Transcript PPT
Can Global Warming trigger rapid climate change? Take Away Concepts 1. 2. 3. 4. 5. Rapid climate change mechanisms The ocean’s role in rapid climate change. Evidence of past rapid climate change. The Pentagon Document (2003) Causes and patterns of drought in the US. Emissions Projections 2xCO2 450 ppm IPCC AR4 Warming Projections 2xCO2 = +3°C 2.0 - 4.5 °C range Modern warming is unusual, Future warming is “another world” 3°C warming “Dangerous Climate Change” A future level of warming sufficiently great to push climate to a tipping point. What value is this? Subjective,but can be estimated. • Level = +2°C above modern temperatures • Why? Warmer than any time in last three million years (Pliocene) when world was considerably warmer than today. • 450 ppm CO2 Why Dangerous? Feedbacks! • • • • • • Ice-albedo feedback Ice-sea-level feedback Salinity-Ocean circulation feedback Glacier-water supply link Hydrological shifts “unknowns” Pentagon Report (2003) What climate change really looks like Abrupt Climate Change warm Temperature in Central Greenland cold now then Younger Dryas interval in the Greenland Ice Core Warmer, pure ice Cold, dusty ice “Younger Dryas” cooling event (13 - 11.5 ka BP) warm cold present Warming Warming Cold reversal Last ice age past Flowers of “Dryas” plant Tundra flowering plant Pollen found in European lake sediments during YD What happened ? 15°C ∆T within a decade. Deep ocean circulation was shut down. Within years-to-centuries this signal was communicated around the world What happened? Glacial Lake Agassiz The Global Ocean Conveyor FRESH WATER …shutdown during the Younger Dryas cold period Past climate has seen MANY of these abrupt climate changes WARM then now COLD Sea-surface salinity Notice that Atlantic is saltier than Pacific Atlantic Thermohaline circulation AAIW NADW AABW South North Density of Seawater (due to salinity and temperature) Thermohaline (∆temperature and ∆salinity) How the ocean can respond non-linearly to freshwater forcing + 1 Deep water ventilation rate 5 2 4 3 - Fresh water addition to surface ocean Changes in deep water flow NADW “On” NADW “Off” An overview of the mechanism.. Steady increase in greenhouse gases… Steady warming of ocean and land surface … Invigorates hydrological cycle … High-latitude ice melt and increased river outflow causes high-latitude freshening … At some point, the high-latitude oceans become stratified --> no convection… The North Atlantic is freshening now Why? Warmer world speeds up hydrologic cycle Dry place become drier Wet places become wetter Also, melting ice contributes fresh water Red = saltier Blue = fresher How ice sheets melt How likely is this scenario? • Each step is physically plausible • Each step is occurring • BUT - we’re still a long way away from rapid (i.e. catastrophic) climate change. • The past is instructive because it shows that climate changes rapidly. This “Day” won’t happen Another face of Climate Change: Drought The Dust Bowl (1932-1939) 6-8 years long; $1 Billion (1930s dollars) What is Drought? What caused the ‘30s dust bowl? Observations Climate Model Tropical Pacific Ocean temperatures “La Niña” conditions Richard Seager and Ed Cook (Lamont) US Drought (7 years so far) Lake Powell levels (1963-Present) The New York Times MAY 2, 2004 Drought Settles In, Lake Shrinks and West's Worries Grow PAGE, Ariz. - At five years and counting, the drought that has parched much of the West is getting much harder to shrug off as a blip. Some of the biggest water worries are focused here on Lake Powell . . . © Ron Niebrugge © Ron Niebrugge Recall the Medieval Mega-Droughts * * Same pattern as Dust Bowl. … except, they lasted 20-40 years! Lake Powell Today 2002 2003 The Future is now: … “imminent drying of the southwest US” American West drought index Wetter Rainfall projections for the Southwest US. Based on historical climate and future CO2 18 of 19 climate models show this trend Drier Richard Seager (2007; Lamont) Summary 1. 2. 3. 4. Most warming due to greenhouse gases Climate sensitive to even weak forcing Climate changes can happen very quickly. Not just a “temperature” issue - “water” too. Bottom line: Climate is sensitive to “small nudges” GHG forcing is a “big push” Surprises are likely.