Climate Change & Tropospheric Temperature Trends

From a review of their methods and results, several comments can be made ¹³.

First, it is odd that Tett and Thorne (hereafter, TT) base their comparison study on the tropics only. This is precisely the latitude band for which lapse rates are largest and trends are most variable for the period they studied. Extant radiosonde and reanalysis products are poorly characterized in this region as well. It is not clear why they did not extend their analysis to include a study of global and high latitude trends, and they offered no explanation for this. Such a study would have been particularly useful because the northern latitudes in particular are where their chosen radiosonde and reanalysis products are relatively well characterized and have good coverage. Furthermore, the high southern latitudes are where we expect the biggest differences between UAH and RSS products prior to correction by the Fu et al. method, and where we expect the largest contamination of the TLT record from sea-ice and summer melt pools signals. A test of the Fu et al. method in these regions would have been far more revealing than the region they chose. TT use ERA-40 for their reanalysis product, and this analysis has made great strides over the earlier ERA-15 product in dealing with issues like sea-ice and snow cover, particularly during the satellite era (Bromwich and Fogt, 2004). Comparisons with this product in these regions might have shed light on potential problems with the TLT record, but were not investigated.

Even so, their criticisms of the tropical record are flawed as well. TT rightly point out that the Fu et al. method is most reliable when stratospheric trends are relatively stationary by region and period. But then they point to the Quasi-Biennial Oscillation (QBO) as evidence that they are not and claim that Fu et al. are aliasing QBO trends into their MSU2 correction. Figure 8 shows the stratospheric QBO signal compared to monthly anomaly time series for 6 vertical layers averaged over several upper-air products. Included in this comparison are LKS, HadRT, RIHMI, Angell 63, Angell 54, and UAH Versions D and 5.0. All six time series shown are global, and the QBO signal was determined using 50-hPa altitude zonal wind patterns from radiosonde data at Singapore (Seidel et al., 2004). Because these time series’ draw upon a variety of products including both radiosonde and MSU, they are less subject to the idiosyncracies of any particular dataset, and as they are global in nature they present a better comparison to the Fu et al. methods than the tropical data used by TT. Three things are apparent. First, it can be seen that apart from a slight upturn prior to 1981 (at the beginning of the satellite era) and the upward punctuations of the El Chicon and Pinatubo eruptions, the MSU4 and 50-100 hPa time series’ are fairly monotonic and stable for the entire period TT examine, so this requirement is met.

It may be argued that the two volcanic events destroy this continuity, but they are also reflected in the tropospheric MSU2 and 850-300 hPa records as proportionally large dips in those records shortly after the stratospheric spikes. Therefore, both layers will reflect this activity in comparable proportions with regard to trend comparisons. Second, a close examination of the stratospheric global MSU4 and 50-100 hPa layer records reveals that at best, the QBO impact on them is barely noticeable. The tropics where TT chose to do their analysis, is the one region where we expect the most significant QBO impact, but this region tells us the least about the applicability of the Fu et al. method to the global trends it was used for (Seidel et al., 2004). Finally, the QBO time series is highly periodic, and therefore self-canceling. Even if it did alias a significant signal into the tropospheric record, that signal would be largely removed by the trending process (Fu et al., 2004b). Furthermore, TT’s criticisms assume that the Fu et al. weighting function goes negative above 100 hPa and will therefore alias QBO effects into the free troposphere record that are not there currently. In fact, this is true only of the Fu et al. global weighting function. The revised Fu and Johanson tropical weighting function does not go negative until around 75 hPa. Figure 29 shows this function compared to its MSU2 counterpart. It is evident that the MSU2 weighting receives more signal from this layer than the Fu et al. weighting, and for the latter the layer above 100 hPa will cancel out while the MSU2 contribution will not (Fu and Johanson, 2004; Fu et al., 2004b). This can even be seen in the global Fu et al. and MSU2 weightings shown in Figure 53.

