Is the North Atlantic thermohaline circulation in danger of shutting down in the near future and if so, what are the likely consequences? What do observations, current understanding of ocean processes, and models tell us about the likelihood of such an event in the near and distant future?
What is the current understanding of long-term droughts and what drives them? How well are historic droughts faithfully reproduced in the current generation of models? Is there sufficient forecasting skill to estimate the likelihood of long-term droughts in the Southwestern U.S. and in the sub-tropics, in general? If so, what is the outlook?
Dr. Anthony Socci, Senior Science Fellow, American Meteorological Society
Dr. M. Susan Lozier, Professor and Chair, Earth and Ocean Sciences, Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC
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Dr. Richard Seager, Doherty Senior Research Scientist, Lamont Doherty Earth Observatory, Columbia University, Palisades, NY
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Abrupt Changes in Ocean Heat Transport
Much of the concern surrounding abrupt climate change seems to be based on 1) an expectation that global warming will slowdown or halt the overturning circulation of the ocean and 2) paleo evidence that abrupt changes to Earth s climate have occurred in the geologic past. The overturning circulation of the ocean redistributes heat between the tropics and the Polar Regions; significant changes to this circulation are expected to create disruptions of regional climates that are potentially greater than the global warming that initiated the change in the circulation. Hence, the concern has arisen that a small amount of global warming could trigger, abruptly, much larger regional changes in climate.
Though geologic evidence and modeling studies lead oceanographers to expect changes in the ocean's overturning circulation, there is no firm evidence to date that the circulation has slowed down due to recent global warming. Oceanographers' ability to detect changes in the overturning circulation is hampered by the strong variability of ocean currents and properties in time, coupled with the fact that the ocean is severely under sampled. Continuous monitoring of the ocean flow field is necessary for oceanographers to detect changes in the overturning circulation, and, importantly, to understand and predict how ocean circulation will respond to future climate change. Finally however, a clear human impact on the oceans is evident from the uptake of anthropogenic carbon dioxide, from the devastation of marine fisheries due to overfishing, and from rising sea levels.
The Case for an Imminent Transition to a
More Arid Climate in Southwestern North America
Drought is one of the most costly of natural hazards to trouble the United States. With the exception of the Midwest and Southeast, much of American agricultural production lies in areas such as the Plains and Southwest that are prone to severe, multi-year droughts. Recent climate modeling work has made clear that these severe droughts, such as the Dust Bowl and the 1950s Southwest drought, are to a significant extent forced by small changes of tropical Pacific (and, to a lesser extent, Atlantic) sea surface temperatures (SSTs). Work is now underway to determine the extent to which these climate variations are predictable.
Severe though these modern day droughts are, including the one that began in 1998, they are dwarfed by a series of multidecadal megadroughts that struck the West between about 800 and 1400 A.D. These megadroughts have also been related to small variations in tropical SSTs that were potentially forced by small changes in solar irradiance and by variations in explosive volcanism. The well established existence of megadroughts raises great concern as to whether southwestern North America could ever move back to an even more arid climate than it currently has.
It turns out that there is widespread agreement amongst the 19 coupled general circulation models that participated in the IPCC Fourth Assessment Report that the Southwest begins a transition to a drier climate at the end of the 20th century and that a new, drier, climate is well established early in the current century. This climate change is part of a general subtropical drying that also strongly impacts the Mediterranean-North Africa-Middle East region. Subtropical drying is a consequence of the impact that global warming has on atmospheric water vapor transport and atmospheric circulation. At this point it seems to be dynamically distinct from the causes of Medieval megadroughts but the implications are similar. According to the state-of-the-art models, conditions equivalent to, say, a perpetual 1950s style drought is likely to become the new climate of the Southwest in the coming years to decades. This will have serious implications for development and water resource allocation in the region and, since the drier climate also impacts northern Mexico, for cross border relations.
Dr. Susan Lozier is Professor and Chair of Earth and Ocean Sciences in the Nicholas School of the Environment and Earth Sciences at Duke University. After receiving her Ph.D. in physical oceanography from the University of Washington in 1989, Professor Lozier conducted postdoctoral studies at the Woods Hole Oceanographic Institution, then joined the Duke faculty in 1991. She is the recipient of a National Science Foundation Career Award, serves as the University Liaison for NASA s Education and Outreach for the Aquarius Satellite Mission, is a member of the Ocean Observatory Initiative Steering Committee and is an Adjunct Scientist at the Woods Hole Oceanographic Institution.
Professor Lozier's research lies in the field of physical oceanography. The overriding direction of her research is toward an evaluation of the ocean as a reservoir for climate signals. By understanding the extent to which climatic anomalies spread from their source region, and the rapidity of that spreading, Professor Lozier aims to determine the effectiveness of the deep ocean as a climatic reservoir for heat and carbon dioxide. A current research emphasis is on how climate signals are exported from the Labrador and Mediterranean Seas. She is also interested in the physical constraints placed on ocean nutrient supply and productivity. Professor Lozier has published 25 peer-reviewed articles, given over 30 invited talks, and participated in six research cruises.
Dr. Richard Seager is a Doherty Senior Research Scientist at Lamont Doherty Earth Observatory of Columbia University in Palisades, New York. He gained his undergraduate degree at Liverpool University in England and came to the United States 1983, as a graduate student at Columbia. His Ph. D work was completed in 1990 under the supervision of Professor Mark Cane and involved using tropical atmosphere and ocean models to understand key features of the tropical climate. In 1991-2 he completed a postdoctoral appointment at the University of Washington before returning to Lamont.
Throughout his career Dr. Seager has used numerical models, observations and proxy reconstructions of past climates to understand the physical mechanisms responsible for climate variability and change on seasonal to glacial-interglacial timescales. He has a particular interest in how the variation of the tropical atmosphere-ocean system organize climate on a global scale. His recent work has focused on the mechanisms of persistent North American drought and its relation to tropical Pacific and tropical Atlantic ocean temperature variations. This work has led him into studies of Medieval megadroughts in the American West and studies of the hydrological future of the West. He has around 50 published peer-reviewed papers in scholarly journals and a small number of popular articles in addition.
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