Sir:
In his article, “Don’t Panic: A Movie Review of The Great Global Warming Swindle” (Brown Spectator, May 2007), Andrew Kurtzman attempts to refute the scientific consensus that humans, in burning fossil fuels, cause global warming. Kurtzman does not explicitly deny a link between greenhouse gas emissions and rising global temperature; he leaves that to the “documentary” film under review. His claim is simply that climate change is horribly misunderstood. Therefore, we should not assume that anthropogenic carbon dioxide emissions are deleterious, and therefore, governments should not bother to reduce societal CO2 emissions. It is true that climate change needs further scientific study. However, we already know that humans are significantly increasing the concentration of atmospheric CO2 through fossil fuel burning and deforestation. Computer models of the global climate predict that, over the next century, elevated CO2 concentration will raise the temperature by a few degrees Celsius, affecting water availability, sea level, and biodiversity.1 Kurtzman is wrong to suggest that we know too little to improve anything.
Kurtzman’s first argument against a link between anthropogenic emissions and global warming is to reference 1970s predictions of “Global Cooling.” 1970s data indeed suggested a very modest downward trend in the average earth surface temperature. However, when the data from 1940 to 1970 are viewed in the context of temperature data before 1940 and after 1970, the so-called “global cooling” becomes a temporary plateau in an upward trend.2 Popular 1970s fear of a new global ice age came largely from exaggerated, sensational media coverage that misinterpreted scientific arguments that the Earth may enter a glacial period within a geologically short time – on the order of fifty thousand years.3
The CO2 isotope distribution in tree rings is indisputable evidence that anthropogenic emissions increase atmospheric CO2. Trees, which use CO2 for photosynthesis, take CO2 out of the atmosphere and layer the carbon in rings as they grow. Thus trees can serve as archives of the different types (isotopes) of carbon that were present in the atmosphere over time. Combustion of fossil fuels adds a mixture of carbon isotopes to the atmosphere that has a characteristic lack of 13C relative to 12C. The heavy isotope deficit exists in fossil fuel because it consists mostly of fossilized plants that used photosynthesis when they were alive.
Photosynthesis is a process that selects for lighter carbon isotopes. Volcanic, oceanic, and other nonliving input of CO2 into the atmosphere lacks the distinctive 13C/12C ratio. The tree-ring record shows a decline in the 13C/12C ratio toward the present day, which can be explained only by input of CO2 from the biosphere. The decline begins in earnest at about 1850, when industrialization became important in Europe and North America. We even know that the carbon input is from old fossil fuels and not dying organisms because of an observed decrease in 14C in the tree-ring record. This radioactive carbon isotope is unstable over long periods of time, so it is absent from fossil fuels, which are hundreds of millions of years old.
In 1982 scientists in Antarctica began drilling the Vostok ice core, ultimately to recover about 3.6 km of ice. The ice that the scientists sampled had been deposited over the past 420,000 years, and as it deposited, air bubbles were trapped inside, preserving the chemical composition of the atmosphere at the time of deposition. By looking at the chemical makeup of ancient air bubbles, scientists have deduced a 420,000-year record of CO2 concentration and temperature in Antarctica. As Kurtzman points out in his second argument, CO2 concentrations in the ice core record start to rise 800 years after the temperature begins to rise. However, each warming period in the record lasts about 5000 years. Although rising CO2 did not trigger the (interglacial) temperature rises, it likely amplified the remaining ~4200 years of relative warmth.4 Earth is a system of feedback loops. Small perturbations can be dramatically amplified. Changes in the amount of sunshine that the Earth receives due to ~20,000-year cycles in the Earth’s orbit (Milankovitch cycles) were the likely perturbations that triggered the glacial and interglacial times. The Vostok ice core record is a confirmation of the global climate computer models used to predict the temperature in the coming centuries. We cannot explain the temperature intensity of the interglacial times without taking into account the CO2 greenhouse effect.5
Kurtzman implies that the 800-year lag is explained solely by reduced oceanic uptake of CO2. Although this misconception contains a grain of truth,6 the reality is much more complex. The Earth is a system of interdependent components and conditions: CO2 in the ocean, CO2 in the atmosphere, and the effect of temperature on solubility is one example of a sub-system. CO2 in the atmosphere, surface temperature, and the greenhouse effect is another important sub-system. Incoming sunlight warms the earth and is re-radiated at infrared wavelengths. Atmospheric CO2 absorbs this outgoing long-wave radiation, and re-radiates some of it back toward the surface, creating a greenhouse effect. The more atmospheric CO2 there is, the greater the greenhouse effect will be: see frigid Mars and scorching Venus as examples of planets with CO2 pressures much lower and much higher, respectively, than that of the Earth. It is too simple to say, as Kurtzman does, “global warming causes carbon emissions, and not the reverse.” It is true that temperature affects the ocean’s uptake of CO2; it is also true that CO2 concentration affects temperature. These sorts of relationships are what global climate computer models take into account. In order to talk about the effect of increasing atmospheric CO2 on global temperature, it is necessary to understand the complexity of the Earth system. Some aspects of this complexity are still debated among earth scientists. But there is enough understanding to say that our burning of fossil fuels will raise the global temperature in the next century.7
Finally, Kurtzman (and the Swindle film) invoke research by Eigil Friis-Christensen to argue that “heightened solar activity… has led to the warmer conditions that we see today.” In an April 2007 statement8, Friis-Christensen and a colleague detail the distortions that Swindle made in order to make fallacious claims about the link between solar activity and Earth temperature. Friis-Christensen also says: “…the results presented in [the] graph used by the documentary do not exclude the impact of other climate forcing agents on the climate at any period in the last 400 years, including anthropogenic greenhouse gases. To suggest as much is incorrect.”
Combustion of fossil fuels has caused the concentration of atmospheric CO2, a greenhouse gas, to rise to levels well beyond the range of natural variability. That much is certain. The debate among earth scientists is not over whether the elevated CO2 causes significant warming. The debate now concerns how bad the warming will be.
Sincerely,
- Kump, Lee R., et al., The Earth System, Second Edition. Pearson / Prentice Hall, New Jersey, 2004. [↩]
- Brohan, P., J.J. Kennedy, I. Haris, S.F.B. Tett and P.D. Jones (2006). “Uncertainty estimates in regional and global observed temperature changes: a new dataset from 1850”. J. Geophysical Research 111: D12106. DOI:10.1029/2005JD006548 [↩]
- Loutre and Berger, Climatic Change, 46: (1-2) 61-90 2000 [↩]
- C Lorius, et al., The ice-core record: climate sensitivity and future greenhouse warming. Nature 347, 139 – 145 (13 September 1990). [↩]
- http://www.realclimate.org/index.php/archives/2007/04/the-lag-between-temp-and-co2/#more-430 [↩]
- http://www.realclimate.org/index.php/archives/2004/12/co2-in-ice-cores [↩]
- http://ipcc-wg1.ucar.edu/wg1/wg1-report.html [↩]
- http://folk.uio.no/nathan/web/statement.html [↩]
