What will affect the earth’s temperature the most in the next 10 years?

Aerosols

“Aerosols” is a fancy name for “tiny particles in the air”. Aerosols are the most potent climate lever in the world, because they act near instantly.

Some aerosols warm the planet, for example, soot (‘black carbon’). Some aerosols cool the planet, for example, sulfur dioxide.

CO2 and methane have to accumulate for decades to cause the warming impact shown above. But sulfur has a short lifetime in the atmosphere, so its cooling effect is determined only by our recent emissions. It’s more like a flow rather than an accumulated stock. Any increase or decrease will quickly affect the earth’s temperature. Changing the amount of sulfur dioxide in the atmosphere is the most direct temperature control that we have over the planet.

Sulfur dioxide cools the planet a lot. In fact, it’s currently masking around 0.5 degrees of warming. Without it, we’d already be close to 2 degrees of warming.

The amount of sulfur dioxide in the atmosphere is changing for two reasons:

Reason #1: we are removing sulfur from fossil fuels

Sulfur dioxide is put into the atmosphere by burning fossil fuels.

But in 2020, the International Maritime Organisation mandated that ships must use low-sulfur fuels.

This has reduced sulfur dioxide in the atmosphere, which is un-masking existing warming. This is bad!

This single policy change by the IMO will cause 0.07 °C of temperature rise. That’s about the same as the warming caused by all of France’s emissions, in history [footnote 1]. Such is the leverage of aerosols.

Reason #2: someone might geoengineer the earth and add more sulfur.

Humans could artificially inject more sulfur into the atmosphere quite  easily. This would be risky, as we have high uncertainty about the side effects of geoengineering. But we do have certainty that it would be effective at reducing temperatures.

You don’t need an opinion on geoengineering to appreciate that it is the most potent temperature control that we have. If someone decided to deploy it, it would eclipse the importance of other climate solutions by an order of magnitude.

I think that it’s quite likely that someone deploys geoengineering. I explain why here.

Methane

Here’s the warming impact of different greenhouse gases (GHGs).

Methane doesn’t stick around as long as CO2, but the short term matters. We could hit many climate tipping points in the next 20 years.

Unfortunately, methane’s trajectory is much more worrying than CO2. Our CO2 emissions are relatively stable, even if they haven’t started falling yet. By contrast, methane is soaring, with no signs of slowing down.

Figure adapted from Spark Climate, data from NOAA Global Monitoring Lab, UNEP, and CCAC

The primary cause of this is rising fugitive emissions. Natural gas is methane. Because we use a lot of natural gas, a lot more leaks out.

Additionally, methane has some strong feedback loops.

1. As the earth gets warmer, our natural methane emissions increase. This isn’t included in climate models (even CMIP6), which means that we’re underestimating future methane emissions by quite a bit.
2. The more methane that we add, the longer that the existing methane takes to decay. If methane concentrations double, the atmospheric lifetime of all methane increases by roughly 35% (h/t Climateer).

What to do? Thankfully there are some easy methane solutions:

• Clean up fugitive emissions (methane leaks). You really can just fix up some pipes, compressors, and fracking wells, with virtually no downsides. Incredibly high climate impact per $.
• Capture methane from landfills. This ain’t hard.

Other solutions are tricky. Cows are 1/3 of methane emissions and it’s hard to feed them things that reduce their burps (although these additives do exist).

Methane removal is grossly underexplored. Methane gets naturally “removed” from the atmosphere by oxidation into CO2 (which is much less potent as a GHG). Methane removal means finding ways to accelerate this. It’s difficult, because methane is very dilute in the atmosphere - only 2 parts per million.

Solutions here are very early, and very under-resourced:

• The world is full of organisms called methanotrophs, which break methane down. Enriching soils and crops with methanotrophs would remove more methane.
• We could enhance natural methane oxidation throughout the atmosphere by adding powders like Iron Salt Aerosols.
• For more, read Spark Climate’s excellent Atmospheric Methane Removal Primer.

CO2

All solutions to reduce CO2 rely on one of three ingredients: clean energy, biomass, and carbon capture.

Almost every proposed CO2-reduction technology falls into one of these buckets.

This was the first graphic I ever made for this blog, back in 2023. Forgive the graphic design.

The main requirements of carbon capture and engineered carbon removal are a lot of energy, so our critical ingredients mostly reduce to just two: clean energy and biomass.

Therefore, we must scale:

Clean energy (grid-connected)

We are going to electrify everything, from vehicles to heating. Energy that previously flowed through pipelines will flow through the power grid. Anyone who says that electric vehicles and heat pumps won’t strain the power grid is incorrect. We must use flexible demand to match renewables, and streamline permitting for new generation.

Clean energy (off-grid)

Solar will get too cheap to connect to the power grid. A lot of future energy will be off-grid, for hydrogen production, synthetic fuel production, and for data centres.

Biomass

Engineering plants could help us scale clean fuels and carbon removal. I do not work in Biotech, but I am very excited by what’s possible. As Niko McCarty puts it:

“this is really the first generation where direct molecular observation and manipulation of living cells is possible. [...]  The cost to sequence a nucleotide of DNA fell from about $20 in 1990 to fractions of a penny today. The cost to “write”—or synthesize—a base of DNA fell by four orders of magnitude between 2000 and 2017. It’s now relatively cheap to sequence and make strands of DNA that can, in turn, be used to engineer cells.”

Conclusion

Our levers to reduce temperature are simple. We should:

• Spend more effort reducing uncertainties on aerosol effects
• Urgently deploy carrots and sticks for methane leak reduction
• Spend more effort investigating enhanced methane oxidation
• Integrate clean power into the grid - reduce permitting and use energy flexibility
• Scale off-grid clean power
• Work on solutions that increase the abundance of biomass.

Footnotes

Footnote 1 - back of the envelope on France historical emissions

(Rough calculation alert!!)

• France cumulative CO2 to date: around 40b tCO2
• Weight other GHGs at an extra 30%: 52 tCO2e total
• 52 tCO2e = roughly 6.7 ppm concentration change
• Radiative forcing including France = 5.35 * ln(420/278) = 2.207 W/m^2
• Radiative forcing w/o France = 5.35 * ln((420 - 6.7)/278) = 2.122 W/m^2
• Forcing caused by France: 0.085 W/m^2
• Assume climate sensitivity of 0.8°C per W/m^2: 0.068 °C.
• So warming caused by France’s cumulative historical emissions: ~0.07 °C.