Can stratospheric aerosols directly affect global precipitation?

What is the effect of stratospheric aerosol geoengineering on global precipitation? If we were to inject sulphate aerosol into the stratosphere it would reflect some sunlight and cool the Earth, but the atmosphere’s CO₂ levels would remain high. This is important, because CO₂ actually has an effect on precipitation even when it doesn’t affect surface temperature. In a recent paper with a summer student, I’ve shown the aerosols can contribute a similar effect.

Three climate models (CanESM2, HadGEM2-ES, MPI-ESM-LR) did simulations of the future with and without geoengineering. The simulations with stratospheric aerosols (G3 and G4) show greater temperature-independent precipitation reductions than the simulations without them (RCP4.5 and G3S).

Precipitation as energy flow

Precipitation transfers energy from the Earth’s surface to its atmosphere. It takes energy to evaporate water from the surface. Just as evaporation of sweat from your skin cools you off by taking up heat from your skin, evaporation from the Earth’s surface cools it through energy transfer. Precipitation occurs when this water condenses out in the atmosphere. Condensation releases the heat energy stored when the water evaporated, warming the atmosphere. Globally, precipitation transfers about 78 Watts per square metre of energy from the surface to the atmosphere. Multiplying that by global surface area that’s a total energy transfer of about 40 petajoules (that’s 40 with 15 zeros after it) of energy every second! To put that in a bit of context, it’s about 40% of the amount of energy the Sun transfers to the Earth’s surface.

If precipitation changes, that’s the same as saying the atmospheric energy balance changes. If we warm the atmosphere up, it is able to radiate more energy (following the Stefan-Boltzmann law). To balance that, more energy needs to go into the atmosphere. This happens through precipitation changes.

Direct effects of gases on precipitation

Now imagine we change the amount of CO₂ in the atmosphere. This decreases the amount of energy the atmosphere emits to space, meaning the atmosphere has more energy coming in than out. To restore balance the atmospheric heating from precipitation goes down. This means that the global precipitation response to global warming from increasing CO₂ has two opposing components: a temperature-independent effect of the CO₂, which decreases precipitation, and a temperature-dependent effect which arises from the warming the CO₂ subsequently causes. In the long run the temperature-dependent effect is larger. Global warming will increase global precipitation – although there could be local increases or decreases.

But what happens if we do geoengineering? Say we get rid of the temperature-dependent part using aerosols to reduce incoming solar radiation. The temperature-independent effect of CO₂ remains and global precipitation will go down.

Detecting the effect of stratospheric aerosol

CO₂ isn’t the only thing that has a temperature-independent effect. Any substance that modifies the energy balance of the atmosphere has one. In our new study, we ask whether stratospheric sulphate aerosol has a detectable effect on global precipitation. Theoretically it makes sense, but it is difficult to detect because usually there are temperature-dependent effects obscuring it.

We used a common method to remove the temperature-dependent effect. We calculated the precipitation change for a given surface temperature change from a separate simulation, then used this to remove the temperature-dependent effect in climate model simulations of the future. We did this for future scenarios with and without geoengineering.

As expected, we found a temperature-independent influence which reduced precipitation. Importantly, this effect was bigger when geoengineering aerosols were present in the stratosphere. This was detectable in three different climate models. The figure above shows this. The non-geoengineered ‘RCP4.5’ simulation shows a precipitation decline when the temperature effect is removed. This comes mainly from the CO₂.  The ‘G3’ and ‘G4’ geoengineering simulations (blue and green lines) have an even greater decline. The aerosol is acting to decrease precipitation further.

How does aerosol affect precipitation?

The temperature-independent effect wasn’t present when geoengineering was done by ‘dimming the Sun’. The ‘G3S’ simulation  (orange lines in the figure) does this, and it has a similar precipitation change to RCP4.5. So what causes the precipitation reduction when stratospheric aerosols are used? We calculated the effect of the aerosol on the energy budget of the troposphere (where the precipitation occurs). We separated this in two: the aerosol itself, and the stratospheric warming that occurs because of the effect of the aerosol on the stratosphere’s energy budget.

Black bars show the temperature-independent precipitation changes simulated by the models. Orange bars show our calculation of the effect of the stratospheric warming. Green bars show our calculation of effect of the aerosol itself. Grey bars show our calculation of the total effect, which is very close to the actual simulated result.

