HISTORICAL GLOBAL TEMPERATURE TRENDS

It has been claimed that the last decade of the 20th century was the warmest of the past hundred years and possibly the warmest of the entire past millennium (Mann et al., 1998, 1999). It has also been claimed that this observation is a cause for much concern, because the temperatures in question are supposedly so unprecedented. In fact, those who would have us believe that these supposedly high air temperatures are the result of anthropogenic CO2 emissions resulting from the burning of fossil fuels contend that the near-surface air temperature of the planet is currently so high that we must radically reformulate the energetic basis of the entire industrialized world in order to avoid a host of unwanted climatic consequences. But is this climatic characterization correct? ... and is its adherents' call to action truly prudent? 

Fully cognizant of the seriousness of this situation, Petit et al. (1999) described their analysis of the deepest ice core ever recovered from the Russian Vostok drilling station in East Antarctica, from which they extracted a 420,000-year history of the earth's near-surface air temperature and atmospheric CO2 concentration. This record covered the current interglacial period in which we now live, i.e., the Holocene, as well as the preceding four such climatic intervals. And why is this fact so important? Simply for what it tells us about the uniqueness or non-uniqueness of the Holocene; and that is that the current interglacial is by far the coldest of the five most recent such periods. In fact, the four interglacials that preceded the Holocene were, on average, more than 2°C warmer than the one in which we currently live. Also, atmospheric CO2 concentrations during all four prior interglacials never rose above approximately 290 ppm, whereas the air's CO2 concentration today (mid-2013) stands at about 400 ppm. Therefore, if there was anything unusual, unnatural or unprecedented about late-20th-century air temperatures, it was that they were so low in the presence of such high CO2 concentrations.