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Historical%20Changes%20in%20Climate

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Title: Historical%20Changes%20in%20Climate


1
Historical Changes in Climate
  • Temperature change over last few thousand years
  • Less than 1C
  • Highly variable from region to region
  • Archives for climate data
  • Mountain glaciers
  • Tree rings
  • Corals
  • Historical observations
  • Instrumental data only during last lt1K years
  • General trend
  • Dramatic 20th century warming
  • Cooler climates prior to 20th century
  • Little Ice Age (1400-1900 AD)

2
Medieval Climatic Optimum
  • Evidence for relative warmth in high latitude
    northern hemisphere
  • Approximately 1000-1300 AD
  • Nordic people settled southwest Greenland
  • Fringes of ice sheet
  • Agricultural evidence for warmth wheat crops
  • Little mention of sea ice from this region
  • Settlements abandoned during Little Ice Age
  • Suggests marginal environment became inhospitable

3
Little Ice Age
  • Cooling during 1400-1900 AD
  • Well documented in Europe
  • Colder winters
  • Failed crops
  • Shorter growing seasons
  • Lakes, rivers and ports frozen
  • Advance of alpine glaciers
  • Frequency of sea ice along coastal Iceland
  • Too much ice to fish
  • Not a true ice age

4
Ice Growth during Little Ice Age
  • Evidence for ice growth in Canadian Arctic from
    lichen halos
  • Lichens grow on rock surfaces at known rates
  • Lichen halos interpreted as lichen killed by ice
    cover
  • Size of living lichens give time since lichen
    death
  • Small size indicates only 100 years of growth

5
Extent of Ice Growth in Canada
  • Baffin Island shows large expansion of ice
  • Ice killed lichens during Little Ice Age
  • Retreat allowed them to grow only 100 years ago
  • Mapping lichen halos gives distribution of ice

6
Climate Extent Temperature Trend?
  • Few observational data and small change
  • Restrict determining if global or regional
    cooling
  • Cooling trend not known
  • Culmination of slow orbital-scale cooling
  • Most recent in a series of millennial-scale
    oscillations

7
Proxy Measures of Historical Changes
  • Instrumental records prior to 1900 largely do not
    exist
  • Quantitative information is scarce
  • Problems for archives are several fold
  • Records must extend from 1900s back 1000-2000
    years
  • High resolution required high deposition rate
  • Must be extremely sensitive to detect small
    changes
  • Archives
  • Alpine glacier ice cores
  • Tree rings
  • Corals

8
Alpine Glaciers
  • Small ice caps on mountains and valley glaciers
  • Make excellent climate archives
  • Range from few hundred to thousands of years
  • Back to LGM
  • Annual layering at surface
  • Degrade to decadal resolution at depth

9
Alpine Glaciers
  • Drilling is difficult task
  • Haul equipment to summit
  • Lack of oxygen
  • Lack of electricity
  • Freezing to subfreezing temperatures
  • Few mountain glaciers drilled

10
Alpine Glaciers
  • Multiple ice cores drilled
  • Through entire thickness of glacier (100-200 m)
  • Photograph shows drill core extruded
  • Solar powered drill

11
Alpine Glaciers
  • Harsh conditions can prevail
  • Cores and equipment must be hauled down after
    drilling
  • Produce similar records as Greenland and
    Antarctic ice cores
  • No mention of gases?

12
Quelccaya Ice Cap, Peru
  • Peruvian Andes, at 18,500 feet elevation
  • From 1980s show annual-scale d18O and dust
    changes
  • More positive d18O and less dust near 1900
  • Warmer temperatures and weaker winds
  • More negative d18O and more dust 1600-1900
  • Colder temperatures and stronger winds

13
Quelccaya Ice Cap, Peru
  • Little Ice Age showed more negative d18O between
    1550 and 1900 AD
  • Accumulation record more complex
  • Pronounced wet period before 1700 followed by
    significantly drier conditions thereafter

14
Quelccaya Ice Cap, Peru
  • Return expedition in 1991
  • Annual layering at top of core destroyed by
    meltwater percolation
  • Previous record indicated 1500 years without
    melting
  • Meltwater percolation presumably due to 20th
    century warming
  • Rate that was unprecedented compared to last
    millennium

