Historical Changes in Climate Temperature change over last few thousand yearsTemperature change over last few thousand years

Less than 1°CLess than 1°C Highly variable from region to regionHighly variable from region to region

Archives for climate dataArchives for climate data Mountain glaciersMountain glaciers Tree ringsTree rings CoralsCorals Historical observationsHistorical observations

Instrumental data only during last <1K yearsInstrumental data only during last <1K years General trendGeneral trend

Dramatic 20Dramatic 20thth century warming century warming Cooler climates prior to 20Cooler climates prior to 20thth century century

Little Ice Age (1400-1900 AD)Little Ice Age (1400-1900 AD)

Medieval Climatic Optimum Evidence for relative warmth in high latitude Evidence for relative warmth in high latitude

northern hemispherenorthern hemisphere Approximately 1000-1300 ADApproximately 1000-1300 AD Nordic people settled southwest GreenlandNordic people settled southwest Greenland

Fringes of ice sheetFringes of ice sheet•Agricultural evidence for warmth – wheat Agricultural evidence for warmth – wheat

cropscrops•Little mention of sea ice from this regionLittle mention of sea ice from this region

Settlements abandoned during Little Ice AgeSettlements abandoned during Little Ice AgeSuggests marginal environment became Suggests marginal environment became

inhospitableinhospitable

Little Ice Age Cooling during 1400-1900 ADCooling during 1400-1900 AD

Well documented in EuropeWell documented in EuropeColder wintersColder wintersFailed cropsFailed cropsShorter growing seasonsShorter growing seasonsLakes, rivers and ports frozenLakes, rivers and ports frozenAdvance of alpine glaciersAdvance of alpine glaciersFrequency of sea ice along coastal Frequency of sea ice along coastal IcelandIceland•Too much ice to fishToo much ice to fish

Not a true ice ageNot a true ice age

Ice Growth during Little Ice Age Evidence for ice growth in Canadian Arctic from lichen Evidence for ice growth in Canadian Arctic from lichen

haloshalos Lichens grow on rock surfaces at known ratesLichens grow on rock surfaces at known rates

Lichen halos interpreted as lichen killed by ice coverLichen halos interpreted as lichen killed by ice cover Size of living lichens give time since lichen deathSize of living lichens give time since lichen death

• Small size indicates only 100 years of growthSmall size indicates only 100 years of growth

Extent of Ice Growth in Canada Baffin Island shows large expansion of iceBaffin Island shows large expansion of ice

Ice killed lichens during Little Ice AgeIce killed lichens during Little Ice Age Retreat allowed them to grow only ~100 years agoRetreat allowed them to grow only ~100 years ago Mapping lichen halos gives distribution of iceMapping lichen halos gives distribution of ice

Climate Extent & Temperature Trend?

Few observational Few observational data and small data and small changechange Restrict determining Restrict determining

if global or regional if global or regional coolingcooling

Cooling trend not Cooling trend not knownknown Culmination of slow Culmination of slow

orbital-scale coolingorbital-scale cooling Most recent in a Most recent in a

series of millennial-series of millennial-scale oscillationsscale oscillations

Proxy Measures of Historical Changes

Instrumental records prior to 1900 largely do Instrumental records prior to 1900 largely do not existnot exist Quantitative information is scarceQuantitative information is scarce

Problems for archives are several foldProblems for archives are several fold Records must extend from 1900’s back 1000-Records must extend from 1900’s back 1000-

2000 years2000 years High resolution required high deposition rateHigh resolution required high deposition rate Must be extremely sensitive to detect small Must be extremely sensitive to detect small

changeschanges ArchivesArchives

Alpine glacier ice coresAlpine glacier ice cores Tree ringsTree rings CoralsCorals

Alpine Glaciers Small ice caps on Small ice caps on

mountains and valley mountains and valley glaciersglaciers Make excellent climate Make excellent climate

archivesarchives Range from few Range from few

hundred to thousands hundred to thousands of yearsof yearsBack to LGMBack to LGM

Annual layering at Annual layering at surfacesurfaceDegrade to decadal Degrade to decadal resolution at depthresolution at depth

