Richard J. Behl, PNAS, 108(15), Glacial demise and methane's rise

Proceedings of the National Academy of Sciences, April 12, 2011, Vol. 108, No. 15, pp. 5925-5926; doi: 10.1073/pnas.1101146108

Glacial demise and methane's rise

1. Richard J. Behl*

Department of Geological Sciences, California State University, Long Beach, CA 90840, U.S.A.

1. Department of Geological Sciences, California State University, Long Beach, CA 90840

The historical sciences—geology, archeology, and cosmology—test hypotheses differently than the experimental, laboratory sciences. When the process or event being investigated took place long ago or at a great distance from the investigators, hypotheses are tested by assembling key data that can support or refute the likelihood of the proposed explanation. In this way, we have developed understanding and agreement on many major events in Earth history, such as the Cretaceous–Tertiary extinction or the glacial–interglacial cycles of the Pleistocene epoch. A consilience of findings is required, and in a mechanistic, causative model, timing is of critical importance. If a key factor in the explanation can be shown to have occurred at a time inconsistent with the model—too early or too late—the hypothesis has to be modified or rejected. In PNAS, Reyes and Cooke (1) apply a refined dating approach to assess the timing of deglacial environmental change across the high-latitude circumpolar Arctic and its relation to increases in a major atmospheric greenhouse gas, methane. In their study, these investigators use improved methods for presentation and interpretation of the initiation dates for a very large dataset of Arctic peatland, tundra, and thermokarst sites to demonstrate that their development occurred too late to be the principal cause of impressively abrupt and large methane increases in atmospheric methane abundance at the beginning of the Bølling and the end of the …

*Correspondence e-mail:   behl@csulb.edu

Link to extract:  http://www.pnas.org/content/108/15/5925.extract

A. V. Reyes & C. A. Cooke, PNAS 108 (2011), Northern peatland initiation lagged abrupt increases in deglacial atmospheric CH4

Proceedings of the National Academy of Sciences, (March 22, 2011), Vol. 108, No. 12, pp. 4748-4753; doi: 10.1073/pnas.1013270108

Northern peatland initiation lagged abrupt increases in deglacial atmospheric CH₄

1. Alberto V. Reyes¹,²,³ and 
2. Colin A. Cooke¹,²,⁴

1. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada T6G 2E3

1. Edited by James P. Kennett, University of California, Santa Barbara, CA 93106, and approved January 14, 2011 (received for review September 13, 2010)

Abstract

Peatlands are a key component of the global carbon cycle. Chronologies of peatland initiation are typically based on compiled basal peat radiocarbon (¹⁴C) dates and frequency histograms of binned calibrated age ranges. However, such compilations are problematic because poor quality ¹⁴C dates are commonly included and because frequency histograms of binned age ranges introduce chronological artefacts that bias the record of peatland initiation. Using a published compilation of 274 basal ¹⁴C dates from Alaska as a case study, we show that nearly half the ¹⁴C dates are inappropriate for reconstructing peatland initiation, and that the temporal structure of peatland initiation is sensitive to sampling biases and treatment of calibrated ¹⁴C dates. We present revised chronologies of peatland initiation for Alaska and the circumpolar Arctic based on summed probability distributions of calibrated ¹⁴C dates. These revised chronologies reveal that northern peatland initiation lagged abrupt increases in atmospheric CH₄ concentration at the start of the Bølling–Allerød interstadial (Termination 1A) and the end of the Younger Dryas chronozone (Termination 1B), suggesting that northern peatlands were not the primary drivers of the rapid increases in atmospheric CH₄. Our results demonstrate that subtle methodological changes in the synthesis of basal ¹⁴C ages lead to substantially different interpretations of temporal trends in peatland initiation, with direct implications for the role of peatlands in the global carbon cycle.

