There is reasonable evidence to suggest that the Earth is warming. The most probable cause appears to be increased CO2 in the atmosphere, which has been reasoned about for quite some time.
There appear to have been some volcanic eruptions in the 1200s that lead to a mini-ice age. The cause appears to be increased sulfur dioxide (S02) in the atmosphere.
Could we engineer this?
My question is: Would adding sulfur dioxide to the atmosphere have a global cooling effect?
There have been a range of studies on the issue published in Atmospheric Chemistry and Physics and other scientific journals.
The short answer is "yes." The longer answer is "yes, but...."
In Climatic impacts of stratospheric geoengineering with sulfate, black carbon and titania injection, Jones, et. al. (2016) note:
In this paper, we examine the potential climatic effects of geoengineering by sulfate, black carbon and titania injection against a baseline RCP8.5 scenario. We use the HadGEM2-CCS model to simulate scenarios in which the top-of-the-atmosphere radiative imbalance due to rising greenhouse gas concentrations is offset by sufficient aerosol injection throughout the 2020–2100 period. We find that the global-mean temperature is effectively maintained at historical levels for the entirety of the period for all three aerosol-injection scenarios, though there is a wide range of side-effects which are discussed in detail.
Those side-effects would vary with the size of the particles used and the method and location of atmospheric injection, but would include:
There would be no amelioration for other climate impacts such as ocean acidification.
It is therefore important to note that the safest possible solution to avoiding the sort of climate change instantiated by, e.g., Fig. 6a of this report is to effectively mitigate greenhouse-gas emissions.
Other issues not addressed by this physics-based paper include:
SO2 is a gas in the atmosphere, which does not cool the atmosphere. However, SO2 oxidizes to H2SO4, which gets involved with aqueous chemistry and often leads to sulfate (SO4). Sulfate is an aerosol that will reflect sunlight (like many aerosols) and block energy input to the Earth. If you increase the amount of sunlight being reflected back to space (reducing the amount of sunlight absorbed by the surface of the Earth), there is a net cooling affect.
So, yes, adding sulfur dioxide will have a net cooling affect, since a lot of it will convert to sulfate. However, increasing sulfur dioxide in the troposphere is the equivalent of polluting the air, which leads to health problems and acid rain problems. If you could inject sulfates into the stratosphere (which a very explosive volcanic eruption can do) then it won't affect the air we breathe (or cause acid rain) nearly as much. The real solution, though, to global warming is to limit carbon emissions (e.g. GHGs) so that significant polar ice and snow pack can be retained and do its job of reflecting light back to space.
http://www.sciencedirect.com/science/article/pii/S0021850296000638 http://www4.ncsu.edu/~franzen/public_html/Poland/Poznan08a/Sulfur_Cycle.pdf https://en.wikipedia.org/wiki/Stratospheric_sulfate_aerosols_(geoengineering)
One of the issues with adding sulfur dioxide to the air is that it produces acid rain, which in the past has had a detrimental effect on forests. This was a major issue during the 1960s to 1990s and it is still an issue.
One area that was badly affected, and is still dealing with the effects of acid rain is the Black Forest.
Sulfate aerosols already reduce the overall warming from raised GHG's, however the effect is more closely proportional to the rate fossil fuel burning that makes them, a process that also makes CO2; ie when that burning stops the cooling effect fades away quickly in days to weeks. But CO2's climate impact is more closely proportional to the accumulation of emissions over time; when the burning stops the warming effect continues and declines only slowly, over decades to centuries.
A hypothetical case of starting a whole lot of high sulfur coal plants all at once would expect to see an immediate cooling effect at full strength within days to weeks, with CO2 initially adding zero warming but increasing over time; at some point the warming will be equal to the aerosol cooling, taking temperatures back to where they started and after that, exceeding it. Stopping the coal burning would cause a rapid spike in temperatures, yet continuing it for the sake of the aerosols continues to add yet more warming. There is no fixing the warming by adding more coal burning, but it can give the appearance of reducing it for a time. It isn't quite a Zeno's race paradox, but has similarities; you don't get to a finish line, ever. And it doesn't address the effects of raised CO2 on ocean pH.
Fossil fuel aerosols remain predominately in the lower atmosphere and don't stay long.
Volcanic aerosols reach the stratosphere and the aerosols that do stay there longer - up to a few years - but would need to keep being added to compensate for the CO2 accumulation; it would take dedicated efforts that, unlike coal burning, makes no other income, that must be funded by levies or taxes that will be, as they always are, resisted and evaded.
In parallel with phasing out fossil fuel emissions doing so can reduce regional and global temperatures, by reducing sunlight intensity. Which can have other (unintended) consequences, including to agricultural yields, which may not be justly distributed.
As an alternative to phasing out fossil fuel burning it must be done at rates that not only exceed the current warming from accumulated GHG's, but must exceed that from further GHG accumulations.
Stratospheric sulfate aerosol dispersal remains a hypothetical possibility but it lacks support, including from a large bloc that sees the growth of low emissions alternatives as the most effective and cost effective response to global warming, that opposes actions that provide justifications for continuing to use fossil fuels.
It is unlikely to gain sufficient support and funding to make any difference to the climate problem within the time frames we are facing.
