Angus Ferraro

A tiny soapbox for a climate researcher.

How do we decide whether geoengineering is worth it?

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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 climate model simulations using the risk matrix. The simulation uses stratospheric aerosols to balance the surface warming from a quadrupling of carbon dioxide.

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.

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.

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Author: Angus Ferraro

Trainee secondary physics teacher and former climate research scientist.

2 thoughts on “How do we decide whether geoengineering is worth it?

  1. Recent volcano eruption at Mount Kelud, Indonesia spewing sulphur into the sky could tell us what geoengineering actually means. What is your view on this?

    • Volcanic eruptions were the original inspiration for the idea of stratospheric aerosol geoengineering. The sulphur gets thrown up high into the atmosphere where it forms a layer of reflective particles, cooling the Earth. The cooling effect lasts for a few years, until all the particles fall out of the atmosphere. The amount of cooling depends on the amount of sulphur produced. The eruption of Mt Pinatubo in 1991 produced about 0.5 degrees C of cooling. It’s probably too early to tell how much of a cooling effect Mt Kelud could have. Meanwhile we can all hope that those affected by the eruption can stay safe.

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