Link to abstract:  http://www.pnas.org/content/108/12/4748

Northern peatland initiation lagged abrupt increases in deglacial atmospheric CH4 by A. V. Reyes & C. A. Cooke, PNAS,

Proceedings of the National Academy of Sciences, 108(12) (2011); doi:10.1073/pnas.1013270108

Northern peatland initiation lagged abrupt increases in deglacial atmospheric CH₄

1. Alberto V. Reyes¹,²,³ and 
2. Colin A. Cooke¹,²,⁴

Abstract

Peatlands are a key component of the global carbon cycle. Chronologies of peatland initiation are typically based on compiled basal peat radiocarbon (¹⁴C) dates and frequency histograms of binned calibrated age ranges. However, such compilations are problematic because poor quality ¹⁴C dates are commonly included and because frequency histograms of binned age ranges introduce chronological artefacts that bias the record of peatland initiation. Using a published compilation of 274 basal ¹⁴C dates from Alaska as a case study, we show that nearly half the ¹⁴C dates are inappropriate for reconstructing peatland initiation, and that the temporal structure of peatland initiation is sensitive to sampling biases and treatment of calibrated ¹⁴C dates. We present revised chronologies of peatland initiation for Alaska and the circumpolar Arctic based on summed probability distributions of calibrated ¹⁴C dates. These revised chronologies reveal that northern peatland initiation lagged abrupt increases in atmospheric CH₄ concentration at the start of the Bølling–Allerød interstadial (Termination 1A) and the end of the Younger Dryas chronozone (Termination 1B), suggesting that northern peatlands were not the primary drivers of the rapid increases in atmospheric CH₄. Our results demonstrate that subtle methodological changes in the synthesis of basal ¹⁴C ages lead to substantially different interpretations of temporal trends in peatland initiation, with direct implications for the role of peatlands in the global carbon cycle.

Link:  http://www.pnas.org/content/108/12/4748.abstract

J. D. Stanford et al., Global and Planetary Change, Sea-level probability for the last deglaciation: A statistical analysis of far-field records

Global and Planetary Change, doi: 10.1016/j.gloplacha.2010.11.002 

Sea-level probability for the last deglaciation: A statistical analysis of far-field records

J. D. Stanford^a, , , R. Hemingway^a, E. J. Rohling^a, P. G. Challenor^b, M. Medina-Elizalde^a and A. J. Lester^c

^a School of Ocean and Earth Science, University of Southampton, National Oceanography Centre, Southampton SO14 3ZH, United Kingdom

^b National Oceanography Centre, Southampton, SO14 3ZH, United Kingdom

^c The Chamber of Shipping, 12 Carthusian Street, London, EC1M 6EZ

(Received 4 March 2010;  

accepted 8 November 2010.  

Available online 26 November 2010.) 

Abstract

Pulses of ice-sheet meltwater into the world ocean during the last deglaciation are of great current interest, because these large-scale events offer important test-beds for numerical models of the responses of ocean circulation and climate to meltwater addition. The largest such event has become known as meltwater pulse (mwp) 1a, with estimates of about 20 m of sea-level rise in about 500 years. A second meltwater pulse (mwp-1b) has been inferred from some sea-level records, but its existence has become debated following the presentation of additional records. Even the use of the more ubiquitous mwp-1a in modelling studies has been compromised by debate about its exact age, based upon perceived discrepancies between far-field sea-level records. It is clear that an objective investigation is needed to determine to what level inferred similarities and/or discrepancies between the various deglacial sea-level records are statistically rigorous (or not). For that purpose, we present a Monte Carlo style statistical analysis to determine the highest-probability sea-level history from six key far-field deglacial sea-level records, which fully accounts for realistic methodological and chronological uncertainties in all these records, and which is robust with respect to removal of individual component datasets. We find that sea-level rise started to accelerate into the deglaciation from around 17 ka BP. Within the deglacial rise, there were two distinct increases; one at around the timing of the Bølling warming (14.6 ka BP), and another, much broader, event that just post-dates the end of the Younger Dryas (11.3 ka BP). We interpret these as mwp-1a and mwp-1b, respectively. We find that mwp-1a occurred between 14.3 ka BP and 12.8 ka BP. Highest rates of sea-level rise occurred at ~ 13.8 ka, probably (67% confidence) within the range 100-130 cm/century, although values may have been as high as 260 cm/century (99% confidence limit). Mwp-1b is robustly expressed as a broad multi-millennial interval of enhanced rates of sea-level rise between 11.5 ka BP and 8.8 ka BP, with peak rates of rise of up to 250 cm/century (99 % confidence), but with a probable rate of 130 -150 cm/century (67 % confidence) at around 9.5 ka BP. When considering the 67 % probability interval for the deglacial sea-level history, it is clear that both mwp1a and 1b were relatively subdued in comparison to the previously much higher rate estimates.

