by Michael Mann, Stefan Rahmstorf, Gavin Schmidt, Eric Steig, and William Connolley
Senator James Inhofe (R) of Oklahoma recently provided us with an update of his views on the issue of climate change in a speech given on the opening senate session, January 4, 2005. His speech opened with the statement:
As I said on the Senate floor on July 28, 2003, “much of the debate over global warming is predicated on fear, rather than science.” I called the threat of catastrophic global warming the “greatest hoax ever perpetrated on the American people,” a statement that, to put it mildly, was not viewed kindly by environmental extremists and their elitist organizations.
Cutting through much of his polemic, Inhofe’s speech contains three lines of scientific argument which, according to him, provide “compelling new scientific evidence” that anthropogenic global warming is not threatening. We here submit his statements to scrutiny.
(1) The Paleoclimate Record
“We are also in the midst of a natural warming trend that began about 1850, as we emerged from a 400 year cold spell known as the Little Ice Age.”
Scientific studies come to the opposite result. All published scientific investigations of the causes of 20th century warming have consistently found that natural factors alone cannot explain the warming. Model simulations of large-scale temperature changes in past centuries , for one, can only reproduce the post-“Little Ice Age” warming through the inclusion of non-natural, anthropogenic forcing. The IPCC concluded in its 2nd asessment report that “that there is a discernible human influence on global climate”, based in fact on a variety of different techniques, including so-called “Detection and Attribution” studies. These studies involve detailed analyses of the spatial patterns of the observed 20th century changes, which differ for different causes of warming (e.g. anthropogenic factors such as increased greenhouse gases or industrial aerosols, or changes in land use, and natural factors such as changes in solar output or explosive volcanism), each of which have their own unique spatial pattern or “fingerprint”. Another simple reason that natural causes cannot explain recent warming is that none of the natural factors which could potentially cause warming (e.g., the combined solar+volcanic forcing or even the somewhat more dubious hypothesized forcing by cosmic ray flux changes) show a trend since the mid 20th century.
Inhofe then launches into a slew of criticisms of the “Hockey Stick” reconstruction of past temperature changes in this speech, touching on every one of our documented “myths”. Consider, for example, his reference to
“the well-known phenomena of the Medieval Warming [sic] Period–when, by the way, it was warmer than it is today”
All quantitative paleoclimate reconstructions of the past millennium published in the scientific literature have come to the opposite conclusion. They consistently find that late 20th century warmth is anomalous in the context of at least the last 1000 years for the Northern Hemisphere on the whole. Though certain regions appear to have exhibited mild conditions during the so-called “Medieval Warm Period”, there is no credible evidence we are aware of that average temperatures for the Northern Hemisphere or globe were as warm as (let alone warmer than) the late 20th century.
Inhofe then attempts to criticize the “Hockey Stick” reconstruction by citing a modeling study by the German GKSS group that actually supports the “Hockey Stick” conclusion that late 20th century warmth is anomalous in the context of the past 1000 years (see GKSS curve in Figure ) as well as the conclusion that this warmth can only be attributed to anthropogenic influences.
Furthermore, he attempts to criticize the methodology underlying the Mann et al (1998) reconstruction (one of more than a dozen estimates coming to essentially the same “Hockey Stick” conclusion), based on a reference to a comment by “three geophysicists from the University of Utah” [Chapman et al (2004)--see the response here] related to a modeling study by Mann and Schmidt (2003). That study has nothing to do with the Mann et al (1998) temperature reconstructions or methodology whatsoever (and did not even reference them) , but instead analyzed the factors governing the difference between ground surface temperature and land air temperature change in a climate model simulation of the latter 20th century.
(2) Global Sea level Rise
Estimates from tide gauges indicate that sea level has changed at the rate of 1.8 to 2.4 mm/yr over the last century. Satellite altimeter estimates currently show a global sea level change of 2.8+/- 0.4 mm/yr over the last 12 years. Due to the different methodologies involved, a direct comparison of the two values is not straightforward, but the satellite results provide absolutely no support for Inhofe’s contention that “there is a total absence of any recent acceleration in sea level rise”.
(3) Recent Arctic warming
Inhofe contends that “current Arctic temperature is no higher than temperatures in 1930s and 1940s” and cites many studies that appear to agree with him. However, the context for those studies is important and was well covered in the Arctic Climate Impact Assesment. In particular, natural variability in the climate system is particularly large in the high latitudes, such as the Arctic. This implies that temperatures from any one or two years may not be very representative of a long term trend. Arctic temperatures did indeed have a peak around 1940, but the decadal mean temperatures are now (1995-2004) warmer than the mean over 1935-1944. The variations in temperature in the Arctic resemble the global mean changes over the last century but are larger, clearly demonstrating the effect of polar amplification. More important are the causes of these temperature changes, and this cannot be determined simply by looking at one time series (this is further discussed here). The current consensus view is that warming in the 1940s was likely a combination of increasing GHG and solar forcing combined with a significant amount of internal variability, particularly associated with the North Atlantic. The subsequent cooling was related to the post-war increase in (mainly) sulphate aerosols. Subsequent to the 1970′s greenhouse gas forcing has become dominant, leading to the recent warming.
As is made very clear in the ACIA report, the Arctic is a complex and dynamic environment. It is well known that changes to atmospheric circulation associated with the North Atlantic Oscillation (NAO) can have important consequences for temperatures in the region. Over the last 35 years, the NAO has strengthened (implying more westerly winds), and this has lead to enhanced warming over Eurasia, and consequent cooling over southern Greenland, particularly in winter. Thus the Arctic is warming despite this dynamical trend, which itself may be related to anthropogenic forcing (Gillett et al, 2002).
Finally it is worth providing a bit of context for this latest speech. Inhofe has a history of making inflammatory and incorrect claims about the science of climate change. He previously gave a speech on the senate floor in July 2003 on “The Science of Climate Change” (partial transcript here) in which he stated that “catastrophic global warming is a hoax” and made a rather substantial number of false claims about the science. In fact, Senator John McCain (R) of Arizona subsequently provided two climate scientists mentioned specifically by Inhofe, Dr. Stephen Schneider of Stanford University (whom Inhofe referred to as “the father and promoter of the catastrophic global warming fearmongers”) and Dr. Tom Wigley of the National Center for Atmospheric Research, the opportunity to respond to several of these false assertions in the Senate record–see the account provided in the article “Earth Last” by science journalist Chris Mooney. In this speech, Inhofe repeated many of the standard contrarian arguments challenging the mainstream, consensus view of the climate research community that the activity of human beings now has had a discernable impact on global climate and that this warming is likely to continue as anthropogenic greenhouse gas concentrations continue to increase. Most of these arguments are debunked on the pages of RealClimate. Inhofe, for example, once again promoted each of the “myths” we have documented about the “Hockey Stick” reconstruction of past temperature changes, citing contrarian criticisms that have since been thoroughly discredited.