Geoengineering: Can we hack our way out of climate change?

In the quest to minimise the effects of climate change, the idea has been proposed to try to reduce the amount of sunlight reaching the earth. This can be done by injecting huge amounts of sulphur compounds into the high atmosphere. There, the sulphur compounds form thin clouds that reflect some of the sunlight. Complexity researchers from Utrecht University have studied the economic costs and benefits of this idea and published their results in the journal Earth System Dynamic.

Claudia Wieners, one of the authors, wrote a short article to elaborate on the research.

The global mean temperature is rising, and the risks of unmitigated climate change are becoming ever more ominous. In 2015, politicians decided to keep the warming "well below 2 degrees", but the time window to achieve this is closing; we would need to reach net-zero emissions by around 2050.  Yet greenhouse gas emissions are far from decreasing. National pledges for CO2 reduction efforts fall short of achieving the goal, even if all pledges are kept. Could geoengineering - large-scale human interventions to cool the planet despite elevated greenhouse gas concentrations - solve our global warming problem? 

Volcanic eruptions

Out of several Geoengineering technologies proposed in recent years, we focus on Solar Radiation Management (SRM) using stratospheric sulphate aerosol, which is most likely to become feasible within a few years or decades. The method involves injecting sulphur compounds such as SO2 into the stratosphere, i.e. the atmospheric layer at 15-20km height. The sulphur compounds will then react to create sulphuric acid, which forms droplets that reflect sunlight - just like clouds. A thin veil of sulphuric acid droplets could reflect a few percent of the incoming solar radiation, thereby cooling the Earth. We know that the method works in principle: explosive volcanic eruptions have the same effect (e.g. Pinatubo in 1991). However, since the sulphate aerosol will rain out after about a year, we would have to keep injecting sulphur gas on a regular basis.

Currently, we do not have the technical means to perform SRM, but engineers are confident that special airplanes could be built for the purpose, and the costs for maintaining and operating them might be, very roughly, of the order of 2-10 billion $ per megaton of injected gas. Mimicing the Pinatubo eruption of 1991 (20 Mt(SO2)) would therefore cost 40-200 billion. World GDP is currently around $ 80 trillion, so geoengineering seems a major, but still affordable undertaking. On top of these implementation costs, geoengineering might also cause environmental and hence economic damage, for example through shifts in precipitation patterns, acid rain, and a delayed healing of the ozone hole. 

Economy and the climate system

So, what is the optimal balance between Geoengineering and CO2 reduction? To get a rough answer, we used a highly simplified model, the Dynamic Integrated model of Climate and Economy (DICE).

The model has two components: the world economy and the climate system. In the economic sector, capital and labour are used to generate economic output; part of this output is invested to generate new capital; part is consumed. Consumption makes the population happy; and the aim is to keep happiness (called "utility" by economists) at a high level now and in the future. Unfortunately, production also leads to CO2 emissions. The climate model estimates how those emissions influence global mean temperature. Global warming is assumed to damage the economy (reducing economic production); therefore humanity might choose to sacrifice some of their wealth to reduce CO2 emissions (called "abatement").

We extended the model by introducing geoengineering: It can reduce global mean temperature (and hence climate damage) even in a high CO2 world; but its implementation costs money and causes damages itself. In addition, we assume two sources of uncertainty: geoengineering might suddenly have to be abandoned (say, due to unexpected environmental dangers), but also global warming may turn out more dangerous than anticipated (e.g. sudden disintegration of West-Antarctic ice sheet). The model is run in an optimisation mode, i.e. at each time it is determined how much abatement and geoengineering should be done to optimise happiness over time. Remember that the DICE model is highly simplified: The following results should be interpreted in a qualitative fashion, not as exact numerical values.

Red lines show the mean over 10,000 simulations; blue lines indicate sample simulations, and the light blue shade the range of possible values. Blue stars mark simulations where geoengineering has to be abandoned (SRM=0). Abandoning geoengineering leads to a hurried increase of abatement, but nonetheless temperature jumps rapidly. 

Do both

The optimal climate policy, in a world where both CO2 abatement and geoengineering are available, is to do both. In those simulations where geoengineering remains available, the economy will be completely carbon-free in 2242; 27 years later than in a scenario where geoengineering is not available.

In other words: The availability of geoengineering leads to reduced efforts in CO2 reduction, but only slightly. Geoengineering does not replace CO2 reduction. Rather, it is used - at an equivalent of 3 Pinatubo eruptions / year - as a complement to abatement, in order to achieve additional cooling. It might also be useful as an emergency measure in case that, for example, unexpected feedback loops (such as sudden release of CO2 and methane from thawing permafrost) exasperate global warming.

Interestingly, in a society that cares strongly about the future (i.e., is more willing to spend money now for the benefit of future generations), the balance shifts to stronger CO2 reductions and less geoengineering. After all, geoengineering is a cheap short-term measure, while each ton CO2 avoided now will also reduce global warming in the future.

Further investigation

There are several issues worthy of further investigation. For example, the damage function - specifying the economic damage brought by climate change - is suspected to be far too low in DICE; so in reality the optimal policy would involve stronger climate action (more abatement or more geoengineering or both). Also, DICE is rather pessimistic about the costs for CO2 reduction, especially in the light of the recent price drops for solar cells and wind energy. It also wrongly models abatement as a permanent cost, whereas in reality it is mainly an, albeit huge, investment (once sustainable infrastructure is built, maintaining it is likely not more expensive than maintaining fossil infrastructure). So in reality, it might be better to have more abatement and less geoengineering than DICE suggests. Finally, DICE does not take into account political costs, for example the possibility of conflict about the magnitude of geoengineering.

We are planning to do a follow-up study on some of these issues. Meanwhile, we conclude two things:

• Geoengineering might be a useful addition to CO2 reductions under some circumstances, so it deserves further investigation (and if it does not work, this would also be good to know soon...)
• However, geoengineering can not replace CO2 reduction. Abatement must remain our priority!

Publication

Koen G. Helwegen, Claudia E. Wieners, Jason E. Frank, Henk A. Dijkstra. Complementing CO₂ emission reduction by solar radiation management might strongly enhance future welfare. Earth System Dynamics, published on 12 July 2019, https://doi.org/10.5194/esd-10-453-2019