Geoscientist Online

Sir, I write in response to the critique submitted by S.W. Foster (16.10.13) of my letter of 24.09.13. Mr Foster creates confusion by conflating two separate things – the reports of the UN’s Intergovernmental Panel on Climate Change (IPCC), and the Geological Society’s statement on climate change. We wrote the latter because we did not like how the IPCC had addressed past climate change. Our drafting group made a completely independent report based on our own research experiences and a survey of the literature. We became convinced that CO₂ plays an important role in governing climate change, as expected from the physics of greenhouse gases, and that adding significant amounts of CO₂ to the atmosphere risked warming the Earth. Our arguments are supported by an extensive checkable geological literature.

If anyone disagrees with the findings of our statement, they must clearly state their grounds. What other agent caused the rise in temperature at the Palaeocene-Eocene boundary, if not the remarkable rise in CO₂? What else caused the cooling of the Cenozoic, if not the remarkable fall in CO₂? Geologists know that volcanic activity produces CO₂, that weathering mops it up, and that the CO₂ ends up as carbonate sediments or organic deposits. Volcanism and weathering are not always balanced, so changes in climate may result from changes in CO₂ with time. The geological record shows that climate and CO₂ are related. Correlation is not always causation, but one cannot let it pass without a plausible explanation.

CO₂ is not the only cause of climate change.  Volcanoes also provide aerosols that can cool the Earth. Eruptions in Large Igneous Provinces can affect climate temporarily. Milankovitch cycles in the Earth’s orbit change insolation, so affecting climate. Short term variations in solar output do too, though with smaller effects. Orbital variations made Earth’s climate cool significantly over the past 10,000 years. Superimposed on that decline were small warmings and coolings (e.g. Medieval Warm Period and the Little Ice Age) related to fluctuations in solar output ¹. Muscheler’s analysis of solar output, based on ¹⁴C and ¹⁰Be data, discovered significant positive peaks in 1100, 1790, and 1960. I see that most solar output since 1960 has been significantly less than it was in 1960 ², yet global temperatures are much warmer. Our geological findings suggest that the most likely culprit is rising CO₂. If not that, then what?

Mr Foster asked me to comment on the workings of the IPCC. Not being a member of, or contributor to, the IPCC, I would rather stick with evaluating the geological record.

Colin Summerhayes, Scott Polar Research Institute