climate change a chapter 4_pastclimate.ppt

climate change

1. 4. Paleoclimatereconstructions
and contemporary climate trends
4.1 Techniques for climate reconstruction
4.2 Proxy evidences of past climate changes
4.3 Climatic trends since the advent of
instruments

2. Introduction
Time present and time past
Are both perhaps present in time future.
And time future contained in time past.
TS Euot (1888-1965)

3. 4.1 Techniques for climate reconstruction
•The knowledge about Earth’s climate change
is based on
–Knowledge of how the climate has behaved in the
past
–Ability to explain past climate changes
•Based on understanding of past climate , it is
possible to build conceptual and then detailed
computer models, in turn possible to predict
about the future

4. Cont’d
•Two main types of climatic data for analysing
past climatic variation:
–Direct or instrumental measurements (started ca
200 yrs ago)
–‘Proxy’ or indirect data

6. 1. Historical methods
A. Direct Quantitative Measurements
–Most direct and desirable methods for assessing
climate change is to compare statistics
•Since mid-1800 for surface observation and since mid
1900s for the present more extensive surface and
upper air networks
•Still several gaps (far from adequate number of
synoptic station for both surface based and upper air
observation)
•Serious gaps in atmospheric network

7. Cont’d
B. Descriptive accounts of General Environmental
condition
-several sources before the advent of scientific
meteorological instrumentation
•Biblical sources, historiography, literature and arts
(primitive cave paintings) etc
•Sequentially recorded climatic data in the form of
personal diaries and records of lake levels, times of
freezing, spring plantings, harvests, frosts, and other
agriculturally significant events

8. 2. Surficialbiogeologicproxy evidence
•Provide most reliable & extensive climatic
indicators even for a large part of the period
of historical past
•Several types:
–Annually layered life forms (tree rings, corals)
–Surface geomorphic evidences (e.g., sea & lake
level changes, sand formation & movement, ice
and glacier signatures)

9. Annually layered life forms
•Most useful method is a tree ring analysis
•The concentric annual deposits of tree trunk
material, the thickness and density of which are
proportional to the relative warmth and moisture
availability
•Time resolution and span: annual; centuries to
millennia
–E.g., certain trees can leave for up to 5000 yrs permitting
the inference of fairly long-term qualitative climatic
variation
•Other living biological indicator is annual growth
layers of the corals

10. 3. Surface geomorphic evidence
•At least four classes of surface geomorphic evidence
that are of value in paleoclimatic reconstruction:
–The structure of marine shoreline as evidence of sea level
height
–Lake shorelines and beds and evidence of past regional
hydrologic conditions
–Soils and sand. The weathering of rocks and
decomposition of organic material into soil and sand, and
their chemical and their physical properties are highly
dependent on climatic conditions
•E.g., red beds –of soil are related to high temperature and some
dryness in the past and the presence of evaporities (salt, potash,
gypsum) is a strong indicator of extended periods of aridity
•Kaolin-rich soil and peat bogs are indicators of a past history of
humid conditions

11. Cont’d
•sand formations found in inland desert regions are
usually the result of arid climatic conditions prevailed
for many years. Mainly dune indicates the prevailing
winds for many yrs
–Ice and glacier effects. The advance and
subsequent retreat of vast ice sheets over a given
terrain leave a strong signature on the terrain as
exemplified by
a)scratch –due to moving ice and rocks encased in the ice and are
imprinted on the underlying surface rock
b)The form of the valleys through which the ice is progresses
(preglacialV-shaped to post glacial U shaped)
c)Basal water flow beneath & surrounding ice sheet. The
formation of debris, moraines, rounded boulders
d)etc

12. 4. Conventional nonisotopicstratigraphicanalysis of
sedimentary rocks
•The most important means of estimating long-term
climate changes is analysing the geological records
resulting from the deposition in oceans (and lakes) of
sediments (layered sedimentary rock)
•The analysis involves:
–Physical structure of sediments
–Climate dependent biological content of the layers
–Isotopic composition of the biogeneic materials
–Nonisotopic chemical methods applied to the deep ocean

13. Physical indicators
•On land surfaces, the composition of the rock assemblages in
the strata can give a first clue to the climate condition of
deposition, particularly with reference to glacial conditions –
identifying layers containing glacial debris or moraine
materials
•Sedimentary layers containing much coarser grained detrital
materials are indicators of humid , high energy water drainage
conditions
•Layers containing residues from evaporating shallow sea
waters are indicative of hot and arid conditions and calcium
carbonates (limestone layers) can be indicative of nutrient
rich surface waters supporting abundance of organisms

14. Paleobiological indicators (Fossil Faunal types and
abundances)
•The stratigrapgic layers contain much fossil evidence
of the climate dependent life forms that existed at
the time of sedimentary deposition. Several forms of
deposition
–Pollen and spores
–Imprints of plant macrofossil or floral assemblages, or organic residue
(peat, coal, oil, etc) –imprints of cooling trend over the cenozoic era
–Fossil faunal assemblages mainly in the form of bone material or large
animals
–Calcareous and siliceous shells of marin organisms, including
planktonic plants and animals, and larger molluscan and shelfish forms

15. 5. Isotopic methods
•Among the most important indicators of climate change are
oxygen isotope ratio (
18
O/
16
O) and the carbon isotop ratio
(
13
C/
12
C) of the calcareous material in sedimentary cores.
These ratios are usually expressed as departures, δ, from a
laboratory standard value (in parts per thousands)
•Major types
–Oxygen isotopes
–Deuterium and Beryllium in ice core
–Stable carbon isotopes
–Strontium and Osmium isotopes

