Blokerer solen for at kontrollere den globale opvarmning

Det lyder som noget ud af en dårlig science fiction-film - kunstig blokering af sollys for at forhindre global opvarmning i at overophede Jorden. Alligevel, en lille kadre af forskere studerer muligheden - så hvis menneskeheden nogensinde får brug for den, det vil være en informeret beslutning.

Den seneste rapport fra Intergovernmental Panel on Climate Change (IPCC), udgivet i begyndelsen af ​​august, gjort det klart, at menneskeheden er nødt til at tage øjeblikkelig handling for at bremse den globale opvarmning. Der er håb om, at internationale klimaforhandlinger i Glasgow i november endelig kan resultere i stærke nok drivhusgasemissionsgrænser til at gøre en forskel.

Men for en sikkerheds skyld, en international gruppe af forskere, herunder NTNU's Helene Muri, har studeret en teknologi kaldet solar geoengineering som en sidste udvej.

Solar geoengineering er præcis, hvad det lyder som, hvor forskellige teknologier bruges til at blokere sollys og afkøle Jorden. Typisk, tre hovedtilgange – hvoraf ingen i øjeblikket er teknologisk klar – bliver undersøgt for deres evne til at blokere for sollys og sænke jordtemperaturen. (Se boks)

Muri, en seniorforsker ved universitetets industriel økologiprogram, har brugt det sidste årti på at se på, hvordan solar geoengineering måske – eller måske ikke – fungerer.

I juni, hun og hendes kolleger fra USA, Kina og Storbritannien udgav et papir i Natur mad der brugte computermodeller til at vurdere solar geoengineering's potentielle effekter på landbruget i en verden med høj emission. Deres resultater udløste international mediedækning, fordi de fandt ud af, at solar geoengineering i disse scenarier faktisk kunne have en positiv effekt på afgrødevækst fra højere luftfugtighed.

Andre undersøgelser, der brugte enklere modeller, fandt enten en begrænset effekt eller tab for regnfodrede afgrøder, da der kan falde mindre nedbør med de lavere temperaturer, der følger med solar geoengineering - afhængigt af den måde teknologien bruges til at afkøle Jorden.

Nu, mens verden forbereder sig på at debattere grænser for CO ₂ emissioner under novembers klimaforhandlinger, det er værd at se på de foranstaltninger, der undersøges af forskere som Muri – og en vurdering af deres mulige risici og faldgruber.

Band-aid eller tourniquet?

Enhver diskussion af solar geoengineering må erkende, at det er langt fra en perfekt løsning, siger Muri.

"Solar geoengineering, uanset hvor godt vi gør det, vil aldrig perfekt opveje virkningerne af klimaændringer, " hun sagde.

Problemet er, at solar geoengineering kan afkøle Jorden, men slipper ikke af med det overskydende kuldioxid og andre varmefangende stoffer i atmosfæren. Og kuldioxid gør mere end blot at opvarme Jorden.

Det gøder planter - hvilket kunne være en god ting - men fordi meget af det bliver opløst i havvand, det gør havene mere sure.

"Der vil altid være ting, som du ikke kan fikse med solar geoengineering, specifikt havforsuring, " sagde hun. "Et mere surt hav påvirker alt i fødekæderne i havet, herunder koralrevsdød, hvilket er forfærdeligt for økosystemet som helhed. Det bliver tydeligt, så snart du virkelig begynder at se på det. Der er ingen sølvkugle. Det er ikke den ene løsning, der kan ordne alt."

Muri siger, at enhver diskussion om geoengineering også forudsætter, at CO ₂ emissioner vil blive behandlet samtidig med, at enhver solar geoengineering er indsat.

Alan Robock, en klimaforsker ved Rutgers University i USA, der er leder af et internationalt samarbejdsprojekt ved navn GeoMIP, som Muri er en del af, var enige.

"Det er ikke en løsning på den globale opvarmning, når det er bedst. Hvis det nogensinde blev brugt som et plaster - eller en mundkurv - løser det ikke rodproblemet, " han sagde.

Mange ukendte, men skal stadig vide det

Muri siger, at der stadig er meget, der er ukendt om solar geoengineering, dels fordi det meste af klimaændringsforskningen er fokuseret på andre emner end geoengineering.

"Bare for at sætte forskningsniveauet ind i kontekst, i de sidste fem til 10 år, der har været omkring 100 til 130 artikler udgivet om året om solar geoengineering, " sagde hun. "Når det kommer til klimaændringer, er det mere som 30, 000 papirer om året i den periode. Det vigtige er, at det er en meget, meget forskelligt beløb. Det er kun et mindretal af indsats og finansiering, der går til forskning i solar geoengineering."

På samme tid, hun siger, US National Academies of Sciences, Engineering and Medicine udgav en omfattende rapport om solar geoengineering, der sagde, at de presserende risici ved klimaændringer betød, at "USA burde forfølge et forskningsprogram for solar geoengineering - i koordinering med andre nationer, underlagt styring, og sammen med en robust portefølje af klimaafbødnings- og tilpasningspolitikker." Rapporten anbefalede amerikansk finansiering på omkring $100 millioner-$200 millioner over de første fem år.

