Transcript Document

Meteo 1020 – Lecture 4
The Natural Variability of the Earth-Atmosphere System
• We have discussed evidence that suggests the earth is
warming due to human outgassing of carbon dioxide
• Could we be wrong? Consider the major criticisms of the
global warming hypothesis:
• Instrumental error
• Urban heat island
• Others
• A potential cause of the present warming may be due to
natural variability in the climate system.
Meteo 1020 – Lecture 4
The Natural Variability of the Earth-Atmosphere System
•
Natural variability can be classified by its forcing type (internal or
external to the climate system) and whether it is periodic/cyclic or
episodic.
Modes of Natural Variability that we know of:
timescale
1. Seasonal
90 days
2. El nino
3-5 yrs
3. North Atlantic Oscillation
decadal
4. Volcanism
none
5. Ice ages
40,000 yrs
6. Pacific Decadal
20-50 yrs
form forcing
cyclic external
cyclic internal
cyclic internal??
episodic external
cyclic internal
cyclic ????
http://sealevel.jpl.nasa.gov/science/images/el-nino-la-nina.jpg
Meteo 1020 – Lecture 4
The Natural Variability of the Earth-Atmosphere System
•
Natural variability can be classified by its forcing type (internal or
external to the climate system) and whether it is periodic/cyclic or
episodic.
Modes of Natural Variability that we know of:
timescale
1. Seasonal
90 days
2. El nino
3-5 yrs
3. North Atlantic Oscillation
decadal
4. Volcanism
none
5. Ice ages
40,000 yrs
6. Pacific Decadal
20-50 yrs
form forcing
cyclic external
cyclic internal
cyclic internal??
episodic external
cyclic internal
cyclic ????
•
North Atlantic Oscillation
The NAO index is defined as the
anomalous difference between the polar low
and the subtropical high during the winter
season (December through March)
The Negative NAO
•
The negative NAO index phase
shows a weak subtropical high and
a weak Icelandic low.
•
The reduced pressure gradient
results in fewer and weaker winter
storms crossing on a more westeast pathway.
•
They bring moist air into the
Mediterranean and cold air to
northern Europe
•
The US east coast experiences
more cold air outbreaks and hence
snowy weather conditions.
Positive NAO Index
•
The Positive NAO index phase
shows a stronger than usual
subtropical high pressure center
and a deeper than normal Icelandic
low.
•
The increased pressure difference
results in more and stronger winter
storms crossing the Atlantic Ocean
on a more northerly track.
•
This results in warm and wet
winters in Europe and in cold and
dry winters in northern Canada and
Greenland
•
The eastern US experiences mild
and wet winter conditions
Meteo 1020 – Lecture 4
The Natural Variability of the Earth-Atmosphere System
•
Natural variability can be classified by its forcing type (internal or
external to the climate system) and whether it is periodic/cyclic or
episodic.
Modes of Natural Variability that we know of:
timescale
1. Seasonal
90 days
2. El nino
3-5 yrs
3. North Atlantic Oscillation
decadal
4. Volcanism
none
5. Ice ages
40,000 yrs
6. Pacific Decadal
20-50 yrs
form forcing
cyclic external
cyclic internal
cyclic internal??
episodic external
cyclic internal
cyclic ????
Volcanic eruptions and climate:
•
The present atmospheric
composition, Nitrogen-78%,
Oxygen-21%, Argon-<1%, water
vapor-0.4%, carbon dioxide0.036%
•
How did the present atmospheric
composition evolve?
•
Assume outgassing from early
volcanoes provided the first
atmosphere.
•
Composition of volcanic gasses:
Water vapor-80%, Nitrogen-1%,
Oxygen-0%, carbon dioxide-12%,
sulphur compounds and others-7%
Volcanism and the early atmosphere:
So how did the atmosphere evolve from the volcanic composition to
our present composition?
1. Water began to precipitate early – forming oceans
2. Carbon Dioxide dissolved rapidly in the early oceans reaching
saturation and leading to precipitate of Calcium carbonate to the
deep ocean
3. Nitrogen and argon built up slowly since it does not dissolve in sea
water
4. Oxygen built up in the atmosphere due to by product of
photosynthesis.
Earth’s Atmosphere Develops
http://www.globalchange.umich.edu/globalchange1/current/lectures/first_
billion_years/first_billion_years.html
The climate effects of volcanic eruptions:
What makes an eruption climatically significant?
•
nature of the eruption – lava vs. ash (ash is more significant)
•
composition – need high sulfur dioxide gas content
•
location – Tropical eruption spread globally
In most eruptions, the particulates have only a minor effect. If the
sulfur dioxide gas can reach the stratosphere, it converts to small
sulfuric acid droplets that have long residence times in the stable
stratosphere. It is this cloud of particles that spread and influence
climate over long periods.
Note Figure 4.3 in the book.
•
Natural variability can be classified by its forcing type (internal or
external to the climate system) and whether it is periodic/cyclic or
episodic.
Modes of Natural Variability that we know of:
timescale
1. Seasonal
90 days
2. El nino
3-5 yrs
3. North Atlantic Oscillation
decadal
4. Volcanism
none
5. Ice ages
40,000 yrs
6. Pacific Decadal
20-50 yrs
form forcing
cyclic external
cyclic internal
cyclic internal??
episodic external
cyclic internal
cyclic ????
Pacific Decadal Oscillation
Pacific Decadal Oscillation
warm phase
cool phase
How does ocean circulation affect local climates?
Answer: Heat release locations are warmer!
Warm surface current- Less Dense
Cold deep water current–More Dense
Salty water anywhere- More Dense
Intergovernmental Panel on Climate Change (IPCC), "Climate Change 2001: The Scientific Basis"
What directions do warm and cold
water currents travel?