Alpine Glaciers Drilling is difficult taskDrilling is difficult task

Haul equipment to Haul equipment to summitsummit

Lack of oxygenLack of oxygen Lack of electricityLack of electricity Freezing to Freezing to

subfreezing subfreezing temperaturestemperatures

Few mountain glaciers Few mountain glaciers drilleddrilled

Alpine Glaciers Multiple ice cores Multiple ice cores

drilleddrilled Through entire Through entire

thickness of thickness of glacier (100-200 glacier (100-200 m)m)

Photograph shows Photograph shows drill core extrudeddrill core extrudedSolar powered Solar powered drilldrill

Alpine Glaciers Harsh conditions can Harsh conditions can

prevailprevail Cores and Cores and

equipment must be equipment must be hauled down after hauled down after drillingdrilling

Produce similar Produce similar records as records as Greenland and Greenland and Antarctic ice coresAntarctic ice coresNo mention of No mention of

gases?gases?

Quelccaya Ice Cap, Peru Peruvian Andes, at ~18,500 feet elevationPeruvian Andes, at ~18,500 feet elevation From 1980’s show annual-scale From 1980’s show annual-scale 1818O and dust changesO and dust changes

More positive More positive 1818O and less dust near 1900O and less dust near 1900 Warmer temperatures and weaker windsWarmer temperatures and weaker winds

More negative More negative 1818O and more dust 1600-1900O and more dust 1600-1900 Colder temperatures and stronger windsColder temperatures and stronger winds

Remarkably similar to Little Ice Age in EuropeRemarkably similar to Little Ice Age in Europe

Low dust early part of Little Ice AgeLow dust early part of Little Ice AgeMedieval Climate Optimum high dustMedieval Climate Optimum high dust

Quelccaya Ice Cap, Peru Little Ice Age showed more negative Little Ice Age showed more negative 1818O between O between

1550 and 1900 AD1550 and 1900 AD Accumulation record more complexAccumulation record more complex

Pronounced wet period before 1700 followed by Pronounced wet period before 1700 followed by significantly drier conditions thereaftersignificantly drier conditions thereafter

Quelccaya Ice Cap, Peru Return expedition in Return expedition in

19911991 Annual layering at top Annual layering at top

of core destroyed by of core destroyed by meltwater percolationmeltwater percolation

Previous record Previous record indicated 1500 years indicated 1500 years without meltingwithout melting

Meltwater percolation Meltwater percolation presumably due to 20presumably due to 20thth century warmingcentury warming Rate that was Rate that was

unprecedented unprecedented compared to last compared to last millenniummillennium

Quelccaya Ice Cap, Peru - 1976

Quelccaya Ice Cap, Peru - 2000

Dunde Ice Cap, Tibet Remote region on mountain range separating China's Remote region on mountain range separating China's

highest desert, the Qaidam Basin, from the Gobihighest desert, the Qaidam Basin, from the Gobi Snow accumulations for 40,000 years on a 60 kmSnow accumulations for 40,000 years on a 60 km22

ice cap deep in sparsely inhabited interiorice cap deep in sparsely inhabited interior US and Chinese expedition in 1987US and Chinese expedition in 1987

Dunde Ice Cap, Tibet Record averaged over Record averaged over

50-year intervals50-year intervals More positive More positive 1818O O

before 1500before 1500 More negative More negative 1818O O

during Little Ice Ageduring Little Ice Age Transition to more Transition to more

positive positive 1818O at 1700O at 1700 Last 50 year interval is Last 50 year interval is

more positive than any more positive than any other yearother year Core taken to Core taken to

12,000 year12,000 year

Ice Core Summary Climate on low latitude Climate on low latitude

mountainsmountains Colder during the Little Colder during the Little

Ice AgeIce Age Uniquely warmer during Uniquely warmer during

the 20the 20thth century century Antarctica and Greenland Antarctica and Greenland

corescores No distinctly cold No distinctly cold

pattern during Little Ice pattern during Little Ice AgeAge

No unique 20No unique 20thth century century warmthwarmth

Variation in climate Variation in climate recorded in these archives recorded in these archives has been regionalhas been regional

Dendroclimatology Records from tree ringsRecords from tree rings

Regions where trees are Regions where trees are sensitive to climate stresssensitive to climate stress