16. 6. Nonisotopicgeochemical methods
•Many other nonisotopic chemical properties
of sedimentary rock and ice stratigraphy are
potentially useful. Examples
–Cadmium analysis
–Greenhouse gas analysis of trapped air in ice cores
–Chemical and biological constituints and dust
layers in ice cores

17. Cont’d
•Dating the proxy evidences
–The most useful method for establishing
chronologies for the past 40ky is the radiocarbon
method based on the unstable isotope

18. Proxy indicators of climate related variables
Indicator Property
measured
Time
resolution
Time span Climate related
information
obtained
Tree rings
Lake and bog
sediments
Coral growth
rings
Width, density,
isotopic ratios,
trace elements
Deposition
rates, species
assemblages
from shells,
and pollen,
macrofossils,
charcoal
Density,
isotope ratios,
fluorescence
Annual
Annual
annual
Centuries to
millennia
Millennia
centuries
Temperature,
rainfall, fire
Rainfall,
atmospheric
water balance,
vegetation
type, fire
Temperature,
salinity, river
outflows
4.2 Proxy evidences of past climate changes

19. IndicatorProperty measured Time
resolution
Time span Climate related
information
obtained
Ice cores
Ocean
sediment
cores
Boreholes
Isotopes, fractional
melting, annual layer
thickness, , dust grain
size, gas bubbles
Species assemblages
from shells& pollens,
deposition rates,
isotopic ratios, air born
dust, pollen
Temperature profile
Annual
Usually
multidecadal
or centuries
decadal
Millennium
Millennium
centuries
Temp, snow
accumulation
rate, windiness,
gas
concentrations
Sea temp.,
salinity, acidity,
ice volumes, &
sea levels, river
outflows,
aridity, land
vegetation
Surface
temperature

20. IndicatorProperty measured Time
resolution
Time span Climate related
information
obtained
Old
ground
water
Glacial
moraines
Sand
dunes
Coastal
land form
Document
ary
evidence
Isotopes, noble gases
Maximum glacier
length
Orientation, grain size
Ledges, former beach
lines, debris lines
Reports of extremes,
harvests, datesof
breakup of river or lake
ice
Centuries
Decades
Centuries
Decades to
centuries
annual
Millennia
Centuries to
millennia
Millennia
Decades to
millennia
Centuries to
millennia
Temperature
Temperature
and
precipitation
Wind direction
and speed
Former sea-
level, tropical
cyclones
Temperature,
precipitation

21. 4.3 Climatic trends since the advent of
instruments
•Temperature: (see figure from IPCC)
-The details of surface global-mean temperature variations
for the period since 1750 or 1860, used as a reference for
current climate change impact studies.
-Values for each hemisphere and the globe all show the
general upward trend in temperature. Based on IPCC estimate
global mean surface temperature has increased about 0.6
degrees C since the beginning of the 20
th
century. It is also
considered, with very high statistical confidence, that the last
decade (1990-1999) has been the warmest decade of the
period
•it is considered that the increase in temperature in the 20th
century has been larger than that for any century in the past
1 000 years.

23. Figure SPM.3. Observed changes in (a) global average
surface temperature, (b) global average sea level from
tide gauge (blue) and
satellite (red) data and (c) Northern Hemisphere snow
cover for March-April. All changes are relative to
corresponding averages for
the period 1961–1990. Smoothed curves represent
decadal average values while circles show yearly values.
The shaded areas are the
uncertainty intervals estimated from a comprehensive
analysis of known uncertainties (a and b) and from the
time series (c). {FAQ 3.1,
Figure 1, Figure 4.2, Figure 5.13}

26. Precipitation
•Precipitation variability has quite different features from
temperature variability.
•From the past 90 years data all over the world (main land), it
is observed, decadal variability is quite evident as it was for
temperature.
•The data suggests systematic variations of land area
precipitation with latitude with increases in precipitation in
the middle-and high-latitudes of the Northern Hemisphere
and decreases in the tropics.
•It is difficult to see correlations between the temperature and
precipitation variability. Also the precipitation does not have
a clear trend.
•See Ethiopian case (NMSA)

27. Severe weather
•Daily extreme weather (e.g. see paper)
•Drought frequency (meteorological,
hydrological, agricultural droughts)
(see the article for historical drought events in
Ethiopia and associate with agricultural
production)
•Floods
•Tropical storms

28. Expert Team on Climate Change Detection and Indices
(ETCCDI)

29. •Pronounced inter-annual variability exists in the
tropical Pacific sea-surface temperature as part
of ENSO
•Oceanic variability at decadal scales has been
hard to detect and describe because of the short
observational record
•Recently decadal variability in sea-surface
temperature has been detected in the central
and northern Pacific Ocean area.
Ocean condition

30. Cont’d
•The decadal component of the North Atlantic
Oscillationcorrelates with decadal variations in sea-
surface temperature, currents, salinity and sea ice
in the north Atlantic. The involvement of deep
thermohaline currents can link this variability with
other parts of the ocean system.
•Sea level itself is of interest to all shore land areas
and ocean island inhabitants. Long-term records for
sea level show a general rising trend in the major
ocean regions in the range of 1-3 mm/yr along with
decadal variations as large as 200 mm