We found the main effect was from the aerosol itself. The aerosol’s main effect is to reduce incoming solar radiation and cool the surface. But we showed it also interferes a little with the radiation escaping to space, and this alters the energy balance of the troposphere. The precipitation has to respond to these energy balance changes.

This effect is not huge. We had to use many model simulations of the 21st Century to detect it above the ‘noise’ of internal variability. In the real world we only have one ‘simulation’, so this implies the temperature-independent effect of stratospheric aerosol on precipitation would not be detectable in real-world moderate geoengineering scenario. This also means climate model simulations not including the effects of the aerosol could capture much of the effects of geoengineering on the global hydrological cycle.

This effect could be more important under certain circumstances. If geoengineering was more extreme, with more aerosol injected for longer, precipitation would decrease more. But, based on these results, the main effect of geoengineering on precipitation is that the temperature-dependent changes are minimised. This means the temperature-independent effect of increasing CO₂ concentrations is unmasked, reducing precipitation.

Take a look at the paper for more details – it’s open access!

Ferraro, A. J., & Griffiths, H. G. (2016). Quantifying the temperature-independent effect of stratospheric aerosol geoengineering on global-mean precipitation in a multi- model ensemble. Environmental Research Letters, 11, 034012. doi:10.1088/1748-9326/11/3/034012.

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On a personal note, this paper is significant because it is the culmination of the first research project I truly led.  Of course I managed my own research as a PhD student and post-doc, but my supervisors secured the funding. They also acted as collaborators. Here I came up with the idea, applied for funding, supervised Hannah (the excellent student who did much of the analysis) and wrote up the results. It’s a milestone on the way to becoming an independent scientific researcher. For this reason this work will always be special to me. Thanks also to Hannah for being such a good student!

Stratospheric aerosol geoengineering and the polar vortex

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.

Strength of the polar vortex as measured by winds at 60N, 10 hPa. Each grey line shows the wind speed over 1 year. The mean of the Control simulation is shown by the dashed black lines. The means from the other simulations are shown by solid black lines.

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.

Changes in probability density function of North Atlantic jet latitude in (a) December-January-February, (b) March-April-May, (c) June-July-August, and (d) September-October-November. Grey shading shows the interquartile range of the Control simulation with the median marked with a white bar.

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.

How do we decide whether geoengineering is worth it?

Citation: A J Ferraro, A J Charlton-Perez, E J Highwood (2014) PLOS ONE, doi:10.1371/journal.pone.0088849

Some have proposed we take a different approach to climate change and attempt to stop global warming by reflecting sunlight. We have a new paper out today which asks the question: how do we decide whether such geoengineering would be effective?

Maps of effectiveness of geoengineering using a risk approach. The simulation uses stratospheric aerosols to balance the surface warming from a quadrupling of carbon dioxide.

Does geoengineering have the potential to reduce climate risk?

One way to exert a cooling influence on the climate would be to pump tiny particles up into the stratosphere, where they would reflect a small amount of the Sun’s energy. Should we consider intentionally modifying our environment in this way in order to affect the climate? Some argue there is a chance of unintended side effects, and that such meddling is too risky. Others argue the opposite: that it is too risky to allow global warming to continue.

What are these risks? A basic way to think about it is that people are adapted to our present climate. They are used to a particular mix of warm and cold, wet and dry. As climate changes, this mix will also change, posing a risk to those not prepared for it. For example, a warmer climate might be seen as a risk for healthcare systems not equipped to deal with medical problems associated with heat waves. A wetter climate might increase the risk of flooding. Risks like this could be costly – which is essentially why climate change could pose a problem.

Could geoengineering be used to help? Geoengineering with stratospheric aerosols might pose risks of its own: reduced rainfall, depletion of the ozone layer. It might also produce benefits: reduced warming and enhanced agricultural productivity. We need a way to compare the risks and benefits of geoengineering with the risks and benefits of not geoengineering (here, we are assuming we don’t do a good job of reducing greenhouse gas emissions).

How do we weigh up different kinds of risk?

Consider this: you are diagnosed with a medical condition which may deteriorate in future and cause you difficulty. You are given the option of a treatment which might stop the symptoms of the disease but may also have other side-effects. Do you take the treatment? You have to weigh up the risks.

