Pre-Industrial Rise in Greenhouse Gases Had Natural and Anthropogenic Causes For years scientists have intensely argued over whether increases of potent methane gas concentrations in the atmosphere - from about 5,000 years ago to the start of the industrial revolution - were triggered by natural causes or human activities. oregonstate.edu/ua/ncs/archives/2013/nov/pre-industrial-rise-greenhouse-gases-had-natural-and-anthropogenic-causes A new study, which will be published Friday in the journal Science, suggests the increase in methane likely was caused by both. Lead author Logan Mitchell, who coordinated the research as a doctoral student at Oregon State University, said the early anthropogenic hypothesis, which spawned hundreds of scientific papers as well as books, cannot fully explain on its own the rising levels of atmospheric methane during the past 5,000 years, a time period known as the mid- to late-Holocene. That theory suggests that human activities such as rice agriculture were responsible for the increasing methane concentrations. Opponents of that theory argue that human activities during that time did not produce significant amounts of methane and thus natural emissions were the dominant cause for the rise in atmospheric CH4. We think that both played a role, said Mitchell, who is now a post-doctoral researcher at the University of Utah. The increase in methane emissions during the late Holocene came primarily from the tropics, with some contribution from the extratropical Northern Hemisphere. Neither modeled natural emissions alone, nor hypothesized anthropogenic emissions alone, are able to account for the full increase in methane concentrations, Mitchell added. Combined, however, they could account for the full increase. Scientists determine methane levels by examining ice cores from polar regions. Gas bubbles containing ancient air trapped within the ice can be analyzed and correlated with chronological data to determine methane levels on a multidecadal scale. Mitchell and his colleagues examined ice cores from the West Antarctic Ice Sheet Divide and the Greenland Ice Sheet Project and found differences between the two. Ice cores from Greenland had higher methane levels than those from Antarctica because there are greater methane emissions in the Northern Hemisphere. The difference in methane levels between the hemispheres, called the Inter-Polar Difference, did not change appreciably over time. If the methane increase was solely natural or solely anthropogenic, it likely would have tilted the Inter-Polar Difference out of its pattern of relative stability over time, Mitchell said. Since coming out of the ice age some 10,000 years ago summer solar insolation in the Northern Hemisphere has been decreasing as a result of the Earths changing orbit, according to Edward Brook, a paleoclimatologist in Oregon States College of Earth, Ocean, and Atmospheric Sciences and Mitchells major professor. This decrease affects the strength of Asian summer monsoons, which produce vast wetlands and emit methane into the atmosphere. Yet some 5,000 years ago, atmospheric methane began rising and had increased about 17 percent by the time the industrial revolution began around 1750. Theoretically, methane levels should have decreased with the loss of solar insolation in the Northern Hemisphere, or at least remained stable instead of increasing, said Brook, a co-author on the Science article. They had been roughly on a parallel track for some 800,000 years. Mitchell used previous models that hypothesized reasons for the methane increase - both natural and anthropogenic - and compared them to the newly garnered ice core data. None of them alone proved sufficient for explaining the greenhouse gas increase. When he developed his own model combining characteristics of both the natural and anthropogenic hypotheses, it agreed closely with the ice core data. Other researchers have outlined some of the processes that may have contributed to changes in methane emissions. More than 90 percent of the population lived in the Northern Hemisphere, especially in the lower latitudes, and the development of rice agriculture and cattle domestication likely had an influence on methane emissions. On the natural side, changes in the Earths orbit could have been responsible for increasing methane emissions from tropical wetlands. All of these things likely have played a role, Mitchell said, but none was sufficient to do it alone. The study was supported by the National Science Foundations Office of Polar Programs, with additional support from the Oregon National Aeronautics and Space Administration (NASA) Space Grant Consortium. References: (*) Humans Fueled Global Warming Millennia Ago Science 22 November 2013: Vol. 342 no. 6161 p. 918 DOI: 10.1126/science.342.6161.918 sciencemag.org/content/342/6161/918.summary News & Analysis: Climate Change People were already pumping greenhouse gases into the atmosphere 5000 years before the Industrial Revolution, air bubbles in Antarctic ice suggest. The new evidence supports a paleoclimatologists provocative idea that humanity began warming the world early, as methane bubbled out of early rice farmers paddies. 1. L. Mitchell, E. Brook, J. E. Lee, C. Buizert, T. Sowers. Constraints on the Late Holocene Anthropogenic Contribution to the Atmospheric Methane Budget. Science, 2013; 342 (6161): 964 DOI: 10.1126/science.1238920 Constraints on the Late Holocene Anthropogenic Contribution to the Atmospheric Methane Budget Science 22 November 2013: Vol. 342 no. 6161 pp. 964-966 DOI: 10.1126/science.1238920 sciencemag.org/content/342/6161/964.abstract Editors Summary Bipolar Signature Atmospheric methane has increased approximately 2.5-fold since the start of the industrial revolution, a consequence of human activity. However, a smaller and more gradual rise began around 6000 years ago, near the time when human agriculture began to develop and expand. Mitchell et al. (p. 964) present two, high-resolution ice core methane records of the past 2500 years, one from each pole. Methane emissions were primarily from the tropics, with secondary contributions from the higher latitudes where most humans lived. Thus, both natural and human sources are needed to explain the late-Holocene atmospheric methane record. Abstract The origin of the late preindustrial Holocene (LPIH) increase in atmospheric methane concentrations has been much debated. Hypotheses invoking changes in solely anthropogenic sources or solely natural sources have been proposed to explain the increase in concentrations. Here two high-resolution, high-precision ice core methane concentration records from Greenland and Antarctica are presented and are used to construct a high-resolution record of the methane inter-polar difference (IPD). The IPD record constrains the latitudinal distribution of emissions and shows that LPIH emissions increased primarily in the tropics, with secondary increases in the subtropical Northern Hemisphere. Anthropogenic and natural sources have different latitudinal characteristics, which are exploited to demonstrate that both anthropogenic and natural sources are needed to explain LPIH changes in methane concentration. Supplementary Materials sciencemag.org/content/suppl/2013/11/21/342.6161.964.DC1/Mitchell.SM.pdf
Posted on: Fri, 22 Nov 2013 21:17:54 +0000
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