From a citizen perspective, there are three main questions in climate change science: What happened? Who done it? How bad is it going to get? Those questions roughly correspond to the scientific issues of detection, attribution, and climate sensitivity.
All scientists, including skeptics, agree with the IPCC’s assessment that warming of the climate system since the 1950s is “unequivocal.” In contrast, IPCC climate sensitivity estimates are increasingly controversial due to the growing divergence between climate model projections and observed temperatures. The chart below by University of Alabama in Hunstville (UAH) atmospheric scientist Roy Spencer illustrates — and satirizes — the divergence:
Earlier this week, I reviewed a comment letter by Spencer’s colleague John Christy on the scientific basis of EPA’s Clean Power Plan. To recap, EPA’s climate change endangerment analysis is largely based on IPCC climate models. Christy challenges not only the IPCC’s climate sensitivity estimates, but also the IPCC’s claim that most warming since 1951 is anthropogenic. That “attribution” assessment is the core of what IPCC-affiliated scientists and their allies call “the consensus.”
Today’s post offers an historical perspective on Christy’s critique of the consensus position.
In his comment letter, Christy magnifies and analyzes a section of a chart (Figure 10.SM.1) “buried . . . without comment” in Supplementary Material for the IPCC Fifth Assessment Report chapter on climate change detection and attribution.
The chart reveals that, during 1979-2011, the range of observed temperatures in the tropical troposphere (the area between the white lines) falls within the range of climate simulations from models forced with natural variability only (the blue area) and outside the range of simulations from models forced with both natural variability and greenhouse gas emissions (the red area). Christy comments:
This IPCC figure shows that the white-bordered observational envelope lies completely within the blue envelope of models which have no extra greenhouse gas forcing. Thus, the proper scientific conclusion here is that the models demonstrate that CO2 has had no discernable impact in the atmospheric region where models assert greenhouse gas impacts should be largest.
In other words, natural variability accounts for all warming of the bulk tropical atmosphere since the start of the satellite record (the past 34 years). Remarkably, the IPCC’s analysis implies that Nature done it!
Christy’s finding clashes with the IPCC’s official story arc of ever-greater scientific certainty. Let’s review the IPCC’s climate change attribution assessments from 1990 to the present. Bear in mind that, in IPCC parlance, “likely” means a probability greater than 66%; “very likely,” greater than 90%; and “extremely likely,” greater than 95%.
The IPCC’s First Assessment Report (FAR), published in 1990, did not declare anthropogenic global warming to be a fact. Rather, it stated (p. 6):
The size of the warming is broadly consistent with predictions of climate models, but it is also of the same magnitude as natural variability. Thus the observed increase could be largely due to this natural variability; alternatively this variability and other human factors could have offset a still larger human-induced greenhouse warming. The unequivocal detection of the enhanced greenhouse effect is not likely for a decade or more.
The Second Assessment Report (SAR), published in 1995, famously concluded (p. 22):
The balance of evidence, from changes in global mean surface air temperature and from changes in geographical, seasonal and vertical patterns of atmospheric temperature, suggests a discernible human influence on global climate.
Although expressing greater certainty than the FAR, the SAR’s iconic formulation is still not an assertion of what is demonstrably true, only what the “balance of evidence” “suggests.” As the SAR explained:
Our ability to quantify the human influence on global climate is currently limited because the expected signal is still emerging from the noise of natural variability, and because there are uncertainties in key factors.
As expected, the IPCC’s Third Assessment Report (TAR), published in 2001, proclaimed increasing certainty that most recent warming is anthropogenic (p. 10):
There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities. . . . In the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.
The Fourth Assessment Report (AR4), published in 2007, expressed even more certitude (p. 10):
Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.
The Fifth Assessment Report (AR5), published in 2013, essentially declared the matter settled (p. 17):
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.
However, if Christy’s analysis is valid, climate change attribution may be less certain now than it was in 1990. In the FAR, the IPCC said observed warming was “broadly consistent” with climate model predictions although of the same magnitude as natural variability. In the AR5 information unpacked by Christy, observed warming remains of the same magnitude as natural variability but is inconsistent with model predictions.
I realize that IPCC attribution statements refer to a period longer than the satellite record, and are based on analyses of several components of the climate system, not just temperature change in the tropical troposphere (AR5, Ch. 10).
Nonetheless, as Christy points out, the tropical troposphere is the portion of the atmosphere where models project a “highly consistent and significant” warming response to rising CO2 concentrations. If the models do not show a discernible human influence in the tropical troposphere, how can they show that more than half the warming is anthropogenic for the planet as a whole?
Christy’s critique of the core “consensus” position is serious and substantial. Suppose IPCC-affiliated scientists cannot refute it. How uncertain does the claim that most recent warming is anthropogenic become?
Should “extremely likely” be demoted to “very likely” or just “likely”? Are we back to a debate about what the “balance of evidence” “suggests”? Or would an honest broker say that “unequivocal detection of the enhanced greenhouse effect is not likely for a decade or more”?
By law, agencies are required to respond to every unique and salient comment they receive on their regulatory proposals. Alas, it is more than “extremely likely” that no amount or quality of scientific criticism will dissuade EPA from promulgating the Clean Power Plan.