15
Quelccaya Ice Cap, Peru - 1976
16
Quelccaya Ice Cap, Peru - 2000
17
Dunde Ice Cap, Tibet
  • Remote region on mountain range separating
    China's highest desert, the Qaidam Basin, from
    the Gobi
  • Snow accumulations for 40,000 years on a 60 km2
    ice cap deep in sparsely inhabited interior
  • US and Chinese expedition in 1987

18
Dunde Ice Cap, Tibet
  • Record averaged over 50-year intervals
  • More positive d18O before 1500
  • More negative d18O during Little Ice Age
  • Transition to more positive d18O at 1700
  • Last 50 year interval is more positive than any
    other year
  • Core taken to 12,000 year

19
Ice Core Summary
  • Climate on low latitude mountains
  • Colder during the Little Ice Age
  • Uniquely warmer during the 20th century
  • Antarctica and Greenland cores
  • No distinctly cold pattern during Little Ice Age
  • No unique 20th century warmth
  • Variation in climate recorded in these archives
    has been regional

20
Dendroclimatology
  • Records from tree rings
  • Regions where trees are sensitive to climate
    stress
  • Near limit of natural range
  • Climate stress revealed by narrow rings
  • Century to millennia-length tree-ring
    chronologies are useful for evaluating
  • Frequency and magnitude of droughts and wet
    periods
  • Placing ecosystem changes into a long-term,
    historical context of climate change

21
Stable Isotopes in Dendroclimatology
  • The d18O of rain varies with changes in
    temperature and rainout
  • Rain is absorbed by the growing tree, the d18O of
    water recorded in the tree rings, and become
    climate indicators
  • The d18O of organic material is determined by
  • The isotopic composition of the source or soil
    water
  • Enrichment taking place in the leaf water due to
    transpiration, resulting in an increased d18O of
    leaf water compared to d18O of soil water
  • Biochemical fractionations

22
Tree Ring Climate Calibration
  • Tree ring data must be cross-calibrated with
    instrumental data
  • Before can used to interpret ancient climate
  • The character of the relationship between climate
    and tree growth is assessed
  • Statistical model is derived to describe that
    relationship

23
Arctic Tree Rings
  • Results synthesized from circumarctic region
  • Covers middle and end of Little Ice Age
  • Inferred temperature change of 1C
  • Not a time of extreme cold
  • Warming of the Arctic apparent from mid-20th
    century
  • Reaching highest temperatures during 320 year
    record
  • But temperatures not significantly higher than
    18th century

24
Tree Ring Studies in Central Asia
  • Climate on large continent less moderated by
    oceans
  • Still, similar to Arctic region
  • Intervals of warmth during Little Ice Age
  • Mid-18th century and earlier
  • Colder temperatures in late 16th, late 17th and
    mid-to late 19th centuries
  • Warming in mid- to late 20th century unprecedented

25
Tree Rings on Tasmania
  • Records from Huon Pines extend back 2000 years
  • Best tree ring records of S. hemisphere
  • Little Ice Age is cooler than late 20th century
  • Does not stand out as uniquely cold
  • 20th century warmth stands out in the rate of
    increase
  • Temperature matched by earlier times

26
Tree Ring Summary
  • Tree ring studies indicate
  • Climate variable from region to region
  • Over last several hundred years
  • No one record fully describes climate trends in
    all area
  • Similar to ice cores
  • Climate varied significantly within the Little
    Ice Age
  • In some area even warming to early 20th century
    levels
  • Last few decades generally show maximal warmth

27
Coral and Tropical SST
  • Decadal resolution SST
  • Warm tropical environments
  • Pacific Ocean atolls
  • Ideal for measuring the occurrence and intensity
    of El Nino events
  • Oxygen isotopic and trace elemental compositions
    yield important climate records

28
El Nino
  • Marked by appearance of unusually warm waters in
    the eastern Pacific in December every 2 to 7
    years
  • Radical alteration of the entire Pacific oceanic
    and atmospheric system occurs in two phases
  • In a cool phase, strong SE trade winds push
    eastern Pacific surface waters westward, allowing
    cool nutrient-rich bottom waters to upwell

