Most-recent articles

New evidence that CO2 may have driven cooling 50-30 million years ago

Author: Geoff Zeiss - Published At: 2016-10-02 18:06 - (333 Reads)
The paleoclimate history of atmospheric CO2 provides a perspective for assessing the relationship between climate cooling and atmospheric CO2 level. It has been argued that decreasing CO2 was the main cause of a cooling trend that began 50 million years ago and led to Antarctica glaciation which began about 34 million years ago and continues to the present day. Prior to this the Earth was nearly ice-free. Since then parts of the Earth in the high latitudes have been covered with continental-scale ice sheets. New evidence from analyzing fossil plankton shells has revealed that CO2 concentration in the Earth's atmosphere was about triple current levels around 52 million years ago. It then declined to levels close to current atmospheric CO2 concentration 34 million years ago when Antarctica began to glaciate. Changing atmospheric CO2 concentration was the primary driver of early Cenozoic climate, Eleni Anagnostou et al., Nature 533, 380–384 (19 May 2016) doi:10.1038/nature17423

Fingerprinting the source of rising CO2 during the last deglaciation

Author: Geoff Zeiss - Published At: 2016-09-30 15:27 - (250 Reads)
During the last deglaciation, about 20,000 to 12,000 years ago, the concentration of CO2 in the atmosphere increased from about 190 ppm to 270 ppm. In this study the stable carbon isotope ratio (ratio of carbon-13 to carbon-12) in the atmosphere is used to fingerprint the source of the CO2 increase. Since plants preferentially absorb the lighter isotope carbon-12, the stable isotope ratio is less in plants than in the atmosphere. In this study the stable carbon isotope ratio is reconstructed over the past 24,000 years from Antarctic ice cores. The time series reveals that during the first period of increasing atmospheric CO2 from 17,000 to 15,000 years ago the stable isotope ratio dropped precipitously indicating that a source of the CO2 was a large pool of carbon-13 depleted carbon of organic origin. There is independent evidence that this old carbon resulted from upwelling of deep water in the Southern Ocean. Carbon Isotope Constraints on the Deglacial CO2 Rise from Ice Cores, Jochen Schmitt et al., Science 11 May 2012:Vol. 336, Issue 6082, pp. 711-714 DOI: 10.1126/science.1217161

Evidence for CO2 outgassing from the Southern Ocean at the beginning of the last deglaciation

Author: Geoff Zeiss - Published At: 2016-09-27 21:15 - (287 Reads)
A Southern Ocean radiocarbon (carbon-14) record for Southern Ocean deep and intermediate depths covering 35,000 to 10,000 has been reconstructed from deep-sea corals. The record shows that deep water was radiocarbon-depleted during the last ice age, but this depletion and the deep stratification of the southern ocean disappeared by about 14,600 years ago. During this time there was a substantial reduction in radiocarbon in the atmosphere. Since the oceans contain ten times as much carbon as the atmosphere, it has been suggested that this was the result of a transfer of old organic carbon from the deep ocean to the atmosphere via a Southern Ocean ventilation event. The radiocarbon record reveals that radiocarbon-depletion in deep water, which originated in the last ice age, disappeared during the first 2000 years of the last deglaciation. This is consistent with a Southern Ocean outgassing event that resulted in a significant drop in atmospheric radiocarbon between 17,000 and 14,500 years ago. The Southern Ocean’s role in carbon exchange during the last deglaciation, A. Burke and L. F. Robinson Science 335, 557 (2012). doi:10.1126/science.1208163

Estimating greenhouse gas emissions from melting permafrost

Author: Geoff Zeiss - Published At: 2016-09-27 21:00 - (276 Reads)
During the last ice age organic carbon was captured in permafrost soils as a result of the decay of plants and fauna. As the Earth warms, permafrost soils melt and this old carbon is released into the atmosphere as methane and CO2. If a significant amount of this carbon were to reenter the atmosphere, it would accelerate the rise in atmospheric greenhouse gases. In this article the authors use radiocarbon dating of methane bubbles and soil organic carbon from lakes formed by melting thermafrost in Alaska, Canada, Sweden and Siberia combined with remote sensing determination of the growth of lakes to estimate the amount of old carbon released from permafrost soils in the Arctic over the past 60 years. Based on these results it is estimated that 0.2 to 2.5 Pg (0.2 to 2.5 gigatonnes) of permafrost carbon was released as methane and carbon dioxide in the Arctic region during the past 60 years. This is much less than the CO2 contributed annually from anthropogenic and other sources. Methane emissions proportional to permafrost carbon thawed in Arctic lakes since the 1950s, Katey Walter Anthony et al, Nature Geoscience 9, 679–682(2016) doi:10.1038/ngeo2795

CO2 lagged rising temperature in the Southern Hemisphere during last deglaciation

Author: Geoff Zeiss - Published At: 2016-08-20 14:56 - (325 Reads)
In this study the EPICA Dome C (Antarctica) ice core has been used to measure a high resolution record of atmospheric CO2 and methane concentrations over the last deglaciation about 19,000 to 11,000 years ago. Comparing the CO2 record to the Antarctic surface air temperature reveals a close correlation, but the resolution of the record is not sufficient to determine whether there is a lag between temperature and CO2. However, the times at which temperature and CO2 began to rise can be distinguished. The ice core record revealed that start of the CO2 increase lagged rising temperature by about 800 years. An uncertainty analysis suggests that the lag could have been as low as 200 or as much as 1400 years. This result is consistent with the Southern Hemisphere playing a dominant role in the rise in atmospheric CO2. Methane was found to increase at about the same time as CO2, but the rise in methane is thought to have been determined by Northern Hemisphere processes. Atmospheric CO2 Concentrations over the Last Glacial Termination, Eric Monnin et al., Science 05 Jan 2001: Vol. 291, Issue 5501, pp. 112-114

