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