Near limit of natural Near limit of natural rangerange

Climate stress revealed Climate stress revealed by narrow ringsby narrow rings

Century to millennia-length Century to millennia-length tree-ring chronologies are tree-ring chronologies are useful for evaluatinguseful for evaluating Frequency and magnitude Frequency and magnitude

of droughts and wet periodsof droughts and wet periods Placing ecosystem changes Placing ecosystem changes

into a long-term, historical into a long-term, historical context of climate change context of climate change

Stable Isotopes in Dendroclimatology The The 1818O of rain varies with O of rain varies with

changes in temperature and changes in temperature and rainoutrainout

Rain is absorbed by the growing Rain is absorbed by the growing tree, the tree, the 1818O of water recorded O of water recorded in the tree rings, and become in the tree rings, and become climate indicatorsclimate indicators

The The 1818O of organic material is O of organic material is determined by determined by The isotopic composition of The isotopic composition of

the source or soil waterthe source or soil water Enrichment taking place in Enrichment taking place in

the leaf water due to the leaf water due to transpiration, resulting in an transpiration, resulting in an increased increased 1818O of leaf water O of leaf water compared to compared to 1818O of soil waterO of soil water

Biochemical fractionationsBiochemical fractionations

Tree Ring Climate Calibration Tree ring data must be cross-calibrated with Tree ring data must be cross-calibrated with

instrumental datainstrumental data Before can used to interpret ancient climateBefore can used to interpret ancient climate The character of the relationship between climate and The character of the relationship between climate and

tree growth is assessedtree growth is assessed Statistical model is derived to describe that Statistical model is derived to describe that

relationship relationship

Arctic Tree Rings Results synthesized from circumarctic regionResults synthesized from circumarctic region

Covers middle and end of Little Ice AgeCovers middle and end of Little Ice Age Inferred temperature change of 1°CInferred temperature change of 1°C

Not a time of extreme coldNot a time of extreme cold Warming of the Arctic apparent from mid-20Warming of the Arctic apparent from mid-20thth century century

Reaching highest temperatures during 320 year Reaching highest temperatures during 320 year recordrecord

But temperatures not significantly higher than 18But temperatures not significantly higher than 18thth centurycentury

Tree Ring Studies in Central Asia

Climate on large continent less moderated by oceansClimate on large continent less moderated by oceans Still, similar to Arctic regionStill, similar to Arctic region Intervals of warmth during Little Ice AgeIntervals of warmth during Little Ice Age

Mid-18Mid-18thth century and earlier century and earlier Colder temperatures in late 16Colder temperatures in late 16thth, late 17, late 17thth and mid-to and mid-to

late 19late 19thth centuries centuries Warming in mid- to late 20Warming in mid- to late 20thth century unprecedented century unprecedented

Tree Rings on Tasmania Records from Huon Pines Records from Huon Pines

extend back 2000 yearsextend back 2000 years Best tree ring records Best tree ring records

of S. hemisphereof S. hemisphere Little Ice Age is cooler Little Ice Age is cooler

than late 20than late 20thth century century Does not stand out as Does not stand out as

uniquely colduniquely cold 2020thth century warmth century warmth

stands out in the rate of stands out in the rate of increaseincrease Temperature matched Temperature matched

by earlier timesby earlier times

Tree Ring Summary Tree ring studies indicate Tree ring studies indicate

Climate variable from region to regionClimate variable from region to regionOver last several hundred yearsOver last several hundred years

•No one record fully describes climate No one record fully describes climate trends in all areatrends in all area

Similar to ice coresSimilar to ice coresClimate varied significantly within the Little Climate varied significantly within the Little

Ice AgeIce Age•In some area even warming to early 20In some area even warming to early 20thth

century levelscentury levelsLast few decades generally show maximal Last few decades generally show maximal

warmthwarmth

Coral and Tropical SST Decadal resolution SSTDecadal resolution SST Warm tropical Warm tropical

environmentsenvironments Pacific Ocean atollsPacific Ocean atolls

Ideal for measuring the Ideal for measuring the occurrence and intensity occurrence and intensity of El Nino eventsof El Nino events

Oxygen isotopic and Oxygen isotopic and trace elemental trace elemental compositions yield compositions yield important climate recordsimportant climate records

