By Tim Radford, Climate News NetworkThis piece first appeared at Climate News Network.

LONDON — The geo-engineers just cannot win, it seems. First, scientists demonstrated that ambitious plans to cool the planet by dimming solar radiation could have unintended and unwelcome consequences. And now they have shown something even more alarming: any programme to block the sunlight could precipitate even more dramatic global warming once it stopped, according to Environmental Research Letters.

Geo-engineering as a fallback strategy has been on the climate science agenda for decades. Almost all climate researchers argue, and have argued for 30 years, that the most effective response to global warming and the threat of climate change is a drastic reduction in the use of fossil fuel, everywhere on the planet.

Although many governments have agreed, and have even introduced attempts to control greenhouse gas emissions, levels of carbon dioxide and other greenhouse gases keep rising.

So researchers have suggested other possible solutions: “artificial trees” to absorb atmospheric carbon dioxide; the fertilisation of the oceans to accelerate algal photosynthesis; and even the injection of sulphate particles into the stratosphere to block the incoming sunlight. Such things happen naturally, during occasional violent volcanic eruptions, and global cooling has been observed to follow.

This last deliberate technological response might create other kinds of unwelcome climate change, among which would be an alarming change in rainfall patterns, other scientists have argued.

But now Kelly McCusker of the University of Washington in the US and colleagues have proposed yet another reason for limiting discharges into the upper atmosphere. Whatever benefits might follow the technique known as solar radiation management or SRM would be wiped out once the management stopped.

Worse when you stop

Quite simply the technology could ultimately make climate change more dramatic and global warming more alarming. If solar radiation management techniques were applied for a few decades and then halted, global temperature increases would more than double. The consequential heat would be worse than that expected if the sun-block had never been applied at all.

All such research is conducted with computer simulations. There remains no practical way of modelling such an experiment in real time and in the real world on a pilot scale.

But climate simulations seem to accurately “hindcast” real climates of the last 50 years, and the bet is that therefore they can forecast with reasonable accuracy the climate patterns to come.

The University of Washington team modelled a future climate in which the world went on increasing the rate of emissions in such a way that global average temperatures were 1°C above the average for 1970-1999.

Then, in the same simulation, the hypothetical geo-engineers stepped in and kept on loading the upper atmosphere with sulphate particles for 25 years. And then, for some hypothetical political or economic reason, or some notional technological failure, the solar radiation management plan came to a sudden halt.

And at that point, the models showed, global average temperatures would rise steeply, by 4°C. Over the same time frame, without the sun-block experiment, global average temperatures would have risen only 2°C.

Penalising the poor

The new rate of temperature rise would be on average 1°C per decade, about six times faster than the average over the last 40 years. This sustained warming, the authors say, would be “well outside 20th century climate variability bounds.”

The researchers say the temperature changes would be greatest in winter in the high latitudes – this seems to be the same in all global warming scenarios – but even in the tropics in the summer, once the sun-block was abandoned, the changes would be pronounced.

This would hit the world’s poorest, who are concentrated in the tropics, and at the same time pose a threat to the world’s richest centres of biodiversity, once again in the tropics. There would be a pronounced increase in rain and snow fall, but half of all land areas would also experience an increase in aridity.

All this of course is based on a set of simulations, and these incorporate the assumption that the injection of sulphate particles into the upper atmosphere would be technically possible, economically feasible, and truly effective: none of this is for certain.

The research does not deliver an accurate picture of the future. But it certainly does not deliver any encouragement to believe that a “technological fix” could counter the consequences of the steady increase in greenhouse gas emissions in the atmosphere.

“The only way to avoid creating the risk of substantial temperature increases through SRM, therefore, is concurrent strong reductions of GHG emissions”, says McCusker.

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