29
Non-El Nino Years
  • The western Pacific during cool phases is
    typified by
  • A pool of warm water stretching eastward to
    170W, and an accompanying belt of low pressure
    and high precipitation
  • Indonesian Low - covers parts of Asia and
    Australia
  • A belt of high precipitation, the ITCZ, lies
    several degrees north and south of the Equator
    and east of the Date Line

30
El Nino
  • In the warm phase, trade winds weaken and less
    eastern Pacific surface water is pushed westward
  • Upwelling in the eastern Pacific slows
  • Warm waters spread across the Pacific increasing
    SST by 3-5C in the Galapagos Islands

31
El Nino
  • ITCZ moves S and W, while the Indonesian Low
    follows the warmer waters east
  • Barometric pressure in Darwin, Australia rises as
    higher pressure replaces the Indonesian Low
  • During particularly severe warm events, winds in
    the western Pacific reverse and become mild
    westerlies

32
Southern Oscillation
  • El Nino years are times of
  • Unusually high pressure and dry conditions over
    N. Australia
  • Low pressure and high rainfall in the
    south-central Pacific
  • Non-El Nino years are time of
  • Low pressure and moist conditions over N.
    Australia
  • Higher pressure and reduced rainfall in the
    south-central Pacific
  • Linking of these two circulation systems in a
    large-scale flow known as El Nino Southern
    Oscillation (ENSO)

33
ENSO
  • Long-term changes in atmospheric pressure show
  • Warm El Nino years are time of
  • Drier conditions and higher pressure in N.
    Australia
  • Wetter conditions and lower pressure in
    south-central Pacific

34
Atmosphere-Ocean Linkage
  • Strong east-to-west trades common in non-El Nino
    years
  • Pile warm surface water in western Pacific
  • Warm water is natural source of moisture
  • Rising moisture off ocean creates low pressure
  • Creates high precipitation in N. Australia and
    Indonesia
  • Rising air cools and flows eastward in the
    east-central Pacific
  • Contributes to cooler and dry conditions near S.
    America

35
Atmosphere-Ocean Linkage
  • Trade winds in eastern Pacific weaken in El Nino
    years
  • Pool of warm western Pacific water diminished
  • Water flows eastward
  • As warm water replaces cool water in central and
    then eastern Pacific
  • Becomes the source of moisture and low pressure
  • As the warm water flow hits western N. America
  • Flow diverted N and S bring heavy rainfall to
    California and Peru
  • Loss of warm water in western Pacific create
    dryer conditions in Australia and Indonesia

36
Teleconnections
  • Unusual oceanic and atmospheric conditions
  • Tropical regions can affect circulation patterns
    outside tropics
  • Flooding in Peruvian Andes and SE US common
  • Droughts in Indonesian, central India and
    Australia

37
Coral Calibration
  • Calibration of coral d18O with SST measurements
  • Slight mismatch probably due to salinity
  • Galapagos corals record low d18O values during
    warm El Nino years

38
400 Year Coral SST Record
  • Little long term trend obvious
  • Generally more negative values near start and end
    of record
  • Perhaps just before 1700 and 1800
  • No hint of Little Ice age or 20th century warming
  • Some Pacific corals show gradual ocean warming
    and more rainfall towards present

39
Summary
  • Despite efforts, coverage of climate over last
    1000 years remains incomplete
  • Synthesis of N. hemisphere temperature change
  • Show a gradual decline for 900 years
  • Ending in a dramatic warming in 20th century

40
Mechanisms Producing Trend
  • Cooling from 1000 to 1900 AD
  • Follows orbital cooling pattern
  • Follows the millennial-scale pattern
  • Little Ice Age cooler than preceding and
    following intervals
  • Began with abrupt cooling

41
Mechanisms Producing Trend
  • Most obvious trend between 1000-900 years
  • Match with gradual orbital cooling

42
20th Century Warming
  • Stands out as a unique feature
  • Rate of warming highest
  • Temperatures just now rising above uncertainty
    levels of the reconstruction

43
20th Century Warning
  • Reconstruction suggests that 20th century warming
  • Not simply another in long series of natural
    climate oscillations
  • Something unprecedented for the entire millennium

