1Insights from paleoclimate studies• Is 20th century unprecedented in terms ofrecent global warming?– For past millennium, yes– For past 10,000 yrs, probably…• Is 20th century representative of recenthydrologic variability?– No - natural variability is large!• What is the natural behavior of the climatesystem?– Nonlinear! Abrupt transitions frequent– Mechanisms can be apparent, or not.Global warming: natural cycleor unusual trend?We use paleoclimatic methods toexpand our view of past climateGlaciersLake sedimentsTree ringsCorals“Multiproxy” temperature reconstructionsMann et al. 1999, Nature• Use annual (or nearly annual) resolution data• Combine them using a clever statistical method• Calibrate to modern temperature• Result is yearly history of temperature change• Many authors have done this…2The original “hockey stick”Mann et al. 1999The original “hockey stick”• Multiproxy• Used by IPCC 3• Low variance in earlypart of record,followed by sudden rise• A lightning rod!– Triggered aCongressional inquiry– Mainly upheld by NRCreportTemperature of past millennium• Sources of difference - proxies, assumptions• Borehole vs multiproxyBorehole temperature estimates• Measure temperature of rock down a drilledborehole• This temperature mainly reflects the conduction ofheat out from the planet's hot interior (geothermalgradient)• Perturbations to temperature at the surface showup in this profile as deviations from thegeothermal gradient• Must model (define) the expected geothermalgradient for a given region, then remove that toget at the perturbations that came from thesurface.3Borehole temperature estimates• Resolution of this technique is low (century-scale)• Modeling the expected geothermal gradient is challenging,and non-temperature factors can influence it (e.g. hydrologicchanges, seasonal snow cover)• Proponents argue that this is nearly pure geophysics• Detractors note likely effects of trends in hydrologicvariables, e.g. snow coverSouthern Hemisphere-1.0-0.50.01500 1600 1700 1800 1900 2000YearNorthern Hemisphere-1.0-0.50.0Global-1.0-0.50.0Southern Hemisphere-1.0-0.50.01500 1600 1700 1800 1900 2000YearNorthern Hemisphere-1.0-0.50.0Global-1.0-0.50.0Was there a “Medieval warm period”?• Define - a period ofglobal warmth in theMedieval interval(~800-1400 AD)• A big issue (redherring?) in skeptic’sarguments againstanthropogenic globalwarming• Issues:– Was it global?– Was it warmrelative to 20thcentury?Bradley et al. 2003Was there a “Medieval warm period”?• Define - a period of globalwarmth in the Medievalinterval (~800-1400 AD)• A big issue (red herring?)in skeptic’s argumentsagainst anthropogenicglobal warming• Issues:– Was it global?– Was it warmrelative to20th century?• IPCC - 8records thatcorrelate wellto moderntemperature…4How do past andfuture compare?• Scaled to match both verticallyand horizontally• IPCC forcings end 2100• Unprecedented nature ofexpected future change2300Causes of 20th centuryclimate changeStott et al. 2000 Nature• Use model driven bynatural-only andnatural+anthropogenicforcings• Natural+anthropogenicforcings matchobserved record best• Mismatch with onlynatural forcingsTake-away messages• 20th century is warmer than any time in thepast millennium, due at least in part tohuman influence.• Strength of past century-scale variability isstill debated, but 20th-century warmth isnot.• Anthropogenic forcings are required toexplain recent temperature changes; theyare not natural.Abrupt climate change andthermohaline circulation• What is abrupt change? Why care?• The ocean thermohaline example:– processes– records– current status– Predictions• Recent abrupt changes: drought5Characteristics of abrupt change• Transition between states• More rapid than cause– (although cause often not fully known)– Cause may be undetectably small (chaoticsystem)• Most critical: changes that are– Persistent– At least subcontinental in scale– Unexpected and/or for which adaptation(ecologic, economic…) is difficultWhy be concerned?• Abrupt changes are unpredictable, may beunprecedented• Capacity to adapt will be key:– Economic resources– Ecological resilience• Most economic studies of climate change focuson responding to gradual changesNonlinear and threshold responsesForcingResponseForcingResponseForcingResponse• Contrast linear and nonlinear responses to forcing:• One type of nonlinear response is a thresholdresponseThe climate system showsall of these behaviors invarious ways!ForcingResponseAbrupt change theory• Trigger– May be fast or slow; may be detectableor not (chaotic)• Amplifier/globalizer– Turns small change into large one• Source of persistence– Makes new state somewhat stable6Abrupt climate changeand the future• A natural feature of the climatesystem, but…• Appears more likely when climateforcings are changing• Not predictable with currenttechnology!• High risk• How to recognize it?• How to be resilient?Thermohaline circulation: Important source of abrupt change inpast, thus maybe in future?• Density driven vertical circulation of the ocean• Cold & salty waters are denser than warm & freshThermohaline circulation• If you make surface water denser, it will sink• North Atlantic is key region todayNADWThermohaline circulation• Atlantic is saltier than Pacific• Gulf Stream brings salty water northwards, where it chills,becomes dense and sinks: N Atl. Deep water (NADW)• Drives a global pattern of thermohaline circulation• NADW formation very sensitive to freshwater/salt balance7Thermohaline circulation:Role in past abrupt changesMillennial changes seen in many other locations:China, Oman, Arizona, western and eastern Pacificoceans, Indian Ocean, Antarctica…Thermohaline circulation:Role in past abrupt changes• Example: Younger Dryas interval (11.5-13 k yrs ago)• Abrupt transitions in andespecially out– Greenland T incr 10°C in 10yrs!• LOTS of data in support• Ocean data indicatethermohaline involvement• Mechanism:– Trigger = freshwater influx to N Atlantic associated with glacialmelting– Amplifier/Persistence = NADW shutdownhttp://www.ncdc.noaa.gov/paleo/abrupt/• Abrupt changes likethe Younger Dryascan be nearly globalin scope– Tropical imprintshere in trade windstrength, methaneproduction• Most likelymechanism is oceancirculation change– (recent