TT’s reported disparities between Tsub>fjws and Tsub>850-300 trends in the tropics are also less revealing than they believe. In addition to the large lapse rates and temporal variability characterizing this region, the tropical tropopause often dips as low as 300 hPa. Tropopause trends are poorly characterized across all upper-air products and can significantly affect lower altitude trends if it is not excluded from the sampling (Seidel et al., 2004). For this region, Tsub>fjws is representative of the entire troposphere from the surface up to 100 hPa rather than the 850-300 hPa layer alone. This can be seen clearly in TT’s own dataset. Their 1000-100 hPa layer trends agree quite well with their reported Tsub>fjws trends (Fu et al., 2004b). Their ERA-40 derived trend for Tsub>850-300 is 0.03 deg. K/decade. The ERA-40 vertical trend profile in this region is revealing. It is positive below 775 hPa, negative between 700 and 400 hPa, and strongly positive between 300 hPa and the tropopause – which itself may occur anywhere between 300 and 100 hPa in this region for the period TT analyze. Therefore, for this region the T^tr_850-300 trend may be much smaller than its Tsub>fjws counterpart simply because of the vertical variability of this region (Fu et al., 2004b). But the global record will not reflect this.

TT state that modeled results agree with Tsub>fjws trends only for the atmosphere-only runs, but once again there are serious omissions. They use HadAM3 (atmosphere-only) and HadCM3 (coupled ocean-atmosphere) forced with natural and anthropogenic inputs for their model comparisons. But like Douglass et al. (2004b) they did so using model components and regional constraints that are not representative of the method they are testing. TT report that while their atmosphere-only and atmosphere-ocean coupled model runs gave similar results, their couple model runs (HadCM3) yielded a higher range of trends and only were consistent only with the corrected Tsub>fjws trend of RSS Version 1.0. However, TT’s HadCM3 coupled model run was not based on a true deep ocean model, but on HadlSST which is a simple analysis of observed SST’s (Tett, 2004). As such, like the Ocean A component of the GISS SI2000 model, it neglects the moderating effects of deep ocean latent heat advection and will thus overestimate atmospheric trends. In light of this, it is instructive to compare TT’s use of HadCM3 with that of Douglass et al. (2004b) and the corresponding runs of Ocean A and Ocean B forced GISS SI2000 (Hansen et al., 2002). DEA obtained SST forced HadCM3 data (Tett et al., 2002) directly from Tett and used it to generate vertical trend profiles for the tropics (30 deg. S to 30 deg. N Latitude). These vertical profiles are directly comparable to the data used by TT, the sole exception being that whereas TT report 1979-2001 trends by layer, DEA truncate their analysis to 1979-1996 so as to create the surface-troposphere trend disparity that their case depends on.

Figure 30 shows DEA’s 1979-1996 vertical trend profile for the same tropical region analyzed by TT (Douglass et al., 2004b). Included is a direct comparison of HadCM3 for the period 1975-1995 and GISS SI2000 forced with Ocean A for a similar period (1979-1998). Like TT, DEA use HadCM3 runs that were forced with natural and anthropogenic inputs, and did the same for their GISS SI2000 results. For this region and these periods, it is evident that both SST forced models give strikingly similar results indicating that they are largely comparable for this region and period. Extending the record to 2001 would not be likely to change this result significantly, as both models can be expected to capture the large 1997 ENSO event which dominates this portion of the record. Given the similarities between the two models, the impact of replacing the SST driven Ocean A component of GISS SI2000 that was used by DEA with a true deep ocean component like Ocean B. Figure 20B shows the difference. The left side figure shows the 1979-1998 vertical trend profile for the tropics and extra-tropics (40 deg. S to 40 deg. N Latitude - a region slightly wider than that used by TT) that is obtained using Ocean A SST forcing. The right side shows the comparable trend profile obtained from Ocean B forcing. A clear moderating effect of roughly -0.07 to 0015 deg. K/decade can be seen in the Ocean B run, demonstrating what we saw earlier when examining DEA’s results – the neglect of deep ocean latent heat advection leads to a model induced overestimation of atmospheric trends.