44
Historical Records of El Nino
  • Records from ships logs date to 1525 AD
  • SST and sea level
  • Catch of anchovy and other fish
  • Sea bird abundance
  • Heavy rain and floods
  • Disease (malaria and cholera)
  • Records ranked qualitatively

45
Historical Records of El Nino
  • 115 events in 456 years event every 8 years
  • Events cluster
  • No correlation with Little Ice Age
  • Record provides a limited glimpse of climate
  • Local changes are difficult to extrapolate to
    global scale
  • El Nino events limited to N. hemisphere winter

46
Instrumental Temperature Records
  • Records over last 200-300 years
  • Air and SST
  • Limited regions prior to 1900
  • Methods limitation on temperature accuracy
  • Population growth affects local temperature
  • Change in albedo
  • Asphalt and vegetation
  • Can change temperature by as much as 30

47
20th Century Global Temperature
  • Overall trend shows 0.6C temperature rise
  • Year to year variability present
  • Temperature estimates form satellite disagree
    with thermometer
  • Abundant observations support significant warming
    in 20th century

48
Glaciers
  • Retreat of alpine glaciers indicates climate
    warming
  • Alpine glaciers show varying responses
  • Due to heterogeneity of climate system
  • Most glaciers have been in retreat
  • Rate of melting has accelerated in recent decades
  • Most plausible explanation is warming of climate

49
Global Average Sea Level
  • Difficult nut to crack
  • Local rebound and tectonic movements
  • Displacement of groundwater into oceans
  • Overall agreement of slow rise in sea level
  • 12-15 cm in last 100 years

50
Cause of Sea Level Rise
  • Ice on land has melted adding water to oceans
  • Water in the oceans has expanded
  • Surface warming of oceans accounts for about 1/3
    of rise
  • Melting ice remains the most probable explanation
    for remaining rise

51
Melting Glaciers
  • Alpine glaciers estimated to contribute 3 cm to
    sea level rise
  • Thermal expansion and alpine glacier retreat
  • Make up more than 50 of sea level rise
  • Melting of Antarctic and Greenland ice sheets
  • Antarctica cold and dry environment
  • Unlikely contributed to sea level rise
  • Some suggestions that ice sheets grew
  • Removing up to 10 cm of sea level rise
  • Many uncertainties
  • Greenland ice sheet other possible source

52
Greenland Ice Sheet
  • Ruddiman reports that Greenland Ice could have
    grown or shrunk
  • Recent reports suggest both occurred during 20th
    Century
  • Patterson and Reeh (2001, Nature 41460-62)
    report
  • Small changes in eastern Greenland
  • Western Greenland showed
  • Significantly higher thinning rates
  • Thinning rates extending to higher elevation
  • Compared to earlier studies

53
Patterson and Reeh (2001)
  • Survey using trigonometric leveling
  • Measured elevation at 300 stations on 1200 km
    transect
  • 1953-1954
  • Radar altimetry 1994-1995
  • Digital elevation model
  • Measured surface elevation changes

54
Model Results
  • Interpreted ice thickness changes
  • Band A shows ice thickening of 9.78.4 cm y-1
  • Bands B-D no significant change
  • Band E showed average thinning of 16.511 cm y-1
  • Band F showed average thinning of 31.010.7 cm y-1

55
Implications of Results
  • 41-year record measured dynamic response of
    glacier
  • Long-term trend in ice thickness
  • West Greenland thinned significantly
  • This thinning contributed to global sea level
    rise
  • Paterson and Reeh (2001) do not provide estimate
    of West Greenland Ice sheet thinning
  • To global sea level rise
  • Only point out that it is important

56
Cloud Cover
  • Estimated extent of cloud cover
  • Begin in 1900
  • Cloud cover increased in both hemispheres
  • Especially since 1940
  • Reports do not specify what kinds of clouds
    increased
  • Limited usefulness
  • Due to rise in surface temperatures or
  • Due to increase in the number of particles in air
  • Increases cloud condensation nuclei

57
Length of Growing Season
  • Monitored at Earth stations or by satellite
  • Measurements in central Alaska
  • Indicate erratic increase in length of growing
    season by 2 weeks in 50 years

