Prioritize...
After completing this section, you should be able to:
- Define "solar radiation management" and describe two ways it can theoretically be done.
- Explain the potential drawbacks of emitting sulfate aerosols either near the surface or high up into the atmosphere.
Read...
Solar radiation management (SRM) is one of the most widely discussed and controversial geoengineering approaches. Unlike methods such as carbon capture and storage (CCS) or direct air capture, which focus on reducing greenhouse gas concentrations (in other words, lowering the amount of carbon dioxide in the atmosphere), SRM aims to counteract the warming effects of these gases by reducing the amount of solar radiation that reaches the Earth's surface. Remember our energy budget model from earlier in this class: we can "turn down" the sun's energy input and offset warming that way, too. This can be achieved in two primary ways: decreasing the total solar energy received (effectively reducing the solar constant) or increasing the Earth's ability to reflect radiation back into space (increasing the planet's albedo).
So, how might we do this? One potential method involves introducing sulfate aerosols into the lower troposphere (the region of the atmosphere closest to the surface), which has already happened unintentionally as a byproduct of industrial activity. These aerosols act like tiny mirrors, reflecting sunlight, and have offset some greenhouse warming over the past few decades. In some ways, this gives us a glimpse into how intentional manipulation of the Earth's shortwave (i.e., solar) radiative balance might work. For example, removing sulfur scrubbers from smokestacks or intentionally not installing them in emerging economies like China could sustain or increase the current sulfate aerosol burden. This cooling effect could, in theory, act as a "free pass" for some portion of the CO₂ emissions we’ve already added to the atmosphere.
Hopefully, your immediate reaction is: "Adding more particulate matter and aerosols to the atmosphere seems like a bad idea." You’d be right—there are some serious drawbacks to this approach. Sulfate aerosols contribute to problems such as acid rain and air pollution. This approach might be the definition of "robbing Peter to pay Paul."
Another, more ambitious strategy involves injecting sulfate aerosols into the stratosphere rather than the troposphere, mimicking the natural cooling effect of large volcanic eruptions, such as Mount Pinatubo in 1991. The idea is to periodically load the stratosphere with enough aerosols to reflect sunlight and offset the warming caused by greenhouse gases—essentially, artificially creating the cooling effects of a volcanic eruption on a regular schedule. For example, if CO₂ levels are limited to double pre-industrial concentrations, a Pinatubo-scale injection would be required approximately every six years to keep the planet's temperature stable. For higher CO₂ levels, more frequent injections would be necessary.
Technological proposals for stratospheric aerosol injection include launching containers that release sulfate aerosols at high altitudes via balloons or dispersing them directly from aircraft flying in the lower stratosphere.
But how sensible and safe is the idea of solar radiation management? Just because an approach mimics a natural process doesn’t guarantee it’s harmless—sometimes, it can be "too much of a good thing." Some scientists support this approach, arguing that the urgency of the climate crisis might make it necessary. If we cannot implement emissions reductions, carbon capture, or air capture quickly enough to avoid crossing the threshold of dangerous anthropogenic interference (DAI) with the climate system, geoengineering methods like stratospheric sulfate aerosol injection might be needed as a last resort.
However, there are significant challenges and risks. For one, the cooling effect of SRM is not uniform across the globe, much like the uneven impacts of a large volcanic eruption. Changes in atmospheric circulation could lead to uneven temperature effects, cooling some regions significantly while others, such as parts of the Arctic, might continue warming. This uneven warming could accelerate Arctic sea ice loss or Greenland ice sheet melting, compounding the very problems SRM seeks to mitigate. Precipitation patterns could also shift dramatically, with many continental areas drying out, threatening water supplies and agriculture. Additionally, sulfate aerosols worsen stratospheric ozone depletion, presenting a clear environmental tradeoff.
While other SRM approaches could avoid some of these issues—such as placing reflective mirrors in space or increasing the Earth's albedo by painting roofs and roads white—these alternatives face their own challenges. Many are prohibitively expensive or logistically unfeasible on the massive scale required. For instance, launching enough mirrors into space to counteract existing warming would cost more than one trillion U.S. dollars, not including the cost of positioning and maintaining those mirrors. Furthermore, none of these strategies address rising CO₂ levels, leaving ocean acidification—a critical problem—entirely unmitigated.
One notable advantage of SRM is its rapid deployability, offering a faster response compared to the slow pace of greenhouse gas mitigation. However, this quick implementation comes with its own dangers. If the world becomes reliant on SRM to offset warming, any disruption—whether due to war, economic collapse, or sabotage—could abruptly halt its deployment. Imagine developing a strategy to release sulfate aerosols into the stratosphere for decades, only to suddenly stop. This immediate cessation would almost certainly unmask decades of accumulated greenhouse warming in a matter of months, resulting in climate changes far more rapid and severe than what would have occurred otherwise. Reliance on SRM, therefore, introduces a precarious dependency.