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AT: Warming Kills Gulf Stream
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AT: Warming Kills Gulf StreamGlobal warming makes the ocean saltier – stabilizing the Gulf Stream Brahic 07 — (New Scientist’s environmental reporter , Saltier North Atlantic should give currents a boost.” http://environment.newscientist.com/article/dn12528) The surface waters of the North Atlantic are getting saltier, suggests a new study of records spanning over 50 years. And this might actually be good news for the effects of climate change on global ocean currents in the short-term, say the study's researchers. This is because saltier waters in the upper levels of the North Atlantic ocean may mean that the global ocean conveyor belt – the vital piece of planetary plumbing which some scientists fear may slow down because of global warming – will remain stable. The global ocean conveyor belt is the crucial circulation of ocean waters around the Earth. It helps drive the Gulf Stream and keeps Europe warm. The density of waters which drives the flow of ocean currents is dependent on temperature and salinity, so any change in saltiness may have an impact. Tim Boyer of the US National Oceanographic Data Center and colleagues compiled salinity data gathered by fisheries, navy and research ships travelling across the North Atlantic between 1955 and 2006. They found that during this time, the layer of water that makes up the top 400 metres has gradually become saltier. The seawater is probably becoming saltier due to global warming, Boyer says. "We know that upper ocean is warming in the North Atlantic, so it stands to reason that there should be more evaporation, making waters more salty," he says. Polar 'pulse' The global ocean conveyor belt is in part driven by salty and relatively dense subpolar waters sinking and flowing south to the equator. So when a huge "pulse" of less dense freshwater was found to have been dumped into the sub-polar waters of the North Atlantic in the mid-1960s, researchers speculated the sub-polar waters might just stay floating where they were and cause circulation to stagnate. The freshwater pulse probably came from a combination of increased rainfall and melting ice, as well as big chunks of ice suddenly pushing through the Fram Straight into the Atlantic. When in their recent study Boyer and his colleagues zoomed in on the subarctic Atlantic, they found that the waters there became much less salty in the 1960s, as expected. But since the 1990s, they have been getting saltier again, and are now about as salty as they were in the 1970s. Backing up this finding, when the team looked at the salinity of deeper waters, those flowing more than 1300 metres beneath the surface, they found that these have been getting less salty since the late 1980s. They see this as a sign that the pulse of freshwater has been slowly making its way south. It takes roughly 10 to 15 years for subpolar water to move away from the Arctic and down to the equator. Even if they’re right only minor climate changes will occur, Global warming won’t lead to an ice age or a collapse of the AMO Weaver and Hillaire 2004 — (Andrew Weaver and Claud Hillaire, Gordon head of the School of Earth and Ocean Sciences at the University of Victoria and a Canadian geoscientist of great distinction and a world leader in Quaternary research. He is known for his groundbreaking research on the environment, climate change, and oceanography. He is a Fellow of the Royal Society of Canada and professor at l'Université du Québec à Montréal, 4/16/2004, “Global Warming and the next Ice Age,” http://web.ebscohost.com/ehost/detail?vid=1&hid=14&sid=5e63d5e2-5a5a-4141-a53a-7826e5e7c1bb%40sessionmgr2) Models that eventually lead to a collapse of the AMO under global warming conditions typically fall into two categories: (i) flux-adjusted coupled general circulation models, and (ii) intermediate-complexity models with zonally averaged ocean components. Both suites of models are known to be more sensitive to freshwater perturbations. In the first class of models, a small perturbation away from the present climate leads to large systematic errors in the salinity fields (as large flux adjustments are applied) that then build up to cause dramatic AMO transitions. In the second class of models, the convection and sinking of water masses are coupled (there is no horizontal structure). In contrast, newer non — flux-adjusted models find a more stable AMO under future conditions of climate change ( 11, 13, 14). Even the recent observations of freshening in the North Atlantic ( 15) (a reduction of salinity due to the addition of freshwater) appear to be consistent with the projections of perhaps the most sophisticated non — flux-adjusted model ( 11). Ironically, this model suggests that such freshening is associated with an increased AMO ( 16). This same model proposes that it is only Labrador Sea Water formation that is susceptible to collapse in response to global warming. In light of the paleoclimate record and our understanding of the contemporary climate system, it is safe to say that global warming will not lead to the onset of a new ice age. These same records suggest that it is highly unlikely that global warming will lead to a widespread collapse of the AMO — despite the appealing possibility raised in two recent studies ( 18, 19) — although it is possible that deep convection in the Labrador Sea will cease. Such an event would have much more minor consequences on the climate downstream over Europe. Ice Age - CO2 KeyCO2 emissions are the only thing preventing the next ice age P. C. Tzedakis, et al J. E. T. Channell,D. A. Hodell,H. F. Kleiven & L. C. January 2012 Skinner Determining the natural length of the current interglacial http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1358.html Climate modelling studies show that a reduction in boreal summer insolation is the primary trigger for glacial inception, with CO2 playing a secondary role3, 5. Lowering CO2 shifts the inception threshold to higher insolation values1, but modelling experiments indicate that preindustrial concentrations of 280 ppmv would not be sufficiently low to lead to new ice growth given the subdued insolation minimum2, 3, 4. However, the extent to which preindustrial CO2 levels were ‘natural’ has been challenged10, 11 by the suggestion that anthropogenic interference since the mid-Holocene led to increased greenhouse gas (GHG) concentrations, which countered the natural cooling trend and prevented a glacial inception. The overdue glaciation hypothesis has been tested by climate simulations using lower preindustrial GHG concentrations, with contrasting results, ranging from no ice growth5 to a linear increase in ice volume4 to large increases in perennial ice cover6. Empirical evidence from intervals characterized by similar boundary conditions to the current interglacial may also be used to infer the timing of the next ‘natural’ glacial inception, assuming that, for a given insolation and CO2 forcing, ice-volume responses between two periods are also similar. Here, we limit the search for potential Holocene analogues to the past 800 kyr, for which ice-core records of atmospheric GHG concentrations are available12, 13. We then explore approaches to constraining the timing of glacial inception and assess the relevance of this information to the current interglacial.
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