“Climate change is the greatest challenge currently faced by mankind”.
This quote or one like it has been uttered by many of the leading academics, politicians and celebrities in recent years. So what is climate change? It tends to be currently thought of as the multitude of effects caused by global warming. As a society we dislike climate change. We are aware that its effects are likely to be detrimental, sometimes seriously, to a wide range of organisms, and to many humans, particularly those who have done little to induce the change and who are least able to deal with the effects. And we feel a responsibility for it. We are aware that recent climate change has been caused by human activities increasing the amount of greenhouse gases in the atmosphere. Knowing the cause of the problem, emissions of greenhouse gases, a solution should be simple, stop emitting these gases. No? Well for socio-political reasons that I cannot begin to address in this article, the simple solution of reducing the amount of greenhouse gases we emit has proved to be somewhat, inconvenient.
Since the obvious solution has so far proved impossible we try to look for alternative solutions. The basic criterion of these solutions is that they must reduce the average temperature at the surface of the Earth. Such attempts to manipulate the climate on a regional to global scale have been dubbed ‘geo-engineering’. At first glance many of these solutions seem like effective ways to both have our cake and eat it, to continue to pump more greenhouse gases into the atmosphere but not suffer the consequences of our actions. However, like most attempts to right one thing not by undoing it but by doing something else, we shall see that this could lead to unexpected and unwanted consequences.
One of the most well known geo-engineering solutions put forward is the pumping of large amounts of sulphate aerosol into the stratosphere. This sulphate aerosol would reflect some of the sunlight back to space before it reaches the surface. This is similar to the effect of a large volcanic eruption which also injects large amounts of sulphate aerosol into the stratosphere. However, research suggests that this temporary solution to global warming may have unexpected consequences. In this Snack I will focus on two. But first we will look at why sulphate aerosol geo-engineering should work.
A very basic model of energy in the earth system is that energy is received from the sun, in the form of short wavelength, high energy radiation. This is absorbed by the Earth, which then re-radiates this energy back to space as low energy, long-wavelength radiation. However if things were really this simple the surface temperature of the Earth would be about -18°C. Actually the average surface temperature of the earth is about 15°C. So why is it that we are not living in a permanent winter wonderland? The answer is that greenhouse gases in the atmosphere (mainly H2O) cause some of the sun’s energy to be trapped in the Earth system for a time. Instead of the long-wave radiation from the surface heading straight back out to space, some is absorbed by these gases and re-radiated back in all directions, some of it back down to the surface. It can bounce up and down like this for a while before finally escaping back to space (Figure 1). This increases the total amount of energy in the system and hence the temperature at the surface of the Earth. This can be considered the ‘natural greenhouse effect’. The ‘anthropogenic greenhouse effect’, which leads to global warming and climate change, follows exactly the same principle, enhancing this natural effect.
The simple idea behind sulphate aerosol geo-engineering is that these aerosols reflect sunlight back to space before it can reach the surface of the Earth. This reduces the total amount of energy in the earth system. This sounds like a solution which addresses the original problem. Using this method we should be able to bring the surface temperature back to pre-industrial levels. However, nothing is ever that straightforward. Though we have reduced the total amount of energy in the system, energy comes in different forms (short-wave and long-wave in our simple model). The ‘greenhouse effect’ has reduced the amount of long-wave radiation being emitted back to space. The sulphate aerosol solution seeks to bring the total energy back down by reducing the amount of short-wave radiation from the sun reaching the surface. From a purely total energy view this could work. However, although short-wave and long-wave radiation are both forms of energy, some of their properties are very different. While long-wave radiation generally just warms things up, short-wave radiation from the sun is integral to many chemical processes occurring in the atmosphere and biological processes occurring on the Earth’s surface.
The first process I will consider is photosynthesis. This is the mechanism by which plants convert CO2 and H2O to the carbohydrates which are essential to all life. Plants use light of very specific wavelengths for photosynthesis: 400 – 475 nm and 650 – 700 nm. Light of these wavelengths is received in radiation from the sun, whereas radiation back from the earth is all at longer wavelengths than this. Intuition would suggest that decreasing this shortwave radiation would decrease photosynthesis. However, plant photosynthesis in some areas actually increased after the eruption of Mt. Pinatubo in 19911. This is because as well as reflecting some of the incoming solar radiation, some of the radiation that does get through is scattered by the aerosols. This ‘diffuse’ radiation is treated differently by the plants than ‘direct beam’ radiation from the sun and they can actually use it more efficiently for photosynthesis. But while plants in some areas may benefit it seems likely that those already receiving little light, i.e. those in high latitudes, may well see a negative effect.
Secondly, sulphate geo-engineering may further deplete stratospheric ozone.This would lead to a worsening of the Antarctic ozone hole and possibly a far more significant Arctic ozone hole, which currently is rare. The large scale ozone depletion that is seen in the Antarctic in winter, and to a much lesser extent in the Arctic, is caused by reactions associated with chemistry that occurs on aerosol surfaces. Generally these are nitric acid particles found in polar stratospheric clouds but it is also known that sulphate aerosol can have the same effect. After the Pinatubo eruption in 1991, ozone depletion in both the Antarctic and the Arctic was observed to be greater than normal.The Antarctic ozone hole is beginning to recover as the amount of chlorine in the stratosphere has begun to slowly decrease over the past few years. It is predicted to have returned to 1980 levels by 2050. The use of sulphate aerosol for geo-engineering is predicted to put this recovery back by 30 – 70 years2 and to result in more frequent and greater ozone holes in the Arctic, where currently they are rare and far less severe than those in the Antarctic.
So, while sulphate aerosol geo-engineering is indeed a solution to our original problem, that of climate change caused by global warming, it will almost certainly lead to a cascade of unexpected consequences which would then require further solutions. Does all this mean that it will never happen or that it shouldn’t happen? Not necessarily. Notwithstanding all the potential side effects it is a solution that could be put into action quickly and would have immediate effects. Because of this we should not rule it out as an emergency measure. However it is not and never can be a permanent solution to climate change. If we truly want a solution that reverses the changes that we have effected on the atmosphere it needs to increase the amount of long-wave radiation that the Earth is emitting. Perhaps a novel way to do this will emerge, some sort of giant heat sink for transferring heat from the Earth directly out to space. But until then, the only obvious way to get rid of that excess energy is by removing the greenhouse gases which we have put in the atmosphere since the industrial revolution.
1 L. Gu, et al., Response of a deciduous forest to the Mount Pinatubo eruption: enhanced photosynthesis, Science, 299, 2035-2038, 2003
2 S. Tilmes, et al., The sensitivity of polar ozone depletion to proposed geo-engineering schemes, Science, 320, 1201-1204, 2008