Debating climate data

“Temperatures rising” (feature, March 17) outlines views about climate and climate change from four Princeton faculty members who are global-warming skeptics. The information in many quotes and paraphrased statements from these scientists is wrong. This letter corrects the most serious errors.

Perhaps the most egregious error is the statement that “human activity has little to do with rising CO2 levels.” There is redundant and overwhelming scientific evidence, from our work and that of many other scientists, that the ongoing increase in the CO2 concentration of air is essentially all anthropogenic, and mostly due to burning of fossil fuels (some is from deforestation).   The atmospheric CO2 concentration, known from ice-core studies to be roughly constant at 270 ppm (parts per million) from 10,000 years ago to 1800 A.D., has since risen to 385 ppm. Isotopic studies, which fingerprint the CO2 source, show that the rise is due mainly to burning fossil fuels.   The geographic pattern of CO2 concentration changes in air shows the same.  

Second, the idea that “plants and early humans … evolved when CO2 levels were about 1,000 parts per million” goes beyond what can be supported by observations and is partly inconsistent with the data where we do have them. We have only the vaguest idea of CO2 concentrations as plants evolved hundreds of millions or billions of years ago. Ironically, the best evidence for high CO2 is that we simply cannot explain what kept the Earth from freezing over during much of this time, when the sun shone less brightly than today, unless we assert the existence of a powerful greenhouse effect. When Homo sapiens evolved, about 200,000 years ago or later, CO2 was between 180 to 290 ppm. This we know definitively from studies of fossil air in ice cores. If we go back to the time of the last common ancestor of humans and chimpanzees, perhaps 7 million years ago, indirect evidence suggests that CO2 was 250 to 400 ppm. There is no evidence for levels around 1,000 ppm then, or at any intervening time.  

Third, contrary to a skeptic’s view, climate-change scientists hardly “ignore the fact … that temperatures in fact cooled during the period from roughly 1940 to 1970.” This cooling, of about 0.2˚F, has received extensive attention from climate scientists around the world, including some of us. The dominant cause is still uncertain, but natural fluctuations of the climate system probably played a role. These natural fluctuations are likely to be large enough to occasionally mask a decade or two of warming due to increasing levels of CO2, but not to negate the long-term trend. Such fluctuations are also seen in climate-model projections.

Fourth, it is not correct to say that “(sea levels) have been rising since the end of the last Ice Age, and there is no evidence that the rate is increasing.” Nearly all of the sea-level rise from the demise of the great ice sheets had occurred by 8,000 years ago. We know, from measurements, that sea level is now rising at the rate of more than 20 cm/century (8 inches/century), at least five times higher than the average rise (if any) of the past 4,000 years.  

Fifth, the IPCC does not “develop” climate models, as was stated incorrectly. The most elaborate climate models are developed by about 20 laboratories around the world. This effort is anything but monolithic, and there is a wide range of projections about how much warming we can expect from a given CO2 increase. Hundreds of other researchers collect and analyze data, or work on climate models or theories that focus on specific aspects of the climate system. The role of the IPCC is to review the relevant literature and assemble information from climate models and many other ways of studying the climate system, to provide the best explanations for climate changes in the recent past, and to make the best estimates of the consequences of different emissions scenarios for the future.

Sixth, the idea that “CO2 levels have been much higher in the past, but this did not induce a tipping point …” has no basis in the available evidence. According to our best indirect estimates, CO2 was about 1,000 ppm (with a large uncertainty) from about 170 million until 30 to 35 million years ago, when it fell near-present levels. This high-CO2 world at times had polar temperatures about 20˚F warmer than present, equatorial temperatures 10˚F warmer than present, and sea level about 200 feet higher than present (for reference, Princeton is 215 feet above sea level). Recent research has determined that, during an extraordinary high-CO2 event about 55 million years ago, seawater temperatures on the Jersey Shore reached nearly 90˚F (about 18˚F warmer than now). Clearly, the high-CO2 world of past times was very different from today’s.

The list above is by no means a complete summary of climate skeptics’ statements in the article with which we disagree.

Two things should give the Princeton community concern about the judgment of the climate skeptics described in PAW. The first is the many errors in what they present as facts. The second is the loss of objectivity exemplified by Dr. Happer’s quote about climate scientists who believe that carbon dioxide causes global warming : “This is George Orwell. This is the ‘Germans are the master race. The Jews are the scum of the earth.’ It’s that kind of propaganda.”

Editor’s note: A condensed version of this letter was published in the April 28, 2010, issue of PAW. Isaac Held is a climate scientist who is senior scientist at the NOAA Geophysical Fluid Dynamics Laboratory and lecturer with rank of professor in the Atmospheric and Oceanic Sciences Program. The other signers are climate scientists and carbon cycle scientists who are senior Princeton faculty members in the Atmospheric and Oceanic Sciences Program, Woodrow Wilson School, and the departments of civil and environmental engineering, ecology and evolutionary biology, and geosciences.