El Nino Marked by appearance of unusually warm waters in the Marked by appearance of unusually warm waters in the

eastern Pacific in December every 2 to 7 yearseastern Pacific in December every 2 to 7 years Radical alteration of the entire Pacific oceanic and Radical alteration of the entire Pacific oceanic and

atmospheric system occurs in two phasesatmospheric system occurs in two phases In a cool phase, strong SE trade winds push eastern In a cool phase, strong SE trade winds push eastern

Pacific surface waters westward, allowing cool Pacific surface waters westward, allowing cool nutrient-rich bottom waters to upwellnutrient-rich bottom waters to upwell

Non-El Nino Years The western Pacific during cool phases is typified by: The western Pacific during cool phases is typified by:

A pool of warm water stretching eastward to 170°W, and an A pool of warm water stretching eastward to 170°W, and an accompanying belt of low pressure and high precipitationaccompanying belt of low pressure and high precipitation

Indonesian Low - covers parts of Asia and AustraliaIndonesian Low - covers parts of Asia and Australia A belt of high precipitation, the ITCZ, lies several degrees A belt of high precipitation, the ITCZ, lies several degrees

north and south of the Equator and east of the Date Linenorth and south of the Equator and east of the Date Line

El Nino In the warm phase, trade winds weaken and less In the warm phase, trade winds weaken and less

eastern Pacific surface water is pushed westwardeastern Pacific surface water is pushed westward Upwelling in the eastern Pacific slowsUpwelling in the eastern Pacific slows Warm waters spread across the Pacific increasing Warm waters spread across the Pacific increasing

SST by 3-5°C in the Galapagos IslandsSST by 3-5°C in the Galapagos Islands

El Nino ITCZ moves S and W, while the Indonesian Low follows ITCZ moves S and W, while the Indonesian Low follows

the warmer waters eastthe warmer waters east Barometric pressure in Darwin, Australia rises as Barometric pressure in Darwin, Australia rises as

higher pressure replaces the Indonesian Lowhigher pressure replaces the Indonesian Low During particularly severe warm events, winds in the During particularly severe warm events, winds in the

western Pacific reverse and become mild westerlieswestern Pacific reverse and become mild westerlies

Southern Oscillation El Nino years are times of:El Nino years are times of:

Unusually high pressure and dry conditions Unusually high pressure and dry conditions over N. Australiaover N. Australia

Low pressure and high rainfall in the south-Low pressure and high rainfall in the south-central Pacificcentral Pacific

Non-El Nino years are time ofNon-El Nino years are time of Low pressure and moist conditions over N. Low pressure and moist conditions over N.

AustraliaAustralia Higher pressure and reduced rainfall in the Higher pressure and reduced rainfall in the

south-central Pacificsouth-central Pacific Linking of these two circulation systems in a Linking of these two circulation systems in a

large-scale flow known as El Nino Southern large-scale flow known as El Nino Southern Oscillation (ENSO)Oscillation (ENSO)

ENSO Long-term changes in atmospheric pressure showLong-term changes in atmospheric pressure show

Warm El Nino years are time ofWarm El Nino years are time of Drier conditions and higher pressure in N. AustraliaDrier conditions and higher pressure in N. Australia Wetter conditions and lower pressure in south-Wetter conditions and lower pressure in south-

central Pacificcentral Pacific

Atmosphere-Ocean Linkage Strong east-to-west trades common in non-El Strong east-to-west trades common in non-El

Nino yearsNino years Pile warm surface water in western PacificPile warm surface water in western Pacific

Warm water is natural source of moistureWarm water is natural source of moisture•Rising moisture off ocean creates low Rising moisture off ocean creates low

pressurepressure•Creates high precipitation in N. Australia Creates high precipitation in N. Australia

and Indonesiaand IndonesiaRising air cools and flows eastward in the Rising air cools and flows eastward in the

east-central Pacificeast-central Pacific•Contributes to cooler and dry conditions Contributes to cooler and dry conditions

near S. Americanear S. America

Atmosphere-Ocean Linkage Trade winds in eastern Pacific weaken in El Nino Trade winds in eastern Pacific weaken in El Nino

yearsyears Pool of warm western Pacific water diminishedPool of warm western Pacific water diminished

Water flows eastwardWater flows eastward As warm water replaces cool water in central As warm water replaces cool water in central

and then eastern Pacificand then eastern Pacific•Becomes the source of moisture and low Becomes the source of moisture and low

pressurepressure•As the warm water flow hits western N. As the warm water flow hits western N.