It is reasonable to expect similar behavior in HadCM3. Had TT used a true deep ocean component in their HadCM3 runs we would expect their modeled tropical trends to be lower by a similar spread. This would have put them in a range where given the uncertainties in forcing and model component responses, they would be adequate representations of either UAH or RSS corrected tropospheric trends. With regard to the uncertainties inherent in an exercise like this one, note also that neither Had CM3 or GISS SI2000 with either forcing scenario reproduces the positive-negative-positive vertical trend variability that is observed in the tropics as we saw earlier (Fu et al., 2004b). Something approaching this behavior is somewhat noticeable in the high southern latitudes (Figure 20B, lower right), but is not captured in the tropics and extra-tropics. This alone should lead us to exercise caution when using model runs for comparisons with observation in localized latitude bands like the tropics that are highly variable and not well characterized in upper-air products. TT raise many important question regarding the use of the Fu et al. method and have shown that care must be used in applying it. But their specific criticisms are at best a poor representation of how the method is used, and thus they do not stand up to scrutiny.

With regard to tests of the Fu et al. method, there is one more that needs to be considered. In the same December 2004 issue of Nature, alongside of TT, Nathan Gillett and Andrew Weaver of the University of Victoria, BC and Ben Santer (hereafter, GWS) published the results of their application of the Fu et al. method to AOGCM derived upper-air temperatures for the period 1958-1997 (Gillett et al., 2004). GWS used global upper-air temperatures from a four-run ensemble of the DOE PCM coupled ocean-atmosphere model forced with natural and anthropogenic inputs (Santer et al., 2003c; Washington et al., 2000) and used the Fu et al. method to derive values for the asub>0, asub>2, and asub>4 coefficients. These were then applied to MSU2 and MSU4 brightness temperature trends that had been obtained by applying the respective weighting functions to PCM temperatures and using least squares methods to obtain the corresponding layer trends. The resulting Tsub>FT (free troposphere) trends were then compared with the equivalent Tsub>850-300 and TLT trends that had been derived from PCM. Results are shown in Figure 31.

GWS found that the Fu et al. derived Tsub>FT trends agree with the model “observed” Tsub>850-300 trends to within +/- 0.016 deg. K/decade. Similar agreement was found for the northern and southern hemispheres and the tropics (Gillett et al., 2004). It is interesting to note that GWS’s Tsub>FT trends also agree with their simulated TLT trends for the same period and regions, indicating that the two do reflect similar upper-air layers. The significance of this test as compared to others is that the PCM modeled climatology is precisely known, and is therefore not subject to the observational uncertainties that plague the existing satellite, radiosonde, and reanalysis records (e.g. sampling noise, incomplete coverage and temporal record, differences in merge method, etc.). Whether or not it is accurate in its finest details compared to observations is beside the point. PCM does in fact reproduce the large scale behavior of the surface and troposphere and captures most of its more significant features. Therefore, it represents a valid “upper-air” environment against which the Fu et al. method itself can be tested. Because the objective of the GWS study was to test this method rather than reproduce observed climate variables, the robustness of the Fu et al. technique was demonstrated.

Thus, though a number of challenges have been made to the Fu et al. method, none of them withstands scrutiny. Given the relative stability of the stratospheric record and the independent test of the method’s robustness using modeled and multi-dataset applications, what criticisms remain regarding the statistical characterization of the method’s trend analysis are not likely to stand the test of time. As the quality of radiosonde, rawinsonde, and AMSU products grows, better characterizations of Wsub>FT and Tsub>FT will emerge that will allow for more complete investigations of the Fu et el. Weighting function and TLT products. There is no reason to expect that future investigations will not continue the general trend of closing even further the gap between surface and upper-air products.

But if past history gives any indication of what to expect, it is even more certain that the Fu et al. methods will not silence global warming skeptics any time soon. The same criticisms will likely continue and if anything, they will become even more strident – and even more irrelevant. As increasing numbers of climate scientists are now acknowledging, this is the last piece of the puzzle that makes the perceived disparity between surface and troposphere trends a red herring - and with it, the case of global warming skeptics.

