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
Add to Briefcase
See more stories about...
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.

What We're Following See More »
Morning Consult Poll: Clinton Decisively Won Debate
2 days ago

"According to a new POLITICO/Morning Consult poll, the first national post-debate survey, 43 percent of registered voters said the Democratic candidate won, compared with 26 percent who opted for the Republican Party’s standard bearer. Her 6-point lead over Trump among likely voters is unchanged from our previous survey: Clinton still leads Trump 42 percent to 36 percent in the race for the White House, with Libertarian nominee Gary Johnson taking 9 percent of the vote."

Trump Draws Laughs, Boos at Al Smith Dinner
2 days ago

After a lighthearted beginning, Donald Trump's appearance at the Al Smith charity dinner in New York "took a tough turn as the crowd repeatedly booed the GOP nominee for his sharp-edged jokes about his rival Hillary Clinton."

McMullin Leads in New Utah Poll
2 days ago

Evan McMul­lin came out on top in a Emer­son Col­lege poll of Utah with 31% of the vote. Donald Trump came in second with 27%, while Hillary Clin­ton took third with 24%. Gary John­son re­ceived 5% of the vote in the sur­vey.

Quinnipiac Has Clinton Up by 7
2 days ago

A new Quin­nipi­ac Uni­versity poll finds Hillary Clin­ton lead­ing Donald Trump by seven percentage points, 47%-40%. Trump’s “lead among men and white voters all but” van­ished from the uni­versity’s early Oc­to­ber poll. A new PPRI/Brook­ings sur­vey shows a much bigger lead, with Clinton up 51%-36%. And an IBD/TIPP poll leans the other way, showing a vir­tu­al dead heat, with Trump tak­ing 41% of the vote to Clin­ton’s 40% in a four-way match­up.

Trump: I’ll Accept the Results “If I Win”
2 days ago

Welcome to National Journal!

You are currently accessing National Journal from IP access. Please login to access this feature. If you have any questions, please contact your Dedicated Advisor.