A United Nations climate science panel has concluded global warming is “unequivocal” and there’s at least a 95 percent chance human activities are the main driver of temperature increases over the last six decades.
“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 UN’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 human’s are the main cause, signaling at least 90 percent confidence.
Advocates of more aggressive international and U.S. steps to limit emissions quickly seized on the findings.
UN Secretary General Ban Ki-moon said “the heat is on” and used the report to press for UN-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 video conference, said at an IPCC press conference in Stockholm, Sweden on Friday.
The report finds that continued warming will bring increasingly strong weather.
“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 UN’s World Meteorological Organization, at the press conference.
The report acknowledges that temperatures rises over the last roughly 15 years have been lower than models have simulated.
While the last decade has been the warmest on record, average surface temperatures have scarcely risen, and climate skeptics have seized on this “hiatus” to question both the U.N. panel’s work and, in the U.S., White House climate policies.
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.” However, 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 in trying to quantify them.
It also acknowledges that some models may have overestimated the temperature response to increasing greenhouse gas emissions and other “anthropogenic forcing.”
The report’s architects on Friday sought 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 post was last updated at 6:37 a.m.