Link:  

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VF0-51JPWV6-1&_user=10&_coverDate=11/26/2010&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=f1911315d3f2025ff72450700fb92288&searchtype=a

J. D. Stanford et al., Global Sea-Level Rise at the End of the Last Ice Age Interrupted by Rapid 'Jumps'

Global Sea-Level Rise at the End of the Last Ice Age Interrupted by Rapid 'Jumps'

ScienceDaily, December 4, 2010 — Southampton researchers have estimated that sea-level rose by an average of about 1 metre per century at the end of the last Ice Age, interrupted by rapid 'jumps' during which it rose by up to 2.5 metres per century. The findings, published in Global and Planetary Change, will help unravel the responses of ocean circulation and climate to large inputs of ice-sheet meltwater to the world ocean.

Global sea level rose by a total of more than 120 metres as the vast ice sheets of the last Ice Age melted back. This melt-back lasted from about 19,000 to about 6,000 years ago, meaning that the average rate of sea-level rise was roughly 1 metre per century.

Previous studies of sea-level change at individual locations have suggested that the gradual rise may have been marked by abrupt 'jumps' of sea-level rise at rates that approached 5 metres per century. These estimates were based on analyses of the distribution of fossil corals around Barbados and coastal drowning along the Sunda Shelf, an extension of the continental shelf of East Asia.

However, uncertainties in fossil dating, scarcity of sea-level markers, and the specific characteristics of individual sites can make it difficult to reconstruct global sea level with a high degree of confidence using evidence from any one site.

"Rather than relying on individual sites that may not be representative, we have compared large amounts of data from many different sites, taking into account all potential sources of uncertainty," said Professor Eelco Rohling of the University of Southampton's School of Ocean and Earth Science (SOES) based at the National Oceanography Centre (NOC) in Southampton.

The researchers brought together about 400 high-quality sea-level markers from study sites around the globe, concentrating on locations far removed from the distorting effects of the past massive ice sheets.

Using an extensive series of sophisticated statistical tests, they then reconstructed sea-level history of the last 21 thousand years with a high degree of statistical confidence.

Their analyses indicate that the gradual rise at an average rate of 1 metre per century was interrupted by two periods with rates of rise up to 2.5 metres per century, between 15 and 13 thousand years ago, and between 11 and 9 thousand years ago.

The first of these jumps in the amount of ice-sheet meltwater entering the world ocean coincides with the beginning of a period of global climate warming called the Bølling-Allerød period. The second jump appears to have happened shortly after the end the 'big freeze' called the Younger Dryas that brought the Bølling-Allerød period to an abrupt end.

"Our estimates of rates of sea-level rise are lower than those estimated from individual study sites, but they are statistically robust and therefore greatly improve our understanding of loss of ice volume due to the melting of the ice sheets at the end of the last Ice Age," said lead author Dr Jennifer Stanford of SOES.

"The new findings will be used to refine models of the Earth climate system, and will thus help to improve forecasts of future sea-level responses to global climate change," added Rohling.

The researchers are Jenny Stanford, Rebecca Hemingway, Eelco Rohling and Martin Medina-Elizalde (SOES), Peter Challenor (NOC) and Adrian Lester (The Chamber of Shipping, London).

The research was supported by the United Kingdom's Natural Environment Research Council.

Link:  http://www.sciencedaily.com/releases/2010/12/101201120605.htm

Z. Liu et al., Science, 325 (July 17, 2009), Transient simulation of last deglaciation with a new mechanism for Bølling-Allerød Warming

Science (17 July 2009): Vol. 325, No. 5938, pp. 310-314; DOI: 10.1126/science.1171041

Transient simulation of last deglaciation with a new mechanism for Bølling-Allerød Warming

Z. Liu,^1,2,3,* B. L. Otto-Bliesner,⁴ F. He,³ E. C. Brady,⁴ R. Tomas,⁴ P. U. Clark,⁵ A. E. Carlson,⁶ J. Lynch-Stieglitz,⁷ W. Curry,⁸ E. Brook,⁵ D. Erickson,⁹ R. Jacob,¹⁰ J. Kutzbach,³ and J. Cheng^1,3

Abstract

We conducted the first synchronously coupled atmosphere-ocean general circulation model simulation from the Last Glacial Maximum to the Bølling-Allerød (BA) warming. Our model reproduces several major features of the deglacial climate evolution, suggesting a good agreement in climate sensitivity between the model and observations. In particular, our model simulates the abrupt BA warming as a transient response of the Atlantic meridional overturning circulation (AMOC) to a sudden termination of freshwater discharge to the North Atlantic before the BA. In contrast to previous mechanisms that invoke AMOC multiple equilibrium and Southern Hemisphere climate forcing, we propose that the BA transition is caused by the superposition of climatic responses to the transient CO₂ forcing, the AMOC recovery from Heinrich Event 1, and an AMOC overshoot.

