Separating Science From Spin on the Global-Warming ‘Pause’

What’s causing a temporary slowdown in planetary warming, and why should anyone worry that more warming is coming?

Volcanic eruptions, such as this one of Mount Rinjani in Indonesia, are fueling a "pause" in global warming.
National Journal
Marina Koren and Patrick Reis
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Marina Koren Patrick Reis
Aug. 21, 2013, 10:40 a.m.

“If the plan­et is warm­ing, why have tem­per­at­ures been steady for a dec­ade?”

That ques­tion is now the go-to coun­ter­point for glob­al-warm­ing skep­tics, and it has long been a stick­ing point for sci­ent­ists as they try to ex­plain their cli­mate con­clu­sions to an in­creas­ingly po­lar­ized pub­lic.

The de­bate was re­born anew last week when a leaked draft of the United Na­tions’ In­ter­gov­ern­ment­al Pan­el on Cli­mate Change up­com­ing re­port con­ceded that warm­ing has largely paused over the past dec­ade, prompt­ing out­cry from skep­tics and lead­ing con­ser­vat­ive news out­lets (in­clud­ing Fox News) to play up the pause in their re­port­ing.

Cli­mate sci­ent­ists largely agree that warm­ing has paused over the past dec­ade (es­pe­cially in meas­ure­ments of sur­face tem­per­at­ure), but they say that break is tem­por­ary, and the near-con­sensus on hu­man-caused glob­al warm­ing re­mains un­broken.

“There has been a slow­down or hi­atus in the rate of change of glob­al tem­per­at­ure in the 21st cen­tury, and that’s real,” says Dav­id Gutz­ler, an earth- and plan­et­ary-sci­ences pro­fess­or at the Uni­versity of New Mex­ico who con­trib­uted to the IP­CC re­port. “Most of us think that this is prob­ably a tem­por­ary hi­atus as op­posed to a ces­sa­tion of glob­al warm­ing.”

So if glob­al warm­ing is still the fu­ture, what’s caus­ing the tem­por­ary pause, and why should any­one worry that more warm­ing is com­ing?

The IP­CC re­port at­trib­utes this hi­atus to short-term factors that res­ult in tem­por­ary cool­ing peri­ods, in­clud­ing vol­ca­noes, sol­ar cycles, ab­sorb­ent oceans, non-green­house-gas pol­lut­ants, and a string of oth­er tem­por­ary-yet-power­ful nat­ur­al forces.

Blame the vol­ca­noes

While green­house gases are trap­ping heat, vol­ca­noes are do­ing their best to block it out.

Vol­can­ic erup­tions send large quant­it­ies of sul­fur di­ox­ide in­to the stra­to­sphere, a slice of Earth’s at­mo­sphere that be­gins about eight miles above the earth’s sur­face. These emis­sions — known as vol­can­ic aer­o­sols — block the sun’s light and heat from reach­ing the lower at­mo­sphere and heat­ing the plan­et.

The aer­o­sols do not stay in the stra­to­sphere per­man­ently, but they can linger for years. And de­pend­ing on the fre­quency and size of vol­can­ic erup­tions over a giv­en peri­od, the aer­o­sols’ con­cen­tra­tion in the at­mo­sphere waxes and wanes. In eras of in­creas­ing con­cen­tra­tions, the aer­o­sols form a more ef­fect­ive heat shield around the plan­et and tem­por­ar­ily work against glob­al warm­ing.

That’s ex­actly what’s hap­pen­ing right now, ac­cord­ing to a study re­leased in March. Since 2000, vol­can­ic aer­o­sols in­creased their heat-block­ing abil­ity by between 4 per­cent and 7 per­cent, ac­cord­ing to the study from a team of re­search­ers at NASA, the Na­tion­al Ocean­ic and At­mo­spher­ic Ad­min­is­tra­tion, the Uni­versity of Col­or­ado, and else­where.

But while vol­ca­noes are con­trib­ut­ing to a tem­por­ary slow­down in glob­al warm­ing, they’re not a per­man­ent solu­tion, said Col­or­ado pro­fess­or Bri­an Toon. “Over­all these erup­tions are not go­ing to counter the green­house ef­fect,” Toon said in a state­ment ac­com­pa­ny­ing the study’s re­lease. “Emis­sions of vol­can­ic gases go up and down, help­ing to cool or heat the plan­et, while green­house-gas emis­sions from hu­man activ­ity just con­tin­ue to go up.”

