Turning Down the Sun to Cool the Earth?

Learn about solar radiation management and how it might impact our ocean

We know it’s now an all-hands-on-deck moment for climate. So, it’s a good thing we’re seeing climate action picking up steam here in the United States and around the world because halting climate change is like stopping a large ship—with no brake pedal and a lot of momentum. We either need to slow down or put it in reverse. Globally, we are on track to reach 1.5° Celsius of global warming between 2030 and 2052 which will degrade almost all warm-water coral reefs, inundate coastal areas that currently sustain millions of people’s lives and livelihoods and fundamentally change our relationships with the ocean.

So, curbing planetary warming to 1.5° Celsius, which would help us avoid the worst of climate change’s effects on our ocean, definitely requires drastically slowing down emissions of greenhouse gases. Staying under 1.5° Celsius (or if we overshoot that target) may also require “shifting into reverse”—using methods to cool the Earth system and remove greenhouse gases faster than Nature alone could. A lot of these proposed solutions almost sound like they’re out of a science fiction novel and you may have heard of them as “geoengineering.” This can include reflecting the sun’s energy back into space, filtering the solar energy reaching the Earth’s surface or capturing greenhouse gases from the atmosphere.

But there’s a lot we don’t know about these methods—especially their effectiveness, side effects and scalability. There are currently no implemented geoengineering projects to cool the Earth or remove greenhouse gases from the atmosphere. However, there is a lot of interest in whether this is possible. Climate engineering methods are not well understood—we don’t know how to regulate them and we still don’t know the human or environmental consequences of any of them. That’s why we advocate for a lot more research to be conducted before any geoengineering method is actually implemented, and careful consideration of how these proposals could be tested. It might seem clear on paper how they will help our climate, but there could be all sorts of unknown side effects that result from geoengineering for people and our ocean.

Turning Down the Sun?

It’s universally agreed that geoengineering methods can’t substitute for eliminating greenhouse gas emissions to slow down climate change. They’re being proposed as an add-on to mitigating greenhouse gas emissions. One of the flashier proposals is called solar radiation management (SRM).

Solar radiation management is essentially turning down the sun to cool the Earth’s surface. Scientists have proposed methods that either use sunglasses-like methods to cut down the solar energy coming through the atmosphere or reflector-like methods to turn solar energy away from the lower atmosphere or Earth surface and allow heat energy to exit the atmosphere.

Methods imagined for the high atmosphere, like distributing mineral dust from airplanes, mimic the way volcano ash has broadly dimmed the atmosphere at times and slightly cooled the Earth in the past. Although this approach would likely decrease global temperature somewhat, it would also likely alter rain, snowfall and plant growth in unpredictable ways, and it wouldn’t completely stop some of the complex climate-driven systems on Earth, like Arctic ice melt that contributes to sea-level rise and increased flooding.

And there are also methods proposed for areas closer to the Earth’s surface that act more locally and depend on making a bigger difference over a smaller area. This could translate into brightening marine clouds using ships that shoot sea spray into the lower atmosphere in specific areas, or modifying how reflective natural surfaces are by lightening the color of desert, city or ocean areas to reflect more solar energy away from the Earth. These methods could only affect a tiny percentage of the Earth’s surface, and so they would need to be relatively stronger—meaning having a greater effect over a smaller surface area—than high-atmosphere methods to make a global difference. However, they could still alter local weather patterns in unexpected ways.

A New Report Lays Out Research Principles

The U.S. National Academies of Sciences, Engineering and Medicine (NASEM), a leader in evaluating what we know about solar geoengineering (see their earlier reports here and here ), has just released a report on a research agenda and research governance for solar radiation management. They have another report underway to outline a research strategy on ocean carbon dioxide removal (CDR), due out this fall.

The NASEM report recommends an integrated research agenda that allows research governance and research activities to evolve together while supporting and bringing together multidisciplinary research. The research includes recommendations for safeguards and built-in mechanisms for strong, diverse stakeholder engagement and input. The interconnection of scientific, social and governance considerations makes decision-making about SRM especially difficult, and the NASEM’s research agenda is structured to include a broad array of related social science and natural science research to answer questions on these topics. Periodic reassessment of the research program would allow the program to incorporate new insights and considerations as knowledge grows. The NASEM report also notes that ways to close out SRM research should also be specified, should further studies show that SRM would not be beneficial.

