Geoengineering won't curb sea-level rise

Space mirrors and 'volcanic' blasts are not an easy fix for the rise in sea levels.
Richard Lovett Nature.com 23 Aug 10;

Unless they involve extreme measures, geoengineering approaches to offset the effects of human-driven climate changes won't do much to combat rising sea levels, an international team of scientists reports in the Proceedings of the National Academy of Sciences1.

That is because sea levels respond slowly to changes in Earth's temperature, says John Moore, a palaeoclimatologist at Beijing Normal University and lead author of the study.

"We've got this 150-year legacy of fossil-fuel [burning], land-use changes, et cetera," he says. "You can't just slam on the brakes instantaneously."

Moore and his team examined two proposed geoengineering schemes: mirrors orbiting in space to reduce incoming sunlight, and sulphates being shot into the upper atmosphere to create a bright, sunlight-reflecting haze — similar to the one produced naturally by the 1991 eruption of Mount Pinatubo in the Philippines. Either scheme could reduce incoming solar energy by about 1–4 watts per square metre, enough to offset the atmospheric warming caused by carbon dioxide build-up until at least 2070.

To determine how this might affect sea levels, Moore and his group used a computer model that could track historical sea-level changes over the past 300 years. They found that under a business-as-usual fossil-fuel scenario, even the 4-watt space mirror reduced this century's sea-level rise by only some 39 centimetres out of a projected 'no-intervention' rise of about 1 metre.

Only in combination with fairly aggressive carbon dioxide emissions reductions, Moore and his team calculated, could these geoengineering schemes have a larger effect. Even then, they would not completely stop sea-level rise, with the oceans likely to be about 30 centimetres higher by 2100, depending on the emissions scenario.
Two and a half Pinatubos

To nip sea-level rise in the bud, the scientists calculated, would require either injections of sulphur dioxide aerosol into the stratosphere equivalent to more than 2.5 Pinatubo eruptions every four years, or a commitment to constructing an ever-expanding space mirror.

Another problem, Moore says, is that once started, geoengineering must be continued or temperatures will quickly rebound to what they would have been without intervention. An attendant surge in sea-level rise wouldn't occur quite as quickly, but it would follow soon enough, at a rate of up to 1–2 centimetres per year, he says.

"Those are speeds that were observed during the last deglaciation," says Moore, "so we're not forecasting anything that is out of the geological record."

The economic effects of that would be so extreme, he adds, that a cost-benefit analysis indicates that geoengineering isn't even worth considering if there is more than about a one-in-ten chance that it will be suddenly discontinued. "It's playing economic roulette."

Richard Alley, a glaciologist and climate researcher at Pennsylvania State University in Philadelphia, calls the findings a "nice advance". He notes, however, that it is only the beginning of trying to determine how glaciers might react to geoengineering.

"We don't really have an ice-sheet model that we trust," says Alley, noting that, in addition to global warming, glaciers react to local and regional changes in winds, ocean temperatures and ocean circulation. "In many ways," he says, "this large advance serves to show how far we have to go before climate modelling of geoengineering is really good enough that useful regional projections could be made to guide decision-makers."

Alan Robock, a geophysicist from Rutgers University in New Jersey, agrees, but says that one finding that does come through strongly is that geoengineering has only a relatively minor effect on sea-level rise. "Reducing emissions of greenhouse gases will have a much larger impact," he says.

Moore concurs. "Anything that isn't reducing the amount of carbon dioxide in the atmosphere is like putting [on] a bandage rather than actually solving the problem," he says.

References
1. Moore, J. C., Jevrejeva, S. & Grinsted, A. Proc. Natl Acad. Sci. USA advance online publication doi:10.1073/pnas.1008153107 (2010).

Geoengineering 'not a solution' to sea-level rise
Katia Moskvitch, BBC News 24 Aug 10;

Even the most extreme geoengineering approaches will not stop sea levels from rising due to climate change, a study suggests.

New research proposes that as many as 150 million people could be affected as ocean levels increases by 30cm to 70cm by the end of this century.

This could result in flooding of low-lying coastal areas, including some of the world's largest cities.

The team published the study in the journal PNAS.

Scientists led by John Moore from Beijing Normal University, China, write that to combat global warming, people need to concentrate on sharply curbing greenhouse gas emissions and not rely too much on proposed geoengineering methods.

"Substituting geoengineering for greenhouse emission control would be to burden future generations with enormous risk," said Svetlana Jevrejeva of the UK's National Oceanography Centre, a co-author of the study.

Geoengineering has been talked about for countering some of the effects of climate change for the past several years, with some figures like the billionaire Bill Gates ploughing millions of dollars into the research.

