Climate archives, data, models (Ch. 2)
• climate archives
• dating of climate archives
• timespan & time resolution
• GCMs
Climate archives
-- a climate archive is a source of climate data
types include:
sediments
glacial ice
tree rings & corals
historical records
instrumental records
sediments
Sediments
-- loose material produced by the disintegration of rocks
-- transported by wind or water near Earth’s surface
-- tend to accumulate in layers in low spots
(sedimentary deposition in low areas)
examples:
sand or silt grains at beaches, or in streams
mud / clay particles in lake & ocean bottoms
shells of dead organisms in oceans
pollen
Sediments
-- often trap biologic material
-- can record temperatures (e.g., inferred from
O-isotope data)
-- sediments accumulate in low areas, most recent
at top
-- get time record by taking a core sample
Sedimentary deposition in lakes, seas, ocean:
Some lake core sample locations
time records for as long as deposition in lake persists;
can be ~1000 years
Ocean core sample locations
time records for as long as deposition in ocean persists;
can be ~1 - 10 million years
Glacial ice
-- ice in glaciers or ice sheets
-- deposited in annual layers
-- ices trap gases in bubbles & record
temperatures (O-isotope data)
-- get time sequence by taking ice cores in areas
that are experiencing ice accumulation
Mountain
glaciers
time records
up to
~1000 years
_____________
Ice sheets
(e.g.,
Antarctica,
Greenland)
time records
up to
~100,000 years
Tree rings
-- annual growth of wood layers
time records of ~ 100 - 10,000 years
Corals
-- organisms that live in shallow ocean water
-- secrete annual carbonate layers
time records of ~ 10 - 1,000 years
Some tree ring, coral, and ice core sample locations
Historical records
-- info about climate that was recorded by people
time records over ~ 1000 years
Instrumental records
-- info on climate (e.g. temperature) recorded by
direct measurement
time records over ~ few hundred years
Dating of climate archives
to understand how climate has varied over time,
one needs to be able to determine relative or
absolute (actual) ages
use one or all of the following techniques:
(1) radiometric dating
(2) correlation
(3) counting annual layers
(1) Radiometric dating
-- absolute dating technique
-- depends on the decay of radioactive isotopes
-- usually applied to rocks that solidified from
magma (molten rock), but radiocarbon dates
can be obtained for organic material in
sedimentary materials
What are isotopes?
-- atoms that vary in mass but have the same chemical
behavior (i.e., same element, different isotopes)
Example: there are 3 stable oxygen isotopes
16O
17O
8 protons
8 neutrons
8 protons
9 neutrons
18O
8 protons
10 neutrons
But not all isotopes that exist in nature are stable
(some undergo radioactive decay)
This changes the number of neutrons or protons
in the nucleus of the atom
(can get different element as result)
Example: Carbon has 3 isotopes
12C
- contains 6 protons, 6 neutrons - stable
13C - contains 6 protons, 7 neutrons - stable
14C - contains 6 protons, 8 neutrons - unstable (radioactive)
Carbon-14 decays to Nitrogen-14
14C
(6 protons, 8 neutrons) --> 14N (7 protons, 7 neutrons)
Parent isotope 14C has decayed to daughter isotope 14N.
So: how do we use radioactive decay
to date something?
So: how do we use radioactive decay
to date something?
Answer: If we know the rate of decay and
can measure the amount of parent and
daughter isotopes, we can calculate the time
elapsed.
Half-life = the amount of time needed to
transform 1/2 of the parent into the
daughter isotope
Radioactive decay
D/P = 0/24 = 0
12/12=1
18/6=3
21/3=7
the ratio of daughter to parent is unique at any given time, and
gives us the number of half-lives that have passed
Some half-lifes:
(2) Correlation
-- relative dating technique
-- goal is to understand time sequence of events
even if absolute age not known
-- use in geologic outcrops where cross-cutting
relationships or distinctive features are seen
in sedimentary rock layers, the layer on top is
the youngest, the layer at bottom is the oldest
in order for a rock unit to cut across other rocks,
it has to be younger than the other rocks
Geologic
principles
Relative
ages (oldest
to youngest):
igneous 1
sed layer A
igneous 2?
sed layer B
igneous 3
sed layer C
igneous 4?
To get ages of rocks that cannot be radiometrically
dated, use combination of correlation &
radiometric dating
The circle below represents a point of interest (say a fossil)
found in an undatable rock unit that is bounded above and
below by datable lava flows.
How old is the green dot?
<- lava flow dated at 3.6 my ->
lava flow dated at 3.8my ->
<- lava flow dated at 4.2 my ->
3.9 + 0.3 m.y.
3.7 + 0.1 m.y.
The date on the right hand side is a more precise date.
<- lava flow dated at 3.6 my ->
lava flow dated at 3.8my ->
<- lava flow dated at 4.2 my ->
(3) Counting annual layers
-- relative dating technique
-- can be turned into an absolute age if
additional info known (e.g., if one knows
when layers started or stopped forming)
Timespan & time resolution
-- different climate archives give info on
different timespans
timespan: largest time unit we can measure
-- vary also in time resolution
time resolution: smallest time unit we can
measure
archive resolution
related to the time
span:
longer time span
archives tend to
have worse (larger)
time resolution
...and vice versa
sediments give us
our oldest record of
climate: oldest
sedimentary rocks
are ~ 3.5 b.y. old
oldest ice core layers
from Antarctica:
~400,000 years old
General circulation models (GCMs)
• these are 3-d computer models that provide a
complete numerical simulation of the
climate system
• they simulate response of climate to various forcings
• useful for:
-- understanding climate archives
-- predicting future climate
• can be tested by comparing simulated to real
responses
Steps in models:
subdivide
climate
system
into smaller
pieces
analyze how
these interact
Observed
Model
January
surface
temperature