A matrix showing the different outcomes of geoengineering. On the horizontal axis is the probability of a big climate change under carbon dioxide. On the vertical axis is the probability of a big change in climate under geoengineering. [EDIT: Thanks to the reviewer who suggested this method of presentation!]

In the same way we have to weigh up the risks to decide whether geoengineering is worthwhile. We would want it to reduce climate risk compared to not geoengineering. But there’s another layer of complexity here. Perhaps the reduction in risk happens somewhere that wasn’t actually at high risk of big climate changes in the first place. So perhaps no one cares?

We looked at this by dividing climate risk into four possible outcomes, shown in the diagram on the left. The horizontal axis shows the chance of getting a substantial climate change in the first place from carbon dioxide. The vertical axis shows the chance of getting a substantial change from geoengineering. So, if geoengineering reduces climate risk but there wasn’t much risk to start with (low change of substantial climate change on the horizontal axis). we classify geoengineering as ‘benign’ (it hasn’t really done much). If geoengineering reduces risk where carbon dioxide increases risk we classify geoengineering as ‘effective’. But what if geoengineering increases risk? We classify it as ‘ineffective’ if geoengineering introduces climate risk in a similar manner to carbon dioxide. Finally, if geoengineering introduces climate risk into areas which were not previously at risk from carbon dioxide-driven climate change, we classify geoengineering as ‘damaging’.

This way of looking at things can be used to classify climate changes. The maps in this post give an example: temperature and precipitation from a climate model. The ‘global warming’ case involves a climate with levels of carbon dioxide four times what we have now, and a climate about 4 degrees C warmer. The ‘geoengineering’ case uses stratospheric aerosols to counterbalance this warming. So as expected, if you look at temperature, geoengineering is largely effective. But rainfall looks rather different. Geoengineering is not effective in quite large parts of the globe.

Trade-offs

We have made some subjective choices here, and different choices would give quite different results as to the effectiveness of geoengineering. To further complicate things, I would expect different climate models to paint quite different pictures of regional changes.

Geoengineering isn’t necessarily good or bad. It involves a trade-off between risks. These risks are different for different aspects of climate. As these (and many previous) results have shown, it might not be a good idea to use geoengineering to counterbalance all warming, because this would produce large rainfall changes. Approaches like the one described here could be used to find what the optimum level of geoengineering is that would minimise changes in both temperature and rainfall.

Impact of geoengineering on rainfall could be greater than we thought

Citation: A J Ferraro, E J Highwood, A J Charlton-Perez (2014) Environ. Res. Lett. 9 014001 doi:10.1088/1748-9326/9/1/014001

Aerosol layer (grey stripe in centre) produced by the 1991 eruption of Mt. Pinatubo.

I have a paper out today (with my PhD supervisors Ellie Highwood and Andrew Charlton-Perez) which suggests that the impact of geoengineering on rainfall in the tropics could be greater than we thought.

Geoengineering is a proposed response to climate warming driven by greenhouse gases. Basically, the idea is to mimic the effects of a large volcanic eruption on the Earth’s climate by injecting tiny particles called aerosols into the stratosphere. These particles would reflect a small amount of the energy coming from the Sun, cooling the planet. The basic idea makes sense, and from observing the climate following volcanic eruptions we know it could provide some cooling.

It’s also well understood that using geoengineering to counteract the warming effects of greenhouse gases and bring the surface temperature down would reduce global rainfall to levels lower than those we would get if there was no geoengineering or enhanced greenhouse gas levels. This is because the reduction in solar energy reaching the surface means there is less energy available to evaporate water, so the atmosphere has less water available to fall as rain.

Tropical temperature changes from carbon dioxide and geoengineering

But my research suggests there’s another effect stratospheric aerosols have on rainfall, especially in the Tropics. Here, rain is mainly produced by towering convective clouds which transport heat energy up from the surface to the atmosphere.

Our paper shows that aerosols in the stratosphere emit radiation down into the troposphere below, interfering with this convection. Geoengineering aerosols emit energy (in the form of radiation, as shown in the picture above) downwards into the troposphere, which causes the upper troposphere to warm up. In essence, the heating from the aerosol increases the stability of the tropical troposphere.

We don’t see in the increase in stability when geoengineering is represented by just turning down the Sun (right-hand panel in the picture above) because there isn’t any aerosol in the stratosphere to emit radiation downwards*.

