Transcript Document 469247

Outline
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Review of Ocean Stratification and
Circulation
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Recent historical Climate Change
External Climate Forcings
Natural Climate Variability
Paleoclimatology
Ice Ages
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Recent historical climate change
A. Past 1000 years: evidence from winter severity information, tree
rings, etc. suggests that there was a medieval warm period
about 1000 years ago, then a "Little Ice Age" from about 1400 to
the late 19th Century
B. Past 100+ years: Direct surface weather station measurements
of temperature indicate slowly rising global temperatures from
late 19th Century until about 1940, then weak cooling until
1965, then sharply rising temperatures up to the present
C. Greenhouse gas concentrations have increased steadily since
the beginning of the Industrial Revolution; why has global
temperature not increased monotonically? Can we predict with
confidence that the globe will continue to warm in the future?
D. Must consider other external climate forcings and natural
Recent historical climate change
Must consider other external climate forcings and natural
(unforced, internal, random) variability of climate
system.
I. External climate forcings (other than greenhouse
gases).
A.Solar luminosity variations
B.Volcanic eruptions.
C.Anthropogenic (tropospheric) aerosols include
atmospheric particles and droplets: sulphate, soot,
dust, sea salt.
Recent historical climate change
II. Natural variability.
A.Short time scales (1-2 years): Random weather-related
variations of turbulent, chaotic atmosphere. Global
temperature animation:1971-1999.
B.Interannual (2-8 years): Primarily ENSO; longer time
scale due to interaction of atmosphere with more
massive ocean mixed layer and thermocline.
C.Decadal-to-century scale: Due to changes of
intermediate/ deep ocean circulation and interaction
with atmosphere; unknown magnitude and triggering
mechanisms leave open question of whether climate
Recent historical climate change
II. Natural variability.
A.Short time scales (1-2 years): Random weather-related
variations of turbulent, chaotic atmosphere. Global
temperature animation:1971-1999.
B.Interannual (2-8 years): Primarily ENSO; longer time
scale due to interaction of atmosphere with more
massive ocean mixed layer and thermocline.
C.Decadal-to-century scale: Due to changes of
intermediate/ deep ocean circulation and interaction
with atmosphere; unknown magnitude and triggering
mechanisms leave open question of whether climate
Recent historical climate change
A. Past 1000 years: evidence from winter severity information, tree
rings, etc. suggests that there was a medieval warm period
about 1000 years ago, then a "Little Ice Age" from about 1400 to
the late 19th Century
Why we study past climates
(paleoclimatology)
A. We can measure many
aspects of the climate
system today and, from
these measurements,
understand fundamental
physical and chemical
processes.
B. These measurements can
be used as input to climate
models and the complex
processes and their
feedbacks can be
simulated.
Why we study past climates
(paleoclimatology)
C. We cannot instrument the
past; the complex
atmospheric processes can
not be measured. The
climate system must remain
a black box
D. However, the mysterious
climate system of the past
produces a record and from
this record we learn about
past climate systems.
The kinds of paleoclimatic archives
Proxy Records: naturally record and store climate information.
1.Sensitivity
Best to have present-day analogue for calibration.
Ability to isolate parameter of concern.
Understand reasons for complications.
2.Storage, accumulation.
Sedimentary records.
Growth rings.
3.Dating.
Figure out when things happened.
Are there gaps in the record?
Cross compare with other records, events.
Determine rate of change.
The kinds of paleoclimatic archives
Marine sediments: accumulate slowly (typically 26 cm per 1000 years) but relatively
continuously.
Lake sediments:
Tree rings: annual growth layers in trees and
shrubs.
Peat deposits:
Glacial Ice Cores:
The Milankovitch Mechanism
Pacemaker of the ice ages
A. The Milankovitch or astronomical theory of
climate change is an explanation for the
changes in the seasons which result from
changes in the earth's orbit around the sun. The
theory is named for Serbian astronomer Milutin
Milankovitch, who calculated the slow changes
in the earth's orbit by careful measurements of
the position of the stars, and through equations
using the gravitational pull of other planets and
stars.
The
Milankovitch
Mechanism
There are three main
components to Earth
orbital variability
Eccentricity (100,000 year
period),
Tilt (41,000 year period),
and precession (23,000
and 19,000 year periods).
Only variations in orbital tilt
and precession
significantly affect the
amount of radiation
received during a given
season.
The
Milankovitch
Mechanism
These orbital changes
cause large changes
(up to ±15%) in the
amount of sunlight
received during a given
season.
The key variable is the
amount of summer
radiation at high
northern latitudes.
The climate record
of ice cores
1.Result of snow accumulation
2.Snow contains air
3.Some air gets trapped in bubbles
Bubbles then contain "fossil air."
4.Ice contains water and water contains isotopes of both
hydrogen and oxygen
5.Factors controlling the behavior of hydrogen and
oxygen isotopes
6. Relation of dO18 values to temperature.
The climate record
of ice cores
6. Relation of dO18 values
to temperature.
Then you can
reconstruct time
series of past climate
change...
The climate record of ice cores
Antropogenic CO2 Increase
A. The anthropogenic rise in
CO2 is ~1.5% each year
due to fossil fuel
combustion and
deforestation.
B. The preindustrial CO2 level
was ~280 ppm, it is now
(1998) at 375 ppm By the
time most of you are 30
years old it will be close to
460-470 ppm.
Doubling of atmospheric
CO2 is expected by the
year 2050, perhaps sooner
depending on the emission
scenario which plays out.
Antropogenic CO2 Increase
C. Here is a comparison of the CO2 variations over the full glacial to interglacial
cycle of the last 140,000 years compared to the anthropogenic rise in CO2
just over the past 300 years (Fig. 34).
The climate
record of ice
cores
C. Here is a comparison of
the CO2 variations over
the full glacial to
interglacial cycle of the
last 140,000 years
compared to the
anthropogenic rise in
CO2 just over the past
300 years
Recent historical climate change