58
Length of Growing Season
  • Alaska measurements confirmed by satellite
    observations
  • Observations of color
  • Sense chlorophyll produced by vegetation
  • North of 45N
  • In the mid-1990s
  • Growing season started a week earlier in Spring
  • Growing season ended a half week later
  • Compared to the 1980s

59
Reduction in Snow Cover
  • Decrease in snow cover in northern hemisphere
  • Between 1978 and 1995
  • Shown by two kinds of satellite measurements
  • Mainly due to earlier melting of snow in Spring

60
Reduction in Sea Ice
  • Decrease in Arctic sea ice cover by 6 from
    1970-1990
  • Measurements by submarines indicate
  • Remaining ice thinned by 40 from 1950-1995

61
Summary
  • Records of recent climate change
  • Consistent with warming during 20th century
  • Records include
  • Surface temperature observations
  • Alpine glacier melting
  • Sea level rise
  • Many records are only a few decades in length
  • Must interpret cautiously
  • Must verify long-term climate change
  • Using records that span several more decades

62
Sources of Climate Variations
  • Tectonic scale changes irrelevant for 20th
    Century
  • Rate of temperature change 0.0001C per 1000
    years
  • Orbital-scale changes averaged over the Earths
    surface
  • Temperature cooled by 5C over last 5000 years
  • Equates to 0.2C cooling over last 1000 years
  • Millennial-scale changes difficult to asses over
    last 1000 years
  • Best guess is that millennial-scale changes over
    the last 1000 years
  • Less than those caused by orbital-scale changes

63
Changes in Solar Radiation
  • Changes in strength of Sun
  • First satellite measurements in 1978
  • 11 year cycles of 0.15 (2 W m-2)
  • Cycles this short not likely to cause temperature
    change on Earth
  • Change correlated with sunspot activity
  • Long records of sunspot activity

64
Sunspot Cycles
  • Observations confirm 11-year record existed at
    least to 1600 AD
  • No correlation in record with 11 year climate
    cycle
  • Earths surface temperature seems to correlate
    with average sunspot maxima
  • Over the last 100 years

65
Sunspot Strength
  • Long-term average sunspot strength provides
    enough time for climate to change
  • Changes in the strength of Sun
  • Confirmed using tree ring 14C
  • Estimates suggest Sun weaker by 0.25
  • Over long cycles of known weakness
  • Maunder sunspot minimum (1645-1715)
  • Sporer sunspot minimum (1460-1550)
  • Correlation with coldest intervals of Little Ice
    Age

66
Link to Climate
  • Size of hypothesized Sun-climate link weak
  • N. hemisphere temperature not correlated
  • Maunder sunspot minimum
  • Times of more abundant sunspots
  • Some intervals of climate history
  • Show weak trends with sunspot activity
  • Some show opposite trend expected

67
ENSO Cycles
  • Models predicting ENSO events sophisticated
  • Ultimate cause of ENSO events remains unknown
  • Success in predicting scale of event is difficult
  • Regional warming effects of ENSO large (2-5C)
  • Cause global scale temperature anomalies of 0.1C
  • Cannot contribute markedly to long-term global
    trends
  • Episodic nature of ENSO
  • Adds to year-to-year variability in signal

68
Large Volcanic Eruptions
  • Large scale explosive volcanic eruptions
  • Emit SO2 into atmosphere
  • Forms sulfate particles which block incoming
    solar radiation
  • Cools climate
  • 1991 eruption of Mt. Pinatubo
  • Produced 0.6C cooling one year after eruption
  • Net cooling of 0.3C
  • However, within 2 years back to background levels

69
ENSO and Volcanic Eruptions
  • Gradual increase in 20th century global
    temperature
  • Not explained by ENSO or volcanic eruptions
  • Cannot explain any long-term climate trends

70
Summary
  • Gradual increase in temperature over last 100
    years
  • Cannot be explained by
  • ENSO events or explosive volcanic eruptions
  • Erratic climate effect
  • Orbital-scale changes
  • Produce global cooling, not warming
  • Millennial-scale changes could produce warming
  • Evidence is inconclusive
  • Changes in Suns strength or sunspot activity and
    greenhouse gas concentrations
  • Plausible factors
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