AmericaAmerica– Flow diverted N and S bring heavy rainfall Flow diverted N and S bring heavy rainfall

to California and Peruto California and Peru Loss of warm water in western Pacific create Loss of warm water in western Pacific create

dryer conditions in Australia and Indonesiadryer conditions in Australia and Indonesia

Teleconnections Unusual oceanic and atmospheric conditionsUnusual oceanic and atmospheric conditions

Tropical regions can affect circulation patterns Tropical regions can affect circulation patterns outside tropicsoutside tropics

Flooding in Peruvian Andes and SE US commonFlooding in Peruvian Andes and SE US common Droughts in Indonesian, central India and AustraliaDroughts in Indonesian, central India and Australia

Coral Calibration Calibration of coral Calibration of coral 1818O with SST measurementsO with SST measurements

Slight mismatch probably due to salinitySlight mismatch probably due to salinity Galapagos corals record low Galapagos corals record low 1818O values during O values during

warm El Nino yearswarm El Nino years

400 Year Coral SST Record Little long term trend obviousLittle long term trend obvious

Generally more negative values near start and end of Generally more negative values near start and end of recordrecord

Perhaps just before 1700 and 1800Perhaps just before 1700 and 1800 No hint of Little Ice age or 20No hint of Little Ice age or 20thth century warming century warming Some Pacific corals show gradual ocean warming Some Pacific corals show gradual ocean warming

and more rainfall towards presentand more rainfall towards present

Summary Despite efforts, coverage of climate over last 1000 Despite efforts, coverage of climate over last 1000

years remains incompleteyears remains incomplete Synthesis of N. hemisphere temperature changeSynthesis of N. hemisphere temperature change

Show a gradual decline for 900 yearsShow a gradual decline for 900 years Ending in a dramatic warming in 20Ending in a dramatic warming in 20thth century century

Mechanisms Producing Trend Cooling from 1000 to Cooling from 1000 to

1900 AD1900 AD Follows orbital Follows orbital

cooling patterncooling pattern Follows the Follows the

millennial-scale millennial-scale patternpattern

Little Ice Age Little Ice Age cooler than cooler than preceding and preceding and following intervalsfollowing intervals

Began with abrupt Began with abrupt coolingcooling

Mechanisms Producing Trend Most obvious trend between 1000-900 yearsMost obvious trend between 1000-900 years

Match with gradual orbital coolingMatch with gradual orbital cooling

20th Century Warming Stands out as a unique featureStands out as a unique feature

Rate of warming highestRate of warming highest Temperatures just now rising above uncertainty levels of the Temperatures just now rising above uncertainty levels of the

reconstructionreconstruction

20th Century Warning Reconstruction suggests that 20Reconstruction suggests that 20thth century century

warmingwarming Not simply another in long series of Not simply another in long series of

natural climate oscillationsnatural climate oscillations Something unprecedented for the Something unprecedented for the

entire millenniumentire millennium

Historical Records of El Nino Records from ship’s logs date to 1525 ADRecords from ship’s logs date to 1525 AD

SST and sea levelSST and sea level Catch of anchovy and other fishCatch of anchovy and other fish Sea bird abundanceSea bird abundance Heavy rain and floodsHeavy rain and floods Disease (malaria and cholera)Disease (malaria and cholera)

Records ranked qualitativelyRecords ranked qualitatively

Historical Records of El Nino 115 events in 456 years; event every 8 years115 events in 456 years; event every 8 years

Events clusterEvents cluster No correlation with Little Ice AgeNo correlation with Little Ice Age

Record provides a limited glimpse of climateRecord provides a limited glimpse of climate Local changes are difficult to extrapolate to Local changes are difficult to extrapolate to

global scaleglobal scale El Nino events limited to N. hemisphere winterEl Nino events limited to N. hemisphere winter

Instrumental Temperature Records

Records over last 200-300 Records over last 200-300 yearsyears Air and SSTAir and SST Limited regions prior to Limited regions prior to

