Sulphur dioxide decline adds to global heating

 From Leon Simons

As more and more clean air regulations came into effect from the 1960s onward, CO₂ and sulphur dioxide (SO₂) emissions gradually decoupled. SO₂ emissions reached a global peak around 1980: academic.oup.com/bioscience/a...

There was an almost perfect correlation between CO₂ and SO₂ emissions. But this has turned into a strong anti-correlation:

 

This was made possible by strict clean air regulations, enforcement and compliance. Notably at coal plants, where enormous 'flue-gas desulphurisation systems were installed. The increase in sulphur emissions temporarily stopped global warming for about 40 years:

But it's crucial to take into account where the sulphur is emitted. While the oceans "only" cover about 71% of Earth's surface, about 90% of global warming is ocean warming, while only 1-2% heats the atmosphere:

 

Removing the reflective particles and clouds above the oceans (Earth's main heat sink!) has much stronger climate impacts than most realise. Which is why the very recent desulphurisation of global shipping is crucial to take into account. The IMO [International Maritime Organisation]sulphur regulations made this possible. And there are more regulations in the pipeline! 

Leon Simons notes that the rise in SO2 emissions before 1900 seems suss, and discusses
it in his Bluesky thread (linked above)

Back in 1990, Hansen & Lacis described an "extreme" scenario of half the GHG [Greenhouse gas] forcing being compensated by a negative aerosol forcing. Looking at the same agents, IPCC AR6 WG1 showed it as 66% for the 1850-1989 period. It was likely even more!

The moral of all this is obvious:

1.  With industrialisation taking off post-war, rising SO2 emissions helped offset rising CO2 emissions, and global temperatures actually fell.
2. After 1980, SO2 emissions started to fall, and temperatures started to rise.
3. Since 2020, de-sulphurisation of global shipping has increased, reducing reflective particles and clouds above the oceans, leading to accelerated global temperature rise.

I draw two other conclusions:

1. It is not essential to emit SO2 at great altitudes, as with, say, volcanic emissions or (shudder) geo-engineering for it to cool the planet.  A couple of hundred metres above sea level is enough.
2. The science behind global heating is confirmed.  From Hansen onwards, climate models postulated that SO2 emissions would reduce temperatures, while CO2 emissions would increase them.  This has now been clearly demonstrated by the surge in temperatures as SO2 emissions have been reduced.  The phenomenon of reduced cloud cover (leading to faster heating) is also explained.  The model is consistent.

What is less obvious is what we do about it.  Should we allow global shipping to go back to emitting sulphur dioxide?  Should we deliberately try some sort of geo-engineering fix?  Do we want to go back to acid rain?

No doubt big oil will argue for increased emissions of SO2 with the same vigour it tried to shut down the argument for reducing emissions of CO2, even though the effects on temperatures are derived from the same physics.  Look out for articles funded by stink tanks arguing for SO2 emissions paid for by the taxpayer.

"Relatively low cost" plan to cool the poles

From New Atlas

New research suggests that cooling the poles by 2 °C (3.6 °F), and re-freezing the Arctic and Antarctic, is "feasible at relatively low cost with conventional technologies," using Stratospheric Aerosol Injection (SAI) of heat-reflective particles focused on the poles. The side effects could be nasty, and the politics near-impossible, but the plan offers a way to slow, or reverse the catastrophic sea level rise projected as polar ice collapses.

SAI is an enormously controversial idea inspired by the cooling effects that tend to follow large volcanic eruptions. These natural events eject vast amounts of dust, ash, and often sulfur dioxide into the air. The first two create a shade effect that causes a short-lived cooling effect for a couple of hours, but sulfur dioxide tends to rise high into the stratosphere, where it combines with water molecules to create sulfuric acid particles, and remains for up to three years, reflecting solar radiation away and causing a long-lasting surface cooling effect.

So the idea behind SAI is to load up high-altitude aircraft with sulfur dioxide, and fly around spraying it into the atmosphere at high altitudes, mimicking the cooling effect of a volcano. So far, so good. Mind you, the way that sulfuric acid eventually leaves the atmosphere is by combining into larger and larger droplets that eventually become heavy enough to fall down to earth as acid rain, which is, as you'd imagine, not great for plant life, fish or animals. And all sulfur oxides are nasty to breathe in, harming the lungs and causing asthma and bronchitis if inhaled regularly.

