Transcript Part-1

Recent Climate Changes
and Climate Archives
Martin Visbeck
DEES, Lamont-Doherty Earth Observatory
[email protected]
Outline
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Review of Ocean Stratification and
Circulation
•
Recent historical Climate Change
External Climate Forcings
Natural Climate Variability
Paleoclimatology
Ice Ages
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General Atmosphere Ocean Circulation
The surface energy balance
Top of
atmosphere
seafloor
Imbalance of energy flux at the top can
be balanced by:
Atmospheric Heat Transport
Oceanic Heat Transport
Air-sea
interface
Sea Surface Temperature
•
The link between ocean and climate depends on exchange of energy (mainly
heat and radiation) and materials (water, gases) across the sea surface. T
atmosphere 'sees', influences and responds to the sea surface temperature
(SST), by way of sea-air heat flux. SST generally cools with increasing latitude,
but important deviations from a pure latitudinal dependence occurs. These are
generally due to the movement of sea water in both the horizontal and vertical
directions. Temperature and density of ocean water are related inversely: warm
water means low density, cold water means denser sea water.
Ocean Salinity
• As the range of salt concentration in the ocean
varies from about 3.2 to 3.8%, oceanographers,
who refer to salt content as 'salinity', express salt
concentration as parts per thousand; 34.9 ppt is
Effect of Atmospheric Forcing
on Ocean
Wind Driven Ocean Circulation
Ocean simulation in tank
light
Cool water
Buoyancy Driven Ocean
Circulation
•
The buoyancy forces are capable
of inducing overturning that reach
from the sea surface to the sea
floor.
•
Buoyancy fluxes are those fluxes
between air and water that alter
the density of the sea water.
•
Cooling of the ocean and
evaporation makes the ocean
(colder, saltier) denser, removing
buoyancy.
•
Heating and excess precipitation
has the opposite effect, they add
Overturning
• North Atlantic
Deep Water
• Antarctic Bottom
Water
• Intermediate
Water
Oceanic Heat Transport
The Oceans Role in Climate
The sum of the wind
driven and buoyancy
driven ocean current
transport large amounts
of heat and fresh water
over large distances.
Can you rationalise the
signs?
Recent Climate Changes
and predicting the future
1.What factors
influence
climate
change?
2.On what time
scales do
these factors
operate and
what controls
the time
Review of planetary energy
balance
A. Remember that in equilibrium, absorbed solar energy equals
emitted heat.
B. Absorbed solar energy depends on solar constant (intensity of
Sun at Earth's distance) and planetary albedo (fraction of
incident sunlight reflected)
C. Emitted heat depends on temperature at which Earth radiates to
space; difference between this temperature and surface
temperature is an indicator of the greenhouse effect, which
depends on the concentration of greenhouse gases.
D. Climate change occurs when either side of energy balance is
perturbed.
Climate change occurs when either
side of energy balance is perturbed.
Example 1:
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Increase greenhouse
gases ->
•
decrease IR radiation to
space ->
•
absorbed solar exceeds
emitted thermal ->
•
temperature must increase
to restore balance.
Climate change occurs when either
side of energy balance is perturbed.
Example 2:
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Increase planetary albedo ->
•
decrease absorbed solar ->
•
emitted thermal exceeds
absorbed solar ->
•
temperature must decrease
to restore balance.
•
In general real climate
changes involve changes of
both sides of the energy
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
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
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
Recent historical climate change
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
Recent historical
climate change
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.
Notice the regional difference
between tropics and northern
and southern hemisphere.
Recent historical climate change
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?
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).
II. Natural variability.
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
Solar luminosity variations
Sunspots are dark,
decrease luminosity,
but are surrounded by
bright faculae which
cover a larger area;
thus, Sun is brightest
at peak of sunspot
cycle
Satellite observations
since 1980 indicate
that solar luminosity
oscillates slightly with
the 11-year sunspot
cycle.
Solar luminosity variations
Larger long-term variations
may explain Little Ice
Age (Maunder minimum),
but mechanism is not
understood (related to
sunspot number, cycle
length); unknown
potential contributor to
future climate change.