19001900 Methods limitation on Methods limitation on

temperature accuracytemperature accuracy Population growth affects Population growth affects

local temperaturelocal temperature Change in albedoChange in albedo

Asphalt and Asphalt and vegetationvegetation

Can change temperature Can change temperature by as much as 30%by as much as 30%

20th Century Global Temperature

Overall trend shows Overall trend shows 0.6°C temperature rise0.6°C temperature rise Year to year Year to year

variability presentvariability present Temperature estimates Temperature estimates

form satellite disagree form satellite disagree with thermometerwith thermometer Abundant Abundant

observations support observations support significant warming significant warming in 20in 20thth century century

Glaciers Retreat of alpine glaciers Retreat of alpine glaciers

indicates climate indicates climate warmingwarming Alpine glaciers show Alpine glaciers show

varying responsesvarying responsesDue to heterogeneity Due to heterogeneity

of climate systemof climate system Most glaciers have Most glaciers have

been in retreatbeen in retreatRate of melting has Rate of melting has

accelerated in recent accelerated in recent decadesdecades

Most plausible Most plausible explanation is warming explanation is warming of climateof climate

Global Average Sea Level Difficult nut to crackDifficult nut to crack

Local rebound and tectonic movementsLocal rebound and tectonic movements Displacement of groundwater into oceansDisplacement of groundwater into oceans

Overall agreement of slow rise in sea levelOverall agreement of slow rise in sea level 12-15 cm in last 100 years12-15 cm in last 100 years

Cause of Sea Level Rise Ice on land has melted Ice on land has melted

adding water to adding water to oceansoceans

Water in the oceans Water in the oceans has expandedhas expanded Surface warming of Surface warming of

oceans accounts for oceans accounts for about 1/3 of riseabout 1/3 of rise

Melting ice remains Melting ice remains the most probable the most probable explanation for explanation for remaining riseremaining rise

Melting Glaciers Alpine glaciers estimated to contribute ~3 cm Alpine glaciers estimated to contribute ~3 cm

to sea level riseto sea level rise Thermal expansion and alpine glacier retreatThermal expansion and alpine glacier retreat

Make up more than 50% of sea level riseMake up more than 50% of sea level rise Melting of Antarctic and Greenland ice sheetsMelting of Antarctic and Greenland ice sheets

Antarctica cold and dry environmentAntarctica cold and dry environmentUnlikely contributed to sea level riseUnlikely contributed to sea level riseSome suggestions that ice sheets grewSome suggestions that ice sheets grew

•Removing up to 10 cm of sea level riseRemoving up to 10 cm of sea level riseMany uncertaintiesMany uncertainties

Greenland ice sheet other possible sourceGreenland ice sheet other possible source

Greenland Ice Sheet Ruddiman reports that Greenland Ice could Ruddiman reports that Greenland Ice could

have grown or shrunkhave grown or shrunk Recent reports suggest both occurred during Recent reports suggest both occurred during

2020thth Century Century Patterson and Reeh (2001, Nature 414:60-62) Patterson and Reeh (2001, Nature 414:60-62)

reportreport Small changes in eastern GreenlandSmall changes in eastern Greenland Western Greenland showedWestern Greenland showed

Significantly higher thinning ratesSignificantly higher thinning ratesThinning rates extending to higher Thinning rates extending to higher

elevationelevation•Compared to earlier studiesCompared to earlier studies

Patterson and Reeh (2001) Survey using Survey using

trigonometric levelingtrigonometric leveling Measured elevation Measured elevation

at 300 stations on at 300 stations on 1200 km transect1200 km transect1953-19541953-1954

Radar altimetry Radar altimetry 1994-19951994-1995

Digital elevation Digital elevation modelmodelMeasured surface Measured surface elevation changeselevation changes

Model Results Interpreted ice thickness changesInterpreted ice thickness changes

Band A shows ice thickening of 9.7±8.4 cm yBand A shows ice thickening of 9.7±8.4 cm y-1-1

Bands B-D no significant changeBands B-D no significant change Band E showed average thinning of 16.5±11 cm yBand E showed average thinning of 16.5±11 cm y-1-1