To date, most SAI research and modeling has focused on spreading these aerosol deployments all over the globe. But there's a growing number of scientists starting to look into just doing it at the North and South Poles. The Arctic and Antarctic are feeling the effects of climate change far worse than the rest of the world at this stage; they're warming several times faster than the global average, causing colossal ice structures to collapse and melt. Every climate model factors in the resulting rise in sea levels, which will have catastrophic effects all over the world.

To state the obvious, no scientist wants to fill the air with sulfur, drench the last remaining polar bears and penguins in acid rain, or give carbon emitters any excuses not to clean up their act. But faced with our current trajectory, on which summer sea ice in the Arctic will more or less disappear by 2050 or earlier, humanity finds itself between a rock and a hard place. All options need to be on the table, evaluated, and to some extent ready to go early enough to make a difference.

So research into SAI is progressing quickly, and concentrating it at the poles – an approach referred to as subpolar deployment – may deliver better returns for significantly less money and acid rain than a global model. Prior research has indicated that spring and early summer is likely the most effective season to do it, and that only doing this at one pole could have asymmetrical effects on global weather, so it's probably prudent to target both, with a fleet of planes that travels with the seasons.

A new study from a fairly broad range of contributors digs into what a bipolar SAI program targeted at "refreezing" the Arctic and Antarctic might look like, what it'll cost, and where the equipment and technology gaps might be.

The study proposes a nominal target of cooling the North and South Poles by 2 °C (3.6 °F), noting that Arctic temperatures have already risen by more than 3 °C (5.4 °F) over the last 50 years. It proposes that the aerosol injections be made at the 60th parallels, roughly the latitudes of Oslo, Helsinki, Homer, Alaska and Magadan, Siberia in the Northern Hemisphere, and level with the southern tip of Patagonia in the Southern Hemisphere. At these latitudes, it's possible to get the job done cheaper, since the troposphere sits at a lower altitude and your aircraft don't have to fly so high. This study chooses an altitude of 13 km (42,600 ft). The particles released would drift slowly toward the poles, concentrating their effects.

To achieve a 2 °C result, the plan would inject 6.7 teragrams (6.7 billion kg/14.8 billion lb) of sulfur dioxide per year into each pole, calling for an eye-watering total of 13.4 teragrams (29.5 billion lb) of material annually.

The study goes on to look at logistics, finding that existing aircraft can't carry enough payload to a sufficient height to get the job done. The closest we've currently got are military air-to-air refueling aircraft, but these can't reach the target altitudes without significantly reducing their payloads. The McDonnell Douglas KC-10 Extender, for example, could get up into the spray zone carrying some 128,801 lb (58,400 kg) of payload, but that's just 22% of the payload it's designed to carry, so you'd be carrying a lot of excess weight on every flight.

The study proposes instead a purpose-built stratosprayer called the SAIL-43K, a downgraded version of an aircraft previously specified to fly higher SAI missions closer to the equator. This machine would carry 167,971 lb (76,190 kg) of payload each mission, but its takeoff weight would be some 77,000 lb (35,000 kg) lighter than the KC-10.

To hit the cooling target, this project would need 125 purpose-built SAIL-43Ks, flying a total of 1,458 missions per day during the four-month injection period at each pole. These planes would take off, climb for 30 minutes, vent their entire load of sulfur dioxide within two minutes, then come back down over the following 30 minutes, and spend the next hour loading up again and refueling for the next mission.

In the Northern Hemisphere, there are plenty of airfields suitable for these kinds of operations; virtually the whole 60th parallel falls on land. In the south, things get a bit more fraught as there are really only a few airfields in Southern Patagonia with appropriate runways. These sit at latitudes closer to 54°, but the team calculates they'll get better results just venting the sulfur dioxide at that latitude than flying some 490 nautical miles south to hit 60°.

These airports would need to be upgraded to handle a total of 110 operations per hour, or a little more than the world's busiest current airport – this will be an enormous undertaking in the Southern Hemisphere, since there are so few airfields to start with in Patagonia. This huge infrastructure job would likely take about as long as developing and manufacturing 125 aircraft – about 15 years after making a decision to go ahead with the plan. This, in itself, is hardly a quick or complicated part of the process, and would require some degree of global agreement on a plan that would disproportionately affect people living in the latitudes in question.

