Angus Ferraro

A tiny soapbox for a climate researcher.


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).

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.

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.

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!

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A hiatus in the stratosphere?

During the past few decades the rate at which the Earth’s surface has been warming has decreased. This has been called a ‘pause’ or ‘hiatus’ in global warming. At the same time, the cooling of the lower stratosphere has similarly paused. What’s going on here? Now is a good time to review what we know about drivers of temperatures in different parts of the atmosphere.

Carbon dioxide has a warming effect on the surface and the troposphere (the lowest 10 km or so of the atmosphere) because it absorbs infrared radiation, reducing the amount of energy the troposphere can emit to space. But higher up in the stratosphere (between about 10 and 50 km) carbon dioxide actually has a cooling effect. The reason for this is a bit subtle, but it can essentially be thought of as a result of thin air at high altitudes, which means a lot of the emission from the stratosphere at the wavelengths at which carbon dioxide absorbs is straight out to space; in the troposphere on the other hand there is more reabsorption.

a, Annual global-mean surface and stratospheric temperatures. Surface temperatures from the NASA GISTEMP data set. Stratospheric temperatures are derived from measurements from different channels of the Microwave sounding unit, processed by remote sensing systems. Lower stratosphere (TLS; approximately 14–22 km) and middle stratosphere (C13; approximately 30–40 km). b, Decadal-mean temperatures simulated by seven chemistry–climate models (CCSRNIES, CMAM, LMDZrepro, MRI, SOCOL, UMSLIMCAT and WACCM) for the 14–22 km altitude range relative to 1990–1999 for the CCMVal-2 scenario REF-B2 (All), which uses the IPCC A1B greenhouse-gas scenario. The well-mixed greenhouse gases scenario is the same as REF-B2 but has fixed ODS, and the ODS scenario has fixed greenhouse-gas concentrations. Markers denote the multi-model mean and bars indicate the inter-model range.

The strange case of the two hiatuses

Since 1979 we’ve been able to measure the temperature of the stratosphere using satellite instruments. The lower stratosphere cooled until the mid-1990s, but since then its temperature has barely changed. This flattening of lower stratospheric cooling is happening at the same time as the flattening of surface warming. That’s a little odd – surface warming has paused, and stratospheric cooling has paused as well! Are these things somehow linked? Just looking naively at the temperature data one might be forgiven for thinking something is wrong with our theories of what carbon dioxide does to the atmosphere.

I have a correspondence piece out today in Nature Climate Change with coauthors Mat Collins and Hugo Lambert explaining this little mystery and reviewing some of the great scientific work on understanding drivers of stratospheric temperature change. The ‘pause’ in global surface warming has attracted a lot of attention in recent years, and appears to be mostly a result of natural variations in the amount of heat being taken into the ocean, but at the same time there has been plenty of important scientific research on stratospheric temperature trends that has received rather less attention.

In short, the answer is that the two ‘hiatuses’ are not related to each other, and neither are inconsistent with the scientific basis of global warming by increasing carbon dioxide concentrations.

What drives stratospheric temperature change?

It turns out the main cause of lower stratospheric cooling since 1979 is not carbon dioxide. This is mainly because the lower stratosphere is not very sensitive to a change in carbon dioxide concentrations. It has a much greater effect higher up (the ‘middle stratosphere’ line in the figure above shows strong cooling over the period for which measurements are available). The cooling effect is still there, but it’s not the main culprit for past changes.

The missing piece is what’s been happening to stratospheric ozone. Ozone absorbs solar radiation and warms the air, which means ozone-rich parts of the stratosphere are actually warmer than the upper parts of the troposphere.

Emissions of chlorofluorocarbons (CFCs) and other similar substances have caused the amount of ozone in the stratosphere to decline over past decades. The declining ozone meant less solar radiation was absorbed, so the stratosphere cooled down. It has also led to an increase in harmful ultraviolet radiation from the Sun reaching the surface. Concern about the damage to the ozone layer led to international regulations on the emissions of CFCs and other ozone-depleting substances, starting with the Montreal Protocol in 1989. Now the ozone layer is beginning to show signs of recovery.

So it is ozone, not carbon dioxide, that has been the main driver of lower stratospheric cooling since 1979. The flattening out of the stratospheric cooling trend is because ozone levels have stopped declining.

A delicate balance for the future

Does that mean that, as the ozone layer recovers, we should expect the lower stratosphere to warm up again in the future? In fact it’s a little more complicated than that. Although carbon dioxide isn’t the main cause of past stratospheric cooling, if we keep emitting it at an accelerating rate its effects will start to become more important. In the future we might see carbon dioxide becoming a major influence on the temperature of the lower stratosphere.

Although we know that carbon dioxide causes stratospheric cooling and ozone causes stratospheric warming, the size of their effects is very complicated to calculate. It depends not just on the effects of these substances on radiation but on complex interactions with the atmospheric circulation, and in the case of ozone is also heavily dependent on complex chemical reactions.

This means climate model projections simulate a broad range of possible future temperature trends. The figure shows differences in lower-stratospheric temperature relative to the 1990s in simulations with 8 climate models including the detailed descriptions of changing stratospheric chemistry that are required to accurately simulate changes in ozone. The black bars show the combined effects of both greenhouse gases (mainly carbon dioxide) and ozone-depleting substances (mainly CFCs). The coloured bars show their individual contributions. The simulations show ozone-depleting substances were the main drivers of past stratospheric cooling. In the future the models simulate a large range of influences.

