If we learn how to in­flu­ence the weather, cli­mate change could be slowed. Anna Poss­ner is crunch­ing the num­bers to find out what the con­se­quences might be

BBC Earth (Asia) - - Science -

“Could spray­ing tiny droplets of sea­wa­ter into clouds brighten them enough to cool the planet – without nasty side ef­fects?”

Ie see them al­most ev­ery day, but there’s a lot we don’t know about clouds. Even to me­te­o­rol­o­gists, their in­ner work­ings are some­what hazy. And that mat­ters, be­cause clouds play an im­por­tant role in reg­u­lat­ing the planet’s tem­per­a­ture – both re­flect­ing ra­di­a­tion from the Sun and act­ing like a blan­ket, keep­ing the Earth’s heat in. In fact, some clouds are so ef­fec­tive at the re­flect­ing bit that ‘su­per­charg­ing’ them by mak­ing them even brighter and whiter has been sug­gested as a way to re­duce tem­per­a­tures and fight global warm­ing.

It’s an idea that at­mo­spheric sci­en­tist Anna Poss­ner is very fa­mil­iar with. Her re­search at the Carnegie In­sti­tu­tion for Science in Stan­ford, Cal­i­for­nia, will help to an­swer the ques­tion of whether ‘cloud bright­en­ing’ might ac­tu­ally work. She’s part of the Ma­rine Cloud Bright­en­ing Project, an ini­tia­tive that’s brought to­gether cloud ex­perts in the US and UK with a bunch of re­tired Sil­i­con Val­ley en­gi­neers to find out whether spray­ing tiny droplets of sea­wa­ter into clouds can brighten them enough to cool the planet – and do so without any nasty side ef­fects.

Poss­ner runs ex­per­i­ments with clouds, look­ing to see what hap­pens when she in­jects droplets of sea­wa­ter into them. But the clouds Poss­ner works with are made up of num­bers – they are nu­mer­i­cal rep­re­sen­ta­tions of the real thing cre­ated us­ing al­go­rithms that model how at­mo­spheric sys­tems work. So com­plex are these mod­els that they re­quire su­per­com­put­ers – such as those at the Na­tional Cen­ter for At­mo­spheric Re­search’s (NCAR) cen­tre in Wyoming – to run. “You run your mod­els and you get out­put files that are usu­ally four-di­men­sional – time and 3D space of var­i­ous fields such as tem­per­a­ture, pres­sure, cloud wa­ter and cloud re­flec­tiv­ity,” says Poss­ner.

Now a post­doc­toral re­search sci­en­tist at Carnegie, it was Poss­ner’s re­search for her PhD at ETH Zurich, Switzer­land, that led to her con­nec­tion with cloud bright­en­ing. Here she stud­ied ship tracks, which are the ship­ping equiv­a­lent of air­craft con­trails. These slen­der strips of bright­ened cloud form as ships cross­ing oceans belch out tiny aerosols, such as sul­phate par­ti­cles, from their smoke stacks. It’s around these tiny par­ti­cles that wa­ter vapour in the at­mos­phere con­denses, mak­ing the clouds more re­flec­tive. It’s ex­actly the same prin­ci­ple be­hind cloud bright­en­ing, ex­cept the par­ti­cles that would su­per­charge the clouds would be salt­wa­ter.

Cloud bright­en­ing, like most geo­engi­neer­ing projects, is con­tro­ver­sial. The big­gest con­cern be­ing that med­dling with our weather sys­tems might have un­fore­seen knock-on ef­fects, and could ac­tu­ally make things worse. This con­tro­versy makes fund­ing for such re­search hard to come by. It also makes young at­mo­spheric sci­en­tists like 30-year-old Poss­ner ten­ta­tive about get­ting in­volved. “I’m not say­ing I’m in sup­port or against it re­ally, we have just got to start this now in terms of re­search. This is an idea that’s out there, and if peo­ple ex­pect the sci­en­tific com­mu­nity to make a qual­i­fied state­ment about the pos­si­bil­i­ties and lim­i­ta­tions of this method, it re­quires co­or­di­nated re­search.”

Poss­ner’s vir­tual clouds are help­ing to plan the next stage of the project – where sea­wa­ter will be sprayed into real clouds, rather than nu­mer­i­cal ones, to mea­sure what hap­pens. At first, the plan is to run ex­per­i­ments on land at Moss Land­ing on the Cal­i­for­nian coast be­fore start­ing tri­als out at sea. Ex­actly when these ex­per­i­ments will hap­pen is de­pen­dent on fund­ing. “If you want to see whether this will work, you’ve got to test it in the field,” says Poss­ner. “That’s where ma­rine cloud bright­en­ing has a ben­e­fit – you can test it in the small scale without it hav­ing a long-term im­pact. Sea salt sed­i­ments out of the at­mos­phere quickly, and you’re not spray­ing any­thing that isn’t there al­ready.”

For Poss­ner, the big draw of cloud bright­en­ing re­search is un­der­stand­ing the ba­sic phys­i­cal process of how mi­cro­scopic par­ti­cles, such as sea salt and dust, in­ter­act with the at­mos­phere to gen­er­ate and grow clouds. These aerosol-cloud in­ter­ac­tions, as they are known, are one of the big­gest un­knowns in cli­mate science, and fig­ur­ing them out will give sci­en­tists a much bet­ter idea of what’s in store for our frag­ile planet.

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