In terms of money, the "relatively low cost" of this project would be around US$11 billion a year (2022 dollars), says the team. This might sound like a lot, but it's about a third of the price of a global SAI effort with the same cooling target, and the researchers note that "relative to other possible strategies by which to combat either the impacts or causes of climate change, SAI remains extraordinarily inexpensive."

While calibrated to drop polar temperatures by 2 °C and to begin re-freezing sea ice at the poles, this project will have a number of unwanted side-effects. The researchers note that the sulfur compounds added to the stratosphere may impact ozone concentrations through a number of different effects, and may thus slow or reverse the recovery of the Antarctic ozone hole. It notes that the effects of teragrams of sulfur dioxide and the associated acid rain deposits are risky both to humans and to the wider ecosystem, requiring lots more research. And it expects some stratospheric heating as well.

Furthermore, the planes themselves can only operate by burning jet fuel, using today's technologies. This annoying fact, plus the emissions involved in building out all the required infrastructure on the ground, plus the emissions involved in preparing sulfur dioxide, mean that a polar SAI program would have a pretty damn hefty carbon footprint of its own, although this would only represent a "marginal" increase to overall aviation sector emissions.

Still, the researchers conclude that "while it has yet to be established that the physical or societal impacts of any SAI program would prove to be net positive, it seems clear that a program focused on substantially cooling the world's polar and subpolar regions would be logistically feasible. This could arrest and likely reverse the melting of sea ice, land ice, and permafrost in the most vulnerable regions of the Earth's cryosphere. This in turn would substantially slow sea level rise globally."

So, short answer: yes, we can re-freeze the poles, while minimizing the risk to the bulk of humanity and agriculture. But we'd need the entire world to agree that sea level rise is a worse outcome than the effects of a massive SAI program, especially for the estimated 1% of the population that lives in the areas where the effects will be concentrated.

"There is widespread and sensible trepidation about deploying aerosols to cool the planet," says Wake Smith, lead author of the new study, in a press release. "But if the risk/benefit equation were to pay off anywhere, it would be at the poles. Game changing though this could be in a rapidly warming world, stratospheric aerosol injections merely treat a symptom of climate change but not the underlying disease. It's aspirin, not penicillin. It's not a substitute for decarbonization."

The study is open access in the journal Environmental Research Communications.

Source: Institute of Physics via Phys.org

Mankind has dithered and delayed so long in acting on climate change that there are now no safe methods to undo the damage.   The fact that we even have to think about it is an indictment of those denialists who, obedient to their fossil fuel paymasters, have so poisoned the well that we've delayed decades before acting.  And remember, emissions are still rising.  Temperatures are still rising.  It's not getting better.

Ocean geo-engineering

Naturally, the ppl who brought you fossil fuels and the climate emergency, are in favour of geo-engineering, which will take the carbon dioxide they've put into the atmosphere out again.  Presumably, taxpayers will pay for geo-engineering while oil and coal companies pocket the profits made from fossil fuels.  The trouble is, geo-engineering risks being some airy-fairy pious notion which is really about fossil fuel companies trying to postpone zero- carbon.  The unproven geo-engineering techniques put forward may persuade some that the need to slash emissions is reduced, thus in fact slowing our transition to zero carbon.

In reality, though, we will surely need it, because the world is very unlikely to cut emissions enough to avoid a 1.7 degree rise in global temperatures, and we still run a very serious risk of 2 degrees.   Negative emissions will help undo any 'overshoot' which happens.  The question of who will pay for it remains.   You can depend on it: oil and coal companies and those who made money from pumping CO2 into the atmosphere will not be putting up their hands.

From The Guardian:

Tom Green has a plan to tackle climate change. The British biologist and director of the charity Project Vesta wants to turn a trillion tonnes of CO2 into rock, and sink it to the bottom of the sea.

Green admits the idea is “audacious”. It would involve locking away atmospheric carbon by dropping pea-coloured sand into the ocean. The sand is made of ground olivine – an abundant volcanic rock, known to jewellers as peridot – and, if Green’s calculations are correct, depositing it offshore on 2% of the world’s coastlines would capture 100% of total global annual carbon emissions.

