Albedo modification has been proposed as a way of reducing temperature increases caused by greenhouse-gas emissions. "It works by putting reflective particles into the upper atmosphere, making the Earth more reflective and therefore cooling the surface," Applegate told environmentalresearchweb. "A recent US National Research Council report implies that albedo modification could reduce future sea-level rise, and a 2010 Nature article by Keith et al argues that geoengineering could prevent sea-level rise from collapsing ice sheets."

Applegate and Keller set out to investigate these claims for the Greenland ice sheet.

"We found that the benefits of albedo modification, in terms of avoided sea-level rise, take a while to accumulate, so the short-term benefits may be relatively small," said Applegate. "Our results do depend on the particular temperature scenarios that we investigated, however."

The pair used a simplified 3D ice sheet model and an intermediate-complexity climate model to assess the Greenland ice sheet’s response to solar radiation management over the coming decades and centuries. The researchers looked at business-as-usual – the RCP8.5 emissions scenario – as well as scenarios with albedo modification starting at various points from 2025 to 2475 that either stabilized temperatures or reduced them by 0.1 K per year.

In the most extreme case, commencing temperature stabilization albedo modification in 2025 brought a near-term reduction in total sea-level rise of just 3% of the long-term reduction; the geoengineering avoided 0.2 m of sea-level rise in the first century after implementation and 6.5 m by the end of the millennium.

"Using albedo modification to prevent sea-level rise from the ice sheets may be less effective than intuition suggests, particularly over the first few decades after albedo modification begins," writes the team in Environmental Research Letters (ERL).

According to Applegate, one previous study in ERL had examined albedo modification's ability to reduce sea-level rise from the Greenland ice sheet. "Irvine et al looked at the ice sheet's response over tens of thousands of years," he said. "We chose to focus on the first few decades to centuries after albedo modification begins."

If the entire Greenland ice sheet melted, it would boost sea-level by around 7.3 m. Although surface air temperature affects the ice sheet, feedbacks in the system mean that stabilizing or reducing temperature may not fully prevent or reverse ice loss. For example, if the ice sheet had partially melted before albedo modification began, there would be a smaller area available for accumulation of snow, which could cause additional mass loss even after temperatures fell. What’s more, the accumulation process by which ice sheets grow is slow compared to the melting and calving that shrinks them.

As a result, preventing melt of today’s Greenland ice sheet could require temperatures around 0.5 °C lower than the temperatures of the late 20th century, the researchers believe. Preventing melt in the future, once the ice sheet has shrunk further, would be likely to require lower temperatures still.

The business-as-usual scenario, the team found, would give Greenland a temperature increase of around 4°C by 2100 and 11°C by 2300. This would see rates of sea-level rise from the Greenland ice sheet peak at roughly 13 mm per year around 2300, then drop to below 2 mm by the year 3000, by which time the Greenland ice sheet would almost have disappeared.

Applying albedo modification to the model reduced the Greenland Ice Sheet’s total sea-level contributions and rate of contributions. But with just one exception, melting of the ice sheet continued to boost sea-level after albedo modification had begun. (The exception was the geoengineering scenario with temperature decline beginning in 2025; that saw the ice sheet grow very slightly for a few decades before shrinking.) Depending on the scenario, the ice sheet led to up to 1.2 m of sea-level rise over the first century after solar radiation management began.

For albedo modification scenarios that reduced – rather than stabilized – temperatures, sea-level rise from the Greenland ice sheet generally peaked within 50–150 years after the onset of geoengineering, at which point the ice sheet began to regrow. However, the simulated regrowth was never more than around 1–2 mm per year.

The ice sheet would likely respond similarly to temperature changes caused by either albedo modification or mitigation of greenhouse emissions. But mitigating emissions would have a slower effect on temperatures, so might prove less effective than albedo modification in reducing sea level rise, write the researchers in ERL.

A recent study by McCusker et al found that albedo modification probably wouldn’t stop mass loss from the West Antarctic Ice Sheet, which is driven by rising ocean, rather than air, temperatures. The geoengineering technique couldn’t prevent winds driving warm water to the edges of the ice sheet, the researchers believe.

Now Applegate and Keller are collaborating with colleagues at Penn State on better models of ice sheet behaviour that they can use to extend their results.

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