Download Climate and life zones

Global Climate Patterns and Life
Zone Diversity
• Importance of physical environment
• Causes of global climate patterns
– Temperature
– Rainfall
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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
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)
The next question: What determines the
characteristics of the physical environment,
particularly climate & air/water currents?
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Air & water temperatures?
Seasonality?
Rainfall patterns?
Air & water circulation patterns?
Insolation (heat input to
atmosphere & Earth’s
surface via solar radiation)
maximal at equator, &
declines to 40% of
maximal values at high
latitudes. Insolation drives
mean annual temperatures
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).
Visual illustration of latitudinal gradient of insolation
Moreover, seasonality of insolation arises strictly
because of tilted axis of Earth’s rotation (spin) relative
to plane of Earth’s revolution around sun: Insolation
peaks N. hemisphere June 21, in S. hemisphere
December 21.
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.
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.).
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
Earth’s surface towards
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).
Coriolus Effect, using cartoon of Earth’s
surface dynamics
45º N. latitude:
circumference: 17,000 miles
Equator: 24,000
mile circumference
Earth’s spin (west to
east) faster at equator,
because of greater
circumference traveled
per 24 hour day
Air mass (yellow) pulled
south (e.g., towards
ITCZ) deflects right
relative to Earth’s
surface, because Earth
spinning increasingly
rapidly beneath it
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)
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).
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
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)
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).
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
30º N.
Latitude
Equator
30º S.
Latitude
Classification of basic
vegetation types = biomes
Classification of vegetation
types partially based on kinds
of plants, which tolerate
different climactic conditions.
Holdridge’s life zone
system is one of most
widespread, quantitative
schemes for classification
of vegetation, land types
Holdridge’s 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 > 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)
Holdridge’s 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 Holdridge’s scheme
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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)
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 Earth’s 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 Holdridge’s Life Zone
System is one of best used (particularly at low
latitudes)