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:

sedimentsglacial icetree rings & coralshistorical recordsinstrumental records

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 bottomsshells of dead organisms in oceans

sediments

Sediments

-- often trap biologic material

-- can record temperatures (e.g., inferred fromO-isotope data)

-- sediments accumulate in low areas, most recentat top

-- get time record by taking a core sample

pollen

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 & recordtemperatures (O-isotope data)

-- get time sequence by taking ice cores in areasthat are experiencing ice accumulation

Mountainglaciers

time recordsup to~1000 years

_____________

Ice sheets(e.g.,Antarctica,Greenland)

time recordsup to~100,000 years

Tree rings

-- annual growth of wood layerstime records of ~ 100 - 10,000 years

Corals

-- organisms that live in shallow ocean water

-- secrete annual carbonate layerstime records of ~ 10 - 1,000 years

Some tree ring, coral, and ice core sample locations

Historical records

-- info about climate that was recorded by peopletime records over ~ 1000 years

Instrumental records

-- info on climate (e.g. temperature) recorded by direct measurementtime 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 frommagma (molten rock), but radiocarbon datescan be obtained for organic material insedimentary materials

What are isotopes?

-- atoms that vary in mass but have the same chemicalbehavior (i.e., same element, different isotopes)

8 protons8 neutrons

8 protons9 neutrons

8 protons10 neutrons

16O 17O 18O

Example: there are 3 stable oxygen isotopes

But not all isotopes that exist in nature are stable(some undergo radioactive decay)

This changes the number of neutrons or protonsin the nucleus of the atom(can get different element as result)

Example: Carbon has 3 isotopes

12C - contains 6 protons, 6 neutrons - stable13C - contains 6 protons, 7 neutrons - stable14C - contains 6 protons, 8 neutrons - unstable (radioactive)

Carbon-14 decays to Nitrogen-14

Parent isotope 14C has decayed to daughter isotope 14N.

14C (6 protons, 8 neutrons) --> 14N (7 protons, 7 neutrons)

So: how do we use radioactive decayto date something?

So: how do we use radioactive decayto date something?

Answer: If we know the rate of decay andcan measure the amount of parent anddaughter isotopes, we can calculate the timeelapsed.

Half-life = the amount of time needed totransform 1/2 of the parent into thedaughter 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 eventseven if absolute age not known

-- use in geologic outcrops where cross-cuttingrelationships or distinctive features are seen

in sedimentary rock layers, the layer on top isthe 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

Geologicprinciples

Relativeages (oldestto youngest):

igneous 1sed layer Aigneous 2?sed layer Bigneous 3sed layer Cigneous 4?

<- lava flow dated at 3.6 my ->

<- lava flow dated at 4.2 my ->

lava flow dated at 3.8my ->

To get ages of rocks that cannot be radiometricallydated, 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 andbelow by datable lava flows.

How old is the green dot?

<- lava flow dated at 3.6 my ->

<- lava flow dated at 4.2 my ->

lava flow dated at 3.8my ->

3.9 + 0.3 m.y. 3.7 + 0.1 m.y.

The date on the right hand side is a more precise date.

(3) Counting annual layers

-- relative dating technique

-- can be turned into an absolute age if additional info known (e.g., if one knowswhen layers started or stopped forming)

Timespan & time resolution

-- different climate archives give info ondifferent timespans

timespan: largest time unit we can measure

-- vary also in time resolution

time resolution: smallest time unit we canmeasure

archive resolutionrelated to the timespan:

longer time spanarchives tend tohave worse (larger)time resolution

...and vice versa

sediments give usour oldest record ofclimate: oldestsedimentary rocksare ~ 3.5 b.y. old

oldest ice core layersfrom Antarctica:~400,000 years old

General circulation models (GCMs)

• these are 3-d computer models that provide acomplete 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 realresponses

Steps in models:

subdivideclimatesysteminto smallerpieces

analyze howthese interact

Observed

Model

Januarysurfacetemperature