Pollen studies found that treeless Lake Elgygytgyn in the harsh Siberian Arctic had a temperate climate and was tree lined the last time CO2 was at today's level of 400ppm.
A meteorite crashed into eastern Siberia, just north of the Arctic circle 3.6 million years ago forming a basin that became the deep lake named Lake Elgygytgyn. Sedimentary layers buried deep under the lake contain pollen grains that tell of a much warmer, wetter climate from 3.4 million to 3.6 million years ago when CO2 levels were around today's level of 400ppm. The last time CO2 levels were this high, about 3.5 million years (m.y.) ago, this desolate lake in the Siberian Arctic would have been the perfect place for an upscale summer resort.
After 3.56 m.y., "the pollen stratigraphy provides the best descriptor of paleoenvironmental conditions." From 3.56my to 3.4 my "the mean temperature of the warmest month (MTWM, Fig. 2H ) and PANN (Fig. 2I ) reconstructed from the pollen record imply values of +15 - 16°C and ~600 mm/yr, respectively."
During the warm period 3.5 m.y. ago the climate was insensitive to earth's orbital variations. As CO2 levels began to drop from around 400ppm, the climate became more sensitive to periods of low summer insolation. From 3.4 m.y. to 2.7 m.y. warm and wet periods oscillated with cool and dry ones when summer insolation was strong and weak, respectively. After 2.7 m.y. CO2 continued to drop and the climate cooled into the cold Pleistocene period with its repeated episodes of continental glaciation that came in periods of low northern hemisphere summer insolation.
This week's CO2 levels at Mauna Loa rounded up to an average of 400ppm
The results of this report, the most detailed study of the last 3.6 million years of Arctic climate to date, imply a very high modern climate sensitivity to CO2. These results confirm recent research results from studies of ocean sediments that showed high modern climate sensitivity to CO2. All the guidance to governments, based on IPCC models, is based on lower sensitivity. IPCC models have predicted a mid-range of 2°C rise in temperature from pre-industrial climate for 450ppm CO2. The guidance to governments has been based on this "moderate sensitivity" used by the IPCC. Self proclaimed "climate skeptics", who have ignored the huge uptake of heat by the oceans since 1998, have claimed very low climate sensitivity to CO2 because surface temperatures have barely risen over the past 15 years.
The detailed Arctic Siberian pollen record negates the low climate sensitivity to CO2 hypothesis.
But the record from Lake El'gygytgyn of a very warm Arctic when atmospheric CO2 levels were last at about 400 parts per million (ppm) indicates the opposite, according to Brigham-Grette. "My feeling is we have underestimated the sensitivity, unless there are some feedbacks we don't yet understand or we don't get right in the models."
Prof Robert Spicer, at the Open University and not part of the new study, agreed: "This is another piece of evidence showing that climate models have a systematic problem with polar amplification," ie the fact that global warming has its greatest effects at the poles. "This has enormous implications and suggests model are likely to underestimate the degree of future change."
One hundred years of humans burning fossil fuels has reversed over 2 million years of rapid natural geologic carbon sequestration. The last time atmospheric CO2 was 400 parts per million (ppm) was 3 to 5 million years ago when global temperatures averaged 5°F to 7°F higher and the poles were 17°F warmer. Sea levels ranged from 25 feet to 130 feet higher than today's.
Over the past year CO2 levels jumped up about 2.89 ppm, a rate about 15 times faster than the rate of increase during earth's last great geologic catastrophe 65 million years ago. The asteroid impact, fires and volcanic events that combined to end the age of the dinosaurs caused CO2 levels to rise at the extreme rate of 0.2 ppm per year.
Climate models do not predict that 400 ppm CO2 will cause a return Pliocene warmth. In fact, climate models have been unsuccessful at modeling the distribution of heat in the Pliocene oceans and atmosphere. The Pliocene warming was one and a half to twice as strong as climate models have hindcasted. A recent report (doi:10.1038/nature12003) by Federov et. al., in the journal Nature concludes that climate models underestimated the warming effects of clouds and ocean mixing in the Pliocene.
The last IPCC set a CO2 target of 450 ppm to prevent temperatures from rising more than 2°C (3.6°F). However, recent papers on Pliocene climate show that the future climate might warm as much as 7°F at 450 ppm if the earth returns to early Pliocene warmth - temperatures that were last seen on earth the last time CO2 levels were 400 ppm.
The 2013 reports confirm the high climate sensitivity to CO2 determined by Pagani (DOI: 10.1038/NGEO724) in 2009 based on studies of marine sediments.
Estimated CO2 trends considering probable oceanographic changes.
Moreover, humans have been burning geologically stable carbon that was millions of years old, not the unstable surficial carbon in peat, permafrost and methane ices that have been stored because of cool conditions over the past 3 million years. These vast reservoirs of carbon will be destabilized by warming temperatures. Recent research on Siberian permafrost found that it destabilizes if warming exceeds 1.5°C. Carbon stored very slowly as the earth's climate cooled could be released very quickly during rapid warming.
The enormous thermal inertia of the oceans is presently stabilizing global temperatures from the effects of rapidly rising greenhouse gas levels. The combined effects of Antarctic ozone depletion and a 20% reduction in the oceanic overturning circulation around Antarctica have cooled far southern latitudes. Because less heat has been transported towards Antarctica, more heat has stayed in the tropics, and tropical atmospheric convection has intensified. The strengthening of the tropical convection cell has driven stronger trade winds and stronger cool water upwelling in the equatorial Pacific. The increase in upwelling and La Nina events has increased oceanic heat storage in the western Pacific ocean and the Indian ocean while cooling vast areas of the east Pacific ocean from the coasts of Chile to California.
The recent discovery of the fresh water layer at 100 meters depth around Antarctica has not yet been incorporated into climate models, but will need to be. Massive melting of Antarctic ice shelves from below will affect ocean circulation patterns for many years, slowing southern hemisphere warming while speeding up northern hemisphere warming. It's not as simple as an ice cube cooling a drink, but below surface Antarctic melting is clearly reducing the flow of heat towards Antarctica, keeping it cold. As sea ice disappears in the northern hemisphere, the opposite is happening. The sea surface is warming rapidly in the summer because ice keeps the water cold and is much more reflective than the open ocean. Likewise the rapid greening of Arctic land is leading to earlier snow melt and early spring warming. All of the recent studies are showing that the Arctic is exquisitely sensitive to levels of greenhouse gases in the atmosphere. If CO2 continues to rise rapidly above the 400 ppm level the rates of change will far exceed rates seen in millions of years, straining our attempts to predict the consequences.
"It shows a huge warming – unprecedented in human history," said Prof Scott Elias, at Royal Holloway University of London, and not involved in the work. "It is a frightening experiment we are conducting with our climate."
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