Conclusion

The questions discussed here have never been more important. Advocacy groups skeptical of climate change have used carefully edited presentations on the troposphere temperature record to reposition global warming as “myth”. Their arguments have been enthusiastically embraced by the growing ranks of lawmakers in Congress who are only too happy to base U.S. domestic and foreign policy on them. To this end, have enjoyed much success and undermined many badly needed global warming mitigation efforts. In fact, skeptic arguments based on the troposphere temperature record alone may well comprise the bulk of the Bush Administration’s rationalization for not supporting the Kyoto Protocol. Until the remaining upper-air questions are answered more convincingly, this state of affairs will likely continue.

The potential impacts of climate change are not known with certainty, but as more is learned it is becoming clearer that at best they will be troublesome, and at worst potentially catastrophic. Abrupt climate change is also a possibility that cannot be ruled out (NRC, 2002). Mitigation of these consequences will require the nations of the world to shift their economies away from greenhouse gas emitting technologies and climate disruptive land use activities. These will be long-term changes, and the longer they are delayed, the more risk there will be for future generations. The shift toward wiser, saner economies must begin now. Today’s choices require today’s courage, and that requires the wisdom and foresight to embrace the well-being of our children, and their children, as today’s burden – even if it means costly sacrifice. Yet few things are as unnatural to human beings as delayed gratification, especially when the gratification is a gift to someone else rather than us. This weakness is exacerbated almost beyond remedy when individual conscience becomes collective conscience. Within communities, individuals can dilute personal responsibility with group imperatives and at the same time insulate themselves from the impacts of group policies making denial not only easy, but convincing. It’s no surprise that today’s climate change warnings and the attendant responsibilities are treated with fear and loathing by many, and efforts to replace them with more comforting ideas are widespread.

It’s regrettable, but in recent years science has become highly politicized in the world’s wealthier nations. Nowhere on earth has this been more prominent than in the United States. At the time of this writing (Jan. 2005) the U.S. has just re-elected George W. Bush to another 4-year term as president, and increased the already considerable numbers of its most ultra-conservative factions. This has left America with a Congress and presidential administration that are indisputably the most hostile toward science and the environment than any other in the nation’s history. Never before has the flow of accurate scientific and environmental information to the general public been more distorted and destructive environmental policy more rampant. The scientific community has largely been ill-equipped to prevent this.

Science builds human knowledge one brick at a time, carefully cross-examining its own methods at every step. By its very nature, it is at its best when questions loom larger than answers. Mysteries are investigated. Discoveries are shared openly and subjected to thorough review by peers. Hard questions are asked, ideas are tested by other experiments, other observations and analyses are brought to bear on the challenges, and the uncertainties are worked out over time. Only that which withstands the trial by fire of the peer-review process survives. Though enlightening, this process is not a swift one and it is vulnerable at every point to the lightning rod of emotional appeals. That which has been carefully crafted to speak with passion and certainty to our deepest fears today is far more likely to find a place in our hearts than any seemingly cold and distant knowledge base that offers a return far in the hazy distance – even if the knowledge is far more reliable ¹⁴.

Industry and ultra-conservative advocacy groups have been able to capitalize on these weaknesses. We have already seen numerous instances of where these groups have carefully, and perhaps even deliberately cherry-picked analyses of the upper-air record to yield the conclusions they were after. None of these arguments stands up to even a modest level of scrutiny, and would certainly not survive the peer-review process. It is startling that Douglass et al. (2004, 2004b) even managed to get so far as to publish in Geophysical Research Letters. Unfortunately, these flaws are quite easy to camouflage. By packaging them in visually and emotionally stunning “sound science” presentations and going directly to lawmakers and the general public, advocacy groups have been able to make end-runs around the scientific peer-review process and avoid the trial by fire that would expose their failings. By the time the scientific community can respond within the bounds of its own thorough and thoughtful methods, the fire is already spreading. Consider for instance the attacks on the IPCC. In their Climate Change 1995 report they observed that,

“The balance of evidence suggests a discernible human influence on global climate.”

