Editor’s Note: The following letter by Dr Colin Summerhayes and the response by Professors Bob Carter and Vincent Courtillot are a continuation of their debate on The Geological Perspective of Global Warming which the GWPF published on 14 February. Dr Summerhayes’ letters have also been published by the Geological Society. We welcome this scientific exchange and hope that readers will find it both enlightening and encouraging.
Dr Colin P. Summerhayes, Scott Polar Research Institute, Cambridge
Dear Dr Peiser,
Thank you for the opportunity to respond to the critique by Drs Carter and Courtillot of my note of 14/2/13 on “The Geological Perspective of Global Warming”. I initially wrote to you to draw attention to Geological Society of London’s statement on this topic, because the geological perspective is usually overlooked in discussions about climate change, and it should not be. But, because Drs Carter and Courtillot moved the debate out of just the geological arena, I am responding in my own capacity, not as a representative of the GSL.
Drs Carter and Courtillot took exception to my use of the phrase “The cooling [of the past 50 million years] was directly associated with a decline in the amount of CO2 in the atmosphere”, saying that correlation was not causation. True. What I should have said was “The cooling of the past 50 million years was driven by a decline in CO2 in the atmosphere.” Prior to the Ice Age of the last 2.6 million years the amount of CO2 in the atmosphere resulted from the interplay between the emission of CO2 by volcanoes and its absorption by the weathering of rocks, especially in mountainous areas, as well as by sequestration in sediments. Methods to determine the likely concentration of CO2 in the atmosphere in the geological past have improved in recent years. They include the numbers of pores (stomata) on leaves, the abundance of the mineral nahcolite (stable above concentrations of 1000 ppm CO2), and the carbon isotopic composition of alkenones from marine plankton. Methods for determining global temperature through time have also improved. We now know that the Eocene was a time of greater volcanic output of CO2, and that the rise of major mountain chains after that time pulled CO2 out of the atmosphere. Geochemical models of the carbon cycle simulate the decline in CO2 after the middle Eocene. Convergence between the CO2 data and the output from those models provide confidence that we understand the process. There is no geologically plausible alternative. We are not talking about a loose association where there is uncertainty about cause as Drs Carter and Courtillot imply. Indeed, even Drs Carter and Courtillot accept that CO2 is a greenhouse gas, and that accumulation of greenhouse gas in the atmosphere warms it. Likewise, its loss will cool the atmosphere.
Dr. Vincent Courtillot is a professor of geophysics at the University Paris-Diderot and Chair of paleomagnetism and geodynamics of the Institut Universitaire de France. In the recent lecture below he explains how solar cycles control the climate by influence on cloud formation (the cosmic ray theory of Svensmark et al) and via influence on ocean oscillations and length of day. Dr. Courtillot notes that IPCC climate computer models do not correlate with observations and that temperature trends vary substantially between North America and Europe (which is contrary to IPCC computer model predictions). He also notes that while the total solar irradiance (TSI) only varies by about .1% over a solar cycle, the solar UV varies by about 10% and that secondary effects on cloud formation may vary up to 30% over solar cycles. The IPCC computer models dismiss the role of the sun by only considering the small variations of the TSI and ignore the large changes in the most energetic and influential part of the solar spectrum - the ultraviolet.