Transcript U3A-ClimChange05 7492KB Oct 27 2012

CLIMATE CHANGE
THE GREAT DEBATE
Session 5
PROXY CLIMATE DATA
FROM ICE CORES
Much information about Quaternary Ice Age
climates can be obtained from cores taken
from Greenland and Antarctica
Ice sheets preserve a continuous record of
climate change that may extent for hundreds
of thousands of years. Many characteristics
that relate to environmental conditions can be
measured
ICE ACCUMULATION RATES
The Greenland Ice Sheet experiences much
greater rates of accumulation than the
Antarctic one because it receives much larger
amounts of snowfall
Greenland ice cores provide much greater
time resolution than Antarctic ones but they
do so over much shorter time intervals
The Antarctic records go back 800,000 years
while the Greenland ones go back only
200,000 years
ICE PROPERTIES
Many properties can be measured. They
include the following:
Annual layering
Stable isotope analysis of oxygen and
hydrogen
Concentrations of trace elements
Concentrations of microparticles, eg dust
Air inclusions
ANNUAL LAYERING
Annual layering can be identified in
Greenland ice cores over the last 30,000 years.
Variations in the amount of snowfall and ice
accumulation can be measured for individual
years. It means that when climate changes, we
can measure with a high degree of accuracy
how fast such changes are taking place
STABLE ISOTOPE RATIOS
The stable isotope ratios of oxygen and
hydrogen (18O/16O and D/H) provide a
measure for temperature conditions at the
time when the snow fell. They provide us with
palaeothermometers. Annual variations can be
seen in the Greenland ice cores
The curves show marked similarities to the
stable isotope curves for oxygen derived from
the deep sea cores even though they record
changes in ice volume rather than temperature
Oxygen isotope and carbon dioxide variations
over the last 500,000 years in Vostok ice core
from Antarctica
TRACE ELEMENTS
They are chemical elements that are present
in small but varying quantities. For example:
Nitrogen. Variations may reflect variations in
sunspot activity
Sodium. Variations may relate to the amount
of sea salt present in the precipitation
Calcium. Atmospheric dust is usually rich in
this element
Sulphur. Its concentration may indicate the
level of volcanic activity
MICROPARTICLES
Small particles in the atmosphere are derived
mainly from dust storms and volcanic
eruptions. Strong winds can carry them to the
ice sheets where they are deposited with the
precipitation and incorporated into the ice.
When dust particle concentrations are
analysed throughout the ice core, they are
found to be much more abundant during glacial
times because winds were stronger and there
was less vegetation cover
Variations in
particle
concentrations
in ice cores
over time
INCLUSIONS OF AIR
As snow is compacted to form glacial ice,
small amounts of air are trapped in inclusions
that have become isolated from the
atmosphere. With modern techniques it is
possible to measure concentrations of the
greenhouse gases carbon dioxide and
methane. A surprise discovery is that their
concentrations are closely related to climate.
They are much lower during glacials than they
are during interglacials. It also shows that they
have never been as high during the last
800,000 years as they are today
ABRUPT CLIMATE CHANGE
Greenland ice cores provide a unique
opportunity to discover how quickly climate
change can occur. After the end of the Last
Glacial there was a brief return to full glacial
conditions in Western Europe. This event is
known as the “Younger Dryas” and is well
documented from lake sediment cores. The
name “Dryas” comes from a genus of Arctic
daisies. Its pollen was dominant in the lake
sediment cores at that time
Climate changes recorded in GISP and GRIP
ice cores from 18,000 to 10,000 years ago.
YD indicates the “Younger Dryas”
Thermohaline
Circulation of
Atlantic Ocean
In Northern
Hemisphere
Information obtained from marine cores off
the coast of Venezuela indicates that tropical
climates were also affected
DANSGAARD-OESCHER EVENTS
They are rapid climate fluctuations that
occurred 25 times during the Last Glacial
Period from approximately 120,000 to 15,000
years ago. They are initiated by abrupt
warming followed by rapid cooling
They have occurred at somewhat irregular
intervals and appear to be confined to the
Northern Hemisphere
None have occurred during the present
interglacial (Holocene) that is during the last
12,000 years
Stable isotope variations in ice cores from
Antarctica and Greenland from 140 ka to the present
Source: Wikipedia
Details of Dansgaard-Oescher Events from two
Greenland ice cores between 50 and 30 ka. Note very
abrupt warming followed by rapid cooling
SPELEOTHEMS
They can also provide proxy climate data:
Oxygen isotope data provide information
about past temperatures
Carbon isotope data provide information
about plant productivity
Trace element analysis also provides useful
information about past environments
Figure 1: Oxygen isotope changes
in FT stalagmite
Figure 2: Estimates of temperature
change obtained from Figure 1
Figure 3: Estimates of temperature
change obtained from the Vostok
Ice Core over the same time period
Changes in vegetation activity compared with
changes in summer solar radiation