Reminder: Ending fossil fuel use will cause a short-term increase in temperature as the air clears.

This came up in the comments on my post about the Arctic heat wave, and I think it merits further discussion.

Even ignoring the multiple feedback loops that are likely to see the planet’s warming continue long into the future, if we do everything right in terms of addressing climate change there will be severe warming in our lifetimes. This is not a new concern. Aerosol pollution played a role in the cooling period of the 1970s that climate deniers love to talk about, and after the environmental movement of that era succeeded in reducing air pollution, there was an increase in temperature that followed.

This is one of the reasons I consistently advocate for climate action that includes taking dramatic steps to prepare for a much warmer and more unpredictable world. If we had taken serious action to reduce energy consumption, develop renewable energy, and expand and improve nuclear energy in the 1970s and 1980s, we would have different options available to us today, but we missed that window. Returning to the “pre-industrial norm”, within our lifetimes, no longer seems to be an option. As I discussed in my geoengineering post, we have to learn how to responsibly use the collective power we’ve developed as a species if we want a livable world for future generations.

StevoR linked to this 2005 episode of BBC’s Horizon on global dimming and the overall effects of aerosol pollution on the climate:

The effect was first spotted by Gerry Stanhill, an English scientist working in Israel. Comparing Israeli sunlight records from the 1950s with current ones, Stanhill was astonished to find a large fall in solar radiation. “There was a staggering 22% drop in the sunlight, and that really amazed me,” he says.

Intrigued, he searched out records from all around the world, and found the same story almost everywhere he looked, with sunlight falling by 10% over the USA, nearly 30% in parts of the former Soviet Union, and even by 16% in parts of the British Isles. Although the effect varied greatly from place to place, overall the decline amounted to 1-2% globally per decade between the 1950s and the 1990s.

Gerry called the phenomenon global dimming, but his research, published in 2001, met with a sceptical response from other scientists. It was only recently, when his conclusions were confirmed by Australian scientists using a completely different method to estimate solar radiation, that climate scientists at last woke up to the reality of global dimming.

Dimming appears to be caused by air pollution. Burning coal, oil and wood, whether in cars, power stations or cooking fires, produces not only invisible carbon dioxide (the principal greenhouse gas responsible for global warming) but also tiny airborne particles of soot, ash, sulphur compounds and other pollutants.

This visible air pollution reflects sunlight back into space, preventing it reaching the surface. But the pollution also changes the optical properties of clouds. Because the particles seed the formation of water droplets, polluted clouds contain a larger number of droplets than unpolluted clouds. Recent research shows that this makes them more reflective than they would otherwise be, again reflecting the Sun’s rays back into space.

Reducing CO2 emissions means reducing the production of these other forms of pollution. The global shutdown triggered by the COVID-19 pandemic showed us how dramatically a decrease in fossil fuel use can clear the air of visible pollution, and this will doubtless provide climate scientists with a wealth of data on what we can expect from the kind of rapid, permanent drop in fossil fuel use. Research published in 2019 indicated that climate scientists have been under-estimating the cooling effect of aerosol pollution:

To what extent do aerosols cool down our environment? To date, all estimates were unreliable because it was impossible to separate the effects of rising winds which create the clouds, from the effects of aerosols which determine their composition. Until now.

Rosenfeld and his colleague Yannian Zhu from the Meteorological Institute of Shaanxi Province in China developed a new method that uses satellite images to separately calculate the effect of vertical winds and aerosol cloud droplet numbers. They applied this methodology to low-lying cloud cover above the world’s oceans between the Equator and 40S. With this new method, Rosenfeld and his colleagues were able to more accurately calculate aerosols’ cooling effects on the Earth’s energy budget. And, they discovered that aerosols’ cooling effect is nearly twice higher than previously thought.

However, if this is true then how come the earth is getting warmer, not cooler? For all of the global attention on climate warming, aerosol pollution rates from vehicles, agriculture and power plants is still very high. For Rosenfeld, this discrepancy might point to an ever deeper and more troubling reality. “If the aerosols indeed cause a greater cooling effect than previously estimated, then the warming effect of the greenhouse gases has also been larger than we thought, enabling greenhouse gas emissions to overcome the cooling effect of aerosols and points to a greater amount of global warming than we previously thought,” he shared.

The fact that our planet is getting warmer even though aerosols are cooling it down at higher rates than previously thought brings us to a Catch-22 situation: Global efforts to improve air quality by developing cleaner fuels and burning less coal could end up harming our planet by reducing the number of aerosols in the atmosphere, and by doing so, diminishing aerosols’ cooling ability to offset global warming.

According to Rosenfeld, another hypothesis to explain why Earth is getting warmer even though aerosols have been cooling it down at an even a greater rate is a possible warming effect of aerosols when they lodge in deep clouds, meaning those 10 kilometers or more above the Earth. Israel’s Space Agency and France’s National Centre for Space Studies (CNES) have teamed up to develop new satellites that will be able to investigate this deep cloud phenomenon, with Professor Rosenfeld as its principal investigator.