The plan relies on a natural process called weathering. “Weathering has been working on the planet for billions of years,” says Green, a graduate of Harvard Business School who runs Project Vesta from San Francisco. “When rain falls on volcanic rocks, they dissolve a little in the water, causing a chemical reaction that uses carbon dioxide from the atmosphere. The carbon ends up in the ocean, where it’s used by marine-calcifying organisms like corals and shell-making animals, whose skeletons and shells sink to the bottom of the ocean as sediment and eventually become limestone.”

Olivine weathers easily, and allowing ocean currents to churn it up, says Green, “will make it dissolve much more quickly, to happen on a human-relevant timescale”. It is not a rare mineral: there are beaches in the Galápagos Islands and in Hawaii that are green with olivine-rich sand.

The idea of using the sea to absorb excess carbon is not far-fetched, says Green. Ocean water can hold 150 times more CO2 than air, per unit of volume. “The ocean has already taken up about 30% of the excess carbon dioxide that we’ve emitted as a society,” he says. He and his colleagues are gearing up to test their process in two similar Caribbean coves, one acting as an untouched “control” in the experiment.

There remain many unknowns. Would such an intervention work? Who gets to decide if it should go ahead? Could there be side-effects? It is complex chemistry, and the natural process of weathering would be accelerated to an unnatural pace. Our understanding of the workings of the ocean is a mere drop in the proverbial. But with our race to mend the planet having taken on Sisyphean overtones, there is still hope that the vast, churning seas can be our lifeline.

Increasing carbon capture naturally on land – by planting trees, for example – will not remove enough CO2 to halt global heating. Peter Wadhams, head of the Polar Ocean Physics Group at Cambridge University and author of A Farewell to Ice, says: “If you want to get rid of the industrial emissions from Europe, you’d have to turn Europe into one big primeval forest. It works, but it’s not good enough alone.”

There are many ingenious ideas being discussed. Coastlines could be rewilded with underwater forests of kelp or seagrass, surface water cooled by generating air bubbles to whoosh cold water up from the deep, and marine clouds sprayed with seawater to reflect more heat from the sun.

As the UK prepares to host the UN Climate Change Conference (Cop26) in November, dozens of these projects are being trialled. Most rely on the ocean’s many natural balance-restoring processes: enhancing them to help slow cooling, to lock away carbon, to protect Arctic ice or even to reduce the threat of hurricanes.

Gaurav Sant, director of the UCLA Institute for Carbon Management, [talks about]  another concept, which he is helping to develop just a few hundred miles down the coast from Green, where UCLA engineers have developed a machine that mimics how seashells form. Called a flow reactor, the machine sucks seawater in, and an electrical charge makes it alkaline, which triggers the CO2 to react with the seawater’s magnesium and calcium, producing limestone and magnesite (like forming shells). The water then flows out and, depleted of its captured CO2, is ready to take up more. A byproduct of this process – hydrogen – can be extracted for fuel.

It’s a similar concept to weathering olivine in the ocean, and Sant’s plan is for initial small studies before a gradual scaling up. The team aims to remove between 10 and 20 gigatonnes of CO2 from the atmosphere, starting in 2050.

Sant says it will be a huge challenge to build a system large enough – and then to build thousands more. “Anyone saying ‘we’re going to do this in five years’, is greatly underestimating the challenge,” he says. “We’re talking about an enormous enterprise, the size and scale of which humanity has not seen before.”

Volcanic olivine, which Project Vesta is trialling as a way to capture carbon absorbed in oceans.

Geo-engineering

I am optimistic that the falling costs of renewables and EVs and the rapid improvement in batteries will lead to significant declines in emissions over the next 20 or 25 years.  And if global temperatures continue to rise by 0.2 degrees C per decade (and don't accelerate), that will take us to the 1.5 degrees lower limit set in Paris.  However, even if emissions stop completely in 2040 (and they won't, even if they will be a lot lower than now) temperatures will go on rising after 2040 for another 30 or 40 years, by another 0.6 degrees.   This is because of the thermal capacity of the world's oceans. 

Is there anything we can do, apart from the obvious (roll out renewables even faster), to prevent that?

