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
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.
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.