ENHANCING NATURAL PROCESSES FOR COOLING CLIMATE COULD REDUCE WARMING
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ENHANCING NATURAL PROCESSES FOR COOLING CLIMATE COULD REDUCE WARMING
While the origin of climate change is greenhouse gases, the direct cause of stress on ecosystems, infrastructure, and people is increased heat energy trapped in the atmosphere. Scientific assessments to date, including a 2015 report by the National Academy of Sciences, have found that the most promising approaches to rapidly reducing this heat energy are related to one way that nature regulates the planet’s temperature: the reflection of sunlight from clouds and particles in the atmosphere. Early modeling studies suggest that increasing the reflection of sunlight from the atmosphere by just 1% could offset a doubling of CO2 in the atmosphere, or the equivalent of several degrees of warming.
The most sophisticated modeling study to date of one approach to increasing sunlight reflection used a global climate model to simulate Earth’s surface temperatures in a scenario of high emissions against a scenario in which particles are dispersed in the stratosphere to increase sunlight reflection from 2024-2100. In the scenario with this type of “solar climate intervention”, all parts of the world had temperatures close to today’s and conditions that were safer for people.
There are considerable risks to these approaches, including changes in precipitation, atmospheric circulation and clouds, that must be better understood to determine whether this approach merits serious consideration as a climate response. These risks must be assessed and weighed against the escalating risks of warming.
PROMINENT CLIMATE INTERVENTION APPROACHES ARE BASED ON OBSERVATIONS FROM NATURE
Approaches for increasing the reflection of sunlight, sometimes called “solar climate intervention” or “solar geoengineering,” recommended for research by scientists are based on phenomena that have been observed in the natural system.
Marine Cloud Brightening
Today, particles from pollution and their brightening effect on clouds in the lower atmosphere are thought to be producing a significant cooling effect that offsets some warming. The IPCC estimates this “cloud-aerosol” cooling effect to be somewhere between 0.5 and 1.1°C. Specifically, emissions from ships have a marked effect on the brightness of ocean clouds, that can be seen in observations from space as “ship-tracks.” One proposed approach to cooling climate is to spray salt mist from sea water, from ships or autonomous vessels, into regions of susceptible marine stratocumulus clouds in a way that optimizes the cloud-aerosol effect. Studies suggest that by brightening 10-25% of these clouds (representing 3-5% of the ocean’s surface), it may be possible to offset a doubling of CO2. But the magnitude of the effect is uncertain without more research, and effects on weather and circulation are an area of concern.
Stratospheric Intervention
In 1991, emissions from the eruption of Mount Pinatubo cooled the planet by 0.5 °C for nearly two years, leading to a recovery of Arctic ice during the period. Scientists have proposed that there may be more optimized ways to disperse particles in the stratosphere—possibly with cleaner materials—to produce a similar cooling effect. Historical observations provide a high degree of confidence in the cooling that can be produced in this way, and uncertainties center on the risks, particularly as they might unfold over time periods longer than the lifespan of particles in the stratosphere (a year or two). In particular, scientists hope to explore the effect on the chemistry of the stratosphere that might lead to damage to the ozone layer or changes in atmospheric circulation. Related to this, researchers hope to study both the effects of material released by volcanoes (SO2) and other materials that may have more favorable properties (calcium carbonate or diamonds).
Cirrus Cloud Thinning
Cirrus clouds in the upper troposphere reflect more sunlight back to Earth than they reflect out to space, so by 'thinning' the reflective capacity of these clouds more light could be released from below into space. This could be done by injecting particles into cirrus clouds that catalyze the formation of ice crystals in the clouds, causing them to precipitate and 'thin.' This approach does not have a natural or anthropogenic analog, and thus would require considerable experimentation to determine if the behavior predicted by the theory occurs in the physical world.
IMPERATIVE FOR RESEARCH
There are a number of emerging research efforts to determine the feasibility and risks of climate interventions to help slow Earth's warming. But research is very nascent, and scientists think it could take a decade of research to evaluate these approaches, including both specific research in interventions and accelerating observations and predictions of the Earth system. This work is imperative so that policymakers and members of society are equipped to assess the risk and benefits of these approaches against escalating near-term climate risks and the need to ensure safety.
Explore current climate intervention research efforts here.