Either way, the conclusion is the same. Our current global climate predictions do not correctly take into account the significant effects of aerosols on clouds on Earth’s overall energy balance. Further, Rosenfeld’s recalculations mean fellow scientists will have to rethink their global warming predictions — which currently predict a 1.5 to 4.5-degree Celsius temperature increase by the end of the 21st century — to provide us a more accurate diagnosis — and prognosis — of the Earth’s climate.

An erratum published a couple months later indicated that the degree of mis-calculation may not be as severe as Rosenfeld et al. initially stated, but their overall conclusion remains intact. The overall message remains the same as it has been – the problem of global warming has not been addressed in a manner that allows us to avoid serious consequences this century. That means an unprecedented refugee crisis, crop failures, heat waves, wildfires, and increasingly severe coastal flooding. These are all crises we are capable of dealing with. We have the technology and the understanding to help refugees, relocate or re-design coastal cities, and mage huge changes to global food production.

What we don’t have, currently, is a political and economic system that allows us to respond to the demands of our time. We cannot afford to have global production, distribution, and human movement to be dictated by and for the benefit of a tiny fraction of humanity. The drive for endless growth, and endless capital accumulation, even if we did have a more progressive system of taxation and wealth re-distribution, creates artificial scarcity, and prevents the development of a society that can be sustained in the long term.

This is not just an emergency created by our use of energy, it’s also rooted in the capitalist system that currently governs nearly all human activity on this planet. We need radical democracy in politics and in economies if we’re to have any hope of addressing our environmental crisis, and of saving humanity. We need it soon.

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  1. says

    Thanks for this. I was vaguely aware that aerosols (I would have said “particulates”, but I guess that’s wrong?) had a cooling effect and that ending fossil fuel use would stop adding CO2, but would also stop adding aerosols. I understood that the aerosols last in the atmosphere for a much shorter period of time than CO2 does, and that therefore, even if we quit using fossil fuels all at once, after a few years warming would accelerate again because the particulates would be gone but the CO2 would still be present, doing its warming work, for several centuries yet.

    I was **not** aware that atmospheric scientists had underestimated that aerosol cooling effect and that they weren’t able to accurately model cooling-v-warming as recently as 2019 (and probably right up to this moment, since if the error was only quantified in 2019, and it doesn’t look like it even has been yet with the “deep cloud” bit, it will take a while to get this factor corrected in climate models and then to run them and see the net effect).

    So, if I may ask, what do you think the lag time would be between a hypothetical end of aerosol emissions and the resultant end of the cooling effect?

    And am I right in thinking that the CO2 surplus remains in the atmosphere for several centuries? Or do I have the wrong order of magnitude?

    Also, I assume that even if we have good reason to think that it would stay in the atmosphere for several centuries, actually reducing atmospheric CO2 to 350ppm, the actual reduction relies on biologic actions which might still be frustrated by ongoing human activities and/or climate change itself. For instance, regrowing forests isn’t going to be easy in a period of unpredictable climate where we can’t know which trees to plant where, and when natural forest growth gets interrupted by droughts, etc., or has its climate changing such that after 40 or 50 years conditions no longer favor the current vegetation and the now-optimal vegetation has to fight through the established trees to take over their role (only to perhaps be replaced in turn after another 40-50 years).

    So, I guess that’s all to say that I’ve heard that CO2 stays in the atmosphere for ~500 years-ish, but is it right to be skeptical that we can predict future cycles of natural carbon sequestration from a past record where you didn’t have whaling preventing algal nutrients from rising to the surface when cetaceans feed at depth but poop in the sunlit surface waters where photosynthesis can occur (and water pressure is reduced enough to excrete waste effectively) and clear cutting hinders CO2 sequestration both in woody plants and trees but also in the fungal and other symbiotic organisms that live in the shaded soil as modified by tree roots (including for increased moisture content), but can’t survive in dry, sunbeaten soils?

    Because I’ve got to say that I certainly defer to scientists who study this stuff when they say the CO2 cycle is about 500 years (if I remember that correctly), but I don’t know what relevant information scientists don’t know yet, so I’m not even sure if my concerns are valid. (I mean, my general level of concern is valid, but what if I should be worried less about than forests and algae and more about some other problem/s that I’m not thinking about?)

  2. says

    Seems to me that you’ve got it right. This comment will be a bit rambling because I’m feeling a bit fried, but hopefully there will will be some useful material in it.

    I see “aerosols” and “particulates” used interchangeably a lot in this discussion. I’m not an expert on this, but my understanding is that aerosols are the subset of particulates that are suspended in any gaseous situation. Particulates continue being particulates if they land on a surface, like snow, or enter water, or organisms, but when it comes to this climate effect, it’s specifically the aerosolized particulates that matter.