(IPCC, 1996)

Note the tentativeness and restraint in this statement (e.g. “balance of evidence”, “suggests”, etc.) – despite the fact that it was based on more than 20,000 peer-reviewed papers in a number of climate related fields. Contrast tone and content of that statement with the following one made by Douglass, Singer, and Michaels at Tech Central Station where they announced their 2004 GRL papers,

“How many times have we heard from Al Gore and assorted European politicians that "the science is settled" on global warming? In other words, it's "time for action." Climate change is, as recently stated by Hans Blix, former U.N. Chief for weapons detection in Iraq, the most important issue of our time, far more dangerous than people flying fuel-laden aircraft into skyscrapers or threatening to detonate backpack nukes in Baltimore Harbor.

Well, the science may now be settled, but not in the way Gore and Blix would have us believe. Three bombshell papers have just hit the refereed literature that knock the stuffing out of Blix's position and that of the United Nations and its Intergovernmental Panel on Climate Change (IPCC)…”

So, to all who worry about global warming, to all who think that people threatening to blow up millions to get their political way is no big deal by comparison, chill out. The science is settled. The "skeptics" -- the strange name applied to those whose work shows the planet isn't coming to an end -- have won.”

(Douglass et al., 2004c)

Here there is neither restraint nor humility. We’re told out and out that the skeptics have “knock[ed] the stuffing out of” the IPCC’s conclusions”. As if this statement was not dramatic enough, it is accompanied by a thinly veiled attempt to somehow relate the IPCC’s conclusions to the 9/11 attacks and WMD’s in Iraq. Of course, the two have nothing to do with each other. Yet by making this association in a public and highly emotional appeal, DEA can tap directly into the rage and fears of readers – and lawmakers – and win a foothold in hearts and minds before the merits of their 3 “bombshell” papers are even examined. We have already seen how flawed those papers are. This is exacerbated by the fact that at least some of the advocacy group consultants making claims like these do have scientific credentials, and these groups have grown adept at claiming a scientific “consensus” for their views that does not exist. Even today many are still claiming that over “17,000 scientists and engineers” believe global warming is not happening despite the fact that this claim originated with a petition project and an unpublished paper that were discredited almost as soon as they came out, the paper’s authors were found to have plagiarized the format of a journal that had not reviewed or published it, and nearly all of the signatories have no background in any climate science related field (see Footnotes 2 and 3). Despite shortcomings in these arguments and the checkered history of those who have advanced them, advocacy groups know that once they have been actively promoted to lawmakers and the general public in highly partisan forums like Tech Central Station, it is already too late. The bomb has already exploded and the flaws will not be investigated. The problem is made even worse by the fact in the interest of “balanced coverage”, media outlets will typically give the same degree of coverage to advocacy group consultants that they give to legitimate climate scientists – giving both the appearance of being equally credible. As admirable as this goal is, it is poorly suited to scientific subjects because popular media forums are not generally equipped to discriminate legitimate science from pseudoscience – a fact that industry and ultra-conservative front groups are only too happy to take advantage of ¹⁵.

Tactics like these have allowed climate change skeptics and their benefactors to enjoy successes far beyond what the merits of their arguments justify. As upper-air products are refined and improved, and the knowledge base grows, the effectiveness of these tactics will diminish. In fact, despite their declarations of victory, climate change skeptics have retreated considerably from many of their earlier pronouncements. But until the upper-air knowledge base is greatly expanded from its present state, these tactics will continue to do much damage. It is of the utmost importance that the remaining questions of surface-troposphere temperature trends be addressed, not only so our knowledge of the impacts of our activities can be clarified, but so that impediments to sound environmental and economic policies can be removed.

It is also more important than ever before that scientists take their discoveries directly to the public. We need more popular science writers who can make the discoveries of climate science both interesting and accessible to the general public. The anti-climate science flaws described in this paper need to be more visible and more engaging than they typically have been. Even simple searches reveal that anti-environmental special interest groups have largely dominated the Internet. Effective challenges to this must be put forth. The recent appearance of the Realclimate web site (www.realclimate.org) is a desperately needed step in this direction and has already been an incalculable blessing in the short time it has been up. Such activities need to be expanded and should include print and broadcast media as well. It is true that activities like these detract from the time available for badly needed research, but we have reached a point where they are no longer indispensable. It may sound somewhat trite to say so, but it is easy to forget that we do not own the earth – it is on lease to us from our children and grandchildren. May history find that we were faithful to them in our stewardship.

Footnotes