Band F showed average thinning of 31.0±10.7 cm yBand F showed average thinning of 31.0±10.7 cm y-1-1

Implications of Results 41-year record measured dynamic 41-year record measured dynamic

response of glacierresponse of glacier Long-term trend in ice thicknessLong-term trend in ice thickness

West Greenland thinned significantlyWest Greenland thinned significantly–This thinning contributed to This thinning contributed to global sea level riseglobal sea level rise

Paterson and Reeh (2001) do not Paterson and Reeh (2001) do not provide estimate of West Greenland provide estimate of West Greenland Ice sheet thinningIce sheet thinning•To global sea level riseTo global sea level rise

–Only point out that it is importantOnly point out that it is important

Cloud Cover Estimated extent of cloud Estimated extent of cloud

covercover Begin in 1900Begin in 1900

Cloud cover increased in both Cloud cover increased in both hemisphereshemispheres Especially since 1940Especially since 1940 Reports do not specify what Reports do not specify what

kinds of clouds increasedkinds of clouds increased Limited usefulnessLimited usefulness

Due to rise in surface Due to rise in surface temperatures ortemperatures or

Due to increase in the number Due to increase in the number of particles in airof particles in air Increases cloud Increases cloud

condensation nucleicondensation nuclei

Length of Growing Season Monitored at Earth stations or by satelliteMonitored at Earth stations or by satellite

Measurements in central AlaskaMeasurements in central Alaska Indicate erratic increase in length of Indicate erratic increase in length of

growing season by 2 weeks in 50 yearsgrowing season by 2 weeks in 50 years

Length of Growing Season Alaska measurements confirmed by satellite Alaska measurements confirmed by satellite

observationsobservations Observations of colorObservations of color

Sense chlorophyll produced by vegetationSense chlorophyll produced by vegetation North of 45°NNorth of 45°N

In the mid-1990’sIn the mid-1990’s•Growing season started a week earlier Growing season started a week earlier

in Springin Spring•Growing season ended a half week Growing season ended a half week

laterlater– Compared to the 1980’sCompared to the 1980’s

Reduction in Snow Cover Decrease in snow Decrease in snow

cover in northern cover in northern hemispherehemisphere Between 1978 Between 1978

and 1995and 1995 Shown by two Shown by two

kinds of satellite kinds of satellite measurementsmeasurementsMainly due to Mainly due to earlier melting earlier melting of snow in of snow in SpringSpring

Reduction in Sea Ice Decrease in Arctic sea ice cover by 6% from Decrease in Arctic sea ice cover by 6% from

1970-19901970-1990 Measurements by submarines indicateMeasurements by submarines indicate

Remaining ice thinned by 40% from 1950-1995Remaining ice thinned by 40% from 1950-1995

Summary Records of recent climate changeRecords of recent climate change

Consistent with warming during 20Consistent with warming during 20thth century century Records includeRecords include

Surface temperature observationsSurface temperature observationsAlpine glacier meltingAlpine glacier meltingSea level riseSea level rise

Many records are only a few decades in Many records are only a few decades in lengthlengthMust interpret cautiouslyMust interpret cautiously

Must verify long-term climate changeMust verify long-term climate changeUsing records that span several more Using records that span several more

decadesdecades

Sources of Climate Variations Tectonic scale changes irrelevant for 20Tectonic scale changes irrelevant for 20thth Century Century

Rate of temperature change 0.0001°C per 1000 Rate of temperature change 0.0001°C per 1000 yearsyears

Orbital-scale changes averaged over the Earth’s Orbital-scale changes averaged over the Earth’s surfacesurface Temperature cooled by 5°C over last 5000 yearsTemperature cooled by 5°C over last 5000 years

Equates to 0.2°C Equates to 0.2°C coolingcooling over last 1000 years over last 1000 years Millennial-scale changes difficult to asses over last Millennial-scale changes difficult to asses over last

1000 years1000 years Best guess is that millennial-scale changes over Best guess is that millennial-scale changes over

the last 1000 yearsthe last 1000 yearsLess than those caused by orbital-scale Less than those caused by orbital-scale

changeschanges

Changes in Solar Radiation Changes in strength of SunChanges in strength of Sun

First satellite measurements in 1978First satellite measurements in 1978 11 year cycles of 0.15% (2 W m11 year cycles of 0.15% (2 W m-2-2)) Cycles this short not likely to cause temperature Cycles this short not likely to cause temperature

change on Earthchange on Earth Change correlated with sunspot activityChange correlated with sunspot activity

