(Response to letters from Howard Dewhirst, Tony Bazley, and Stephen Foster)
Dear Editor, The controversy surrounding global warming will remain until ALL the protagonists educate themselves with ALL the information about what it is and how it works. Therein lies a difficulty for geologists: much of the relevant literature comes from the worlds of atmospheric science, oceanography, and solar-terrestrial or planetary physics. So, it is not surprising that scientists outside the realm of climate science may be less well informed than they perhaps could be about the complexities of global warming. Siegert and Lack’s critiques of Mike Ridd’s piece showed some lack of tolerance of that realization. In that sense, Stephen Foster was right to request that in discussing the controversy we should ‘engage’ meaningfully and respectfully with the views of others with whom we may disagree. However, when it comes to the ‘balance’ that Tony Bazley seeks in such discussions we must take care to focus on discussing the merits of hard-won scientific data—not the unsubstantiated opinions voiced so commonly on blog sites.
Howard Dewhirst questions the use of the term ‘consensus’ to describe current understanding of the nature and origins of global warming, and goes so far as to suggest there are ‘substantial errors in the GSL position papers’. The term ‘consensus’ as used here refers to the agreement reached by scientists who contributed to the reports of the Intergovernmental Panel on Climate Change (IPCC). It is common practice for groups of experts on a particular topic to review its status from time to time and to publish their opinions as a ‘consensus view’. Those not part of that process may either accept the consensus view or reject it.
We need to carefully determine any basis for rejecting the climate change consensus. Siegert and Lack believe there is little to question—although there is plenty still to do to diminish uncertainties. Dewhirst, Bazley, Foster and Ridd take a different view, but they lack a rigorously tested alternative scientific explanation of global warming. Climate change and global warming are not simple matters, and I find it disconcerting when Fellows tell me that there is nothing to worry about because ‘the climate is always changing’. Clearly, someone who says that has not even begun to delve into the complexity of climate change. I wonder how geological climate sceptics would explain, for example, how it is that warming continues despite the Sun’s energy, as reflected in sunspot activity, declining since 1990.
The Society recognizes that there is a controversy. To assist in resolving it, in 2009 Council asked a collection of palaeoclimate specialists to provide a climate change statement (published in 2010 and updated in 2013). Some Fellows accept the statement. Others reject it, though their grounds are not clear. Council recognizes that the statement will need occasional review, to ensure it agrees with the current peer-reviewed literature. Such a review is ongoing.
Howard Dewhirst states that ‘The GSL position papers claim all the warming since 1900 is due entirely to human CO2 emissions, but fail to explain why the IPCC makes this claim only for the temperature increase since 1951.’ He is wrong. The IPCC position (5th assessment, 2013) is that the atmospheric concentration of CO2 has increased by >40% since 1750 and by 10% since 1990, and that—in response—globally averaged surface temperature has increased since the beginning of the 20th century and warming has been particularly marked since the 1970s, with 1981-2010 being very likely the warmest 30-year period in the past 800 years. The IPCC states that CO2 is the strongest driver of climate change, far outweighing the contribution from natural drivers, and is experiencing rates of rise unprecedented in the last 800,000 years (as seen in ice core data). It is virtually certain, says the IPCC, that the rising temperature, rising sea level, loss of ice, and melting permafrost are caused by CO2 emissions driven by human activities. Dewhirst says the GSL papers ‘fail to explain how it causes climate change’. He assumes that CO2 does not cause warming, but provides no basis for this claim. In contrast, the GSL statement does make clear the source of its statements, referring to abundant geological evidence that increasing CO2 does warm the climate throughout the Phanerozoic.
Stephen Foster reminds us that within the Vostok ice core the CO2 and CH4 curves lag the temperature curve. But he is wrong to state that scientists have brushed this lag under the carpet. On the contrary, it is widely appreciated that Milankovitch changes in insolation (representing changes in orbital properties and axial tilt) were the primary drivers for ice age change. The French Vostok researchers knew that the Milankovitch changes in insolation were quite small in contrast to the temperature changes, so deduced that the release of CO2 from the warming ocean must have contributed 30-50% of the temperature signal. What caused the lag? The original Vostok data relied on an interpretive model to determine the likely age of the gas bubbles in the ice cores. Did they get the age model right? Recently a French group headed by Frederic Parrenin (Science, 2013) used nitrogen isotopes to show that during the last deglaciation the changes in CO2 and temperature were synchronous, which begs the question: what will we find if we re-analyze the Vostok core using this new technique? These matters are openly discussed in the GSL statement, not brushed under any carpet.
I find clear evidence that if the temperature changes first (e.g. via Milankovitch forcing), CO2 will follow, reinforcing the temperature change, while if CO2 changes first (e.g. from a large input of volcanogenic CO2), then temperature will follow, as is predicted from everything we know about greenhouse gases and infrared radiation. Stephen Foster asks for proof. Other geologists do too. Their desires are met in the atmospheric science literature (e.g. see Chapter 4 of North & Kim, 2017, Energy Balance Climate Models, in the Wiley Series in Atmospheric Physics and Remote Sensing; or Pierrehumbert, R., 2010, Principles of Planetary Climate. Cambridge Uni. Press).
How can Stephen Foster be so confident that the probable cause of most changes in Earth’s past climate was not variation in the composition of the gases in the atmosphere? What caused the Carboniferous ice age, or the exceptional warmth of the mid-Cretaceous? Is it not geologically reasonable to accept, as palynologists and geochemists explain, that the Carboniferous ice age came about from cooling caused by a major fall in atmospheric CO2 (see The Emerald Planet by David Beerling, 2007), and that the massive outpourings of lava that built the mid-ocean ridges of the Cretaceous as the continents moved apart provided a large new source of CO2 to warm the planet? If not, what is his explanation and how rigorously has it been tested?
Colin Summerhayes