Setting the thermostat: an introduction to geoengineering

Over the course of two days starting on 8^th November around 150 people gathered in the hallowed halls of the Royal Society to debate the future. And not just any future. It is a future humanity as a whole has difficulty grasping, and even more difficulty changing. The title of the discussion meeting was ‘Geoengineering: taking control of our climate‘.

Geoengineering, in a nutshell, can be defined as: deliberate intervention in the Earth system, manipulating conditions to achieve a desired effect. Generally, the desired effect is a reduction in the Earth’s global-mean temperature, but of course it is never as simple as that. What is the optimum temperature? Who decides? Questions of this sort pop up with uncomfortable frequency.

The meeting had a clear investigative structure. The first thing to think about when it comes to geoengineering is what circumstances would justify its use. Prof. John Mitchell gave a presentation on ‘tipping points’ in the climate system beyond which drastic change is unavoidable. For example, this could occur if the meridional overturning circulation (MOC) shut down. The MOC transports heat poleward in the North Atlantic ocean, warming north-west Europe and parts of the Arctic. If the MOC were to shut down it would be very difficult to restart. Prof. Mitchell also pointed out that, depending on our temperature target (keeping the global-mean temperature below a +2°C anomaly from pre-industrial temperature, for example), negative greenhouse gas emissions might be required in the future. In other words, we would need to start actively removing carbon dioxide from the atmosphere. Both these cases would need active intervention in the climate system.

Prof. Andrew Watson presented a more extreme scenario of runaway climate change caused by a closing of the Earth’s longwave emission window due to an increase in atmospheric water vapour. The planet would warm until it is able to emit in the near-infrared and even the visible, meaning Earth would be glowing and the oceans would have long boiled away into a 1400K atmosphere. This is what is believed to have happened on Venus. James Hansen believes it could happen here if we burned all the coal and tar on Earth, but Prof. Watson was more conservative in his predictions, saying it was impossible to achieve this runaway effect by greenhouse gas emission alone. However, he does believe there is a risk of a ‘moist greenhouse’ scenario, where the longwave emission window is partially closed by increased evaporation into the atmosphere.

Prof. David Keith proposed we consider geoengineering as a form of risk management. He pointed out that the climate sensitivity (the temperature response to radiative forcing) is highly uncertain. Geoengineering can be viewed as ‘insurance’ against it being very high. He said we should still cut emissions, but have the option of geoengineering should the climate turn out to be more sensitive than our best estimate. This is similar to using geoengineering as an emergency ‘brake’ to stop catastrophic climate change. Both approaches require we research methods now in preparation for the future. It is, however, a concern that by the time we realised the climate system is out of control it may be too late, and the tipping point may have already been passed.

The Royal Society divides geoengineering into two categories: solar radiation management (SRM) and carbon dioxide removal (CDR). The former involves changing the incoming short-wave radiation, the latter the outgoing long-wave. Speakers presented details of a number of different methods during the meeting.

Here is a quick summary:

• Stratospheric particle injection. Aerosols in the stratosphere reflect solar radiation. This is why volcanic eruptions (which produce stratospheric sulphate aerosol) have a cooling effect on the planet for several years until the aerosol is removed. If humans can maintain a cloud of aerosols of the right size this effect can be made permanent.
• Increasing cloud albedo. Seeding marine stratocumulus clouds with the goal of increasing droplet concentration and hence albedo and cloud lifetime.
• Land-use change. Dr. Joy Singarayer presented some preliminary results from a study of changing crop albedo. Crops cover 10% of the Earth’s surface, so a small cooling effect would result.
• CDR from the air. Humans have known how to remove CO₂ from the air (despite its very low concentration) using sodium hydroxide since the 1950s. Prof. David Keith described how such a system would work, the costs and the technical challenges involved.
• Terrestrial carbon capture using peatlands. Peatlands already store hundreds of gigatonnes of carbon. Prof. Chris Freeman explained how this sink could be enhanced by further inhibiting decomposition in these ecosystems.
• Enhanced weathering. Acceleration of natural weather processes which act as a sink for carbon.
• Ocean fertilisation, which enhances the uptake of CO2 into the ocean.

All of these methods have flaws. Some are expensive, some are technically challenging, some relatively ineffective, and all are still relatively unproven. The danger with SRM is that it really is just more ‘meddling’ with the system. The climate achieved might approximate our pre-industrial one, but only in the global mean. The regional impacts (such as changes in population in key areas such as the Amazon) might lead to demands for compensation for the damage. It would also do nothing to stop ocean acidification through CO₂ absorption. It would have to be maintained for centuries, and if it were stopped for some reason a very rapid (several degrees in a matter of decades) global temperature increase would result.

CDR is a ‘softer’ technology, treating the cause of climate change rather than the effect. In many ways it is inaccurate to lump together all these methods under the umbrella term of ‘geoengineering’. In some sense afforestation is also a form of geoengineering. But then so is putting mirrors in space. The two are plainly very different in practice.

Such is the weird world of geoengineering. It is a world where ethics and philosophy are inextricably linked with science, engineering and economics. It is impossible to discuss the science without some kind of consideration of the ethics or ideas about how it should be put into practice; or indeed, whether it should ever be put into practice at all. In my next blog post in a month’s time I’ll be delving a bit deeper into the ethical considerations of geoengineering. Meanwhile, for more information and a considered, balanced viewpoint, see the Royal Society’s 2009 report.

EDIT: follow-up post on ethics here.