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Methane from cat­tle and rice pad­dies next global cli­mate change threat

pub­lished 12 Decem­ber 2016 | mod­i­fied 27 Decem­ber 2016

In a new analy­sis, sci­en­tists raise the alarm about grow­ing con­cen­tra­tions of methane in the atmos­phere — and also high­light the oppor­tu­ni­ties for slow­ing emissions.

Global con­cen­tra­tions of methane, a pow­er­ful green­house gas and cause of cli­mate change, are now grow­ing faster in the atmos­phere than at any other time in the past two decades.

That is the mes­sage of a team of inter­na­tional sci­en­tists in an edi­to­r­ial pub­lished on 12 Decem­ber in the jour­nal Envi­ron­men­tal Research Let­ters. The group reports that methane con­cen­tra­tions in the air began to surge around 2007 and grew pre­cip­i­tously in 2014 and 2015. In that two-​year period, con­cen­tra­tions shot up by 10 or more parts per bil­lion annu­ally. It’s a stark con­trast from the early 2000s when methane con­cen­tra­tions crept up by just 0.5 parts per bil­lion on aver­age each year. The rea­son for the spike is unclear but may come from emis­sions from agri­cul­tural sources and mainly around the trop­ics — poten­tially from farm sites like rice pad­dies and cat­tle pastures.

methane cycleHuman activ­i­ties dom­i­nate the world’s methane cycle, con­tribut­ing roughly 60% of the new methane added to the atmos­phere every year, accord­ing to the 2016 Global Methane Bud­get.
Graphic: Global Car­bon Project, Fon­da­tion BNP Paribas

The find­ings could give new global atten­tion to methane — which is much less preva­lent in the atmos­phere than car­bon diox­ide but is a more potent green­house gas, trap­ping 28 times more heat. And while research shows that the growth of car­bon diox­ide emis­sions has flat­tened out in recent years, methane emis­sions seem to be soaring.

The lev­el­ling off we’ve seen in the last three years for car­bon diox­ide emis­sions is strik­ingly dif­fer­ent from the recent rapid increase in methane
Robert Jack­son, co-​author, Pro­fes­sor in Earth Sys­tem Sci­ence, Stan­ford University »

The results for methane “are wor­ri­some but pro­vide an imme­di­ate oppor­tu­nity for mit­i­ga­tion that com­ple­ments efforts for car­bon diox­ide,” added Jackson.

The authors of the new edi­to­r­ial pre­vi­ously helped to pro­duce the 2016 Global Methane Bud­get. This report pro­vided a com­pre­hen­sive look at how methane flowed in and out of the atmos­phere from 2000 to 2012 because of human activ­i­ties and other sources. It found, for exam­ple, that human emis­sions of the gas seemed to have increased after 2007, although it’s not clear by how much. The methane bud­get is pub­lished every two to three years by the Global Car­bon Project, a research project of Future Earth.

Methane, Jack­son says, is a dif­fi­cult gas to track. In part, that’s because it can come from many dif­fer­ent sources. Those include nat­ural sources like marshes and other wet­lands. But the bulk, or about 60 per­cent, of methane added to the atmos­phere every year comes from human activ­i­ties. They include farm­ing sources like cat­tle oper­a­tions — cows expel large quan­ti­ties of methane from their spe­cialised diges­tive tracks — and rice pad­dies — the flooded soils make good homes for microbes that pro­duce the gas. A smaller por­tion of the human bud­get, about a third, comes from fos­sil fuel explo­ration, where methane can leak from oil and gas wells dur­ing drilling.

Unlike car­bon diox­ide, where we have well described power plants, almost every­thing in the global methane bud­get is dif­fuse,” Jack­son says. “From cows to wet­lands to rice pad­dies, the methane cycle is harder.”

But a range of infor­ma­tion — such as from large-​scale inven­to­ries of methane emis­sions, mea­sure­ments of methane in the air and com­puter mod­els — sug­gests that this cycle has shifted a lot in the last two decades. Jack­son and his col­leagues, for instance, report that the growth of methane in the atmos­phere was mostly stag­nant in 2000 to 2006. But that changed after 2007.

Why this change hap­pened is still not well under­stood,” says Marielle Saunois, lead author of the new paper and an assis­tant pro­fes­sor of Uni­ver­sité de Ver­sailles Saint Quentin and researcher at Lab­o­ra­toire des Sci­ences du Cli­mat et de l’Environnement in France. “For the last two years espe­cially, the growth rate has been faster than for the years before. It’s really intriguing.”

Saunois adds that this run­away pace could threaten inter­na­tional efforts to limit warm­ing from cli­mate change to 2 degrees Cel­sius. The research pro­vides a strong argu­ment that “we should do more about methane emis­sions,” Saunois says. “If we want to stay below 2 degrees tem­per­a­ture increase, we should not fol­low this track and need to make a rapid turn-​around.”

Where does the methane come from?
Pin­point­ing where those methane emis­sions are com­ing from, how­ever, isn’t easy. Many envi­ron­men­tal advo­cates in North Amer­ica have raised con­cerns that expanded drilling for nat­ural gas in recent years could lead to a surge in methane emis­sions. But Saunois says that based on avail­able data, the more likely source, at least for now, is agri­cul­ture. She and her col­leagues aren’t sure what trends may be dri­ving this increase. Accord­ing to the Food and Agri­cul­ture Orga­ni­za­tion of the United Nations, live­stock oper­a­tions around the world expanded from pro­duc­ing 1,300 mil­lion head of cat­tle in 1994 to nearly 1,500 mil­lion in 2014 — with a sim­i­lar increase in rice cul­ti­va­tion in many Asian countries.

Saunois and Jack­son argue, how­ever, that the story isn’t all bad news. A num­ber of researchers have exper­i­mented with dif­fer­ent ways of reduc­ing methane emis­sions from farms. Feed­ing cows a diet sup­ple­mented with lin­seed oil, for exam­ple, seems to reduce the amount of methane they belch out. “When it comes to methane, there has been a lot of focus on the fos­sil fuel indus­try, but we need to look just as hard if not harder at agri­cul­ture,” Jack­son says. “The sit­u­a­tion cer­tainly isn’t hope­less. It’s a real opportunity.”

(Source: Future Earth news release, 12.12.2016)

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