UN climate report: Humans are ‘dominant’ cause of global warming

A United Nations climate science panel has concluded there’s at least a 95 percent chance human activities are the main driver of global warming over the last six decades.

“Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia,” states Friday’s report from the U.N.’s Intergovernmental Panel on Climate Change (IPCC).



{mosads}“Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes,” the report finds. “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”

“Extremely likely” in the U.N.’s parlance means at least a 95 percent chance. That’s greater confidence than the last big IPCC report in 2007, which found it “very likely” that humans are the main cause, which signaled at least 90 percent confidence.



Advocates of more aggressive international and U.S. steps to limit emissions quickly seized on the findings. 



U.N. Secretary-General Ban Ki-moon said “the heat is on” and used the report to press for U.N.-hosted climate talks to reach a binding global climate pact in late 2015, the target for the often-fractious negotiations to conclude. 



The hoped-for pact would go into effect in 2020.



“You have used the world’s best science to address the world’s biggest challenge. This new report will be essential for governments as they work to finalize an ambitious legal agreement on climate change in 2015,” the secretary-general, via videoconference, said at an IPCC press conference in Stockholm Friday.


But the report is certain to become the stuff of Capitol Hill political fights at a time when the White House is pressing ahead with new executive actions on climate change, including carbon emissions standards for power plants.

And the IPCC is under attack from climate skeptics who challenge the scientific consensus on human-induced climate change.

Skeptics accuse the IPCC of pursuing a political agenda and have seized on slower-than-predicted rises in surface temperatures over the last 10 to 15 years. The record-warm 2010 was scarcely warmer than 1998.

In a letter to the State Department’s climate envoy Thursday, four Senate Republicans accuse the Obama administration of “seeking to downplay the current, 15-year hiatus in global temperature increases.”

The letter from Sen. David Vitter (R-La.) and three colleagues alleges the slowdown is at odds with those who are “demanding immediate and costly international and U.S. actions on the basis of these previous IPCC predictions.”
The report projects continued warming that will bring increasingly strong weather and that temperatures are at risk of rising beyond 2 decrees Celsius, the level that many scientists say will trigger the most dangerous climatic changes.



{mossecondads}“Global surface temperature change for the end of the 21st century is projected to be likely to exceed 1.5°C relative to 1850 to 1900 in all but the lowest scenario considered, and likely to exceed 2°C for the two high scenarios,” said Thomas Stocker, co-chair of an IPCC working group that produced the report. 



“Heat waves are very likely to occur more frequently and last longer. As the Earth warms, we expect to see currently wet regions receiving more rainfall, and dry regions receiving less, although there will be exceptions,” he said in a statement.

The report provides a projection of future temperature rises in several groupings. 



The lower end of the lowest range is 0.3 degrees Celsius by the end of this century, while the highest range peaks at 4.8 degrees Celsius. That’s a range of 0.5 degrees and 8.6 degrees Fahrenheit. 



But it notes that lower ranges of temperature rise would require major cuts to greenhouse gas emissions.



“If no action is taken, no way will you be in the lower band,” said Michel Jarraud, head of the U.N.’s World Meteorological Organization, at the press conference.

While the last decade has been the warmest on record, the report acknowledges that temperatures rises over the last roughly 15 years have been lower than models have simulated. 



The report notes with “high confidence” that “the long-term climate model simulations show a trend in global-mean surface temperature from 1951 to 2012 that agrees with the observed trend.”

But it adds:
“There are, however, differences between simulated and observed trends over periods as short as 10 to 15 years (e.g., 1998 to 2012).”



It provides several reasons why the slowdown may have occurred, including redistribution of heat within the ocean, volcanic eruptions and solar cycles, but acknowledges considerable uncertainty about the current ability to quantify them.

It also acknowledges that some models might have overestimated the temperature response to increasing greenhouse gas emissions and other “anthropogenic forcing.”

The report’s architects sought Friday to push back against climate skeptics.

“The relationship between warming and these emissions of greenhouse gases is a very robust one, but it is clear there are phases of natural variability,” Stocker said.

Stocker criticized skeptics’ emphasis on the warm 1998 as the starting point when pointing to a lack of warming.



“When one analyzes longer-term periods, it is not such an unusual case,” Stocker said of the slowdown. “In particular, it is interesting to note that people always pick 1998 as the starting date of these trends. Of course if you pick a special year like 1998 that is characterized by very strong, actually the second strongest El Niño year in the 20th century, then of course that contributes to a trend that is different from the long-term, multi-decadal or century timescale trend,” he said in reference to the ocean cycle called El Niño that influences the climate.



“That, coinciding with a series of medium- to low-strength volcanic eruptions over the last five years or so, plus conditions that rather look like La Niña, which have a global impact, a weak global impact, we are in just in a situation where it is like if you cast three dice … there is a chance that you have three faces of six or three faces of one,” he added at the press conference.

— This story was updated at 10:07 a.m.