This effect could be quite important depending on how strongly aerosols interact with radiation in the way I just described. In my climate model simulations I used one particular type of sulphate aerosol with specific radiative properties. However, it’s possible that aerosols in the real atmosphere could behave rather differently. This research shows its important to get the aerosol properties right if you want to correctly predict the effects of stratospheric aerosol geoengineering on the climate.

It’s very difficult to know what the properties of geoengineering aerosols in the real atmosphere might be. It’s not clear how much the aerosols would ‘clump’ together, which would increase their size and increase the amount of energy emitted into the troposphere. This is important because the more energy emitted down into the troposphere, the weaker tropical convection (and rainfall) becomes.

Geoengineering isn’t a ‘quick fix’ to the problem of greenhouse-gas-driven climate change. We’ve know that for a long time. This research shows that there are some important side-effects of geoengineering which should be taken into account when thinking about whether or not it’s a viable option. How important these sides effects are depends on the size and properties of the aerosol, which, as I’ve said, we don’t really know. In order to work how what geoengineering does and doesn’t do, we’d have to crack the tricky problem of understanding how the aerosols behave in the atmosphere.

* EDIT: This is important. Solar dimming geoengineering to counterbalance increasing CO2 concentrations decreases rainfall from pre-industrial levels, but globally this is smaller than the increase that would happen from CO2 alone. So in that sense solar dimming geoengineering gets us closer to the pre-industrial ‘baseline’. Including the aerosol effect on tropical rainfall, however, shows that the reduction in rainfall from aerosol geoengineering to counterbalance increasing CO2 concentrations is about the same size as the increase that would happen from CO2 alone. So sulphate aerosol geoengineering to counteract CO2 takes us about as far from the ‘baseline’ as CO2 alone does.

Clandestine geoengineering is real

On 15 October The Guardian released a news story about an ocean fertilisation experiment, uncovered by the ETC Group, which took place this July. It reported that around 100 tonnes of iron sulphate was dumped into the Pacific off the west coast of Canada. This is an ‘ocean fertilisation’ approach to carbon dioxide removal (CDR) geoengineering.

The idea is that adding nutrients to the ocean encourages algae to form. The algae take in carbon dioxide by photosynthesis, then sink to the ocean floor and ‘lock up’ the CO2 for the foreseeable future. It is very much a speculative idea. There are legitimate concerns about the ecological impact of ocean fertilisation, as well as serious questions about the amount of CO2 that can be removed from the atmosphere in this fashion.

The UN London Protocol regulates dumping of potentially hazardous material into the oceans, and the Convention on Biological Diversity prohibits large-scale geoengineering experiments if there is a risk to biodiversity. Whether this experiment is in violation of these two legal instruments is a question for the lawyers.

Even if there is no legal case against this experiment, there is plenty to raise concern. This was a significant geoengineering experiment by a private individual, Russ George, presumably motivated by the potential profits from selling the carbon credits from CDR. Even if ocean fertilisation does work, without proper regulation carbon pricing effectively incentivises environmental modification to sequester CO2 regardless of the ecological impacts. What’s more, it appears George persuaded the local indigenous people to contribute financially to the tune of $1m.

The village people voted to support what they were told was a ‘salmon enhancement project’ and would not have agreed if they had been told of any potential negative effects or that it was in breach of an international convention

– Guujaaw, President of the Haida nation

George is quoted in the Guardian article dismissing criticism, saying the UN regulations do not apply to this case and claiming his experiment was the ‘most substantial ocean restoration project in history.’

It looks very bad when exploratory research and experimentation on a new, potentially damaging technology is carried out by a controversial private individual with a clear personal profit motive. It looks even worse when the same individual misleads local stakeholders into partly funding such experiments. I hope we can get some clarification on the aims, extent and legality of this project in the near future. The only information source is the Guardian and a smattering of echo-chamber rehashings of the Guardian story elsewhere on the web.

COP11 of the CBD is currently in session in Hyderabad. It is due to finish on 19 October. Will they make a statement on this experiment?

UPDATE (17/10/12): As ever, the public geoengineering discussion group is a good source of information. The legality of the experiment is discussed. It is also pointed out that clear information about the incident is very limited which makes it hard to draw conclusions.