SRM and the Ocean: Not a Match Made in Heaven

From what we know now, SRM doesn’t appear to be a good tool for the ocean. SRM approaches seem to pose poorly-understood risks to both marine life and physical systems, like those that control the weather. And these risks could be high locally, with harm to individuals or communities far outweighing the hoped-for general planetary benefits of SRM. In addition, decision-making about our interconnected global common spaces such as the ocean and the atmosphere is not set up to take those tradeoffs into account equitably. And SRM is a fever-reducing strategy that doesn’t even address the major source of the disease: carbon dioxide. It would allow ocean acidification to continue unchecked. As the ocean takes up about a quarter of the atmospheric carbon dioxide we release by burning fossil fuels, it becomes acidified and harms the growth and survival of several groups of bivalve shellfish, crabs and fish. And time-related considerations related to geoengineering, like effects associated with starting and stopping SRM or any other sort of intervention, have not been well investigated. It’s thought that stopping any implemented geoengineering method that masks warming would result in a “rebound effect” involving sudden, dramatic warming, but the size of that effect and governance approaches to guard against rebound effects are still being studied.

By reducing incoming solar energy to adjust the Earth’s temperature, SRM would reduce the ocean’s temperature somewhat. But local changes in surface ocean temperature in specific regions caused by brightening marine clouds or the surface ocean would likely alter regional weather and rainfall that coastal communities depend on. Even though SRM essentially “turns down the sun,” it would also not affect the large amount of excess heat already stored in the deep ocean. Ocean heat and temperature are key controls of deep ocean circulation and nutrient recycling. The oversupply of heat already in the deep ocean, paired with changes in ocean surface temperature caused by SRM, could influence weather patterns, ocean currents and the supply of nutrients essential to ocean plant and animal life in ways that are difficult to predict. In addition, SRM would do nothing to address sea-level rise, so some of the best-recognized ocean symptoms of climate change—sunny-day flooding, more frequent and severe storm-driven flooding and groundwater salinization—would not be solved.

Solar radiation not only heats the planet but also feeds the planet by driving photosynthesis. Most land and ocean plants that fuel the planet’s food webs do so by capturing sunlight energy. Changes in the amount or quality of solar radiation reaching the ocean because of SRM could alter photosynthesis by marine algae and other plants, with unknown follow-on effects for the marine food web and for the carbon cycle.

Geoengineering and People

Every human depends on ocean health and well-being in direct or indirect ways. The ocean feeds and provides livelihoods for billions of people. Less obviously, the ocean makes the earth habitable by controlling the planet’s temperature, oxygen and carbon dioxide levels, and freshwater availability. Although all humans have a stake in decisions made about our atmosphere and ocean, decision-making about these spaces is not truly inclusive at present. Historically, decisions about the atmosphere, as well as about vast areas of the ocean, have been made at international levels through processes with arcane rules and with limited access for the public. Ocean and coastal governance is a patchwork of policies administered by very different decision-making bodies at scales from local to international and enforced with varying rigor, even though ocean systems are interconnected and so many marine species move among jurisdictions.

Decisions about SRM will involve both atmospheric and oceanic governance. Because of the planetary-scale outcomes of any geoengineering approach, governance of SRM and other methods is confounded by legal and ethical questions about who has the right and responsibility to decide to allow even small-scale experiments in nature, and through what process. Right now, computer model simulations or laboratory-based studies provide the majority of information on SRM, but they don’t offer much information on side effects or long-term results. We have only speculative information about who would benefit most or face the most risk from SRM. Notably, the NASEM report recommends developing a very strong system for stakeholder engagement as part of research governance.

The NASEM study highlights just how much we need to learn still about climate intervention methods like SRM. But this report, like the others that preceded it, cannot be mistaken for blanket approval of SRM, or even encouragement of a particular technique. Just like in biomedical research, these new ideas need to go through computer modeling, laboratory trials, pilot tests of increasing scales and regulatory approval before they can be deployed at large scales. Any of these steps could uncover insurmountable obstacles. Societal considerations and input need to be part of every stage of the process. But considering our current all-hands-on-deck moment, it’s worth checking out every possible solution using open, publicly-supported science methods. And the NASEM study helps lay how to do this in a deliberative way.

Here at Ocean Conservancy, we agree with the new study. There are many unknowns right now about SRM, and very thoughtful coordination is necessary to coordinate and oversee the research that will inevitably come. There are very clear environmental justice risks associated with geoengineering. We must take into account who will suffer from any unforeseen side effects and where these different geoengineering activities will take place and whose land or ocean spaces they will affect. Everyone needs to be included in this, and we haven’t seen that type of inclusive decision-making process for such global activities. We hope that with drastic emissions reductions now we won’t need geoengineering in the future. But if we do need to engineer the climate to keep the planet habitable, we will need a solid base of knowledge about the process, and a well-thought-out approach to regulating it. Now is the time to figure it out, and the NASEM study helps layout how to do that in an equitable, informed way.

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