Sir, In response to John Gahan’s comments of August 21, the Geological Society's Statement on Climate Change was very careful to exclude all comment on the merits of climate models produced by the meteorological and climatological science communities, because they are not in our sphere of expertise. Our statement was based on a careful appraisal of the geological evidence for the role of greenhouse gases, especially CO2, in causing climate change in past times.
Investigations on this topic began in the 1890s, with the American geologist T.C. Chamberlin, but were then suspended for lack of the detailed information required to test the hypothesis. Much of the information needed from spectroscopists about the absorptive properties of atmospheric gases required advanced scientific equipment that was not available until the mid to late 1950s, and geochemists did not get deeply into trying to understand the role of CO2 in past climate change until the early to mid 1980s. Since the year 2000 a great deal of information has become available from ice, plankton, leaves and soils about the past distribution of CO2 and its relation to temperature.
The most recent ice data show that CO2 and temperature evolved in synchrony (not sequentially) with the last deglaciation – that gives us a new perspective. In addition, the recent data from leaves and plankton demonstrate how CO2 and temperature declined from the greenhouse climate of the Cretaceous into the icehouse climate of the Neogene, as the sources of CO2 from volcanism associated with sea floor spreading declined and as mountains and associated weathering drew CO2 out of the atmosphere. Much of this information is sufficiently new that it is not yet widely recognised in basic geological textbooks. Many people, some geologists included, are unaware that following the peak in insolation reached some 11,000 years ago at the end of the last deglaciation, planet Earth experienced a continued steady decline in insolation accompanied, not surprisingly, by cooling that led our planet into what some Earth scientists refer to as the ‘Neoglacial’, marked by the development of the Little Ice Age.
Superimposed on that cooling curve, which, by the way, is projected to continue for another 1000 years or so, we see small warming and cooling events lasting some few hundreds of years that are related to cyclical changes in the sun's output as expressed through sunspots and recognised in Earth materials via the radionuclides 14C and 10Be. According to the insolation curve we should still be in the Neoglacial and experiencing the continued delights of the Little Ice Age – like Frost Fairs on the Thames. However, from the ice cores from the Antarctic coast, as well as from global thermometer records, it is clear that significant warming began to cut across that insolation driven cooling trend in synchrony with the rise in CO2 that began with the industrial revolution in the 1770s. Solar output rose too, for a while, as seen from sunspots, but declined slightly since 1970, while temperature continued to rise. What did continue to rise after 1970 was CO2, a known greenhouse gas. People would like to know why global warming stalled after the year 2000 (note that it did not go into reverse, by the way).
Explanations are already arriving from the oceanographic and meteorological community for those who like to read their literature on the front line. For one thing, more heat is being taken down deep into the ocean interior than before. And for another thing we are also seeing the effect of a natural cycle, the Pacific Decadal Oscillation (PDO). Just as El Niño events in the Pacific raise global temperatures slightly above the global mean temporarily, and La Niña events there cool the planet temporarily, so too the longer-lived PDO can have the same effect. It is now in a cool phase, which helps to explain the current global stand-still (see Kosaka and Xie, in Nature, 19 September 2013). Underlying my remarks is the unchanged conclusion of the GSL’s Statement on Climate Change, which is that we can see from the past that the climate warms when CO2 goes up. It is hardly surprising that meteorologists and climatologists reach the same conclusions, albeit by different methods.