Muri siger, at klimaforskernes hovedfokus skal forblive på selve klimaforandringerne, fordi samfundet har brug for at vide, hvad virkningerne vil være, hvordan man tilpasser sig, og hvordan man afbøder disse effekter. Alligevel, hun siger, forskere er nødt til at studere solar geoengineering for at se, om det kunne være nyttigt som en foranstaltning, mens verden skifter væk fra fossile brændstoffer.

"Spørgsmålet er, om det kunne bidrage til at reducere et vist niveau af skade fra klimaændringer i en vis periode, mens vi forsøger at sortere begge udledninger af CO ₂ og koncentrationer af CO ₂ inden for klimasystemet, " she said. "Nobody sees it as a one and only solution, but it's not clear yet whether it could be helpful or not. I øjeblikket, there are too many unknowns and uncertainties to really say whether it's overall a good idea or a bad idea."

Robock agrees.

His group at Rutgers University is "doing research to evaluate the risks of doing solar geoengineering versus the risks of not doing it. And that's the information that governments will need in the future to decide whether or not to ever implement it, " he said. "I spend millions of dollars of taxpayer money to do my research. And if I find a danger to society, it's my obligation to warn people about it."

A cooler Earth but potentially changed monsoons

Robock's group is looking at the benefits and risks of using stratospheric aerosols to cool the planet, which emulate a volcanic eruption.

"Benefit number one would be, if you could do it, you would reduce global warming, and many of its risks, " Robock said. "We know that if you could get the aerosols up there, it would work because it doesn't involve creating or affecting clouds in the troposphere, it's just putting a shield up there to reflect sunlight."

Researchers know that big volcanic eruptions, like the 1991 eruption of Mount Pinatubo, cooled the Earth. But these natural solar geoengineering experiments have also given them the ability to observe other pitfalls, Robock said.

"We know that there were other things that were not so good; (the eruption) destroyed ozone, " he said. "And you actually get a huge reduction of monsoon rainfall. We observed that after Mount Pinatubo."

Volcanic eruptions only cause the Earth to cool for a year or two, because the aerosols eventually dissipate. Imidlertid, if stratospheric aerosols were to be used as solar geoengineering to cool the Earth, their use could alter monsoon rainfall for a much longer period, which could result in famine, Robock said.

Some modeling has shown that solar geoengineering could in fact have less of an impact on monsoons than global warming, but nevertheless, the issue illustrates just how difficult making these predictions are.

Who decides?

Then there are issues such as insect-borne diseases, like malaria, Muri points out. How would solar geoengineering affect mosquito populations and the potential spread of malaria?

And what if a failure to cut CO ₂ emissions and reduce global warming results in devastating heat waves, where thousands of people die? Is that enough to outweigh other negatives?

"There are still so many areas where we don't know enough, " hun sagde.

Endelig, there are areas that are far outside of what climate scientists who study the physical effects of climate change can predict. The biggest question is who decides what the temperature of the planet should be?

The political decision making surrounding solar geoengineering is daunting, if you consider the difficulty the nations of the world have already had in trying to agree to curb CO ₂ emissioner, Muri said.

"How would one deal with geoengineering in terms of geopolitics and governance?" Muri said. "We need to develop regulations. Who sets the thermostat and how would you go about agreeing on something like that?"

In a companion piece to Muri and her colleague's article on geoengineering and agriculture, Ben Kravitz, an assistant professor at Indiana University's Earth and Atmospheric Sciences Department, summed it up like this.

"Agriculture is one important piece in our understanding of the effects of climate engineering, " he wrote. "Gaining a better picture of the impacts of climate engineering requires looking at numerous effects in addition to food supply, including water security, geopolitik, and environmental justice…. It is important to figure out whether climate engineering would ultimately be more or less risky than climate change (and to whom)."

What is solar geoengineering?

Researchers are studying a number of engineering approaches as possible methods for cooling the planet. The three described here have been identified by a March report by the US National Academies of Sciences, Engineering and Medicine as meriting further study. The three approaches either rely on controlling the amount of sunlight reaching the Earth, or reducing the amount of heat trapped by the atmosphere.

Stratospheric aerosol injection

This technique requires injecting aerosol particles, like sulfates or pre-cursor gases, like sulfur dioxide, into the stratosphere, which is the layer of air 10 to 50 km above the Earth's surface. Most studies are looking at placing aerosols at about 20 km above the Earth, where the particles scatter and reflect solar radiation to cool the planet. This technique mimics what happens with large volcanic eruptions. When Mount Pinatubo erupted in 1991, it sprayed 15 to 20 megatons of sulfur dioxide into the atmosphere, which cooled the Earth by about 0.4 degrees Celsius for two years. I øjeblikket, imidlertid, there are no planes capable of flying into the stratosphere to do this.

Cirrus cloud thinning

This technique involves spraying chemicals into cirrus clouds, at about 6-13 km above the Earth's surface, to cause them to thin or disappear. The clouds trap heat, so thinning them or reducing them cools the planet by allowing heat to escape the atmosphere. The challenge for this technique is that cirrus clouds are in the region of the atmosphere where jets fly, which could make implementing this measure difficult.

Marine cloud brightening

This approach would add particles to low laying liquid clouds over the ocean to make them thicker and more reflective, which would cool the Earth, if it did not have side effects on other clouds. This mimics what happens now under certain conditions when ships spew pollution into the atmosphere. The effect only works for a few days, and sea salt could be sprayed up from the ocean to seed the clouds.