CO2 drove, but did not trigger, warming during last deglaciation

Author: Geoff Zeiss - Published At: 2016-08-15 17:06 - (340 Reads)
In this study global and regional surface temperatures during the last deglaciation have been reconstructed from proxy temperature records from 80 geographically distributed sites. Comparison with the atmospheric CO2 record reveals that atmospheric CO2 is closely correlated with global surface temperature, but the increase in global surface temperature lagged atmospheric CO2 throughout the last deglaciation. It is suggested that rising CO2 amplified the global warming trend. To investigate regional effects separate temperature reconstructions were developed for the Northern and Southern Hemispheres. It was found that in the Southern Hemisphere the rise in temperature preceded rising CO2, consistent with Antarctic ice-core results, whereas in the Northern Hemisphere temperature lagged CO2. Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation, Jeremy D. Shakun et al, Nature, 484, 49–54 (05 April 2012) doi:10.1038/nature10915

Southern Ocean warming preceded rising CO2 during the last deglaciation

Author: Geoff Zeiss - Published At: 2016-08-13 16:19 - (354 Reads)
This important article provides evidence that southern ocean warming pre-dated the rise of atmospheric CO2. Radiocarbon dating of micro organisms living on the deep ocean floor and in surface waters in a marine core collected in the western tropical Pacific was used to determine the relative chronology of warming in the southern ocean near Antarctica and rising CO2 during the last deglaciation. The results provide evidence that that the southern ocean off Antarctica warmed by ~2°C between 19,000 and 17,000 years before the present, about 1,000 years before the rise in atmospheric CO2. Southern Hemisphere and Deep-Sea Warming Led Deglacial Atmospheric CO2 Rise and Tropical Warming, Lowell Stott, Axel Timmermann, Robert Thunell, Science 19 Oct 2007:Vol. 318, Issue 5849, pp. 435-438 DOI: 10.1126/science.1143791

Accounting for global and regional warming during the last deglaciation

Author: Geoff Zeiss - Published At: 2016-08-09 17:40 - (346 Reads)
The last deglacial warming, which stretched from 19 thousand to 11 thousand years ago, was characterized by increases in surface temperatures of 10-15 °C, sea level rise of 80 meters and by increased atmospheric greenhouse gas concentrations. In this study the authors compiled and analyzed sea surface temperatures and precipitation from ice cores, sea floor sediments, pollen, cave calcite records, and sea phytoplankton records. A principal component analysis revealed two important trends responsible for the variability in temperature during this transition. The dominant warming trend started at about the time of increasing Northern Hemisphere summer insolation. This trend, which ultimately brought temperatures to pre-industrial levels, correlates strongly with increasing greenhouse gas levels. The second trend the analysis revealed is responsible for the pronounced millennial-scale variability such as the Oldest Dryas cooling, Younger Dryas cooling and intervening Bølling-Allerød warming period. Evidence from ocean sediments suggest this trend is associated with changes in the strength of the primary Atlantic north/south current. Global climate evolution during the last deglaciation, Peter U. Clark, Jeremy D. Shakun, et al., Proceedings of the National Academy of Sciences 2012 vol. 109 no. 19 E1134–E1142

New evidence affirms interglacial warming linked to small variations in Earth's orbit

Author: Geoff Zeiss - Published At: 2016-08-03 18:04 - (729 Reads)
Approximately 800,000 years ago, something changed within the climate system that led to glacial/interglacial periods with a cycle time of roughly 100,000 years. In 1941, Milutin Milankovitch introduced his orbital forcing theory which hypothesized that this cycle could be linked to variations in incoming solar radiation resulting from small, predictable changes in the Earth's orbit about the sun. In 1976 an analysis of deep sea sediment cores provided the first evidence supporting this theory. However, an influential study of calcite cores from Devils Hole in 1992 concluded that the evidence did not support the theory. In this study the original calcite cores from Devils Hole were reexamined and new cores analyzed. A systematic correction was identified that is required in dating calcite cores in Devils Hole and with this correction the calcite chronology does support the theory, thus vindicating the orbital forcing hypothesis. Reconciliation of the Devils Hole climate record with orbital forcing, Gina E. Moseley, R. Lawrence Edwards, Kathleen A. Wendt, Hai Cheng, Yuri Dublyansky, Yanbin Lu, Ronny Boch, Christoph Spötl, Science 08 Jan 2016: Vol. 351, Issue 6269, pp. 165-168 DOI: 10.1126/science.aad4132

New evidence that changes in ocean currents triggered abrupt Northern hemisphere warming

Author: Geoff Zeiss - Published At: 2016-07-30 19:38 - (933 Reads)
An important climate research question is what causes the warming and cooling cycles observed in the last million years of the Earth's paleohistory. One conjecture suggests that variations in the Atlantic Meridional Overturning Current (AMOC), which draws southern tropical surface waters to the North Atlantic where they become increasingly saline and sink to become deep water and flow south, has played a central role in abrupt warming (8–16 °C) events and cooling periods during the last ice age. This new study reports measurements from ocean floor sediments that provide the first direct evidence that changes in ocean currents have triggered abrupt northern hemisphere warming. North Atlantic ocean circulation and abrupt climate change during the last glaciation, L. G. Henry et al., Science 29 Jul 2016, Vol. 353, Issue 6298, pp. 470-474, DOI: 10.1126/science.aaf5529

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