Transcript MES_17Jan_Lecture

G & R CC: January 17
Oceans, Atmosphere,
Land Surface, Cryosphere
• Follow up on questions from 12 Jan
• Lecture: Oceans, Atmosphere, Land Surface, Cryosphere
• Break
• Discussion: Kolbert, Ganopolski et al 1998, Ganopolski and Rahmstorf 2001
• Other sources for discussion of next week’s readings
• Assigment: Out of class writing #1
Bush set for climate change U-turn
Gaby Hinsliff, Juliette Jowit and Paul HarrisSunday January 14, 2007The Observer
Downing Street says that belated US recognition of global warming could lead
to a post-Kyoto agreement on curbing emissions. George Bush is preparing
to make a historic shift in his position on global warming when he makes his
State of the Union speech later this month, say senior Downing Street
officials.
Tony Blair hopes that the new stance by the United States will lead to a
breakthrough in international talks on climate change and that the outlines of
a successor treaty to the Kyoto agreement, the deal to curb emissions of
greenhouse gases which expires in 2012, could now be thrashed out at the
G8 summit in June.The timetable may explain why Blair is so keen to remain
in office until after the summit, with a deal on protecting the planet offering
an appealing legacy with which to bow out of Number 10.
Electromagnetic
Spectrum
Visible light: 400-700 nm
Transmittance and reflectance spectra
for the atmosphere
Solar irradiance
Groupe InfraRouge de Physique Atmosphérique et Solaire
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The spectrum of the radiation
emitted by the sun is close to
that of a black body at a
temperature of 5,900K.
•
8% of the energy is in the
ultra-violet
44% is in the visible region
48% is in the infra-red region
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CH4 CO2
CH4
+
N2O
O3
CO2
• Different GHGs
absorb energy
in different
parts of the
spectrum
• H2O, CO2, CH4,
N2O, and O3
are efficient
GHGs because
of the portions
of the spectrum
they absorb
energy
Mostly IR
Mostly near IR/H2O
Mostly near IR/H2O
Dec-Feb
Jun-Aug
Oceans
•
The oceans influence climate over long and short timescales (seasons to centuries)
•
The oceans and the atmosphere are tightly linked and
together form the most dynamic component of the climate
system
•
The oceans play a critical role in storing heat and carbon
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The oceans’ waters are constantly being moved about by
powerful currents
•
These currents influence the climate by transporting heat
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Currents involved in "deep-water formation" are particularly
important for climate
•
An apparently small change in just one aspect of the oceans’
behavior can produce major climate variations over large
areas of the earth.
The oceans influence climate over
long and short time-scales
• Patterns of ocean circulation and upwelling change over seasons to
centuries.
• These changes result in climate
variations and fluctuations.
• Paleo and instrumental records of
global and regional climate show
periods (years to centuries) during
which the climate was systematically
different from earlier and later periods.
• This behavior is related to changes in
the way the oceans store and
transport heat.
PDO
El Niño
The oceans play a critical role in storing
heat (and carbon)
• When the earth's surface cools or is heated by the sun, the
temperature change is greater - and faster - over the land than
over the oceans.
• The ocean is fluid, so it diffuses the effects of a temperature
change for great distances via vertical mixing and convective
movements.
• One consequence of the ocean's ability to absorb more heat is
that when an area of ocean becomes warmer or cooler than
usual, it takes much longer for that area to revert to "normal"
than it would for a land area.
• This also explains why "maritime" climates tend to be less
extreme than "continental" ones, with smaller day-night and
winter-summer differences.
Currents
Ocean currents 1943,
Currents
• Surface currents are largely wind-driven, but rotation of
the earth, the continents, and the oceans' internal
dynamics also have a strong influence
• Deep-ocean flow (and, to a lesser extent, surface flow)
is driven by density differences produced by heating
and cooling and by precipitation and evaporation (cool
saline water is denser than warm fresh-water).
