“The Earth’s climate has been gradually cooling for most of the last 50 million years. At the beginning of that cooling (in the early Eocene), the global average temperature was about 6-7 ˚C warmer than now [1,2]. About 34 million years ago, at the end of the Eocene, ice caps coalesced to form a continental ice sheet on Antarctica [3,4]In the northern hemisphere, as global cooling continued, local ice caps and mountain glaciers gave way to large ice sheets around 2.6 million years ago [4]
Over the past 2.6 million years (the Pleistocene and Holocene), the Earth’s climate has been on average cooler than today, and often much colder. That period is known as the ‘Ice Age’, a series of glacial episodes separated by short warm ‘interglacial’ periods that lasted between 10,000-30,000 years [5,6]. We are currently living through one of these interglacial periods. The present warm period (known as the Holocene) became established only 11,500 years ago, since when our climate has been relatively stable. Although we currently lack the large Northern Hemisphere ice sheets of the Pleistocene, there are of course still large ice sheets on Greenland and Antarctica [1]...During parts of the previous interglacial period, when polar temperatures reached 3- 5°C above today’s [7] global sea levels were higher than today’s by around 4-9m [8]
About 55 million years ago, at the end of the Paleocene, there was a sudden warming event in which temperatures rose by about 6˚C globally and by 10-20˚C at the poles [9] Carbon isotopic data show that this warming event (called by some the Paleocene-Eocene Thermal Maximum, or PETM) was accompanied by a major release of 1500-2000 billion tonnes or more of carbon into the ocean and atmosphere. This injection of carbon may have come mainly from the breakdown of methane hydrates beneath the deep sea floor [10], perhaps triggered by volcanic activity superimposed on an underlying gradual global warming trend that peaked some 50 million years ago in the early Eocene. CO2 levels were already high at the time, but the additional CO2 injected into the atmosphere and ocean made the ocean even warmer, less well oxygenated and more acidic, and was accompanied by the extinction of many species on the deep sea floor. Similar sudden warming events are known from the more distant past, for example at around 120 and 183 million years ago [11,12].
In total, human activities have emitted over 500 billion tonnes of carbon (hence over 1850 billion tons of CO2) to the atmosphere since around 1750, some 65% of that being from the burning of fossil fuels [13,14,15,16,17]. Some of the carbon input to the atmosphere comes from volcanoes [18,19] but carbon from that source is equivalent to only about 1% of what human activities add annually and is not contributing to a net increase.
In the coming centuries, continued emissions of carbon from burning oil, gas and coal at close to or higher than today’s levels, and from related human activities, could increase the total to close to the amounts added during the 55 million year warming event – some 1500 to 2000 billion tonnes.”
References
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2 Miller, K.G., Wright, J.D. and Browning, J.V., 2005, Visions of ice sheets in a greenhouse world. Marine Geology 217, 215-231. 12 Barrett, P. J., 1996, Antarctic paleoenvironment through Cenozoic times—a review. Terra Antarctica, 3, 103–119.
3 Cooper, A.K. and O’Brien, P.E., 2004. Leg 188 synthesis: transitions in the glacial history of the Prydz Bay region, East Antarctica, from ODP drilling. In Cooper, A.K., O’Brien, P.E. and Richter, C. (eds.), Proceedings of the Ocean Drilling Programme, Scientific Results, 188. Available from
www-odp.tamu.edu/publications/188_SR/VOLUME/SYNTH/SYNTH.PDF
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