Albedo is the measure of how much sunlight the Earth reflects back to space. It is a significant factor in global warming due to the growth of darker surfaces that retain more heat.
The loss of albedo increases the imbalance known as radiative forcing. Due to human influence, incoming sunlight now exceeds outgoing radiation.
Increased radiative forcing due to greenhouse gas emissions is partly reduced by anthropogenic cooling, mainly from aerosols that interact with the stratosphere and clouds to increase albedo.
Overall, the world is darkening due to the loss of snow and ice, soot pollution and decreased ocean cloud cover.
The melting of sea ice is a major albedo loss. Satellite and lunar reflection data show more than 0.5 per cent decline in total albedo this century.
Many new technologies have been proposed to enhance albedo. Marine cloud brightening would make salty mist from sea water to increase the albedo of ocean clouds, cooling the water beneath and reducing cyclone intensity.
Australia has led the world in field testing of marine cloud brightening to reduce coral bleaching on the Great Barrier Reef.
Stratospheric aerosol injection could mimic the cooling effect of volcanic eruptions, adding about 1 per cent of the highly reflective sulphur dioxide that used to come from burning coal without scrubbers.
These and other geoengineering methods could mitigate climate impacts such as rising sea levels, biodiversity loss and extreme weather.
Importantly, they could help avoid climate tipping points, while promoting international cooperation.
Advocates contend that research shows the cooling benefits will justify deployment if these technologies are well governed.
Challenges include the need for international governance to ensure deployment is safe, equitable and based on the best science, while overcoming the political hurdles in the transition from fossil fuels.
Risks include disrupting weather patterns, international disputes, sudden termination and allowing ongoing failure to address CO2 impacts such as ocean acidification.
The IPCC is opposed to action to increase albedo, mainly on the view that brightening the planet would undermine decarbonisation. This view ignores the security risk that tipping points such as ice melt could cause sudden cascading warming feedbacks in the Earth system.
The practical impact of neglecting albedo is highly risky, allowing ongoing warming while emissions continue.
Action to cut CO2 will take decades, whereas brightening the planet could have rapid cooling effects, especially by refreezing the Arctic.
Using technology to increase albedo may be the only feasible way to mitigate global warming in the short term.
Unlike net zero emissions, a climate goal of net zero heating achieved by enhancing albedo could stabilise the planetary system quickly and cheaply, by balancing the warming effects with equal and opposite cooling.
- Robert Tulip is convener of the Planetary Restoration Action Group climate thinktank.
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