"Long before it's in the papers"
January 27, 2015


New figures suggest global warming not so hopeless

Jan. 27, 2013
Courtesy of Research Council of Norway
and World Science staff

Glob­al warm­ing may be slightly milder than pre­vail­ing es­ti­mates pre­dict, some sci­en­tists are re­port­ing. And though the dif­fer­ence is small, they say, it points to a de­cid­ed im­prove­ment in the con­trol­la­bil­ity of the prob­lem.

“These re­sults are truly sensa­t­ion­al,” said cli­mate sci­ent­ist Car­o­line Leck of Stock­holm Uni­vers­ity, who was not part of the new stu­dy. “If con­firmed by oth­er stud­ies, this could have far-reach­ing im­pacts on ef­forts to achieve the po­lit­i­cal tar­gets for cli­mate.”

The study says cur­rent trends point to a most-likely in­crease in glob­al sur­face tem­per­a­tures of 1.9 de­grees Cel­si­us (3.4 de­grees Fahr­en­heit) by mid-century. Pre­vi­ous, widely ac­cept­ed es­ti­mates put that in­crease at around 3 de­grees Cel­si­us.

Car­bon di­ox­ide, a gas re­leased as a re­sult of hu­man in­dus­t­ri­al ac­ti­vi­ties, is con­sid­ered the most im­por­tant fac­tor caus­ing glob­al warm­ing. A key ques­tion that sci­en­tists have ex­am­ined is how much the av­er­age air tem­per­a­ture will rise if the car­bon di­ox­ide lev­els in the at­mos­phere dou­ble with re­spect to the world’s pre-in­dus­t­ri­alised lev­el around 1750. That’s ex­pected to hap­pen by roughly 2050.

In the new work, re­search­ers at the Uni­vers­ity of Os­lo and the Cen­ter for In­terna­t­ional Cli­mate and En­vi­ron­men­tal Research-Os­lo reached the 1.9 de­grees fig­ure as the most likely tem­per­a­ture in­crease.

“We have worked on find­ing out the over­all ef­fect of all known feed­back mech­a­nisms,” said proj­ect man­ag­er Terje Berntsen. Berntsen and col­leagues said they de­vel­oped a mod­el that con­sid­ered all fac­tors con­tri­but­ing to man-made cli­mate “forc­ings” since 1750. They al­so ac­counted for cli­mate fluctua­t­ions caused by nat­u­ral fac­tors such as vol­can­ic and so­lar ac­ti­vity, and en­tered tem­per­a­tures mea­sure­ments tak­en in the air, ground, and oceans. They used a cli­mate mod­el that re­peat­ed cal­cula­t­ions mil­lions of times.

When they ap­plied the mod­el and sta­tis­tics to an­a­lyze tem­per­a­ture read­ings for the pre-2000 pe­ri­od, they found that the tem­per­a­ture in­crease would most likely be 3.7 de­grees Cel­si­us. But when they en­tered da­ta from 2000 to 2010, the es­ti­mate slid to 1.9 de­grees Cel­si­us. Berntsen said this in­crease would be up­on us af­ter we reach the dou­bled lev­el of car­bon di­ox­ide con­centra­t­ion com­pared to 1750. The tem­per­a­ture will then be sta­ble for a while, as the oceans de­lay the ef­fect by sev­er­al dec­ades, he said.

The 1.9 de­grees fig­ure is just an av­er­age: in more de­tail, the in­ves­ti­ga­tors con­clud­ed with 90 per­cent like­li­hood that warm­ing would lie be­tween 1.2 and 2.9 de­grees Cel­si­us. The pre­vi­ous, high­er es­ti­mate of 3 de­grees—with a prob­a­ble range of 2 to 4.5 de­grees—came from the Intergo­vernmen­tal Pan­el on Cli­mate Change, un­der the aus­pic­es of the Un­ited Na­t­ions.

