17ClimateObservations

17ClimateObservations - ESM 203 Climate Change— Change—...

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Unformatted text preview: 11/29/2007 ESM 203: Climate Change— Change— Observations Jeff Dozier & Thomas Dunne Fall 2007 Climate change at time scale 10-100 million 10years Reminder from Tom’s lecture on Global Tectonics ◦ Slide #20, 11/01/2007, Solid Earth 1 Fundamental differences between continental crust and ocean crust: lithology, density, elevation Global distribution of continents and ocean basins affects patterns of net radiation and ocean circulation 3 What makes a climate model believable? Crucial question in distinguishing natural from anthropogenic variability Ability to explain ◦ Long-term trends in global climate ◦ Spatial variability in climate ◦ Response to short-term forcing Sea floor Ages Paleoclimatology and historical climatology ◦ Record of past changes 2 http://www.ngdc.noaa.gov/mgg/image/crustageposter.jpg 4 ESM 203: Climate change–observations 1 11/29/2007 Climate change at scales 10 thousand to a few million years: the Milankovitch (1920) hypothesis Long-term variations in climate depend on seasonal and geographic variations in solar radiation which are radiation, caused by orbital variations LongLong-term variability in ocean δ18O: why the shift from 41 Kyr to 100 Kyr? Kyr? 41 Kyr cycle dominated 1.5-2.5 million years ago 100 Kyr cycle dominated in last million years R. A. Muller & G. J. MacDonald, Glacial Cycles and Astronomical Forcing, Science, 277, 215-218, July 11, 1997 5 7 Orbital causes of climate variability Eccentricity ◦ period 95–125 kyr 147M km 152M km ◦ orbit varies between more eccentric and more circular Obliquity ◦ period 41 kyr ◦ tilt of Earth’s axis varies from 21.8–24.4o Precession ◦ period 21-23 kyr ◦ date of perihelion cycles through calendar Jan 5 July 4 There is also a 100 kyr variation in the orbital inclination, 2°+ from the plane of the ecliptic Synchronous behavior of the ice sheets despite asynchronous radiation forcing P. U. Clark, R. B. Alley, & D. Pollard, Northern Hemisphere Ice-Sheet Influences on Global Climate Change, Science, 286, 1104-1111, Nov 5, 1999 Variability at suborbital scale Ice sheets easily flowed on soft bed, were thin, thus covered large area 6 8 ESM 203: Climate change–observations 2 11/29/2007 Spectral analysis of ocean δ18O, ice volume, atmospheric CO2 A. B. C. C D. Details of rapid warming (9º±3ºC) 14,762 (9º years ago Rapid increase in δ18O and accumulation in Greenland Increase in δ15N shows rapid temperature change Increase in CH4 lagged 20-50 years ◦ Thus warming started in North Atlantic Severinghaus & Brook, Science, 29 October 1999 believe CH4 from tropical wetlands 11 Deep Pacific temperature Ocean δ18O (ice volume) Vostok D/H ratio (Antarctic temperature) CO2 in the Vostok core Deep ocean temperature and CO2 are in phase with 100 Kyr cycle Ice volume and Antarctic temperature affected strongly by 41 Kyr and 23 Kyr cycles (N. J. Shackleton, The 100,000-Year IceAge Cycle Identified and Found to Lag Temperature, Carbon Dioxide, and Orbital Eccentricity, Science, 289, 1897-1902, September 15, 2000) 9 Sudden climate changes (at time scales finer than orbital variability) Interstadials ◦ 24 sudden and short-lived warm periods (a few decades, ~10°C warmer) in Greenland between 10,000 and 120 000 years ago 10 000 120,000 (“Dansgaard-Oeschger events”) ◦ Accompanied by changes in Santa Barbara Channel sediments (Kennett) Causes of sudden climate change: freshwater input to North Atlantic? Heinrich events ◦ Short cold periods (3-6°C colder) ◦ A “Younger D As “Y Dryas event” 12,900-11,500 years ago ended, ” 12 900 11 500 d d atmospheric methane increased 10 Clark et al., Freshwater forcing of abrupt climate change during the last glaciation, Science 13 July 2001. 