Solar radiation management: look before you leap

One of the key factors that influences the temperature of the Earth’s surface is what is called ‘radiative forcing’. This refers to the effect of greenhouse gases like carbon dioxide which hinders the release of heat from the terrestrial surface. I read somewhere that in the year 1900 the carbon dioxide level was around 280 ppm. It must have been a derived number because back in those days tracking CO2 levels in the air was certainly not a priority. At that level it was helpful in keeping the earth warm & supporting the substantial forest cover that existed back then. Cut to 2021, the CO2 level is 414 ppm & climbing & we are scrambling to cut emissions to ensure we can cap the temperature rise to below 1.5C. The recent instances of extreme weather events around the world are a reminder – if we still needed one - that we are running out of time. Seen in that light, any idea that has any realistic probability of success is worth a discussion – even if it might seem a little risky, like Solar Radiation Management.

The Earth receives an average yearly solar insolation of 235 W/m2. The effect is compounded by the radiative forcing effect of greenhouse gases to the extent of about 2.7 W/m2 – which, viewed in isolation may not seem like much. But when you reflect on the fact that back in the pre-industrial era when CO2 level in the atmosphere was 200 ppm, this was about half as much, the co-relation between greenhouse gas level and global warming becomes a lot more obvious. Now imagine we could reduce the incoming solar insolation by say 3.5 W/m2 – which is just about 1.5%, not only would it neutralize the radiative forcing we are experiencing today, it could actually result in cooling down of the earth! This is Solar Radiation Management – one of the technologies within the ambit of Geo-engineering that seeks to address global warming by tweaking climactic parameters at planetary scale.

Redirecting sunlight out to space to counter global warming may sound like science fiction but it may be already happening without us knowing about it. Following the volcanic eruption of Mt Pintabu in the Philippines in 1991, global temperatures dropped by 0.5deg C. Scientists believe that this was a result of some of the sulphur dioxide vented from the volcano that condensed & settled in the upper layers of the atmosphere. These microparticles, acting like optical lenses, dispersed some of the incoming sunlight, effectively reducing the amount of heat that reached the Earth’s surface. So in theory it can be done - the question is, can humans be relied upon to replicate a natural phenomenon of this scale, that too, it in a controlled manner?

The lowest level of the atmosphere is Troposphere which stretches roughly up to 12 km from sea level. All weather changes happen here and it basically contains all the air that we breathe. Stratosphere, which comes next, stretches beyond for another 38 km & it is significant because it contains the ozone layer. Solar Radiation Management (SRM) works by seeding high albedo clouds in the Stratosphere to reflect sunlight and therefore reduce the amount of heat received by the Earth. The underlying assumption is that since the Stratosphere has no role to play in weather events, any intervention at that level may not affect the normal weather cycle.

Albedo is the ability of the surfaces to reflect light. If you see the Physical map of the World you will see light and dark patches. The white regions are the areas covered with snow & therefore reflect most of the sunlight they receive whereas the dark portions are the jungle and vegetation which absorb most of the incident sunlight, which they use for photosynthesis. So obviously, the polar regions have high albedo while Amazon rain forests have low albedo. The reason why this is significant is, with increase on global temperatures as glaciers melt, the snow cover on mountains will get depleted, exposing the rocks – which have low albedo & therefore will lead to faster warming: the increase thereafter will not be linear, rather it will be like falling off a cliff.

But let us come back to SRM. There are broadly two routes that have been explored till now – stratospheric aerosols and marine cloud whitening. Marine cloud whitening is about spraying a fine mist of sea water in the upper layers of the troposphere. The salt particles become seeding sites for clouds – which become denser and reflect sunlight better - effectively acting as a shade for the earth’s surface. Stratospheric aerosols involve releasing Sulphur dioxide particles into the stratosphere, which help disperse incoming sunlight. On the face of it, neither is complicated – although getting large quantities of sea water to such heights does seem a rather expensive proposition – but the challenges with SRM go beyond the technology. ?