    My impression is that there has been some debate/disagreement about the effect of aerosols of different forms at different levels in the atmosphere. The question of whether clouds have a net cooling or warming effect is one that Richard Lindzen used for a very long time to spread doubt about climate projections, by claiming that higher temperatures would mean more cloud cover, and that in turn would reflect more heat, canceling out the warming. As I understand it, his predictions have consistently failed to approach accuracy.

    From what I can tell, clouds are generally thought to be neutral – reflecting sunlight during the day, and trapping heat at night.

    I was not aware that there was any reason to think that the cloud-seeding effect of particulate pollution might be expected to have a net cooling effect, and I think there’s a degree to which that’s still unclear. I also don’t know if there’s any evidence that particulate-seeded cloud formation happens more during the day, or whether they’re merely claiming that it “brightens” the clouds, making them more reflective overall.

    When it comes to CO2, I think you’re overshooting by a bit. There’s a fair amount of variation, but CO2 generally stays in the atmosphere for a up to 200 years. The uncertainty comes in with the degradation of natural CO2 sinks. The three big ones are ecosystem destruction (on land), the decline of phytoplankton in water, and the ocean acidification process.

    The oceans are the biggest sink, for all people like to focus on things like the Amazon, and there has been a decrease in phytoplankton that has been at least loosely tied to the rise in water temperature. It’s not certain that that’s the only cause, but it does seem to be a factor. For acidification, as more CO2 diffuses from the atmosphere into the oceans, the relative concentration in the oceans rises, lowering pH, and also lowering the rate at which the water can absorb more CO2.

    Those three factors are the fastest ways for CO2 to leave the atmosphere, and so degrading them increases its lifetime, as the other major natural carbon sink is rock weathering, and that takes millennia. I think that messes with the average a bit.

    One of the feedback loops we get to worry about is a decrease in the speed at which CO2 is pulled out of the atmosphere as the heat messes with photosynthesizing organisms. Overall your grasp of the picture seems solid to me. Again, not an expert here 😛

    In terms of the aerosol effect, I think it’s pretty quick. I’m expecting that we’re going to get some useful data out of just the short decrease we’ve seen from the COVID-19 lockdown, and maybe even a measurable bump in temperature.

    If we stopped all major aerosol production all at once, it would take about two years for the cooling effect to totally fade, at least based on what we’ve seen from major volcanic eruptions. Pinatubo’s eruption in 1991 is generally the go-to example. It was a major event in the climate science field, because it demonstrated the “skillfulness” of their models to deal with an event like that.

    Given that I don’t think any end to fossil fuels will happen faster than a period of a couple decades – and even that seems overly optimistic to me (I REALLY want to be wrong, but I’m afraid the political/power dynamics are not in our favor), so what I would expect is an increase in the rate of warming that lags major decreases in emissions by a year or two, but that accelerates as the constant production of particulate pollution declines.

    Hell, it’ll probably be used by deniers as “proof” that it wasn’t about the CO2 anyway, so that’ll be fun.

    I think worrying about forests and algae are valid. For the forests, it’s less about the carbon uptake, and more about the carbon released. Indonesia, for example, has consistently been among the top emitters of CO2 for a while now, and it’s largely been because of slash and burn agriculture aimed at producing palm oil. That has not only been releasing carbon trapped in the trees into the atmosphere, but there’s also a lot of peat in the region, and THAT has been burning. There are a lot of stores of carbon aside from fossil fuels that are much easier to release even without significant human effort. Removing the forests uncovers some of those, and releases others directly.

    Your concern about regrowing forests under these circumstances is why I think that plant-based carbon capture efforts should probably focus more on things like fast-growing grasses or kudzu. Rather than growing big trees and hoping they stay alive, we could grow, harvest, dehydrate, and store plants that don’t rely on multiple years to get the job done. More labor, but also better results over time, assuming we can do it without burning more fossil fuels in the process.

    I don’t expect the warming to stop any time in the next couple centuries, hence my interest in reshaping society for a radically different planet, rather than focusing on reversing the process and keeping things largely as they are other than changing our power sources.

    That might have worked if we’d gotten serious about it in the 1980s, but we’ve got a different reality to deal with now.

  3. StevoR says

    Thanks Abe Drayton. Glad that article was interesting and useful here and thanks for expanding on it.

    (BTW. It is StevoR with just the one ‘e’ in it but that’s okay.I know I certainly can’t talk when it comes to having lots of typos.)

  4. says

    I’m worried that storing CO2 in plant matter is not as efficient as it was back in the Carboniferous when most coal was laid down. A lot of burrowing animals have evolved since then to break up masses of plant matter. I suspect we really need to stop burning coal.

  5. says

    I’m entirely in agreement that we need to stop burning coal as soon as possible.

    Even if the rate of carbon uptake from plants is as high as it was in the past, it took eons for all that plant matter to turn into coal, and we’re burning it far, far faster than that.

    That said, any serious plant-based carbon capture project is going to have to involve harvesting and storing the plant matter artificially in one way or another.

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