• The behaviour of the atmosphere strongly affects
these density differences. For example, clouds can
cool the sea by blocking solar radiation or precipitation
can reduce surface salinity. Winds influence
evaporation rates.
Currents influence climate by transporting heat
• Gulf stream and Kuroshio current transport petawatts of energy
from equatorial waters to the poles
• San Francisco (37 N, but has the cool, upwelling California
Current) is about as warm as Dublin (53 N, but is at the business
end of the Gulf Stream)
Currents, "deep-water formation" and climate.
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In winter, surface cooling causes water to become more dense. (Cooled fresh water
starts to expand below 4 C, salt-water compresses down to its freezing point of -2 C.)
In areas where evaporation exceeds precipitation, the resulting rise in salinity also
increases density. When the surface water becomes denser than the underlying water,
"convective overturning" occurs and the dense surface water mixes downwards.
In certain places this downward mixing can extend all the way to the bottom, even in
deep oceans. The dense, deep water thus formed spreads out over ocean floor.
As a result, when downward mixing takes place at high latitudes it creates a circulation
pattern in which warm water from tropical and subtropical regions moves poleward,
surrenders heat to the atmosphere, cools and sinks, and flows back towards the equator.
The net result is a transport of heat poleward.
How to shut down the NADW
• Recent observations, ocean core
records, and some modeling results
indicate that North Atlantic deep-water
formation and its associated ocean heat
flow fluctuate substantially over timescales ranging from years to millennia.
• The system is vulnerable because even
a relatively small decrease in surface
salinity prevents water - no matter how
cold it is - from sinking. This could occur
if there is a flood of fresh-water run-off
from the Arctic due to global warming.
Surface melt on Greenland ice sheet
descending into moulin, a vertical shaft
carrying the water to base of ice sheet. Photo
credit: Roger Braithwaite
Atmosphere
• Solar radiation on a sphere
results in unequal distribution;
more at the equator (where
angle of incidence is about
perpendicular), less at the
poles
• This results in convective
currents because of the “heat
flow” from warm to cool.
• If the earth didn’t rotate, this
would be simple: warm air
rises at the equator, heads to
the poles, sinks as it cools,
and returns.
• Coriolis force breaks this up,
and the dynamical nature of
the atmosphere is complex,
not simple
Atmosphere and Climate
• The ocean/atmosphere system, coupled with the
hydrological cycle, basically completes the circulation
system.
• The main influence of the atmosphere is heat
transport (mostly via water vapor) and to couple the
oceans, land surface, and cryosphere.
• The greenhouse effect and aerosols
Clouds
•
•
Cirrus clouds are composed entirely of ice crystals. They usually
form at 6,000 + meters. Cirrus clouds trap more heat in the
atmosphere than they reflect from the sun, so a change in the
percentage of this cloud type could affect global change
scenarios.
Cumulus, stratocumulus and other low elevation, thicker clouds
are better at reflecting more radiation than they absorb, thus
cooling the surface.
Aerosols
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Direct forcing (reflectance or absorbance) and indirect (through influences
on cloud formation)
Natural and human sources; SO2 sources are coal burning and volcanoes
20th century warming due to GHGs: +2.5 W m-2
20th century cooling due to SO2: -1.3 W m-2
Other particulates/aerosols are implicated in changes in circulation
Geo-engineering: using SO2 to cool the surface while warming the middle
atmosphere
Land Surface
• Vegetation, configuration of
continents, geomorphology
• Surface topography
• Albedo: ice, vegetation
• Gas exchange (water, CO2,
etc)
• Land use change is very
important
MODIS Northern Mexico, southwest US.
Composite of data from April 14 - 22, 2000.
Landsat image of PAD Peixoto 1990 Landsat image of PAD Peixoto 1999, R. Lorena UCL
• http://www.uoguelph.ca/~rmckitri/research/trc.html
• http://www.realclimate.org/
• http://www.climateaudit.org/