Unformatted text preview: Chapter 14: Climate Change The earth's changing climate Possible causes of climatic change Global warming The Earth's Changing Climate 18,000 years ago The earth's climate is always changing 20,000 years ago the sea level was so low that the English Channel didn't even exist Determining Past Climates fossil evidence: pollen sediment cores: ocean, lake ice cores: Antarctic, Greenland oxygen isotope ratios dendrochronology: tree-ring Isotopes are atoms whose nuclei have the same number of protons but different numbers of neutrons. As the global warming debate has intensified in recent years, many methods of reconstructing past climates have undergone close scrutiny. Climate During Past 1000 Years and since 1860 "Hockey Stick" graph The global warming of the past 100 years has not been constant Climate Change and Feedback Mechanisms Feedbacks cause climate changes to be either amplified or reduced. positive feedback mechanisms: reinforce the interaction negative feedback mechanisms: weaken the interaction water vapor-greenhouse feedback: positive snow-albedo feedback: positive Cloud feedback: uncertain; overall negative Possible Causes of Climate Change external: changes in incoming solar radiation external (e.g., volcano) or human (e.g., CO2): changes in the composition of the atmosphere external (e.g., mountain uplifting) or human (e.g., land use): changes in the earth's surface Emissions of CO2 and other greenhouse gases are by no means the only way to change the climate. Climate Change and Atmospheric Particles (decadal-century) sulfate aerosols volcanic aerosols Sulfate aerosols are thought to cool the climate and therefore counteract global warming to some extent. Climate Change and Variations in Solar Output (decadal-century) sunspots Climate Change and Variations in the Earth's Orbit (10K-100K years) Milankovitch theory: - eccentricity (100K yr) - obliquity (41K yr) - precession (23K yr) Climate Change, Plate Tectonics, and Mountain-Building (100M yr) theory of plate tectonics Landmasses 150M years ago Today Warming is Unequivocal Rising atmospheric temperature Rising sea level Reductions in NH snow cover And oceans.. And upper atmosphere.... Carbon dioxide is causing the bulk of the forcing. On average, it lives more than a hundred years in the atmosphere and therefore affects climate over long time scales. without air, the earth surface temperature would be 33C colder Human and Natural Drivers of Climate Change Natural versus human-caused temperature increase Climate models are needed: aerosols; cloud; precipitation; land processes; .... Understanding and Attributing Climate Change Anthropogenic warming is likely discernible on all inhabited continents Observed Expected for all forcings Natural forcing only What's in the pipeline and what could come Warming will increase if GHG increase. If GHG were kept fixed at current levels, a committed 0.6C of further warming would be expected by 2100. More warming CO2 Eq would accompany more emission. 3.4oC = 6.1oF 850 600 2.8oC = 5.0oF 1.8oC = 3.2oF 0.6oC = 1.0oF 400 A1B is a typical "business as usual" (2090-2099) scenario: Global mean warming 2.8oC; Much of land area warms by ~3.5oC Arctic warms by ~7oC; would be less for less emission What else happens in a hotter world? Observations of sea level rise from satellites, 1993-2003. The global average SLR for the 20th century was about 6 inches (0.17m), mostly from expansion of the hot ocean, and with contributions from glacier melt (Alaska, Patagonia, Europe....). Future changes just from these processes could be up to 1.5 feet (0.5 m) by 2100, and up to 3 feet (1 meter) within about 2-3 centuries, depending on how much GHGs are emitted. But what about other processes? Rapid ice flow? Other related issues Drought Tropical forest dieoff Hurricane activities Kyoto Protocol ...
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- Fall '07