Mt Pinatubo eruption, 1991

The most "popular" potential solution is to mimic the effect of volcanoes.  Take the eruption of Mt Pinatubo in 1991:

The 1991 eruption of Pinatubo produced about 5 cubic kilometers of dacitic magma and may be the second largest volcanic eruption of the century. Eruption columns reached 40 kilometers in altitude and emplaced a giant umbrella cloud in the middle to lower stratosphere that injected about 17 megatons of SO2, slightly more than twice the amount yielded by the 1982 eruption of El Chichón, Mexico. The SO2 formed sulfate aerosols that produced the largest perturbation to the stratospheric aerosol layer since the eruption of Krakatau in 1883. The aerosol cloud spread rapidly around the Earth in about 3 weeks and attained global coverage by about 1 year after the eruption. Peak local midvisible optical depths of up to 0.4 were measured in late 1992, and globally averaged values were about 0.1 to 0.15 for 2 years. The large aerosol cloud caused dramatic decreases in the amount of net radiation reaching the Earth's surface, producing a climate forcing that was two times stronger than the aerosols of El Chichón. Effects on climate were an observed surface cooling in the Northern Hemisphere of up to 0.5 to 0.6°C, equivalent to a hemispheric-wide reduction in net radiation of 4 watts per square meter and a cooling of perhaps as large as -0.4°C over large parts of the Earth in 1992-93. Climate models appear to have predicted the cooling with a reasonable degree of accuracy. The Pinatubo climate forcing was stronger than the opposite, warming effects of either the El Niño event or anthropogenic greenhouse gases in the period 1991-93. As a result of the presence of the aerosol particles, midlatitude ozone concentrations reached their lowest levels on record during 1992-93, the Southern Hemisphere "ozone hole" increased in 1992 to an unprecedented size, and ozone depletion rates were observed to be faster than ever before recorded. The atmospheric impact of the Pinatubo eruption has been profound, and it has sparked a lively interest in the role that volcanic aerosols play in climate change. This event has shown that a powerful eruption providing a 15 to 20 megaton release of SO2 into the stratosphere can produce sufficient aerosols to offset the present global warming trends and severely impact the ozone budget.

If we could (somehow) pump sulphur dioxide into the stratosphere every year, mimicking a volcanic eruption, we would be able to reduce global temperatures by 0.5 degrees--at the cost of expanding the hole in the ozone layer (not good).  Who would pay for this?  The same people who now reject the switch to renewables "because it's too costly"?  And the solution is temporary: within a couple of years the SO2 aerosols will have fallen to earth as acid rain (also not good). 

Another kind of geo-engineering (though it's not often thought of that way) is to turn carbon dioxide to rock.  This is a permanent solution but will take massive effort and plenty of cash to make it happen.  The pilot plant in Iceland turned 220 tonnes of CO2 into rock.  But this equals the annual emissions of just 12 people in the US or Australia.  We would need 27 million similarly sized plants to turn the emissions of the USA to rock.

The best preventive to runaway global warming remains to replace fossil fuels in power generation and transport, to find different ways of making cement and steel, and to stop clearing and burning forests.  As fast as possible.

If Mt Agung erupts

Big volcanic eruptions temporarily reduce global temperatures because they release sulphur high into the atmosphere, which combines with water to produce sulphuric acid aerosols which reflect incoming sunlight, thus cooling the earth.  If the eruption is large enough, the cooling effect can last a couple of years. There's an excellent article about it on Carbon Brief.

The last time Mt Agung erupted, in 1963, it reduced global temperatures by 0.2 C, as you can see in the chart below.   The blue line in the chart suggests the possible impact on the underlying upward trend if (when) Mt Agung erupts.  No doubt when this happens, denialists will argue that global temperatures will have stopped rising and we can stop worrying about carbon emissions.  Which will be nonsense.

Source: Zeke Hausfather at Carbon Brief

But it does raise the question:  could we simulate volcanic eruptions by artificially releasing sulphur dioxide at very high altitudes (18 kms plus) which will help offset the impact of global warming? This is called geo-engineering. 

There are questions about geo-engineering: cost (who will pay? will it be cheaper than just switching to a green economy? At what scale would we need to do it? ); environmental impact (remember acid rain? is it safe to have a continuous layer of sulphuric acid in the atmosphere?); and politics (would it stop attempts to reduce carbon emissions?)

The Economist has done a short video on it, which you might find interesting.