12 ESM 203: Climate change–observations 3 11/29/2007 Or did methane cause the warming? Kennett’s (UCSB) “clathrate gun” hypothesis ◦ Influx of warmer, saltier water from tropics ◦ Freeing methane trapped in hydrates (formed at low temperature, high pressure) But … 2 problems ◦ Getting the methane through the water column ◦ Evidence that methane lagged the warming Response of South American vegetation to rapid climate change Planktonic Δ14C showing decline in formation of North Atlantic Deep Water Increased reflectance of sediments, indicating stronger Trade Winds Change from forest to grasslands in C f f Venezuela Hughen et al., Abrupt tropical vegetation response to rapid climate changes, Science 20 May 2004 Dickens’ review of Kennett et al., Methane Hydrates in Quaternary Climate Change:The Clathrate Gun Hypothesis 13 15 But methane emission may be related to some earlier rapid climate changes The past 1,000 years T. J. Crowley, Causes of Climate Change Over the Past 1000 Years, Science, 289, 270-277, July 14, 2000 “Miedeval Warm Period” from 1000-1300 17th Century cold— ”Little Ice Age”, early 19th century also cold Crowley explains the variability as caused by volcanoes, solar variability, and greenhouse gases (the hockey stick) http://en.wikipedia.org/wiki/Clathrate_Gun_Hypothesis 14 16 ESM 203: Climate change–observations 4 11/29/2007 Sea level and glacier changes in last 100 years Temperature changes in the last 100+ years Increases in trace gases 0.3 to 0.6 C increase in temperature ◦ Warming variable by location Regional changes in precipitation 10 to 25 cm sea level changes ◦ Glacial melting, ocean expansion ◦ Measurements difficult because of isostatic rebound From Global Change Electronic Edition 17 19 Top: rate of seasealevel rise, showing recent acceleration Bottom: sea level (0 at 1990) S. Rahmstorf Science 315, 368 -370 (2007) Published by AAAS 18 20 ESM 203: Climate change–observations 5 11/29/2007 Is this change unusual? Temperatures highest seen since 1400s During no other warm period in last 1000 years do we see this large of a variation (see it in Ice Ages frequently or transitions between ice ages and warm periods) Precipitation pattern changes show different trends.... Thinning of the Arctic sea ice cover “Comparison of sea-ice draft data acquired on submarine cruises between 1993 and 1997 with similar data acquired between 1958 and 1976 indicates that the mean ice draft at the end of the melt season has decreased by about 1.3 m in most of the deep water portion of the Arctic Ocean, from 3.1 m in 1958-1976 to 1.8 m in the 1990s.” ◦ Rothrock et al., Geophysical Research Letters, 26, 1-5, December 1, 1999. NY Times graphic, “sea ice in retreat,” 19 Oct 2007 21 23 Persistence in Sahel rainfall, dependence on soil & vegetation Zeng et al., Science, 286, 1537 1540, 1537-1540, Nov 19, 1999 Observations of rain and NDVI (“greenness”) Primitive climate model with fixed soil moisture Model with soil moisture variability i bilit Model with soil moisture and interactive vegetation A changing western snowpack? Less snow? 22 Earlier melt? 24 Service, R. F., As the West goes dry, Science, 20 Feb 2004 ESM 203: Climate change–observations 6 11/29/2007 Generally the snowpack peaks in April at most courses … except at lower elevations, especially in the southern Sierra Trends in April SWE and in April-March difference April- 25 27 Typical trends (Tuolumne R drainage) Important characteristics of the paleoclimatological and historical data Relationship between temperature and: ◦ CO2, long- and short-term solar variability, volcanism CO2 higher now than any time in the last 100,000 yr 100 000 Considerable natural variability in the system Feedback effects important ◦ Oceans, ice sheets, sea ice Precipitation in a warmer climate is uncertain 26 28 ESM 203: Climate change–observations 7 ...
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