But Dr Jevrejeva told BBC News that some proposals such as placing mirrors in space and spraying aerosols - microscopic particles - into the sky would only treat the symptoms, as greenhouse gases would remain in the Earth's atmosphere.

Dr Jevrejeva and her colleagues examined two geoengineering schemes with five different scenarios.
'Not a solution'

The first approach involves limiting incoming solar radiation through the injection of SO2 (sulphur dioxide) aerosols into the stratosphere. Alternatively, giant mirrors could be launched into orbit, they said.

The second approach would involve modifying the carbon cycle by either planting more trees (afforestation), converting organic material into charcoal (biochar) or using renewable energy from materials derived from biological sources (bioenergy).

"We used [a computer model to track] 300 years of tide gauge measurements to reconstruct how sea level responded historically to changes in the amount of heat reaching the Earth from the Sun, the cooling effects of volcanic eruptions, and past human activities," said Dr Jevrejeva.

"We then used this information to simulate sea level under geoengineering schemes over the next 100 years," she added.

The team found that, if taken individually, even the most extreme of these methods would result in severe sea-level rise.

"We suggested that the most effective approach would be a combination of three different techniques for managing the carbon cycle," said Dr Jevrejeva.

She explained that these scenarios relied on biological mechanisms to remove CO2 from the air and store it in biomass, soils or geological storage sites.

For instance, afforestation, or adding forests to places where they have been cut down or never existed, would lower the amount of atmospheric CO2, but only by 45ppm (parts-per-million) - a lot less than the amount humans have already emitted.

Biochar would reduce the CO2 levels by even less - 35ppm.

Biofuel production would be more effective, and the combination of the three methods could eliminate up to 250ppm of CO2 and limit sea level rise to between 20 and 40cm.
Carbon storage

The carbon storage technique also has other advantages, pointed out Dr Jevrejeva. It actually reduces the amount of CO2 in the atmosphere, whereas the method of reflecting sunlight back into space does not.

"If you use a mirror, it's extremely expensive and it's an engineering challenge - you have to place mirrors [weighing] some 20 million tonnes into the Earth's orbit," Dr Jevrejeva explained.

There was also the chance these mirrors might break in orbit, the researcher added.

The same goes for SO2 aerosol injection - a controversial approach that has already been tested on a small scale in Russia by one of the country's leading climatologists Yuri Israel.

He told BBC News that a stratospheric layer of SO2 could cool the planet and would be an effective and none-too-expensive way of tackling climate change.
Continue reading the main story
“Start Quote

What worries me is that it's cheap and do-able and all you need is a country with a rocket”

End Quote Sir David King Former UK Chief Scientific Adviser

But Dr Jevrejeva said that even though injecting a certain amount of SO2 into the atmosphere might lower mean global temperatures by 1C or more over a few decades, the CO2 would still persist there.

The researchers' simulations showed that spraying the stratosphere with aerosols would produce a similar effect to a major volcanic eruption occuring every 1.5 years. Besides reducing global temperatures, this approach would also delay sea-level rise by 40 to 80 years.

"During a natural volcanic eruption, there's usually a cooling effect in the atmosphere and a drop in sea level. We [followed] different scenarios using the amount of aerosols equivalent to the biggest eruption of the 20th Century - the eruption of Mount Pinatubo in 1991," said Dr Jevrejeva.

"Particles from volcanic ashes end up in the stratosphere and reflect the radiation from the sun, but the same amount of CO2 stays in the atmosphere, so you do not solve the problem."

Also, no one knows the effect such spraying could have on the ecosystem, added the scientist.

"It's a huge challenge, no one knows what could happen."
Major concerns

In the Proceedings of the National Academy of Sciences (PNAS), the scientists wrote that SO2 injection into the atmosphere would likely lead to such undesirable consequences as "disruption in precipitation patterns and stratospheric ozone, and do nothing to avert the continued absorption of CO2 by the global ocean leading to rising acidity and ecosystem damage."

There are many opponents of geoengineering techniques, among them former UK chief scientific adviser Sir David King.

He told BBC News about his concerns over geoengineering proposals, especially those involving spraying the stratosphere with aerosols.

"What worries me is that it's cheap and do-able and all you need is a country with a rocket and they can put aerosols up into the stratosphere. We have no confidence in models on what these aerosols would do there.

Sir David explained: "Imagine if the aerosols would in some way cause more aerosol production - because there are a lot of chemicals there including ozone - and in time we find that we're getting more than we anticipated so the planet gets cooler and cooler when we wanted it to be stable"