The first concern is whether the shading effect can be localized & sustained over a certain period of time – if that is not possible, it is a non-starter. The objective can be to reduce the solar insolation at the poles to protect the ice-shelves but it may end up reducing the sunlight available to the rain forests in the tropics – setting off a domino effect on the rainfall profile and vegetative cover. Plus, since these changes will happen over an extended period of time, it would difficult to uniquely attribute them to either the base climate change factors or the detrimental effect of lower solar radiation. Any technology runs the risk of under-performance or breakdown. In this case, given the remoteness of the execution and the spread, it will be practically impossible to undo, once implemented – which makes it imperative that we understand the risks involved. There is also the possibility of the technology failing at some point which could lead to ‘rebound warming’ somewhat similar to the feeling one gets on stepping outside on a hot summer afternoon from the comfort of an AC room. While SRM offers a possible solution to deal with radiative forcing, since the CO2 is not getting physically removed from the atmosphere, problems associated with high CO2 concentration like acidification of the ocean could potentially get worse, which could be particularly harsh for underwater formations like corals. So, even if SRM is deployed, it can at best be a short term fix,which can buy us some time while we get the carbon dioxide removal technologies sorted.

Since the deployment of this technology has the potential to affect large geographies and multiple countries, a governance mechanism to decide on the timing & extent of intervention is critical. The risks of unscrupulous players taking advantage of the relatively low technology barrier was starkly brought into focus when a US based startup, Make Sunset made a claim of having sent up a kilo of Sulphur dioxide into space last April using just a hot air balloon. Not only was it carried out in violation of rules, the company went around selling carbon credits for the same, claiming that 1gm of SO2 had the effect of neutralizing the greenhouse effect of 1T of carbon dioxide! One can only imagine what havoc unregulated use of such technology can cause.

Solar radiation management is currently legal in most nations but there has been a de facto global moratorium in place on geoengineering – which includes SRM – since 2010, when it was agreed by governments under the Convention on Biological Diversity, to limit work small-scale scientific research studies. At the United Nations Environment Assembly (UNEA) in Nairobi in February this year, opinions were divided on how to take it forward, so the status quo will continue for foreseeable future.

Now let us look at the problem from another perspective. It have used a particular technology as an example, but it will give you a sense of the challenge involved. While carbon dioxide capture from point of combustion must and will get implemented, for us to turn back the clock we need to do more, and one of the areas which is getting a lot of attention is Direct Air Capture. 1 ppm CO2 corresponds to approximately 8 gigatons (GT) in terms of absolute quantity so if we were to look at remove 50 ppm of CO2 from the atmosphere it would translate to 400 GT of CO2 – which if we were to achieve over a period of 100 years, would translate to 4GT/yr. DAC plants today are capable of CO2 reductions of around 1 million tons (MT) per year. With this technology, we would need to add 80 such plants every year for the next 100 years plus replace the old ones which become obsolete to be able to reach the target of removing 400 GT of CO2. Each plant total costs 260 million US$ in today’s money terms so we are talking big numbers here. In a world where we can’t agree on basics like who pays & how much for climate change technologies, it seems rather optimistic – to put it mildly - to expect that that we can all stay the course for the next 100 years. But if you try and do it over a much shorter period – say 10 years, so that the current leadership can see it through, we are talking astronomical numbers. Sometimes, the scale of a challenge can paralyse us into inaction – here the cost of doing nothing is much higher.

Solar Radiation Management looks like recklessness or a low cost tactical opportunity depending upon where you stand. Cloud seeding with silver iodide is one of the more common forms of climate modifications practiced around the world, notably in the rainfall deficient countries in Middle East. As recently as last month, Dubai was flooded due to unprecedented rain & the jury is still out on whether this is part of the pattern of extreme climactic events that we see around the world or due to cloud seeding. And therein lies the rub. Unless we can separate science from hubris, technologies like SRM are best avoided – not because it can’t be done but because we can’t trust ourselves to do it right.