Geo-engineering to solve global warming
First, a puzzle.
Atmospheric CO2 was rising all through the 20th century, but the Earth’s temperature didn’t really start rising until sometime around 1975:
From 1940 to 1970, global temperatures were pretty much flat. If anything, there was a small cooling trend.
What happened there? Was that just random noise? Some natural climate swing that masked the warming trend?
Global warming skeptics use this period in history to question the consensus on global warming. They look to a few papers published around 1970 that described global cooling, and say that scientists just can’t make up their minds which direction things are going:
It seems like most of those authors were actually just noting a short cooling trend, not forecasting that cooling would continue forever.
The science of global warming was around earlier than that. Svante Arrhenius predicted that CO2 would warm the planet, way back in 1900.
So, what caused the flat or cooling trend until 1970?
It might have been a result of the clean air act.
When people burn coal, it produces carbon dioxide and other kinds of pollution. Another major component of pollution is sulfur dioxide (SO2). That combines with water in the atmosphere to make sulfate aerosols, which have a cooling effect.
Sometime around 1970, we cleaned up some forms of pollution, creating a lot less smog. But we kept producing more and more CO2, so global temperatures increased more quickly.
SO2 pollution peaked in the early 70's:
Looking at a longer time scale, SO2 explains both the flat temperature trends from 1940 to 1970 and a big temperature drop back in 1815:
That was the year Mount Tambora erupted. It was by far the largest volcanic eruption the world has seen in the last 200 years, more than 10 times larger than any other recent volcano.
Today’s industrial sulfur pollution is concentrated in China, but the cooling effects are felt across the world:
We create a mix of different types of pollution, and each has a positive or negative effect on warming:
The main discussion about global warming is about controlling CO2. Looking at the contributions from all those gasses, we can see there are two other big ways to have an impact on climate — we could reduce methane emissions or we could emit more sulfur dioxide.
Cleaning up the rest of SO2 pollution would actually make global warming worse — if you eliminated all of the sulfur and aerosol pollution, global warming would go up another 0.7 degrees Celsius.
The most interesting possibility is that we could stop global warming by pumping more sulfur dioxide into the atmosphere.
Sulfur Dioxide Geoengineering:
Pumping SO2 into the atmosphere would reflect sunlight and cool temperatures. It could balance out the effects of carbon dioxide.
That would be cheaper and easier than stopping the consumption of all fossil fuels.
It could also be done unilaterally — to stop global warming, we need to get every country to agree to stop burning coal and gas. On the other hand, a single country could solve global warming with a sulfur geoengineering project.
We can guess how much we need, from looking at a natural experiment.
Mount Pinatubo erupted in 1991, sending a huge smoke plume into the sky:
That released 20 million tons of SO2 into the atmosphere. Sunsets around the world got a little bit redder. Direct sunlight got a little bit dimmer. Global temperatures fell by 0.5 degrees Celsius for the next 2 years.
Right now, we’re dealing with 1.3 degrees of global warming. We’ll get to at least 2 degrees of warming before we making any progress reducing fossil fuel emissions. Many people think we’ll eventually get to 3 or 4 degrees.
To combat 2 degrees of warming, we could simply pump 40 million tons of SO2 into the atmosphere every year.
The cost for geo-engineering isn’t exact, because the means we’d use aren’t clear. Different people have proposed using balloons, artillery, airplanes, or other mechanisms for putting SO2 into the high atmosphere.
But it would definitely be cheaper than getting rid of fossil fuels. One paper estimates a cost of 10–100 billion dollars per year to control global warming with SO2.
For comparison, a Stanford plan for renewable energy thinks that the world could go fully green for 62 trillion dollars. That’s an optimistic plan — it assumes battery costs will get much lower than what we have today and also uses less energy storage capacity than we’d need for the grid to be fully reliable. With present technology, the cost might be over 100 trillion dollars for the United States alone.
We could instead simply pollute our way out of the global warming problem. The price would be between 100 and 1,000 times cheaper, and it wouldn’t require any international cooperation.
The plan would have side effects:
Sulfur aerosols only work to lower temperatures for a few years. Carbon dioxide stays in the atmosphere and keeps having a greenhouse effect for thousands of years. If the output of SO2 ever drops, then global warming will come back quickly.
SO2 pollution is historically associated with acid rain — SO2 mixed with water in clouds to make sulfuric acid, which makes rain more acidic. But that’s mostly a problem with SO2 in the troposphere. Geo-engineering would aim to put the gas directly into the stratosphere, for more cooling and to reduce this effect.
Because we’d be putting the pollution into the stratosphere, though, it would deplete the ozone layer. The ozone effect would not be severe. For the overall picture, ozone was rapidly depleting in the 1980’s, when countries were polluting with chlorofluorocarbons (CFC’s):
That would have lead to massive ozone depletion, but an international treaty banned CFC’s, and the ozone layer started to recover around the year 2000. Because the gasses last a long time, it will take 100 years for the ozone layer to recover.
With SO2 pollution, we would set back the recovery by about 20 years, but the overall trend should still be positive, as CFC’s slowly settle out of the atmosphere:
Perhaps the biggest problem with sulfur geo-engineering is that countries might decide they have a free pass to give off as much CO2 as they want, now that global warming is temporarily fixed. Over time, they could pollute enough to cause much more potential global warming that’s only temporarily held in check.
Carbon dioxide pollution would also continue to make the ocean more acidic, and that might have an effect on marine life. Putting SO2 into the atmosphere would stop the warming, but do nothing to stop ocean acidification.
Who would do it? Would would benefit?
Part of the problem with global warming is that the effects are worst for poor countries, while rich countries have the most ability to help.
Today’s aerosol pollution already keeps the planet 0.7 degrees Celsius cooler than it would be otherwise. Here’s the impact that has on various countries:
Russia is harmed by the cold, all else being equal they’d prefer a warmer planet. India, Africa, and South America benefit because they’re less warm. The net effect on the United States is close to zero.
If we raised global temperatures 0.7 degrees from more carbon dioxide, the effect would be the opposite. India would suffer, Russia would benefit.
This explains some of the geopolitics of climate change — Russia is opposed to doing anything, both because they’re an oil exporting economy and a cold country. They likely fund climate misinformation in other countries. The effect on the US is close to neutral, and the US is politically divided on doing anything.
Poorer countries face bigger risks, but they’re not the biggest polluters, so they mostly go along with the decisions of however much CO2 the major economies create.
It’s not obvious why any one country would decide to unilaterally act here. Most of the poorest countries don’t have the money or technology to enact the plan. Maybe India is one of the few heavily affected countries that would be rich enough to act alone.
The US would be unlikely to act alone, since the Democrats would probably not favor a plan for worldwide pollution, and the Republicans don’t think climate change is a problem.
In the end, it would be better for the planet if we fixed the CO2 pollution problem directly.
That said, we’re not making a lot of progress on that front. The UN predicts that the world will be emitting more CO2 in 20 years than today.
At some point, the benefits to a cheap geo-engineering solution might outweigh the risks.
