What causes Earth

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What causes Earths climate and climate change?

• The Ocean

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(No Transcript)
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• Recall that the ocean is a natural thermostat
• annual sea surface temperature variation
• 2 ?C in tropics, 8 ?C in middle latitudes, 4 ?C
  in polar regions
• global average 17 ?C
• releases and absorbs heat over decades to
  centuries, whereas the atmosphere does the same
  but in days to weeks
• Water has a high specific heat
• requires high energy to raise its temperature 1
  calorie (4.18 J) of energy to raise water
  temperature by 1 ?C

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Why is seawater salty?

• Seawater is 96.5 water, the rest is sodium
  chloride (NaCl) (about 3) and other dissolved
  salts
• Why is seawater salty?
• land sediments carried by rivers into oceans
• 2.5 billion ton per year
• dissolved cations (Na, Mg2, Ca2, etc.) leached
  from rocks
• anions such as chloride (Cl-) and sulfate (SO42-)
  have accumulated over centuries from gases
  escaping from Earths interior through volcanic
  eruptions

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• less important are dust blown in from deserts and
  anthropogenic pollutants
• Na resides longer in the sea than Ca2 because
  marine animals remove Ca2 to make carbonate
  skeletons
• Na is removed by adsorption to clay minerals but
  slow process

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The composition of seawater
E.A. Mathez, 2009, Climate Change The Science of
Global Warming and Our Energy Future, Columbia
University Press.
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• Addition of salt into water decreases freezing
  point (-1.9 ?C instead of 0 ?C) and increases
  density (1.026 g cm-3 instead of 1.0 g cm-3)
• In oceans, increased temperature decreases
  density
• Processes that alter salinity
• evaporation removes water, so increases salinity
• precipitation or influx of fresh river water
  decreases salinity
• freezing removes water, so increases salinity
• melting of ice adds water, so decreases salinity
• salinity of oceans varies from place to place

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• Salinity is low at equator and at poles because
  of high precipitation (equator) and low
  evaporation (poles)
• also at mouths of large rivers
• High salinity at semiclosed seas in arid regions
• Persian Gulf, Red Sea, and Mediterranean Sea

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Ocean depths

• Density
• increases with higher salinity
• increases with lower temperature
• So, deep water is typically denser, colder, and
  more saline than shallow water
• Ocean is stratified by density into 3 major
  layers
• 0-20 m thin warm surface layer
• called a mixed layer because it affected by waves
  and temperature changes gt rapid mixing/changes
• 20-500/900 m thermocline
• zone where temperature and salinity change
  rapidly with depth
• depth/thickness varies from location to location
  and season to season

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• below the thermocline
• called the deep zone
• slight variation in temperature and salinity
• 65 of ocean water is in this layer
• but in winter at high latitudes, thermocline can
  extend all the way to the ocean bottom (like in
  Norwegian-Greenland Sea)

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Vertical profiles of density, temperature, and
salinity through the upper several hundred meters
of the ocean
E.A. Mathez, 2009, Climate Change The Science of
Global Warming and Our Energy Future, Columbia
University Press.
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The average annual salinity of ocean surface
water, 2005
E.A. Mathez, 2009, Climate Change The Science of
Global Warming and Our Energy Future, Columbia
University Press.
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A conductivity, temperature, and depth
measurement device
E.A. Mathez, 2009, Climate Change The Science of
Global Warming and Our Energy Future, Columbia
University Press. Photograph by E.A. Mathez
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North Atlantic Deep Water (NADW)
Antarctic Bottom Water (AABW)
The global ocean conveyor system
E.A. Mathez, 2009, Climate Change The Science of
Global Warming and Our Energy Future, Columbia
University Press. Source IPCC, 2001
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The global ocean conveyor system

• The global ocean conveyor system is also known as
  Thermohaline Circulation (THC)
• This circulation is driven by differences in
  density of sea water, which is controlled by
  temperature and salinity
• generally less than 0.1 ms-1
• overturns entire ocean depth every 100-1000 years
• Begins with the downwelling of water in the North
  Atlantic and Southern Ocean
• water flows to and wells up in the Pacific Ocean
  and flows as shallow water to replace the
  downwelling water
• This system exerts/moderates a stabilising
  influence on global climate for hundreds to
  thousands of years
• but can change abruptly as well

