Geoengineering by reducing the amount of solar radiation the Earth absorbs has become a hot topic in the last few years. Of all the impacts geoengineering might have on our climate, why on earth should we care about what goes on in the stratosphere, 10 kilometres above our heads? It turns out what goes on up there has a substantial impact on what goes on down here.
This is the subject of the final paper (open access!) from my PhD work with Andrew Charlton-Perez and Ellie Highwood, at the University of Reading. In it we ask what effect stratospheric aerosol geoengineering might have on the stratosphere, and how those effects might be communicated to the troposphere below.
We used some idealised simulations with a climate model to investigate, placing a layer of aerosol in the model’s stratosphere. Since we don’t know exactly how geoengineering might turn out, we had to make some simplifying assumptions about the size of the aerosol particles and the shape of the aerosol cloud. Not all of these were realistic, so it’s important to think about how our results might be affected if these assumptions changed. That’s a rule that holds true for all science, of course.
In our model simulations we compared three different potential deployments of geoengineering. One used sulphate aerosol, mimicking the effect of natural sulphate aerosols produced by volcanic eruptions. Another used titania (titanium dioxide) aerosol, which is much more reflective than sulphate and may do less damage to the ozone layer. Finally, we looked at the case where geoengineering was represented by simply dimming the Sun. In practice this could only be achieved using mirrors placed in space, but it has also been used as a representation of geoengineering with stratospheric aerosols.
We found that the aerosols intensified the stratospheric polar vortex by warming the tropical stratosphere. The polar vortex is linked to the midlatitude jet streams in the troposphere, which act as guides for weather systems. As the polar vortex gets stronger the jet streams tend to shift further poleward. This would obviously have an effect on the meteorology of a geoengineered world. The jet streams would still wobble and meander about all over the place, but on average they would be located closer to the poles, changing which regions experience the strongest storms and most rainfall.
The link between the stratospheric polar vortex and the jet streams is extremely well documented, and reproduced by models. There is, however, still quite a lot of debate over exactly how the two things are linked, and the extent to which models get it right. For example, the polar vortex intensifies in response to volcanic eruptions, just like it does in simulations of geoengineering, but climate models don’t simulate very well the shifting of the jet streams associated with it.
That said, the shifting of the jet streams under stratospheric aerosol geoengineering should be fairly robust. Stratospheric aerosols are known to intensify the polar vortex. This is because they absorb thermal radiation in the tropics (where they get energy from the warm troposphere below) more than they do at the poles (where the underlying troposphere is colder). This temperature gradient sets up a pressure gradient, intensifying the westerly winds of the polar vortex.
The jet streams will shift in response to this, although exactly how, or how much, is open to question. Those are the questions that are more important to answer.
Unfortunately, our study can’t really help with that, for two main reasons.
The first is that we used a single climate model, which means we can’t generalise our results. In order to test the robustness of our results, we would need to look at a number of different models, with different representations of the dynamics of the atmosphere. We also didn’t delve deeply into the theory behind the linkage between the polar vortex and the jets. This is because the science of stratosphere-troposphere coupling is still rather mysterious, and attempting to come up with a theory explaining it is a huge task.
The second reason we can’t use our results to make predictions is that our representation of geoengineering wasn’t particularly realistic. We placed a huge amount of aerosol into the model. In our set up we could put as much in as we wanted because the aerosol particles don’t interact with the atmospheric circulation, or each other. In model simulations where these interactions are allowed, large aerosol injections caused the aerosols to stick together, grow, and fall out of the stratosphere rather quickly. This means it might not even be possible to put such huge amounts of aerosol into the stratosphere.
Whether it would be or not would depend on the degree to which the aerosols stick together. This process would occur differently for different aerosols. For example, sulphate aerosols are liquid and coagulate quite easily. Titania is a solid ‘dust’-type aerosol, which might be more resistant to this. More research is needed on this, though. As far as I am aware no one has done any simulations of how titania might actually behave in the stratosphere.
Another important caveat to our results is that our model didn’t include the effects of the aerosol on stratospheric ozone. As well as it’s important role in blocking UV radiation, ozone affects stratospheric temperatures. Other studies have shown stratospheric aerosol geoengineering would reduce ozone at higher latitudes, cooling the polar stratosphere. This effect would further enhance the intensification of the polar vortices.
So there are a number of reasons we should take care in interpreting our results. The central message, though, is that stratospheric aerosols influence the midlatitude jets, and they do this via polar vortex changes caused by absorption of radiation by the aerosol particles. If an aerosol that didn’t absorb as much was used these effects could be reduced. This is one of the reasons titania is being investigated as a geoengineering aerosol. Titania reflects more radiation than sulphate and absorbs less, meaning one could accomplish the same surface cooling with less aerosol, and have a smaller impact on the midlatitude jets. If we found an aerosol that didn’t absorb radiation at all (not really likely) we would essentially have a very similar case to our solar dimming simulation, which shows very minimal jet shifts.
Finally, it’s important to emphasise this is all hypothetical. I see research like this as part of an effort to understand what stratospheric aerosol geoengineering is. What are the potential risks as well as the potential benefits? This is the first step in understanding geoengineering as a policy option, but it is not the last. There are plenty of potential problems with geoengineering to do with issues of justice, conflict and ultimately, the human relationship with the natural world.