Environmental Variability and Climate Change

More documents

Recommendations

Info

involved instabilities that developed within the large continental ice sheets when they reached a critical size. The bipolar seesaw During the last ice age and the transition into the present warm period (Holocene) the Earth System exhibited climate swings that provide important clues as to how atmosphere, ocean and ice cover work together. Among the most prominent are the Dansgaard/Oeschger (D/O) events, a series of rapid warmings -- up to 16°C in Greenland -- recorded in ice cores. If climate was unstable in the past, what does that tell us about the future? In order to understand the mechanisms responsible for these instabilities, we need to know how the Earth as a whole reacted to these huge changes. Methane holds one of the keys. The atmospheric concentration of methane peaked at every warm episode. Even in locations far from the North Atlantic, there is a clear atmospheric ”echo” of D/O events. When methane concentrations are used to synchronize high-resolution ice core records from Greenland and central Antarctica, inferred temperatures in Antarctica show a slow warming that preceded the abrupt warming in the north by 1000-2000 years. When the north fi nally warmed (within less than a century), a slow cooling began in the south. This remarkable pattern of warming and cooling that is opposite in phase between Greenland and Antarctica has become known as the bipolar seesaw. This seesaw is a consequence of changes in the surface and deepwater circulation of the Atlantic and associated exchanges of heat. In the modern ocean, the overall transport of heat is from the Southern Atlantic to the Northern Atlantic. Thus, when the circulation in the north collapses, less heat is exported from the south, and the Southern Ocean warms. Re-establishment of the modern-type circulation produces a D/O event and the south starts cooling again. Nature is undoubtedly more complicated than this simple concept suggests; nevertheless, the bipolar seesaw is able to explain a number of characteristic features that are found in paleoclimatic records. The seesaw was predicted by numerical models before it was found in paleoclimatic records. Its discovery demonstrates the powerful insight that can be gained by comparing model results and paleoclimatic information. Will the thermohaline circulation change in the future? If the deep ocean circulation is sensitive to small perturbations, an enhanced fresh water fl ux to the high latitude North Atlantic ocean associated with global warming could drastically reduce the amount of heat that the ocean currently brings to Europe. Such potential future perturbations are many orders of magnitude smaller than fresh water inputs that have been used to drive modeled thermohaline shutdowns associated with D/O events. Although these models provide insight into a major dynamical mode of climate variability, they are not currently capable of accurately estimating how likely is a future shut down of the thermohaline circulation. If there is a shutdown in the future, its largest impact would be in Europe. However the associated cooling may be more than compensated by an even larger warming associated with greenhouse gases. Closeup of Marine Sediments Two important sources of information about past oceanic conditions in the North Atlantic. Sand-sized rock and mineral particles (bottom left) found in mid-ocean sediments indicate transport by icebergs far south of present-day ice limits. Calcareous shells of foraminifera (top right) provide evidence about former oceanographic conditions (water temperature, water chemistry). photo: Anne Jennings 10 IGBP SCIENCE No. 3 PAGES – Earth System Dynamics Figure 4 Rapid events The last major example of rapid climate change due to changes in deep ocean circulation occurred approximately 8200 years ago when a large lake on the edge of the North American ice sheet suddenly drained and sent huge amounts of fresh water into the North Atlantic. The resulting changes in the climate were recorded in sites as far apart as Greenland, Central Europe and Central North America.
An iceberg originating from northwestern Greenland fl oating in the northern North Atlantic. As such icebergs drift into warmer water and melt, debris particles rain down onto the sea fl oor leaving a record of the equatorward extent of iceberg drift. During the last glacial period ice-rafted debris in the North Atlantic was delivered as far south as the Portuguese margin. photo: Anne Jennings PAGES – Earth System Dynamics IGBP SCIENCE No. 3 11
Page 1 and 2: International Geosphere-Biosphere P
Page 3 and 4: 3 Foreword 4 Tools of the Trade - P
Page 5 and 6: IGBP SCIENCE No. 3 Foreword WE LIVE
Page 7 and 8: Cores taken from giant living coral
Page 9 and 10: When colder surface waters were pre
Page 11: warmer colder ‰ 1 2 3 ‰ 4.5 5
Page 15 and 16: Climate Forcing Variations in the i
Page 17 and 18: Natural Environmental Variability a
Page 19 and 20: Improving Climate Predictions Model
Page 21 and 22: Reconstruction of the North Atlanti
Page 23 and 24: surface temperatures, evaporation r
Page 25 and 26: Ecosystem Processes Both past clima
Page 27 and 28: Human Impacts Paleorecords show tha
Page 29 and 30: -1000 Lake depth history Naivasha,
Page 31 and 32: which focuses on modern climate cha
Page 33: IGBP SCIENCE No. 3 Further Reading
Page 36: WE LIVE IN UNUSUAL TIMES. Greenhous

Environmental Variability and Climate Change

Create successful ePaper yourself

Delete template?

Save as template?