOIs).