Christy Crock #3: Internal Variability
by dana1981 & Albatross, Skeptical Science, April 14, 2011
In the recent U.S. House of Representatives Committee on Science Space and Technology climate hearing, Dr. John Christy was the main witness presenting the opinions of the global warming "skeptics." As we previously noted, the quality of Dr. Christy's testimony was extremely disappointing, as he frequently repeated and affirmed climate myths. Among them, Dr. Christy touted the myth that internal variability could be the cause of the current global warming:
"When you look at the possibility of natural unforced variability, you see that can cause excursions that we've seen recently"
As we will see here, this statement is simply false. Natural variability cannot account for the large and rapid warming we've observed over the past century, and particularly the past 40 years.
Swanson and Tsonis
One of the most widely-circulated papers on the impact of natural variability on global temperatures is Swanson et al. (2009), which John has previously discussed.
Although Swanson et al. (2009) was widely discussed throughout the blogosphere and mainstream media, the widespread beliefs that the study attributed global warming to natural variability and/or predicted global cooling were based on misunderstandings of the paper, as Dr. Swanson noted:
"What do our results have to do with Global Warming, i.e., the century-scale response to greenhouse gas emissions? VERY LITTLE, contrary to claims that others have made on our behalf. Nature (with hopefully some constructive input from humans) will decide the global warming question based upon climate sensitivity, net radiative forcing, and oceanic storage of heat, not on the type of multi-decadal time scale variability we are discussing here. However, this apparent impulsive behavior explicitly highlights the fact that humanity is poking a complex, nonlinear system with GHG forcing – and that there are no guarantees to how the climate may respond."
In their paper, Swanson et al. use climate models to hash out the role internal variability has played in average global temperature changes over the past century (Figure 1).
Figure 1. Estimation of the observed signature of internal variability in the observed 20th century global mean temperature in climate model simulations
As you can see, over periods of a few decades, modeled internal variability does not cause surface temperatures to change by more than 0.3 °C, and over longer periods, such as the entire 20th century, its transient warming and cooling influences tend to average out, and internal variability does not cause long-term temperature trends.
Additional Studies
A number of other scientific studies have also examined the impact of internal variability on global temperatures, and arrived at a very similar conclusion to Swanson et al. For example, here are the findings of DelSole et al. (2011) (emphasis added):
"The amplitude and time scale of the IMP [internal multidecadal pattern] are such that its contribution to the trend dominates that of the forced component on time scales shorter than 16 yr, implying that the lack of warming trend during the past 10 yr is not statistically significant...While the IMP can contribute significantly to trends for periods of 30 yr or shorter, it cannot account for the 0.8 °C warming that has been observed in the twentieth-century spatially averaged SST."
This conclusion directly contradicts Christy's statement that natural variability can account for all of the recent warming. This is not a new finding, as it is consistent for example with Stouffer et al. (1994):
"throughout the simulated time series no temperature change as large as 0.5 °C per century is sustained for more than a few decades. Assuming that the model is realistic, these results suggest that the observed trend is not a natural feature of the interaction between the atmosphere and oceans."
and with Wigley and Raper (1990):
"Simulations with a simple climate model are used to determine the main controls on internally generated low-frequency variability, and show that natural trends of up to 0.3 °C may occur over intervals of up to 100 years. Although the magnitude of such trends is unexpectedly large, it is insufficient to explain the observed global warming during the twentieth century."
These studies are also consistent with Bertrand and van Ypersele (2002), Rybski et al. (2006), and Zorita et al. (2008), among others. There is a strong consensus that natural variability cannot account for the observed global warming trend, which raises the question: on what is Christy basing his unsubstantiated claims to the contrary? Although he does not provide any supporting evidence in his congressional testimony, Christy is likely relying on the work of his University of Alabama at Huntsville (UAH) colleague and fellow "skeptic," Roy Spencer.
Spencer's Hypothesis
Dr. Roy Spencer has proposed a hypothesis whereby some unknown internal mechanism causes cloud cover to change, which in turn changes the reflectivity (albedo) of the planet, thus causing warming or cooling. Spencer also attributes most of the global warming over the past century to this "internal radiative forcing." There are some significant flaws in this hypothesis. For one thing, it fails to explain many of the observed "fingerprints" of human-caused global warming, such as the cooling upper atmosphere (stratosphere and above) and the higher rate of warming at night than during the day.
In order for internal variability to account for the global warming over the past century (especially over the past 40 years), it requires that the large greenhouse gas radiative forcing can't have much effect on global temperatures. For this to be true, climate sensitivity must be low. ut as discussed in Swanson et al. (2009), if climate is more sensitive to internal variability than currently thought, this would also mean climate is more sensitive to external forcings, including CO2. This is a Catch-22 for Spencer's hypothesis; it effectively requires that climate sensitivity is simultaneously both low and high.
Dr. Andrew Dessler published a study (Dessler 2010) which casts further doubt on Spencer's hypothesis, as detailed in an email exchange between the two scientists. In short, Dessler argues that cloud cover change is a feedback to a radiative forcing, for example increasing greenhouse gases, while Spencer argues that clouds are changing due to some other, unknown cause, and acting as a forcing themselves. Unlike Spencer, Dessler explains the mechanism and supporting evidence behind his cloud feedback research:
"My cloud feedback calculation is supported by a firm causal link: ENSO causes surface temperature variations which causes cloud changes. This is supported by the iron triangle of observations, theory, and climate models."
El Niño-Southern Oscillation (ENSO)
Although he is very coy about the physical mechanisms behind his hypothesis, Spencer does seem to believe that his hypothesized internal radiative forcing will cause "ENSO-type behavior," such as warming surface air temperatures. However, Trenberth et al. (2002) examined the role ENSO has played in the global warming over the past half-century, and their conclusions do not bode well for Spencer's hypothesis:
"For 1950–1998, ENSO linearly accounts for 0.06 °C of global surface temperature increase."
This 0.06 °C accounts for approximately 12% of the warming trend over the timeframe in question. Foster et al. (2010) also examined the effects of ENSO on global temperature and arrived at the same conclusion.
"It has been well known for many years that ENSO is associated with significant variability in global mean temperatures on interannual timescales. However, this relationship (which, contrary to the claim of MFC09, is simulated by global climate models, e.g., Santer et al. [2001]) cannot explain temperature trends on decadal and longer time scales."
Foster et al. examine a number of previous studies which assessed and removed the effects of ENSO on the global surface temperature (emphasis added):
"In all of these previous analyses, ENSO has been found to describe between 15 and 30% of the interseasonal and longer-term variability in surface and/or lower tropospheric temperature, but little of the global mean warming trend of the past half century."
Pacific Decadal Oscillation (PDO)
ENSO is part of the PDO, which Spencer has also tried to blame for the current global warming. In a post on his blog following up on Spencer and Braswell (2008), Spencer claims:
"The evidence presented here suggests that most of that warming might well have been caused by cloud changes associated with a natural mode of climate variability: the Pacific Decadal Oscillation."
However, as detailed here by Dr. Barry Bickmore in a three-part series, and by Dr. Ray Pierrehumbert at RealClimate, Spencer's attribution of the recent global warming to PDO is no more than an example of how to cook a graph. As Dr. Bickmore put it,
"Spencer's curve-fitting enterprise could (and did!) give him essentially any answer he wanted, as long as he didn't mind using parameters that don't make any physical sense."
Further, as we have previously discussed, like ENSO, PDO physically cannot cause a long-term global warming trend. It is an oscillation which simply moves heat from oceans to air and vice-versa, so even if there were a period of predominantly positive PDO over the long-term, the oceans would cool as a consequence of the transfer of heat to the overlying air. That is not the case: the oceans are warming as well.
It's not Internal Variability
In conclusion, there is simply no supporting evidence or physics behind Christy's unsubstantiated claim that the global warming over the past century could simply be attributed to internal variability. Studies on the subject consistently show that internal variability does not account for more than ~0.3 °C warming of global surface air temperatures over periods of several decades. Internal variability also tends to average out over longer periods of time, as has been the case over the past century, and cannot account for more than a small fraction of the observed warming over that period. Spencer's hypothesis cannot account for numerous observed changes in the global climate (which are consistent with an increased greenhouse effect), does not have a known physical mechanism, and there are simply better explanations for interactions between global temperature and cloud cover.
Dr. Christy was simply wrong to tell our policymakers that natural variability can account for all of the observed global warming over the past century, and particularly the past several decades. One can't help but suspect that Christy was simply telling the Republican policymakers what they wanted to hear. What is more, he knows the state of the science on this issue, and cannot plead ignorance. As such, his false statement was disingenuous and the very antithesis of good science.
NOTE: this post has also been adapted into the Intermediate rebuttal to "it's internal variability"
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