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• Warm, near surface water forms in the Atlantic
  Ocean at about 35 ?N and flow northward at a
  depth of about 800 m
• In the north, the water sinks because it loses
  heat to the atmosphere and being now cold and
  more dense, it sinks, and starts to flow
  southward, all the way to the Southern Ocean
• North Atlantic Deep Water (NADW)
• Water in the Antarctica rises because the seas
  here are less dense, but sinks again as the NADW
  are cooled again
• Water wells up at less salty, warmer, and
  shallower Indian Ocean and Pacific Ocean

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• Deep water forms in North Atlantic, rather than
  North Pacific because North Atlantic is saltier
  (by 5) than North Pacific
• more precipitation in North Pacific than North
  Atlantic (which has higher evaporation)
• the global conveyor system acts to redistribute
  the salt to correct this salt imbalance between
  these two oceans
• Northward water flow into the North Atlantic
  brings enormous amount of heat, equivalent to 30
  of annual solar energy, to Europe
• even though Europe is at the high latitudes, its
  weather is mild because of the conveyor system
  that brings heat here from the equator

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Effect of global warming

• More ice from the Arctic melting, adding
  freshwater into the North Atlantic
• Retreating ice cover exposes more of the ocean
  surface, allowing more moisture to evaporate into
  the atmosphere and leading to more precipitation
  (rain and snow)
• So, the increased freshwater into the North
  Atlantic increases the buoyancy of the ocean and
  makes it harder more the warm water from the
  equator to sink to the bottom
• hence, NADW might slow down or stop!
• ironically, causing global cooling (ice age,
  perhaps?)
• average Europe temperature might fall 5-10 ?C
  (colder)

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• Mediterranean Sea has also high salinity and its
  water flows into Atlantic Ocean, making it
  saltier by 6
• increased use of freshwater means less freshwater
  flowing from rivers into the Mediterranean Sea,
  causing higher salinity in both the Mediterranean
  Sea and Atlantic Ocean

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The water balance of the continents and oceans
Region Evapotranspiration Precipitation
Runoff (in
millimeters/year) Europe/Asia 795 1353 558 Afri
ca 582 696 114 North America 403 645 242 So
uth America 946 1,564 618 All
land 480 746 266 Atlantic Ocean 1,133 761
372 Indian Ocean 1,294 1,043 251 Pacific
Ocean 1,202 1,292 90 All oceans 1,176 1,066
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E.A. Mathez, 2009, Climate Change The Science of
Global Warming and Our Energy Future, Columbia
University Press. Source Hartmann, 1994
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Ocean surface currents

• Ocean currents driven by winds
• their interaction with the atmosphere have
  important consequences for both climate and
  weather
• confined mostly to the upper kilometre or two of
  the ocean
• typical speeds
• horizontal flow or currents are 0.01-1.0 ms-1
• vertical speeds within the stratified ocean are
  0.001 ms-1
• Ocean gyres correspond nearly to the wind gyres
• rotate clockwise in the Northern Hemisphere
• rotate counter clockwise in the Southern
  Hemisphere

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• Continents positions affect wind gyres,
  deflecting them into boundary currents flowing
  poleward, parallel to coastlines
• Gulf Stream in North Atlantic
• Kuroshio Current in North Pacific
• Brazil Current, along coats of South America
• others such as East Australian and Mozambique
  Currents

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http//www.crd.bc.ca/watersheds/protection/geology
-processes/globaloceancurrents.htm
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• Boundary currents are important because
• they carry heat from the equator to the north,
  making the weather in the north milder
• Gulf Stream can carry heat from the equator to
  the mid latitudes in a month (mean flow 100 mil.
  m3 of water per second)
• they carry water vapour
• they help to remove CO2 from the atmosphere
• warm waters has less CO2 than colder waters

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Upwelling and downwelling Ekman transport
In the Northern Hemisphere
Deeper waters are richer in nitrates and
phosphates supports growth of plankton and, in
turn, fish, such as occurring in Peru and Ecuador
The food chain Phytoplankton ? Zooplankton ?
Predatory zooplankton ? Filter feeders ?
Predatory fish
http//www.crd.bc.ca/watersheds/protection/geology
-processes/globaloceancurrents.htm
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Upwelling zones
http//www.absoluteastronomy.com/topics/Upwelling
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El Nino and La Nina