Animation by Jack Cook (Woods Hole Oceanographic Institute)
Is it true that the North Atlantic
current could shut down?
The ocean surface transfers heat to the atmosphere!
Animation by Jack Cook (Woods Hole Oceanographic Institute)
Meteo 1020 – Lecture 4
The Natural Variability of the Earth-Atmosphere System
Air-Sea Interactions:
Ocean and atmosphere communicate with one another:
1. constituents: water (precipitation and evaporation), carbon
dioxide, oxygen, trace gasses, etc
2. Energy – momentum through wind stress driving the surface
currents
3. Heat – sensible heat and latent heat due to evaporation
Ocean structure: warm and relatively fresh mixed layer lying on top
of a nearly isothermal cold and salty water mass
Meteo 1020 – Lecture 4
The Natural Variability of the Earth-Atmosphere System
The global thermohaline circulation: Cold Salty water in the
north Atlantic becomes dense and convects downward, spreads
southward and contributes to vertical overturning of deep ocean
water on millennial timescales.
Importance: Climate of northern Europe and Asia rely on heat
and moisture supplied to atmosphere to keep climate habitable
in extreme northern latitudes.
Meteo 1020 – Lecture 4
The Natural Variability of the Earth-Atmosphere System
Even though the oceans and atmosphere are both fluids, they have
a fundamental difference – their density: Atmosphere 1 kg/cubic
meter, Ocean – approximately 1000 kg/cubic meter.
This density difference leads to large difference in heat capacity.
Heat capacity is defined as the amount of temperature change in
kelvin degrees for a unit input of heat energy.
The ocean’s heat capacity is approximately 41 times that of the
atmosphere.
A 1 degree change in atmospheric temperature is equivalent to an
0.02 change in ocean temperature change.
Meteo 1020 – Lecture 4
The Natural Variability of the Earth-Atmosphere System
El Nino is an example of air-sea interactions that occur on annual
time scales
Note figures 4.8 and 4.9:
Sea surface temperature (SST) patterns change fundamentally
with the el nino cycle
The atmosphere both forces and responds to the el nino cycle.
Normal sst patterns: atmospheric forcing of sst is accomplished
through easterly trade winds cooling the central and eastern
equatorial pacific. Atmosphere responds to warm western pacific
water by the occurrence of strong thunderstorms in that region
Meteo 1020 – Lecture 4
The Natural Variability of the Earth-Atmosphere System
An el nino is characterized by a general warming of the water in the
central and eastern pacific.
Atmosphere forces the ocean by a weakening of the easterly trades
in the central and eastern pacific.
Atmosphere responds by shifting thunderstorm activity eastward to
the central pacific.
Implications: Interruption of fishery along western margins of
South America, failure and/or weakening of the Indian Monsoon
impacting agriculture in that region.
Meteo 1020 – Lecture 4
The Natural Variability of the Earth-Atmosphere System
El nino remote forcing: the modulation of the normal patterns in
the equatorial pacific influence weather patterns around the
world:
In the U.S., the pacific storm track is shifted southward leading
resulting in storminess over the southwestern U.S. and droughts
over the Pacific Northwest. Precipitation is often reduced over
the southeastern U.S.
What process could change the
ocean circulation?
If air from the upper atmosphere is brought down to the surface
in a hurricane, will it freeze us?
Closer to the Earth’s surface, the air pressure is higher than higher up in the
atmosphere. As we bring the air down to the Earth the air pressure increases. As we
apply pressure to air, air warms instantaneously!
Bring air which is -80º C from 15km (tropopause) down to the surface
1. Temperature change because of pressure changes
15km*9.8º/km(for dry air)= +147ºC
– Original temp + temp change = temp at surface
-80ºC+147ºC=67ºC
– Change to Fahrenheit
67ºC*(9/5)+32=152.6ºF
Can ice sheets melt and build
overnight?
– Ice Sheets and Glaciers
• Immense mass prevents rapid
melting of ice sheets.
– Ex. Climate quickly changed at the
end of last ice age
» Decades and centuries later the
ice sheets and deep ocean
adjusted. (NSIDC)
• The major ice sheets over North
America at the peak of the last
ice age took tens of thousands of
years to build up. (NASA)
http://daily.greencine.com/archives/day-after-tomorrow.jpg
What process could most likely change the climate?
More moisture in
the air
Warmer climate
Circulation slows down
and changes
More evaporation at the equator
More moisture can
be held in the air
Water does not sink in the northern latitudes
Melting Glaciers
Ocean is not as
salty and dense
More freshwater
in the ocean
More rain
at higher
latitudes
What is happening with ice shelves
today?
• Today shifting
climate
http://images.amazon.com/images/P/B0007PALRU.01.LZZZZZZZ.jpg
– Antarctic Peninsula
the ice shelves are no
longer stable. The
northern most limit
for ice shelves has
moved south over the
past two decades by
about 100 miles.
(NSIDC)
Three Glaciers Retreating
Denver Glacier in Recession,
Alaska, British Columbia
1912
• Source:
C.L. Andrews. 1912, 1938. Denver Glacier: From
1938
the Glacier Photograph Collection . Boulder, CO: National
Snow and Ice Data Center. Digital Media; Marion T.
Millett. 1958. Denver Glacier: From the Glacier
Photograph Collection . Boulder, CO: National Snow and
Ice Data Center. Digital Media.
How do we know about past
climates?
• Ice sheets reveal
annual layers
– history of precipitation
and air temperatures
100,000 years in the
past.
•
Photo by: Lonnie Thompson, Ohio State University
•
http://www.whoi.edu/institutes/occi/images/occi_abrclimate_jk_lev_en.gif
Sea Ice Concentration
Temperature and Ice Accumulation
vs. Time