What all this means is that the future of lower stratospheric temperature will be determined by a tug-of-war between the warming influence of recovering ozone and the cooling influence of increasing carbon dioxide. It is actually difficult to work out which of these effects will win out. It’s even possible they could cancel each other out and the period of constant lower-stratospheric temperatures could continue for decades. In contrast, the period of flat surface temperatures is likely to end in the next few years, and we are very confident it will end with a period of warming (likely accelerated warming as heat is transferred from the oceans to the atmosphere).

Ferraro AJ, M Collins and FH Lambert (2015), A hiatus in the stratosphere?, Nature Clim. Change 5 497-498, doi:10.1038/nclimate2624.

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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.

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Paper review: Stratospheric versus tropospheric control of the strength and structure of the Brewer-Dobson circulation

Citation: Gerber, Edwin P., 2012: Stratospheric versus Tropospheric Control of the Strength and Structure of the Brewer–Dobson Circulation. J. Atmos. Sci., 69, 2857–2877.
[pdf] from Edwin Gerber’s web page

The stratosphere lies above the troposphere (which is where most ‘weather’ happens) between about 10-50 km altitude. Its circulation is in many ways counter-intuitive, thanks mainly to the influence of waves.

Wave motions are ever-present in the atmosphere on a huge variety of scales – from turbulent eddies to thousand-kilometre planetary waves. They can basically be understood as motions which dump momentum (which changes the wind speed) when they break. Think of waves surging onshore at the beach. Out at sea, the wave doesn’t actually move water horizontally. It just moves water up and down. The wave moves but the water doesn’t. As the shore gets closer and the sea shallower, the wave breaks and water rushes forward. Edwin Gerber’s paper describes the behaviour of the wave-driven Brewer-Dobson circulation in the stratosphere.

The Brewer-Dobson circulation (BDC) transports air poleward from the tropics. This happens because waves from the troposphere are produced by mountains (think about putting a rock in a stream and imagine the swirls this produces), move up into the stratosphere and break. This breaking decreases the wind speed and air drifts towards the pole and then sinks. This is because of the principle of conservation of angular momentum. Basically, if you are spinning in a computer chair really fast, you will find it difficult to pull your arms in. If you slow down (imagine this is the effect of some waves) you will be able to pull your arms towards your body (your ‘pole’). The BDC is driven by the breaking of very large planetary waves (thousands of kilometres long)

The Brewer-Dobson circulation (schematic from University of Frankfurt)

Gerber uses a fairly simple atmospheric model to look at the behaviour of the BDC. The model isn’t meant to be like the real world. Instead it’s meant to simulate the fluid dynamics of the atmosphere in a way which looks something like the real world. It follows the same physical laws as the real world. This means the physics we learn about using the model can be applied to the real world.

This paper picks out two ways in which the BDC can change.

  1. Tropospheric control. Increase the wave activity and the BDC will become stronger.
  2. Stratospheric control. Increase the strength of the polar vortex (westerly winds in the winter hemisphere of the stratosphere) and the upward movement of waves will change.

The effects of the control mechanisms on the BDC can be easily seen by looking at the ‘age‘ of the air. This is the time since the air was at a given level in the atmosphere (in this paper, 100 hPa, or around 20 km). The picture below shows the age for different model setups. Red colours show older air, indicating a slower circulation.

Fig. 1 from Gerber (2012). Age of air. Left: (top) weak wave activity, (bottom) strong wave activity. Right: (top) weak winds in the stratosphere, (bottom) strong winds in the stratosphere.

Tropospheric control is the easier one to understand. If the BDC is driven by waves, increasing the wave activity will make it stronger. What does ‘increasing the wave activity’ mean? Well, the amplitude (‘size’) of the waves can be increased by changing the height of the mountains producing them. This is the what the study does. It sounds silly, because for the foreseeable future the mountains on this planet are going to stay much the same. But this is just the simple approach Gerber took in the paper. In fact, the amplitude of the planetary waves can be changed by large-scale changes in the air pressure in the lower atmosphere (such as the ‘Arctic Oscillation’, which is an expression of the latitude of the jet stream).

The stratospheric control mechanism is a little more subtle than the tropospheric one. Changing the winds in the stratospheric polar vortex changes the way waves behave. If the vortex is very weak, waves are trapped at the base of the stratosphere (under 20 km). This means the bottom part of the BDC speeds up, but the top part slows down. If the vortex is strong, waves can move all the way up into the stratosphere, so the top part speeds up.

Gerber’s experiments agree with some previous work indicating the BDC has two branches: an upper and a lower (you can see this in the first picture: the lower part is around 20 km while the upper part goes up to around 50 km). They seem to affected in different ways. The lower branch is most susceptible to changes in the waves coming from below; the upper branch is modified by changes in the way waves behave once they are in the stratosphere.

Why is this important? The BDC transports ozone to the poles, and ozone is important for protecting living things from harmful ultra-violet sunlight. Changes in the upward motion at the bottom of the stratosphere also change the level of the tropopause (thick blue line in the first picture). This is important because the level of the tropopause governs how much water vapour gets into the stratosphere. In the troposphere, extra water just rains out, but in the stratosphere extra water vapour exerts a warming influence on the Earth’s surface. If the tropopause gets lower it moves to a warmer region of the atmosphere, which allows more water vapour to get into the stratosphere. So it’s important we understand how and why the BDC does what it does.

It is unusual to find a single-author paper of such detail nowadays. It is also unusual to find such a readable paper on a highly technical subject. As I have said I find stratospheric dynamics quite tricky, but this paper really nicely illustrated the important physical principles without giving me a headache. There is a lot more in the paper than in this summary, but I hope it has at least given you a flavour of the clear, focused experimental design and the excellent presentation of its arguments.