The global warming “hiatus,” a controversy that spawned congressional hearings and thousands of skeptical blog posts before being curbed last year, is back.
Global Warming Slowed Down in 2000s? Global Warming is Back
The “hiatus” refers to the observation that global warming has slowed in the past 15 years. The planet is still warming, but just not as quickly as some climate scientists expected it to.
The debate between researchers and doubters reached a crescendo last summer, when scientists at the National Oceanic and Atmospheric Administration updated their temperature records and concluded that global warming has not slowed down in the 2000s (ClimateWire, June 5, 2015).
Now, a group of prominent climate scientists are challenging NOAA’s conclusion in a commentary published this week in Nature Climate Change.
“The interpretation [the NOAA group] made was not valid,” said John Fyfe, a climate scientist at Environment and Climate Change Canada and lead author of the commentary. “The slowdown is there, even in this new updated data set.”
The disagreement may seem esoteric, but it underpins the biggest climate disagreement of the past decades. Climate models, which are virtual representations of our planet, project that temperatures were much higher in the early 2000s than was the case in reality. Scientists have been trying to understand why.
Suggestions abound, from cooling aerosols spewed by volcanic eruptions to natural shifts in the Pacific Ocean that happen every decade.
Meanwhile, skeptics have seized on the mismatch to suggest that global warming stopped in 1998. Almost all scientists disagree with this. But there are questions about the rate of warming. Most recently, the NOAA study suggested that rising temperatures never even slowed.
The NOAA study’s release last summer coincided with a science meeting in Colorado where scientists were discussing how to engineer models to accurately predict climate changes in the coming decades on a regional scale. To do so, they would first have to figure out why models had not projected the global warming slowdown.
With the NOAA study’s release, there was this perception that, “Oh, there’s been no slowdown in warming,” said Gerald Meehl, a climate scientist at the National Center for Atmospheric Research and a co-author of the new commentary.
The scientists decided to counter the narrative in the boxing ring of academia. That is, a science journal.
It is possible that the scientific disagreement could spill over into the skeptic blogosphere. But that is not reason enough to sweep the slowdown under the rug, said Michael Mann, a climatologist at Pennsylvania State University and a co-author.
“As scientists, we must go where the evidence takes us, we can’t allow our worries about climate contrarians and how they might seek to misrepresent our work to dictate what we do and do not publish,” he said.
The blowback against the NOAA study has been some time coming. Tom Karl, lead author of the NOAA study and director of NOAA’s National Centers for Environmental Information, and his colleagues compared warming over the past 15 years with the long-term temperature trend between 1950 and 1998 (a 48-year stretch).
But scientists say Karl’s comparison of a 15-year stretch with a 48-year stretch was somewhat arbitrary. It is meant to answer the question, has global warming stopped in the long run? The answer to that is a resounding “no,” they say.
Scientists are more interested in explaining fluctuations in global temperatures over 10- and 20-year stretches. Throughout Earth’s history, global temperatures have risen and fallen in step with natural fluctuations in the climate system that scientists are only just beginning to unravel.
Karl said that understanding this decadeslong variability is important, and his study had dealt with the long-term trend.
“There is no disagreement that there is decadal variability, and that it is real and needs to be better understood,” he said, referring to natural causes of warming.
‘Wrong type of trees’ in Europe increased global warming
The assumption that planting new forests helps limit climate change has been challenged by a new study.
Researchers found that in Europe, trees grown since 1750 have actually increased global warming.
The scientists believe that replacing broadleaved species with conifers is a key reason for the negative climate impact.
Conifers like pines and spruce are generally darker and absorb more heat than species such as oak and birch.
The authors believe the work has implications for current efforts to limit rising temperatures through mass tree planting.
Europe’s green canopy was dramatically thinned between 1750 and 1850, when the forested area diminished by 190,000 sq km.
Ironically the greater use of fossil fuels, particularly coal, slowed the timber rush, and from 1850 to the present day, Europe’s forests grew by some 386,000 sq km and now cover 10% more land than before the industrial revolution.