“The Earth’s mean tem­per­a­ture rose sharply dur­ing the 1990s. This may have caused us to overes­ti­mate cli­mate sen­si­ti­vity,” Bern­st­sen said. “We are most likely wit­ness­ing nat­u­ral fluctua­t­ions in the cli­mate sys­tem—changes that can oc­cur over sev­er­al dec­ades—and which are com­ing on top of a long-term warm­ing. The nat­u­ral changes re­sulted in a rap­id glob­al tem­per­a­ture rise in the 1990s, where­as the nat­u­ral varia­t­ions be­tween 2000 and 2010 may have re­sulted in the lev­elling off we are ob­serv­ing now.”

Berntsen stressed that the find­ings must not be con­strued as an ex­cuse for com­pla­cen­cy in ad­dress­ing hu­man-induced glob­al warm­ing, but they do in­di­cate it may be more with­in our reach to achieve cli­mate tar­gets than pre­vi­ously thought. Pol­i­cy­makers are try­ing to con­tain glob­al warm­ing at less than 2 de­grees Cel­si­us, he said.

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Global warming may be slightly milder than prevailing estimates predict, some scientists are reporting. And though the difference is relatively small, they say, it points to a decided improvement in the controllability of the problem. “These results are truly sensational,” said climate scientist Caroline Leck of Stockholm University, who was not part of the new study. “If confirmed by other studies, this could have far-reaching impacts on efforts to achieve the political targets for climate.” The new study said current trends point to a most-likely increase in global surface temperatures of 1.9 degrees Celsius (3.4 degrees Fahrenheit) by mid-century. Previous, widely accepted estimates put that increase at about 3 degrees Celsius. Carbon dioxide, a gas released as a result of human industrial activities, is considered the most important factor causing global warming. A key question that scientists have examined is how much the average air temperature will rise if the carbon dioxide levels in the atmosphere double with respect to the world’s pre-industrialised level around 1750. That’s expected to happen by roughly 2050. In the new work, researchers at the University of Oslo and the Center for International Climate and Environmental Research-Oslo reached the 1.9 degrees figure as the most likely temperature increase. “We have worked on finding out the overall effect of all known feedback mechanisms,” said project manager Terje Berntsen. Berntsen and colleagues said they developed a model that considered all factors contributing to man-made climate “forcings” since 1750. They also accounted for climate fluctuations caused by natural factors such as volcanic eruptions and solar activity, and entered temperatures measurements taken in the air, ground, and oceans. They used a climate model that repeated calculations millions of times. When they applied the model and statistics to analyze temperature readings for the pre-2000 period, they found that the temperature increase would most likely be 3.7 degrees Celsius. But when they entered data from 2000 to 2010, the estimate slid to 1.9 degrees Celsius. Berntsen said this increase would be upon us after we reach the doubled level of carbon dioxide concentration compared to 1750. The temperature will then be stable for a while, as the oceans delay the effect by several decades, he said. The 1.9 degrees figure is just an average: in more detail, the investigators concluded with 90% likelihood that warming would lie between 1.2 and 2.9 degrees Celsius. The previous, higher estimate of 3 degrees—with a probable range of 2 to 4.5 degrees—came from the Intergovernmental Panel on Climate Change, under the auspices of the United Nations. “The Earth’s mean temperature rose sharply during the 1990s. This may have caused us to overestimate climate sensitivity,” Bernstsen said. “We are most likely witnessing natural fluctuations in the climate system—changes that can occur over several decades—and which are coming on top of a long-term warming. The natural changes resulted in a rapid global temperature rise in the 1990s, whereas the natural variations between 2000 and 2010 may have resulted in the levelling off we are observing now.” Berntsen stressed that the findings must not be construed as an excuse for complacency in addressing human-induced global warming, but they do indicate it may be more within our reach to achieve climate targets than previously thought. Policymakers are trying to contain global warming at less than 2 degrees Celsius, he said.