^*Correspondence, e-mail: zliu3@wisc.edu

See link to abstract for authors' affiliations.

Link to abstract: http://www.sciencemag.org/cgi/content/abstract/sci;325/5938/310

Link to full article (subscription required): http://www.sciencemag.org/cgi/content/full/sci;325/5938/273

Z. Liu & B. Otto-Bliesner: Oak Ridge Supercomputers Provide First Simulation of Abrupt Climate Change

Oak Ridge Supercomputers Provide First Simulation of Abrupt Climate Change

Published on 16 July 2009, 12:04; last update: 17 hour(s) ago by Insciences

OAK RIDGE, Tenn., July 16, 2009 — At the Department of Energy's Oak Ridge National Laboratory (ORNL), the world's fastest supercomputer for unclassified research is simulating abrupt climate change and shedding light on an enigmatic period of natural global warming in Earth's relatively recent history. The work, led by scientists at the University of Wisconsin and the National Center for Atmospheric Research (NCAR), is featured in the July 17, 2009, issue of the journal Science and provides valuable new data about the causes and effects of global climate change.

This research is funded by the Office of Biological and Environmental Research within DOE's Office of Science and by the National Science Foundation through its paleoclimate program and support of NCAR.

In Earth's 4.5-billion-year history, its climate has oscillated between hot and cold. Today our world is relatively cool, resting between ice ages. Variations in planetary orbit, solar output, and volcanic eruptions all change Earth's temperature. Since the Industrial Revolution, however, humans have probably warmed the world faster than nature has. The greenhouse gases we generate by burning fossil fuels and forests will raise the average global temperature 2-12 °F (1-6 °C) this century, the Intergovernmental Panel on Climate Change (IPCC) estimates.

Most natural climate change has taken place over thousands or even millions of years. But an episode of abrupt climate change occurred over centuries—possibly decades—during Earth's most recent period of natural global warming, called the Bolling-Allerod warming. Approximately 19,000 years ago, ice sheets started melting in North America and Eurasia. By 17,000 years ago, the melting glaciers had dumped so much freshwater into the North Atlantic that it stopped the overturning ocean circulation, which is driven by density gradients caused by influxes of freshwater and surface heat. This occurrence led to a cooling in Greenland called the Heinrich event 1. The freshwater flux continued on and off until about 14,500 years ago, when it virtually stopped. Greenland's temperature then rose by 27 °F (15 °C) in several centuries, and the sea level rose about 16 ft. (5 m). The cause of this dramatic Bolling-Allerod warming has remained a mystery and source of intense debate.

"Now we are able to simulate these transient events for the first time," says Zhengyu Liu, a University of Wisconsin professor of atmospheric and oceanic sciences and environmental studies whose team simulated the abrupt climate changes using DOE supercomputers at ORNL. The Oak Ridge Leadership Computing Facility allocated supercomputing time through DOE's Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. "It represents so far the most serious validation test of our model capability for simulating large, abrupt climate changes, and this validation is critical for us to assess the model's projection of abrupt changes in the future," according to Liu.

The Oak Ridge Leadership Computing Facility is funded by the Office of Advanced Scientific Computing Research in DOE's Office of Science.

Liu, director of the University of Wisconsin's Center for Climatic Research, and his collaborator Bette Otto-Bliesner, an atmospheric scientist and climate modeler at NCAR, lead an interdisciplinary, multi-institution research group attempting the world's first continuous simulation of 21,000 years of Earth's climate history, from the last glacial maximum to the present, in a state-of-the-art climate model. The group will also extend the simulation 200 years into the future to forecast climate. The findings could provide great insight into the fate of ocean circulation in light of continued glacial melting in Greenland and Antarctica.

Three parts to abrupt change

Most climate simulations in comprehensive climate models so far are discontinuous, amounting to snapshots of century-sized time slices taken every 1,000 years or so. Such simulations are incapable of simulating abrupt transitions occurring on centennial or millennial timescales. Liu and Otto-Bliesner employ petascale supercomputers, capable of a quadrillion calculations each second, to stitch together a continuous stream of global climate snapshots and recover the virtual history of global climate in a motion picture. They use the Community Climate System Model (CCSM), a global climate model that includes coupled interactions between atmosphere, oceans, lands, and sea ice developed with primary funding from the National Science Foundation (NSF) and DOE.

