Global Climate Patterns and Life Zone Diversity

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Global Climate Patterns and Life Zone Diversity

• Importance of physical environment
• Causes of global climate patterns
• Temperature
• Rainfall
• Water circulation patterns
• Local modifications of climate
• Global life zone classification
• Major assertion With a few simple physical
  principles, we can explain much of global
  vegetation patterns

2
Species distribution often determined by physical
environment (see lectures on physiological
ecology). Example distribution of coral reefs
limited by water temperature (20º C isotherm of
ocean temperature, coldest month)
3
The next question What determines the
characteristics of the physical environment,
particularly climate air/water currents?

• Air water temperatures?
• Seasonality?
• Rainfall patterns?
• Air water circulation patterns?

4
Insolation (heat input to atmosphere Earths
surface via solar radiation) maximal at equator,
declines to 40 of maximal values at high
latitudes. Insolation drives mean annual
temperatures
5
Two basic causes of greatest insolation at
equator

• Note insolation is different from insulation
• Solar radiation travels shorter distance through
  atmosphere (less absorption, scatter, reflection)
  at equator than higher latitudes
• More direct (maximum 90º angle of input) solar
  radiation (including visible light waves) hitting
  Earth at equator. Thus, Given amount of solar
  radiation illuminates less land (atmosphere) area
  at equator (see next 2 slides).

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Visual illustration of latitudinal gradient of
insolation
7
Moreover, seasonality of insolation arises
strictly because of tilted axis of Earths
rotation (spin) relative to plane of Earths
revolution around sun Insolation peaks N.
hemisphere June 21, in S. hemisphere December 21.
8
Tropic of Cancer (latitude 23.5ºN), Tropic of
Capricorn (23.5ºS) defined by extreme latitudes
at which sun is directly overhead
annually--summer winter solstice, respectively.
This corresponds with 23.5º angle of tilt of
Earth. Thus solar equator (region of maximum
solar input) moves relative to latitude
seasonally.
9
The thermal equator, oscillating latitudinally
with seasons, drives low latitude patterns of
rainfall by establishing zones of low pressure
(high rainfall) and high pressure (low rainfall).
The hadley cell (centered on thermal equator)
depends on convection currents with updrafts that
cause low latitude rainforests, and downdrafts
that cause subtropical hot deserts (20º - 30º N,
S lat.).
10
Major latitudinal displacements of surface air
currents convection currents drive Hadley cells,
pulling air at surface into Inter-Tropical
Convergence Zone, ITCZ) Ferrel Cells driven by
low pressure zone at 20º-30º lat. Midlatitude
westerlies converge into jet stream polar cells
driven by high pressure (cold) flows out of polar
region along Earths surface towards south.
11
The Coriolus Force causes winds moving north or
south latitudinally to deflect to the right in
the Northern Hemisphere, and deflect to the left
in the Southern Hemisphere. This force causes
the trade winds moving from higher latitudes
towards ITCZ to come from northeast direction
north of equator (northeast trade winds) and from
southeast direction south of equator (southeast
trades).
12
Coriolus Effect, using cartoon of Earths surface
dynamics
45º N. latitude circumference 17,000 miles
Air mass (yellow) pulled south (e.g., towards
ITCZ) deflects right relative to Earths surface,
because Earth spinning increasingly rapidly
beneath it
Equator 24,000 mile circumference
Earths spin (west to east) faster at equator,
because of greater circumference traveled per 24
hour day
13
Major patterns of oceanic surface flow at low
latitudes caused by surface drag, caused in turn
by winds. Some specific currents worth
remembering (red cold, black warm)
California Current (7), Humboldt Current (2),
Gulf Stream (13), North Atlantic Current (
Labrador Current 15), Equatorial Counter Current
(4)
14
Ocean currents tend to link up globally into a
giant circulation system, or conveyor belt,
comprised of shallow currents (e.g., Gulf Stream)
and deep currents that tend to be cold, salty
(dense).
15
Oceanic Conveyor Belt Circulation

• Gulf Stream pulls warm, surface water from across
  Southern Africa, Asia
• Where Gulf Stream meets North Atlantic (Labrador)
  Current it is cooled and sinks (aided by its
  saltiness from evaporation of water at lower
  latitudes)
• This sinking current circulates back south and
  east at great depths
• One consequence this current transports heat to
  high latitudes, greatly moderating climate of
  North Atlantic region (e.g., parts northwest
  Europe)
• Global warming could disrupt this current, climate

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Previous slides depict broad, global patterns of
temperature, rainfall, ocean currents what about
more local determinants of climate?

• Rain shadows
• More moisture on windward sides of mountains than
  leeward (e.g., desert areas on southeast side of
  Caribbean Mts., on eastern side Cascades,
  Rockies)
• Adiabatic lapse rate
• Rising air mass (e.g., going up mountainside)
  expands (lower pressure, gas laws), cooling at
  rate of 10ºC (when dry), or at 6ºC (when wet)
  and warms similarly as it falls
• Leads to hotter, drier air in rain shadow side of
  mountain
• Continental vs. marine origin of air mass (next
  slide)

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Third type of local effect, deriving from origin
of air masses Air masses originating in
north tend to be cooler (P Polar), those
originating in south tend to be warmer (T
Tropical). Those originating over land tend to be
dry (c continental), those over water tend to
be wet (m marine).
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The foregoing principles and forces explain much
of the global patterns in vegetation types
(depending on temperature, moisture) Wetter
vegetation (forests) green, drier (grassland,
desert) tan to brown, cold (arctic, alpine) areas
white.
30º N
Equator
30º S
19
30º N. Latitude
Equator
30º S. Latitude
Classification of basic vegetation types biomes
20
Classification of vegetation types partially
based on kinds of plants, which tolerate
different climactic conditions.
21
Holdridges life zone system is one of most
widespread, quantitative schemes for
classification of vegetation, land types
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Holdridges Life Zone System

• Assumptions
• Temperature, rainfall most important factors
  determining ecosystem types (biomes)
• Vegetation assumed to be independent of animals
• Three independent axes
• Mean annual biotemperature average monthly
  temperature for all months with average gt 0ºC
• Annual precipitation (mm), including rain and
  snow
• Potential evapotranspiration ratio ratio of
  potential evapotranspiration ( transpiration
  evaporation) to actual evapotranspiration (which
  is limited by water availability)

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Holdridges Life Zone System, cont.

• Deductions/conclusions
• More types of vegetation found at warmer
  temperatures, because of zero biotemperatures at
  high latitudes
• Deserts occur over wide range of temperatures,
  latitudes
• Rainforests also occur at different latitudes
• Criticisms of Holdridges scheme
• Ignores species interactions (e.g. effects of
  browsers)
• Ignores disturbance types, e.g. fire (prairie,
  longleaf pine)
• Ignores effects of history on soils
• Ignores salinity (salt marsh, mangroves),
  limiting nutrients (serpentine soils), soil
  physical characteristics (pines on porous sandy
  soils)

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Conclusions

• With a few basic physical principles (convection
  currents, radiant energy, gas laws) one can
  explain major patterns of temperature, rainfall,
  seasonality, ocean currents on Earths surface.
• No one ecosystem type dominates globe, but
  instead different types vegetation adapted to
  different climatic conditions
• Classification schemes for vegetation types have
  been developed, of which Holdridges Life Zone
  System is one of best used (particularly at low
  latitudes)