The sun is dim­mer … for now

Dur­ing the 20th cen­tury, sci­ent­ists be­lieved that the sun emit­ted en­ergy stead­ily enough that it couldn’t sig­ni­fic­antly af­fect the Earth’s cli­mate. In 2001, a Sci­ence study found that sol­ar highs and lows co­in­cided with ter­restri­al cli­mate cycles. It was a long-term trend — the cli­mate of the north­ern At­lantic Ocean has warmed and cooled nine times in the last 12,000 years. The sun un­der­goes small-scale changes in strength too, and throughout most of the 1900s, a con­sid­er­able in­crease made sci­ent­ists won­der if short-term hot­ter out­put made for a hot­ter Earth. But then, by the be­gin­ning of this cen­tury, the sun’s strength de­clined, and warm­ing of the Earth’s sur­face tem­per­at­ure stalled.

“If the sun had been warm­ing the Earth, that should have come to an end, and we should have seen tem­per­at­ures start to go the oth­er way,” space en­vir­on­ment phys­ics pro­fess­or Mi­chael Lock­wood told Na­tion­al Geo­graph­ic in 2007. But that didn’t hap­pen; des­pite the ebb and flow of sol­ar out­put, Earth stead­ily con­tin­ued to heat up.

While both mil­len­ni­al and decadal fluc­tu­ations in sol­ar strength can con­trib­ute to changes in Earth’s cli­mate, sci­ent­ists say the ef­fects of in­creas­ing green­house gases in the at­mo­sphere far out­weigh our sun’s ef­fects.

Green­house gases heat the plan­et, oth­er pol­lut­ants cool it

The pro­duc­tion and use of fossil fuels and green­house gases of­ten pro­duces soot and ash, pol­lut­ants that rise in­to the air. There, they can change the phys­ic­al prop­er­ties of clouds by mak­ing them more re­flect­ive. Like vol­can­ic aer­o­sols, these cloud-dwell­ing pol­lut­ants block the sun’s heat.

The clouds re­flect more of the sun’s en­ergy back in­to space, keep­ing the sur­face tem­per­at­ure of the Earth from rising. In an in­ter­est­ing twist, pro­grams that aim to re­duce air pol­lu­tion in heavy in­dus­tri­al areas could end up free­ing some room for the sun’s rays to pen­et­rate the at­mo­sphere.

The oceans have been stock­pil­ing ex­tra heat

As the Earth ac­cu­mu­lates heat, that ad­di­tion­al en­ergy has to go some­where — deep in­side the high seas. The Earth’s oceans have be­come warm­er since 1955, ac­cord­ing to the En­vir­on­ment­al Pro­tec­tion Agency.

The ocean-at­mo­sphere sys­tem works like this: As oceans store heat en­ergy, the rate at which the at­mo­sphere warms slows. Oceans don’t al­ways take in heat the same way — changes in cur­rents and tem­per­at­ures that oc­cur nat­ur­ally can change the rate of up­take every few dec­ades, said Gutz­ler, al­ter­ing the sur­face tem­per­at­ure of the Earth. In the last dec­ade, the oceans’ ab­sorp­tion of heat has con­trib­uted to cool­ing of that tem­per­at­ure rather than warm­ing. But again, this decadal fluc­tu­ation can cloud the big­ger pic­ture.

It’s all really, really com­plic­ated

Cli­mate mod­els in­cor­por­ate vast num­bers of dy­nam­ic factors, everything from melt­ing per­ma­frost in Rus­sia to coal de­vel­op­ment in China to de­for­est­a­tion in the Amazon. Sci­ent­ists are still scram­bling to provide a more nu­anced un­der­stand­ing of what glob­al warm­ing will look like. In­deed, many of the forces that checked warm­ing over the past dec­ade may ac­cel­er­ate it in years to come.

With­in that but­ter­fly-ef­fect-like chaos, Gutz­ler said it’s pos­sible that the pre­dict­ive cli­mate mod­els sci­ent­ists use are par­tially wrong — not about the fact that the plan­et is warm­ing, but about how, when, and where that warm­ing will oc­cur.

“Just like weath­er, once you get a month or two out, in­di­vidu­al events are un­pre­dict­able, like decadal events are un­pre­dict­able,” he said.

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