Based on insights gleaned from their continuous simulation, Liu and his colleagues propose a novel mechanism to explain the Bolling-Allerod warming observed in Greenland ice cores. The three-part mechanism they suggest matches the climate record.

First, one-third of the warming, or 9 °F (5 °C), resulted from a 45 ppm increase in the atmospheric concentration of carbon dioxide, the scientists posit. The cause of the carbon dioxide increase, however, is still a topic of active research, Liu says.

Second, another one-third of the warming was due to recovery of oceanic heat transport. When fresh meltwater flowed off the ice sheet, it stopped the overturning ocean current and in turn the warm surface current from low latitudes, leading to a cooling in the North Atlantic and nearby region. When the melting ice sheet was no longer dumping freshwater into the North Atlantic, the region began to heat up.

The last one-third of the temperature rise resulted from an overshoot of the overturning circulation. "Once the glacial melt stopped, the enormous subsurface heat that had accumulated for 3,000 years erupted like a volcano and popped out over decades," Liu hypothesizes. "This huge heat flux melted the sea ice and warmed up Greenland."

Liu and Otto-Bliesner's collaborators include Feng He, a doctoral student at the University of Wisconsin-Madison who is mainly responsible for the deglaciation modeling, as well as ocean modeler Esther Brady (NCAR), atmospheric scientist Robert Tomas (NCAR), glaciologists Peter Clark (Oregon State University) and Anders Carlson (University of Wisconsin-Madison), paleoceanographers Jean Lynch-Stieglitz (Georgia Institute of Technology) and William Curry (Woods Hole Oceanographic Institution), geochemist Edward Brook (Oregon State University), atmospheric modeler David Erickson (ORNL), computing expert Robert Jacob (Argonne National Laboratory), and climate modelers John Kutzbach (University of Wisconsin-Madison) and Jun Cheng (Nanjing University of Information Science and Technology). "This interdisciplinary team, each member contributing to a different aspect of the project, ranging from a proxy data interpretation to supercomputing coding, has been essential for the success of this project," says Liu.

The 2008 simulations ran on a Cray X1E supercomputer named Phoenix and an even faster Cray XT system called Jaguar. The scientists used nearly a million processor hours in 2008 to run one-third of their simulation, from 21,000 years ago—the most recent glacial maximum—to 14,000 years ago—the planet's most recent major period of natural global warming. With 4 million INCITE processor hours allocated on Jaguar for 2009, 2010, and 2011, they will complete the simulation, capturing climate from 14,000 years ago to the present and projecting it 200 years into the future. "This has been a dream run of both of ours for a long time," says Otto-Bliesner. "This was an opportunity to take advantage of the CCSM, the computing facility at Oak Ridge, and the INCITE call for proposals." No other research group has successfully simulated such a long period in a comprehensive climate model.

Science-based forecasts

More accurately depicting the past means clearer insights into climate's outlook. "The current forecast predicts the ocean overturning current is likely to weaken but not stop over the next century," Liu says. "However, it remains highly uncertain whether abrupt changes will occur in the next century because of our lack of confidence in the model's capability in simulating abrupt changes. Our simulation is an important step in assessing the likelihood of predicted abrupt climate changes in the future because it provides a rigorous test of our model against the major abrupt changes observed in the recent past."

In 2004 and 2005, climate simulations on DOE supercomputers contributed data to a repository that scientists worldwide accessed to write approximately 300 journal articles. The published articles were cited in the Fourth Assessment Report of the IPCC, which concluded that global warming is unequivocal and humans have had a substantial role since the mid-20th century.

Liu and Otto-Bliesner's simulations may soon find their way into IPCC's data repository and reports as other groups succeed in continuous simulation of past abrupt climate changes and demonstrate the results are reproducible. The simulations would thus be a resource for the paleo community at large. Meanwhile, Earth's climate continues to prove that change is an eternal constant. Understanding how we affect the rate of change is a grand challenge of our generation. Petascale computing may accelerate answers that in turn inform our policies and guide our actions.

Contact: Dawn Levy, Communications and External Relations, tel.: (865) 576-6448

Source: Oak Ridge National Laboratory (ORNL)

Link to article: http://insciences.org/article.php?article_id=6175