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Simple rule accounts for onset of ice age deglaciations over the past three million years

Author: Geoff Zeiss - Published At: 2017-03-10 23:29 - (93 Reads)
Climate
The sequence of ice ages followed by warm interglacials has been the dominant force in creating, extinguishing and changing nature and life on Earth for the past three million years. Understanding what causes these climate cycles is fundamental to understanding the global climate in the past and present. A recent study has created a simple mathematical rule that can account for the timing of the onset of interglacials following ice ages over the past three million years. The rule is based on predictable long-term astronomical variations in the Earth's orbit and tilt called Milanković cycles, without any knowledge of atmospheric greenhouse gas concentrations, ice sheet dynamics, volcanism, cosmic rays, dust or other climate data. This study supports the orbital theory for glacial/deglacial cycles, but still leaves many open questions regarding the underlying physical mechanism by which the great ice sheets wax and wane.

Accelerating ocean warming revealed by extensive oceanic buoy network

Author: Geoff Zeiss - Published At: 2017-02-26 15:10 - (190 Reads)
Marine and Oceans
Global warming is driven by Earth’s energy imbalance, the difference between the solar energy absorbed by the Earth and the energy radiated by Earth into space. Since most of the energy imbalance is stored in the oceans, measuring changes in ocean heat content provide the most reliable estimate of the planetary energy imbalance. Improved data coverage and analysis has made it possible to reconstruct temperature profiles across most ocean basins and at all depths to 6000 meters (m) from 1960 to 2015. The reconstructions reveal accelerating heating in the upper layers above 2000 m. Ocean warming is stronger since the late 1980s compared to the 1960s to the 1980s. The rate of warming in the deep ocean below 2000 m has been relatively slow and the heat uptake is only 13% of the estimated total oceanic heat uptake rate between 1993 and 2010. This analysis confirms that most of the Earth's energy imbalance has been stored in the upper ocean layers above 2000 meters.

Solar variability less important than greenhouse gases in global warming

Author: Geoff Zeiss - Published At: 2017-02-19 18:21 - (283 Reads)
Climate
Comparing measured solar irradiance and greenhouse gas concentrations with global surface temperature since 1975 reveals a high correlation between surface temperature and greenhouse gas concentrations and a weakening correlation with the variability in solar irradiance. Based on the trend that greenhouse gases are increasing rapidly whereas solar irradiance is changing much more slowly, the evidence reveals that the climate impact of changes in solar irradiance are much smaller in magnitude than the increase in warming due to greenhouse gases. Therefore, the little ice age projected by some researchers for the middle of the 21st Century is unlikely.

Global warming "hiatus" not supported by new analysis

Author: Geoff Zeiss - Published At: 2016-12-22 11:08 - (343 Reads)
Climate
Newly corrected and updated global surface temperature data do not support a global warming “hiatus". The new analysis shows the trend over the period 1950-1999 is indistinguishable from the trend over the period 2000-2014. An apparent "hiatus" in global warming was reported in the the IPCC Fifth Assessment Report (AR5). In this new study an analysis of new global ocean and land temperature data with corrections to older shipboard measurements reveals that the temperature trend over the period 2000-2014 does not differ from the temperature trend over the period 1950-1999. It is concluded that the "hiatus" reported in IPCC AR5 is most likely an artifact of older measurement techniques. Possible artifacts of data biases in the recent global surface warming hiatus, Thomas R. Karl, Anthony Arguez, Boyin Huang, Jay H. Lawrimore, James R. McMahon, Matthew J. Menne, Thomas C. Peterson, Russell S. Vose, and Huai-Min Zhang, Science 04 Jun 2015 DOI: 10.1126/science.aaa5632
Climate
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

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

Author: Geoff Zeiss - Published At: 2016-10-02 18:06 - (623 Reads)
Climate
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 - (492 Reads)
Climate
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 - (533 Reads)
Climate
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 - (516 Reads)
Climate
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 - (600 Reads)
Climate
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



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