• Every several years, coastal areas in Peru and
  Ecuador experience rapid warming, increasing from
  -2 ? to 4 ?C in a month
• usually occurs around Christmas and lasts until
  May/June
• anchovies and sardines disappear
• birds, fur seals and other animals that depend on
  fish die
• unusual heavy rains in coastal areas, and
  droughts in Andes in south Peru and northeast
  Brazil
• caused by El Nino (boy child or Christ child in
  Spanish)

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• El Nino is not a local phenomenon
• global affects weather around the world
• severe droughts in Australia, Indonesia, southern
  Africa, and Egypt
• milder weather (but stormier winter) in North
  America
• El Nino cycle/occurrence is pseudo-periodic
  cycle not consistent, but usually every 2 to 7
  years
• El Nino is caused by a change in atmospheric and
  ocean circulation across the entire equatorial
  Pacific Ocean

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NOAA/PMEL/TAO
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• In normal years, easterly winds (from east to
  west) in the Northern and Southern Hemisphere
  converge along the equator, blowing westwards
  (toward west)
• this causes the usual upwelling in coastal areas
  such as Peru and Ecuador
• the westward flowing surface water piles up in
  the western Pacific, causing sea level to rise
  about 60-70 cm higher than in the eastern Pacific
• the upwelling brings the thermocline nearer in
  the east, while in the west, the thermocline is
  much deeper

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• In the mean time, the warm, moist air in the
  western Pacific rise, causing rain, and warm
  airs place is replaced by easterly winds
• a convection circulation that starts with rising
  air in the west, flowing toward the east Pacific,
  then sinking in the east Pacific, then flows back
  to the west Pacific

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• In an El Nino year, this coupled air-ocean
  circulation cycle is reversed
• easterly winds weaken
• warm Pacific water flows toward east, sea level
  flattens out (less difference between sea level
  at the west and east Pacific)
• upwelling in coastal areas at South America stops
• thermocline along the equator deepens to tens of
  meters in the east
• equatorial undercurrent stops
• Warm air in the east Pacific rises, causing heavy
  rains in coastal areas in east Pacific
• but no rain in west Pacific, causing drought

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• Changes in circulation patterns are caused by
  changes in atmospheric air pressure
• Normal years high pressure at east Pacific, low
  pressure at west Pacific
• El Nino low pressure at east Pacific, high
  pressure at west Pacific
• this swing in air pressure is known as the
  Southern Oscillation
• El Nino is an ocean phenomenom, but Southern
  Oscillation an atmospheric phenomenom
• but the two (ocean and atmosphere) are coupled to
  cause El Nino, they are known together as ENSO
  (El Nino-Southern Oscillation)

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Atmospheric pressure Normal years low at
Darwin, high at Tahiti, causing easterly
(east-to-west) trade winds El Nino year high at
Darwin, low at Tahiti, weakening easterly winds
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• The Southern Oscillation Index (SOI) measures the
  change in air pressure measured between the
  eastern (Tahiti) and western (Darwin) Pacific
• ve SOI means
• Darwin lt Tahiti
• air pressure is lower in the west, higher in the
  east
• normal years (high index)
• La Nina (girl child in Spanish) (a very high
  index)
• -ve SOI means
• Darwin gt Tahiti
• air pressure is higher in the west, lower in the
  east
• El Nino year (low index)

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http//www.pmel.noaa.gov/tao/elnino/faq.html
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2nd strongest period
strongest period
Climate variability and the global harvest
impacts of El Niño and other oscillations on
agroecosystems by Cynthia Rosenzweig and Daniel
Hillel, Oxford University Press, 2008
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Climate variability and the global harvest
impacts of El Niño and other oscillations on
agroecosystems by Cynthia Rosenzweig and Daniel
Hillel, Oxford University Press, 2008
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El Nino has shown to give low grain yield in
South Asia and Australia, but high grain yield in
the North American prairies
Climate variability and the global harvest
impacts of El Niño and other oscillations on
agroecosystems by Cynthia Rosenzweig and Daniel
Hillel, Oxford University Press, 2008
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El Nino of 1982-83 on Brazil
Climate variability and the global harvest
impacts of El Niño and other oscillations on
agroecosystems by Cynthia Rosenzweig and Daniel
Hillel, Oxford University Press, 2008
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Long term effects

• El Nino and La Nina, however, has no effect on
  global warming in the long run
• effects only on the short term
• droughts or heavy rains will give poor yields in
  a short period
• or may give good yields in some cases such as
  more rains in usually dry areas
• El Nino and La Nina cancel each other out to give
  zero net change over the long run