Solar luminosity variations
Larger long-term variations
may explain Little Ice
Age (Maunder
minimum), but
mechanism is not
understood (related to
sunspot number, cycle
length); unknown
potential contributor to
future climate change.
Volcanic
Eruptions
out of
affect
Large ash and dust particles fall
atmosphere quickly, do not
climate.
Climate impact is favored by
(a) material reaching stratosphere, above altitude of
scavenging by rain,
(b) small particles, which fall out slowly; under these conditions,
volcanic aerosols can reflect sunlight (increase albedo) for
several years and cool climate.
Small particle formation: Injection of sulfur-bearing gases
(e.g., SO2) into stratosphere, photochemical reactions form
small sulfuric acid (H2SO4) droplets.
Volcanic Eruptions
Not all volcanoes affect climate: Mt. St. Helens (1980) exploded
sideways sulfur-poor, thus no climate impact; Mt. Pinatubo
(1991) exploded vertically, sulfur-rich, thus biggest climate
impact of 20th Century.
The eruption of Mount Pinatubo presented modern climatologists
with an opportunity to test the reliability of numerical climate
models to predict cooling from a volcanic eruption. They could
estimate the amount of aerosols propelled into the stratosphere
from this eruption, their spread around the world, and eventual
removal from the stratosphere. Using these estimates as input to
the model they asked the model to predict the effect of these
aerosols on our planets mean global temperature. These
estimates are compared with instrumental measurements of
Earth's temperature over a period of several years after the
Volcanic Eruptions
It is clear from this
figure that the model
did a very good job.
This does not mean
that the model would
do as good a job
with other factors
forcing climate
change such as
increasing CO2
input.
About 10 other
volcanoes have
probably affected
Antropogenic (tropospheric)
Aerosols
Anthropogenic (tropospheric)
aerosols include atmospheric
particles and droplets: sulphate,
soot, dust, sea salt.
Some aerosols increase light
reflection/scattering Sulphur
dioxide transforms to sulfuric acid
which attracts water vapor and
condenses to form sulphate
particles that reflect/scatter light.
Sources include combustion,
volcanoes, biology.
SO2 emissions have probably more
than doubled sulfate aerosol
concentration in 20th Century;
systematic upward trend in
albedo (direct effect) may have
offset part of greenhouse
warming; climate models agree
better with observed temperature
Antropogenic (tropospheric)
Aerosols
Anthropogenic (tropospheric)
aerosols include atmospheric
particles and droplets:
sulphate, soot, dust, sea salt.
SO2 emissions have probably more than
doubled sulfate aerosol concentration in 20th
Century; systematic upward trend in albedo
(direct effect) may have offset part of
greenhouse warming; climate models agree
better with observed temperature trend when
aerosols are included.
Antropogenic (tropospheric)
Aerosols
Other types of aerosols, for example soot from combustion and
biomass burning, also are important, but are darker, so reduce
albedo, absorb sunlight, reduce cloud cover due to atmospheric
heating, mayb cause warming.
Additional indirect effect of sulphate aerosols is to modify cloud
properties Increase cloud brightness because aerosols lead to
larger number and smaller size of cloud droplets, causing higher
cloud albedo Increase cloud cover because smaller droplets
inhibit rainfall and increase cloud liftetime.
Antropogenic (tropospheric)
Aerosols
Aerosol forcing
regional in nature
(mostly Northern
Hemisphere, near
and downwind of
industrialized
areas); some
regions may cool
while others warm.
Biogenic regulation of Climate
(GAIA)
Hypothetical example:
Daisyworld (planet populated by white daisies that like warm
temperatures, black daisies that like cool temperatures); if
climate warms, white daisies thrive, albedo increases, limiting
warming; if climate cools, black daisies thrive, albedo decreases,
limiting cooling.
Real world possibility: dimethylsulfide (DMS) emissions by
plankton, leading to sulfate aerosol formation; primary nucleation
source for oceanic clouds; possible impact on climate change,
but DMS dependence on temperature not established.
Natural Variability
A. Short time scales (1-2 years): Random weatherrelated variations of turbulent, chaotic atmosphere
(NAO)
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
change is predictable.