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3. Climate Change 3.1 Observations 3.2 Theory of Climate Change 3.3 Climate Change Prediction 3.4 The IPCC Process 3.1 Observations Need to consider: • Instrumental climate record of the last century or so • Recent changes in greenhouse gases and other quantities 2 Acknowledgement: Steve Arnold (University of Leeds, UK) Important Questions Concerning the Climate Record • How much is the world warming? • Is the recent warming unusual? • How rapidly is climate changing compared to earlier changes? • Have precipitation and atmospheric moisture changed? • Are atmospheric/oceanic circulations changing? • Has climate variability (e.g., extremes) changed? 3 Observed Climate Variables • Temperature (global mean, regional, diurnal range, upper air...) • Precipitation, humidity, cloud cover • Snow cover • Sea-ice thickness and extent • Natural modes (El Nino, North Atlantic Oscillation, Atlantic Multidecadal Oscillation) • Climate extremes 4 Use of ‘Anomalies’ • Anomalies are changes relative to some particular reference period used to emphasise positive and negative excursions around a long-term mean 5 Global Mean Land-Surface Temperatures • Temperature anomalies (difference from 19611990 mean) • Surface temperature trend ~0.05OC / decade since 1850. • Urban “heat island” effect is negligible (~0.006OC / decade) • Differences between compilations arise from weighting of observation density 6 Global Mean Land-surface and Sea Surface Temperatures • Recent land warming dominated by NH continents • Ocean warming slower • 20th-century T Change ~0.7±0.2 OC – Compare 0.45 OC estimated by IPCC in 1995. Land SST 7 Trends http://youtu.be/e0vj-0imOLw 8 Spatial Pattern of Surface Temperature Changes 1901-2005 1975-2005 Grey areas: not enough data White crosses: significant trend 9 Summary of Surface Temperature Changes • In the 20th century there has been a consistent largescale warming of the land and ocean surface. • Rate of warming appears to be accelerating • Some regional details can be explained by atmospheric circulation changes 10 Upper Air Temperatures • Weather balloons (radiosondes) – T at discrete levels – Difficulties with changes in instruments • Satellite (Microwave Sounding Unit – MSU and stratospheric Sounding Unit - SSU) since 1979 – T over broad regions – Problems with orbit drift, calibration, etc. -> spurious T trends 11 Upper Air Temperatures • Stratosphere cooling at > 0.5 OC/decade • Volcanic eruptions cool troposphere and heat stratosphere • Troposphere up to ~10km has warmed at slightly faster rate than surface since 1950s. • Stratosphere cooled markedly since 1979. 12 Changes in the Cryosphere • Snow cover • Sea-ice extent • Mountain glaciers - Average Arctic temperatures have increased at almost twice the global average rate in past 100 years. WMO International Polar Year 2007-2008 www.ipy.org 13 Observed Northern Hemisphere Snow Cover TOP • Annual snow-cover extent decreased by 10% since 1966 • Mostly accounted for by spring/summer since 1980s BOTTOM • Annual snow cover anomalies correlate with T (dashed line) 14 Observed Northern Hemisphere Sea-Ice Extent • http://svs.gsfc.nasa.gov/vis/a000000/a003400 /a003456/AMSR_E_SeaIce_to_09_14_2007_ 512x288.m1v 15 Observed Northern Hemisphere Sea-Ice Extent Summer minimum Arctic sea ice extent 16 Observed Northern Hemisphere Sea-Ice Extent NASA Goddard Space Flight Center (orange line shows average minimum ice extent (for 1979-2010 period). 2012 has lowest sea ice extent in more than 30 years. Observed Mountain Glacier Length • 169 glacier length records • Glacier retreat is worldwide 18 Summary of Observed Cryospheric Changes • Consistency of cryospheric changes and temperature changes • NH snow cover correlates well with spring temperature changes • Reduced sea-ice extent consistent with increases in spring temperatures • Small changes in Arctic winter ice, despite large changes in winter T • Major retreat of glaciers consistent with 20thcentury T changes 19 Precipitation • Wetter: East North and South America, Eurasia • Drier: Sahel, S Africa, Mediterranean, S Asia • More rain, less snow in northern regions • Increased frequency of heavy precipitation 20 Precipitation • Wetter: East North and South America, Eurasia • Drier: Sahel, S Africa, Mediterranean, S Asia • More rain, less snow in northern regions • Increased frequency of heavy precipitation 21 •Beware of the AMO! Precipitation • Increases in amounts of very heavy precipitation (1958-2007) 22 The map shows percent increases in the amount falling in very heavy precipitation events (defined as the heaviest 1 percent of all daily events) from 1958 to 2007 for each region. There are clear trends toward more very heavy precipitation for the nation as a whole, and particularly in the Northeast and Midwest (updated from Groissman et al,2004) Atmospheric/Oceanic Circulations (El Nino) • Occurrence of strong El Ninos may be higher since 1980s • But significance is low due to strong contribtion of natural variability 23 Atmospheric/Oceanic Circulations (NAO) • North Atlantic Oscillation (NAO) is a measure of circulation patterns in the NH • Positive index implies warmer Europe • Greater occurrence of positive indices since 1980s • NAO trending towards negative values recently. • Difficult to separate natural variability and long-term change http://www.cgd.ucar.edu/cas/jhurrell/indices.html 24 Climate Extremes (extreme temperatures) 10th percentile Trends in days/decade 19512003 (relative to 1961-1990). 90th percentile 25 Climate Extremes (heatwaves) 26 Climate Extremes (precipitation) Contribution from ‘very wet’ days (95th precentile) to total decadal precipitation. 27 Sea-level Rise Annual average global mean sea level Coastal tide gauges Reconstructed Satellite altimetry 28 Sea-level Rise 29 Changes in Greenhouse Gases 30