2024 was Hot Due To Termination Shock*

2024 was the warmest year on record. In part, because humanity stopped a geoengineering effort.

Confused? I’ll explain.

Geoengineering, also known as climate engineering or solar radiation management, involves intentionally altering the Earth’s systems to reduce the greenhouse effect. 

One of the most discussed geoengineering techniques, ‘Stratospheric Aerosol Injection’, or SAI, is based on volcanoes. Past eruptions, such as Mount Pinatubo, have cooled the earth for years afterward by ejecting plumes of sulfur oxides (SOx) into the earth’s atmosphere. These aerosols reflect part of the incoming sunlight and cool the planet. It’s important to note that this does not remove greenhouse gases from the atmosphere - it only covers up their effects.

A commonly discussed issue with geoengineering is ‘Termination Shock’. If you start geoengineering and then stop, all the warming you were covering up happens at once.  (For fans of scifi, I’d recommend Neal Stephenson’s book of the same name). This rapid warming is generally feared to be potentially even more dangerous than the relatively sedate pace of anthropogenic climate change.

One of the likely reasons 2024 was the hottest on record: Termination shock from stopping incidental SAI. 

Since the early 1900s, most oceangoing ships have run on “bunker”, or Heavy Fuel Oil. Bunker is the stuff that’s left over when we distill petroleum - a thick, tar-like fuel containing everything we don’t want in gasoline. Of chief importance to us : up to 3.5% of unadulterated bunker is sulfur, causing the global shipping industry to emit up to 14% of global sulfur oxides (SOx) until 2020.

In 2020, the International Maritime Organization, or IMO, introduced air-quality regulations which limited the sulfur content of most marine fuels to 0.5%. A wonderful thing for everyone who likes their rain pH neutral, this single act of legislation dropped the shipping industry’s emissions of sulfur oxides by 80%.

When this dip was introduced into the UKESM1 climate models by a group of researchers with the UK’s Met Office Hadley Centre Climate Programme, the model showed approximately 2-3 years more worth of warming across this decade. Part of the reason the last few years have warmed so fast : termination shock.

Hang on, though, it gets more interesting. 

Part of the reason SAI is controversial is that the injected aerosols can cause difficult to predict relatively ‘local’ climate effects. This applies also to termination shock - stop injecting a bunch of cooling aerosols in one part of the atmosphere, and you might have damaging local effects.

This graphic, pulled from Jin et Al’s 2018 paper, shows how pre-2020 shipping sulfur emissions, in contrast to natural emissions, were extremely concentrated along travelled routes. 

When a group of scientists with the University of Leeds’ Institute for Climate and Atmospheric Science modelled the effects of the sulfur emissions reduction, they found an interesting effect - a “La Niña-like condition”.

La Niña and El Niño are anomalous weather patterns resulting from an oscillating ocean and wind conditions in the Pacific. During El Niño years, trade winds weaken and warmer water is pushed towards the West coast of the Americas, heating and drying the northern US and Canada, while the south gets wetter. During La Niña years, trade winds are strong and warmer waters are located off the coast of Asia, causing drought in the southern US, cooler and wetter conditions in the north, and stronger hurricanes. 

One of the most active shipping regions of the world is the West Pacific, from where vessels laden with goods depart for both America and Europe. When modelling the stopping emissions of sulfur oxides in this part of the world, the researchers found warming in the West Pacific which closely mimicked a La Niña event.

2020-2023 had a very rare ‘triple-dip’ La Niña, starting in the Spring of 2020. This three-year long event, linked to numerous natural disasters including record-setting tropical cyclone activity, appears to correspond to the La Niña-like condition modelled by the researchers. 

While it’s not possible to definitively link cause and effect here, the timing makes it seem possible the 2020 IMO rule change was a trigger for the 2020-2023 event, or further worsened a La Niña event already impacted by emissions-driven climate change.

Today, there are companies , like Make Sunsets, intentionally injecting sulfur into the atmosphere to try and combat global warming. This is a well-intentioned effort that could be extremely valuable to our climate fight. However, I haven’t seen any of these companies making large-scale modeling efforts to anticipate the potential negative effects of geoengineering, including termination shock.

Geoengineering is tempting because it’s the highest-leverage tool humanity has in our climate quiver. For a few billion dollars a year, we could mask nearly all the warming. It is terrifying, too, because it carries potentially catastrophic risks.

The pre-2020 shipping industry SOx emissions and 2020 IMO rules change are the first large-scale stratospheric aerosol injection experiment we’ve run on the climate. We should carefully study the results before committing to another.

What do you think about geoengineering? Helpful tool, or tampering with forces better left alone? Subscribe and comment to join the conversation!

In summary:

• Sulfur oxides, when injected into the atmosphere, reflect incoming solar radiation and have a cooling effect

• In 2020, IMO rule changes mean humanity stopped injecting 10 million metric tonnes of sulfur oxides over the ocean per year

• In climate models, the termination shock of this change caused 2-3 more years of warming to happen this decade

• It’s possible, if not likely, this change also contributed to the 2020-2023 La Niña event

• Currently ongoing (and proposed) SAI efforts should thoroughly analyze this example to understand the potential effects of their work

Next week, I’ll share my analysis of the best (and worst) US cities to live in as the climate changes.