The data presented regarding the relative timing of temperature and CO2 fluctuations do not contradict anthropogenic climate change theories. The authors point to data from the end of the last ice age to demonstrate that global temperature changes preceded changes in CO2 concentrations by several hundred years, concluding that temperature drives CO2 levels. From this they infer that CO2 does not drive temperature change either in ice ages or during the past century.
The latter conclusion is flawed logic. There are multiple initiators of temperature change in the climate system. The importance of CO2 and methane levels in these ancient climates comes from their role as feedback agents that amplify temperature changes. In fact, models run to replicate ice age conditions cannot make the Southern Hemisphere nearly as cold as data suggests without accounting for drops in CO2 and methane. Ice age data actually reinforce anthropogenic warming theories.
The authors also claim that climate models including CO2 forcing are less accurate than models that do not emphasize the role of CO2, but the evidence suggests otherwise. Figure 1, taken from Chapter 9 of the 4th IPCC Report (Intergovernmental Panel on Climate Change), compares the results of simulations taking into account anthropogenic factors, predominantly the release of CO2, to those simulations taking into account only natural climate variability. The warming trend over the past century is accurately predicted only by the models which take anthropogenic forcing into account. These models are extensively evaluated for their ability to reproduce observations of the atmosphere and oceans. They can simulate the temperature trend of the past century with remarkable accuracy. Uncertainties in climate predictions do exist, but the models unanimously predict higher temperatures as CO2 emissions increase.
The authors go on to claim that the temperature rise in the last 150 years is attributable to the end of the “Little Ice Age”, but this may explain only part of the increase in temperature observed in the first half of the last century. It does not explain the rapid acceleration in warming seen in the past 50 years. Here, Cook and Sloss avoid addressing a fact of basic chemistry: CO2 is a greenhouse gas that traps radiation and warms the Earth’s surface. Human beings release over 30 billion tons of CO2 into the atmosphere annually, and most of these emissions come from the burning of fossil fuels in various activities unique to the industrial age. Correspondingly, atmospheric concentrations of CO2 have risen by 35% since 1832. Climate models can and do account for natural changes since the Little Ice Age, but CO2 changes remain the dominant factor. Figure 2, taken from the IPCC 4th Assessment, shows the global temperature trend in the last 150 years. The rate of warming increases dramatically from the gradual rate seen at the end of the Little Ice Age to the rapid pace observed in the past few decades.
The final argument in the article questions the scientific consensus on climate change, but the evidence presented is remarkably unconvincing. Cook and Sloss cite the existence of a contrarian report issued by the Non-Governmental International Panel on Climate Change, a group of climate deniers supported primarily by the Heartland Institute. They boast that the group’s report garnered 31,478 signatures from American scientists who support the document’s point of view. This is misleading, since the participants merely signed a two-paragraph petition available on the PetitionProject.org website. The signer must self-identify as a holder of a B.S., M.S., or Ph.D. in any scientific field to be considered a “scientist”. As over 80 million Americans hold at least a bachelor’s degree, finding 31,478 self-verified science degree holders willing to sign a petition does not discredit the consensus view of climate change. According to the website, only 578 of the individuals claimed to be trained in atmospheric science, and only 39 in climatology.
The ranks of qualified climate change skeptics are vanishingly thin. A comprehensive study released by Peter Doran and Maggie Zimmerman in January, 2009 polled 10,257 Earth scientists from academic institutions, federal research facilities, national laboratories, and state geological surveys. Of the 3,146 respondents, over 90% of whom held Ph.D.s, 82% answered “yes” to the question: “Do you think human activity is a significant contributing factor in changing mean global temperatures?” Of the actively publishing climate scientists, 96.2% answered “yes.” The study concludes that the debate among climate scientists who publish articles on the subject in peer-reviewed journals is virtually nonexistent.
The data that allegedly contradicts anthropogenic global warming in fact fit neatly into our modern understanding of the climate. The overwhelming majority of climate scientists believe that human activity is impacting the climate. Efforts by Stanford and its students to reduce their carbon footprints are ethically commendable in light of the preponderance of evidence suggesting that human beings bear responsibility for global warming.
Those interested in fighting the flood of pseudoscience criticizing climate change should consult the IPCC Fourth Assessment Report, available here: http://www.ipcc.ch/publications_and_data/publications_and_data_reports.htm, or any of a dozen recent reports of the US National Academies of Science, such as: http://dels.nas.edu/climatechange/understanding-climate-change.shtml and http://dels.nas.edu/climatechange/ecological-impacts.shtml.
Brett Dietz is an alumnus of Stanford University, where he studied Atmosphere and Energy. Other contributors to this piece include: Karim Farhat (Energy Resources Engineering); Stephen H. Schneider (Melvin and Joan Lane Professor for Interdisciplinary Environmental Studies); Carolyn Snyder (Emmett Interdisciplinary Program in Environment and Resources); John Ten Hoeve (Atmosphere and Energy).