Beginning about a million years ago, the Earth began to experience periodic cycles of glaciation followed by warm interglacial periods. Initially the glacial/deglacial cycles repeated every 41,000 years, but about 800,000 years ago a shift occurred to cycles with a period of about 100,000 years.? It is not clear what initiated these periods of warming and cooling and the shift to 100,000 year cycles. This has become an important question because scientists are interested in the impact the increase in global surface temperature since 1750 might have on the 100,000-year cycle, for example, by delaying the expected next global cooling. In this study the global average surface temperature over the past 2 million years has been derived from deep sea cores using a newly developed methodology. It was found that the Earth’s surface temperature gradually cooled until 1.2 million years ago after which it has remained stable when averaged over glacial/interglacial cycles. The results reveal that global cooling occurred about 300,000 years before the rapid ice sheet growth and the development of the first 100,000-year glacial/deglacial cycle about 800,000 years ago. This suggests that global cooling was a key factor, but not the sole cause, in the shift to 100,000-year glacial cycles. Evolution of global temperature over the past two million years, Carolyn W. Snyder, Nature 538, 226–228 (13 October 2016) doi:10.1038/nature19798
This article reports the reconstruction of a continuous record of global average surface temperature over the last 2 million years. Global surface temperature (GAST)is not directly measurable, but requires averaging temperature from sites distributed over the Earth to calculate a global average. Calculating global surface temperature in modern times involves averaging billions of temperature measurements from tens of thousands of weather stations.
The temperature reconstructions over the past 2 million years are mostly from sea surface temperature (SST) proxy records because terrestrial temperature reconstructions are too sparse in spatial distribution to be used for a global reconstruction. In this study a method is developed to estimate global surface temperature from a collection of sea surface temperature estimates. The calculation uses 20,000 measured SST time series from 60 ocean sediment cores. The method corrects for the fact that the SST records do not adequately cover the entire Earth surface and do not include records of temperature change over land. For the last 800,000 years the study compares the reconstructed surface temperature with the record of atmospheric greenhouse gas (GHG) concentration and Antarctica surface temperatures from the Antarctica ice core record.
The period studied includes the middle Pleistocene transition (MPT), which stretched from 1,250,000 to 700,000 years ago. During the MPT the Earth’s climate switched from a cooling/warming cycle of 41,000 years to a -year100,000 cycle of glacial and interglacial periods. During the MPT, long-term average ice volume in Antarctica, Greenland and other parts of the Northern Hemisphere gradually increased by the equivalent of 50 meters of sea-level. There is ongoing debate over what led to the MPT.
The study reports a geographically weighted reconstruction of GAST over the past 2 million years using a database compilation of over 20,000 SST temperature proxies from 59 ocean sediment cores. (The temperature proxies include alkenone unsaturation indices, ratios of magnesium and calcium (Mg/Ca) in plankton, and microfossil abundances.) No data from ice cores was used in the calculation.
The methodology used to calculate global surface temperature involves a novel apporach based on constructing averages for latitudinal zones and then summing the latitudinal zones. Comparison of the global surface temperature determined in this study to Antarctic temperature reconstructions for the past 800,000 years reveals that global surface temperature and Antarctic temperature are closely coupled across glacial cycles. This supports the calculated surface reconstruction determined in this work because it did not include any data from ice core records. (It also makes it possible to estimate polar amplification over the past 800,000 years. Polar amplification is the well known fact that climate change is greater in the polar regions. It is estimated that for every 1?°C change in global surface temperature, Antarctica surface temperatures change by 1.6?°C.)
It is also interesting to compare the global surface temperature reconstruction with the atmospheric greenhouse gas concentration record for the past 800,000 years from Antarctic ice cores. This reveals a close correlation over the entire period which also supports the reconstruction. It also makes it possible to calculate that the global surface temperature changes by 9?°C when atmospheric carbon dioxide doubles. This is larger than the current IPCC estimate of 4?°C because it includes longer term processes and assumes that the climate has at least centuries to equilibrate. This provides evidence that in the longer term the effect of rising CO2 concentration in the atmosphere is more than double the short term estimate of the IPCC.
The results reveal a cooling trend beginning 2 million years ago that ends at the beginning of the MPT about 1.2 million years ago. Before the MPT, global temperature cooled by about 0.34?°C every 100,000 years. But after the beginning of the MPT, global surface temperature (averaged over 100,000 years) has remained essentially constant.
The results also reveal that global cooling occurred about 300,000 years before the rapid ice sheet growth and the development of the first 100,000-year glacial/deglacial cycle about 800,000 years ago. This suggests that global cooling was a key factor, but not the sole cause, in the shift to 100,000-year glacial cycles at the mid-Pleistocene transition.