Long records of sunspot activityLong records of sunspot activity

Sunspot Cycles Observations Observations

confirm 11-year confirm 11-year record existed at record existed at least to 1600 ADleast to 1600 AD No correlation No correlation

in record with in record with 11 year climate 11 year climate cyclecycle

Earth’s surface Earth’s surface temperature temperature seems to seems to correlate with correlate with average sunspot average sunspot maximamaxima Over the last Over the last

100 years100 years

Sunspot Strength Long-term average sunspot strength Long-term average sunspot strength

provides enough time for climate to changeprovides enough time for climate to change Changes in the strength of SunChanges in the strength of Sun

Confirmed using tree ring Confirmed using tree ring 1414CC Estimates suggest Sun weaker by 0.25%Estimates suggest Sun weaker by 0.25%

Over long cycles of known weaknessOver long cycles of known weakness•Maunder sunspot minimum (1645-Maunder sunspot minimum (1645-1715)1715)

•Sporer sunspot minimum (1460-1550)Sporer sunspot minimum (1460-1550)Correlation with coldest intervals of Correlation with coldest intervals of Little Ice AgeLittle Ice Age

Link to Climate Size of hypothesized Sun-climate link Size of hypothesized Sun-climate link

weakweak N. hemisphere temperature not N. hemisphere temperature not

correlatedcorrelatedMaunder sunspot minimumMaunder sunspot minimumTimes of more abundant sunspotsTimes of more abundant sunspots

Some intervals of climate historySome intervals of climate historyShow weak trends with sunspot Show weak trends with sunspot activityactivity

Some show opposite trend expectedSome show opposite trend expected

ENSO Cycles Models predicting ENSO events sophisticatedModels predicting ENSO events sophisticated

Ultimate cause of ENSO events remains Ultimate cause of ENSO events remains unknownunknownSuccess in predicting scale of event is Success in predicting scale of event is

difficultdifficult Regional warming effects of ENSO large (2-Regional warming effects of ENSO large (2-

5°C)5°C)Cause global scale temperature anomalies of Cause global scale temperature anomalies of

0.1°C0.1°C Cannot contribute markedly to long-term Cannot contribute markedly to long-term

global trendsglobal trendsEpisodic nature of ENSOEpisodic nature of ENSO

•Adds to year-to-year variability in signalAdds to year-to-year variability in signal

Large Volcanic Eruptions Large scale explosive Large scale explosive

volcanic eruptionsvolcanic eruptions Emit SOEmit SO22 into atmosphere into atmosphere

Forms sulfate particles Forms sulfate particles which block incoming which block incoming solar radiationsolar radiation•Cools climateCools climate

1991 eruption of Mt. Pinatubo1991 eruption of Mt. Pinatubo Produced 0.6°C cooling Produced 0.6°C cooling

one year after eruptionone year after eruptionNet cooling of 0.3°CNet cooling of 0.3°CHowever, within 2 years However, within 2 years

back to background back to background levels levels

ENSO and Volcanic Eruptions Gradual increase in 20Gradual increase in 20thth century global temperature century global temperature

Not explained by ENSO or volcanic eruptionsNot explained by ENSO or volcanic eruptionsCannot explain any long-term climate trendsCannot explain any long-term climate trends

Summary Gradual increase in temperature over last 100 Gradual increase in temperature over last 100

yearsyears Cannot be explained byCannot be explained by

ENSO events or explosive volcanic eruptionsENSO events or explosive volcanic eruptions•Erratic climate effectErratic climate effect

Orbital-scale changesOrbital-scale changes•Produce global cooling, not warmingProduce global cooling, not warming

Millennial-scale changes could produce Millennial-scale changes could produce warmingwarming•Evidence is inconclusiveEvidence is inconclusive

Changes in Sun’s strength or sunspot activity Changes in Sun’s strength or sunspot activity and greenhouse gas concentrationsand greenhouse gas concentrationsPlausible factorsPlausible factors