Ok, it is now official:
“The long-term climate model simulations show a trend in global-mean surface temperature from 1951 to 2012 that agrees with the observed trend (very high confidence). There are, however, differences between simulated and observed trends over periods as short as 10 to 15 years”
It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together. The best estimate of the human induced contribution to warming is similar to the observed warming over this period.
The IPCC has officially (and anti-climactically) issued the AR5 WG1 Summary for Policy Makers. I haven’t had time to go through the report in detail, I mainly looked for these two statements. Note the changes in these two statements from the final draft discussed last week:
“Models do not generally reproduce the observed reduction in surface warming trend over the last 10 –15 years.”
“It is extremely likely that human influence on climate caused more than half of the observed increase in global average surface temperature from 1951−2010.”
These changes as a result of the ‘conclave’ this week totally dissonates my cognitives. Well, IPCC has thrown down the gauntlet – if the pause continues beyond 15 years (well it already has), they are toast. Even though they still use the word ‘most’ in the attribution statement, they go all out and pretty much say it is all AGW: “The best estimate of the human induced contribution to warming is similar to the observed warming over this period.”
In case you haven’t been paying attention, ‘extremely likely‘ in the attribution statement implies 95% confidence. Exactly what does 95% confidence mean in this context?
A few days ago, Seth Borenstein of AP attempted to explain What 95% certainty of warming means to scientists. Excerpts:
With the U.N. panel about to weigh in on the effects of greenhouse gas emissions from the burning of oil, coal and gas, The Associated Press asked scientists who specialize in climate, physics, epidemiology, public health, statistics and risk just what in science is more certain than human-caused climate change, what is about the same, and what is less.
They said gravity is a good example of something more certain than climate change. Climate change “is not as sure as if you drop a stone it will hit the Earth,” Princeton University climate scientist Michael Oppenheimer said. “It’s not certain, but it’s close.”
Arizona State University physicist Lawrence Krauss said the 95 percent quoted for climate change is equivalent to the current certainty among physicists that the universe is 13.8 billion years old.
The president of the prestigious National Academy of Sciences, Ralph Cicerone, and more than a dozen other scientists contacted by the AP said the 95 percent certainty regarding climate change is most similar to the confidence scientists have in the decades’ worth of evidence that cigarettes are deadly.
“What is understood does not violate any mechanism that we understand about cancer,” while “statistics confirm what we know about cancer,” said Cicerone, an atmospheric scientist. Add to that a “very high consensus” among scientists about the harm of tobacco, and it sounds similar to the case for climate change, he said.
George Washington’s Gray said the 95 percent number the Intergovernmental Panel on Climate Change will probably adopt may not be realistic. In general, regardless of the field of research, experts tend to overestimate their confidence in their certainty, he said. Other experts said the 95 percent figure is too low.
Jeff Severinghaus, a geoscientist at the Scripps Institution of Oceanography, said that through the use of radioactive isotopes, scientists are more than 99 percent sure that much of the carbon in the air has human fingerprints on it. And because of basic physics, scientists are 99 percent certain that carbon traps heat in what is called the greenhouse effect.
But the role of nature and all sorts of other factors bring the number down to 95 percent when you want to say that the majority of the warming is human-caused, he said.
JC comment: Oh, my aching head.
When writing the uncertainty monster paper, I tried to figure out how the IPCC AR4 came up with the ‘very likely’ (90%) confidence level for the attribution statement. Here is what I found (section 9.4):
“The approaches used in detection and attribution research described above cannot fully account for all uncertainties, and thus ultimately expert judgment is required to give a calibrated assessment of whether a specific cause is responsible for a given climate change. The assessment approach used in this chapter is to consider results from multiple studies using a variety of observational data sets, models, forcings and analysis techniques. The assessment based on these results typically takes into account the number of studies, the extent to which there is consensus among studies on the significance of detection results, the extent to which there is consensus on the consistency between the observed change and the change expected from forcing, the degree of consistency with other types of evidence, the extent to which known uncertainties are accounted for in and between studies, and whether there might be other physically plausible explanations for the given climate change. Having determined a particular likelihood assessment, this was then further downweighted to take into account any remaining uncertainties, such as, for example, structural uncertainties or a limited exploration of possible forcing histories of uncertain forcings. The overall assessment also considers whether several independent lines of evidence strengthen a result.”
Looks like the AR5 forgot to do the ‘down weighting.’ But seriously, this doesn’t tell us where the 90% came from for AR4.
The IAC Review of the IPCC recommended the following:
Chapter Lead Authors should provide a traceable account of how they arrived at their ratings for level of scientific understanding and likelihood that an outcome will occur.
The IPCC uncertainty guidance urges authors to provide a traceable account of how authors determined what ratings to use to describe the level of scientific understanding (Table 3.1) and the likelihood that a particular outcome will occur (Table 3.3). However, it is unclear whose judgments are reflected in the ratings that appear in the Fourth Assessment Report or how the judgments were determined. How exactly a consensus was reached regarding subjective probability distributions needs to be documented.
Yesterday, a reporter asked me how the IPCC came up with the 95% number. Here is the exchange that I had with him: