Alternative to reducing emissions? Geoengineering

What are the alternatives to not reducing emissions with climate change? Either, you accept the temperature increase and deal with it, or you stop the temperature increasing. This post concerns the latter; how we can geoengineer the climate to stop the temperature increasing. Geoengineering is deliberately changing, or engineering, planetary-scale processes.

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In their paper, Geoengineering the Climate: Science Governance and Uncertainty, the Royal Society accepts immediately that the most reliable method of controlling temperature increase is to reduce greenhouse gas emissions, and that geoengineering should not be seen as an alternative. But, if greenhouse gas emissions are not sufficiently controlled then geoengineering could be useful to mitigate climate change and effectively buy us more time to reduce emissions. 

There are two types of geoengineering, Carbon Dioxide Removal (CDR) and Solar Radiation Management (SRM). 

CDR methods include (from Royal Society paper):

• Land use management to protect or enhance land carbon sinks;
• The use of biomass for carbon sequestration as well as a carbon neutral energy source;
• Enhancement of natural weathering processes to remove CO2 from the atmosphere;
• Direct engineered capture of CO2 from ambient air;
• The enhancement of oceanic uptake of CO2, for example by fertilisation of the oceans with naturally scarce nutrients, or by increasing upwelling processes.

Caldeira, Bala and Cao provide this graphic showing the different methods CDR.

Caldeira, Bala and Cao

The science behind CDR is removing the quantity and hence reducing the concentration of CO2 in the atmosphere, which reduces the effectiveness of the greenhouse effect. This counteracts the pumping of CO2 into the atmosphere from burning fossil fuels. The main issues with CDR are the cost, scale and the time required for it to be effective. Hence, the scale at which CDR is used will probably rely on the cost. Planting trees to remove CO2 as well as other carbon capture methods are expensive relative to the amount of carbon they remove from the atmosphere.

Unlike SRM methods, CDR geoengineering is relatively uncontroversial as taking CO2 out of the atmosphere has few unknowns. The exception is ocean fertilisation. Ocean fertilisation involves deliberate eutrophication (see here for explanation of eutrophication) of the ocean, meaning algae will grow which will absorb CO2 (similar to afforestation, but in the ocean). In his edited book Coastal Systems, Haslett describes how eutrophication can occur with influxes of nitrogen and phosphorus, usually from fertilisers or human waste. The algae blooms that occur can eliminate all bottom-living organisms. As such, deliberately causing eutrophication in the ocean has been met with stern opposition regarding our lack of understanding of the potential impacts on wider ecosystems. 

Now for SRM. Again, from the Royal Society paper the methods of SRM include:

• Increasing the surface reflectivity of the planet, by brightening human structures (eg by painting them white), planting of crops with a high reflectivity, or covering deserts with reflective material;
• Enhancement of marine cloud reflectivity;
• Mimicking the effects of volcanic eruptions by injecting sulphate aerosols into the lower stratosphere;
• Placing shields or deflectors in space to reduce the amount of solar energy reaching the Earth.

Caldeira, Bala and Cao

SRM is increasing the albedo of the Earth. Hence, less of the Sun's energy will be withheld and instead reflected back out to Space. This will reduce temperatures. 

As explained by Caldeira, Bala and Cao, and perhaps unsurprisingly, in contrast to CDR SRM has many more issues associated with it. There are problems relating to global scale environmental management, whereas CDR generally only has local impacts. With this, comes global scale governance challenges. As mentioned the main issue with CDR is the efficiency of reducing temperature, both in terms of investment and time taken. CDR can take decades to be effective, whereas SRM can have almost instant impacts. The issue with SRM is the risk. Predicting the impacts of pumping particulates (aerosols) into the atmosphere to increase reflectivity seems almost impossible. The climate system is complex enough as it is, such that only in the last few years have any weather forecasts began to get predictions right. Therefore, understanding the risk involved with SRM (both potentially to human health and by how much it would actually reduce temperatures) is a challenge, and in the event of uncertainty should be assumed to be high.

Regarding methods other than pumping aerosols into space; massive mirrors in the sky? To me, that's something that'll probably stay in sci-fi movies. Brightening of clouds to make them more reflective is more plausible. Crutzen suggested this would be done by filling balloons with sulphur, and when they reached high altitudes shooting them with artillery guns. The cooling effect would be felt within six months, and the sulphur would last two years. However pumping sulphur into the atmosphere was something my GCSE Chemistry teacher taught me wasn't a good idea.  But, if the alternative was temperatures rising five degrees then it could be considered. Increasing the albedo of the sea is plausible, but as with ocean fertilisation the impacts of attempting to engineer such a complex system as the World's oceans requires near certainty of the effects before being carried out. Painting buildings white and planting more reflective crops is a much less controversial idea. But it falls short, as many of the CDR do, considering how effective it would actually be at reducing global temperatures.

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The effectiveness of SRM is very high though. Caldeira, Bala and Cao modelled the effect of reflecting 1.84% of the Earth's incoming radiation away, whilst doubling CO2 concentrations.

Modelling by Matthews and Caldeira suggests that within months temperatures could start to fall and be reduced by multiple Kelvin within 10years using SRM. Cooling of this magnitude could prevent catastrophic events, such as retreating Antartic glaciers and the collapse of the Greenland Ice Sheet. Irvine et al.'s work focuses on how we can keep the Greenland Ice Sheet and not make drastic changes to our energy mix by using geoengineering, and specifically SRM.

The Economist

In conclusion, both CDR and SRM have some large flaws. CDR's issues surround how effective it would be and hence, the temporal scale and financial investment required to actually meaningfully reduce global temperatures. That's not to say we shouldn't do it, we should. The CDR methods, with the exception of ocean fertilisation, have almost no side effects, so why not? The same goes for painting buildings white and planting reflective crops (unless the crops are genetically modified to be reflective... definitely a discussion for a different blog). 

SRM is much more effective. It can bring about significant changes within a decade and cool down our planet. But whether it's whitening the clouds, putting aerosols into the outer atmosphere or increasing the albedo of the ocean, the unknowns regarding the consequences are huge. Therefore, the shortcoming of SRM is the risk of making such changes to our planet. The Royal Society conclude by saying 'it would be highly undesirable' for geoengineering to take place that had influences beyond national borders without correct government mechanisms and clear knowledge of potential impacts. As such, further research into geoengineering is definitely required to even make a guess at potential impacts. 

But the risks of climate change are also potentially huge, whether it be changing rainfall patterns, higher temperatures or rising sea levels. It may be that to buy ourselves more time to reduce our greenhouse gas emissions we will require geoengineering. Hopefully, this will not be the case and we can just reduce greenhouse gases emissions, keep temperature increase below 1.5degrees, and so scrap the need to geoengineer the climate. 

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