Thermal Gradients Part 1 : Starting on Land with the Familiar
of the most important features of the continental margin that sets them
aside as a special part of the ocean is the presence of great
gradients. Many environmental properties that are quite important
to life change more rapidly on the margins than i any other part of the
ocean. In later lectures we will consider important factors such
at the oxygen levels and the availability of food for bottom animals.
This first lecture is limited to those gradients which are caused
by the manner in which the Sun heats the Earth and the ways that air
and water respond to that heating. Anyone with any familiarity
with geography appreciates that the polar regions near the equator are
hot, and the polar regions at the top and bottom of the Earth are cold.
The broad patterns of plant and animal life reflect this global
thermal gradient. Somewhat less familiar is the fact that the
atmosphere becomes colder with increased elevation. This vertical
temperature gradient also causes zones of distribution that mimic those
caused over a much larger area by the global gradient. Finally,
there is the deep ocean that is least familiar of all. It becomes
colder with depth. As we try to understand the ecology of the
continental margins, we must ask whether this bathymetric thermal
gradient can be as biologically important as the elevation gradient.
First, however, we need to understand the causes of thermal
gradients both with depth, elevation, and latitude.
Local Mountain Elevation - Why it gets cold going up.
order to understand why environmental gradients are so important
to life on the continental margins it is useful to start with examples
familiar to many land-dwelling people. We begin with mountains.
Standing on lowland gazing at distant mountains the effects of
elevation on biology is readily apparent. We can see lowland
vegetation with its characteristic colors spreading part of the way up
the slopes. Above that there is a darker band of green marking
the presence tress in mountain forests. This darker tree zone
terminates fairly abruptly higher on the slopes as a distinct tree
line. Higher yet there is a nearly treeless zone of low
vegetation and rock termed the alpine zone. Closer towards the
summit is the Arctic-like white snow cap. The causes of species
distribution on land or in the ocean are complicated and often involve
many ongoing and historical processes. What you observe on a
mountain, however, is due on very large part to the change in air
temperature as elevation increases. Mountains along with their
biota are wrapped in air that becomes progressively cooler the higher
above sea level the elevation goes. Large trees are generally
absent at elevations where the summer daytime temperatures are below
temperature is a very good example. This 5,892m high volcano is
located near the equator at one of the warmest parts of the Earth.
In the southern-hemisphere summer, however, freezing conditions
and snow may be encountered at an elevation of about 5000m.
Summit temperatures may reach 5° C during the day but plunge
to -20° C during the night. The plant and animal life are
greatly impacted by these extreme conditions for an equatorial region.
The reason that air temperature decreases with altitude is because
of the manner in which the Sun warms the planet and the physical
response of the atmosphere to that warming. The mixture of major gases that make up the atmosphere are highly transparent to
the sunlight reaching the planet, which means that light passes
through the atmosphere without warming the air very much. When
this light reaches the opaque land, most is absorbed and converted into
heat. Air in contact with this warm ground is then heated.
We could just stop here and say that it makes sense that air gets
cooler as you go up since you are actually geting away from the source
of atmospheric warming. Fortunately, the physics behind the
cooling are not overly complicated. Warmed air expands becoming
less dense and rises in a process
called convection. The stirring of the atmosphere caused by convection creates the troposphere,
a distinctive layer of air reaching from the ground up to about 17km at
the equator and 7km at the poles. The higher up in the
troposphere that warm air rises, the more it cools..
The rate at which air cools with altitude within the troposphere is called the environmental lapse rate. The
cooling is due to two processes. One is associated with the
expansion of gas as atmospheric pressure decreases. The other is
associated with the conversion of water vapor contained in air into
liquid water droplest. As dry air rises it
expands but keeps the same amount of heat energy. Since that
energy is spread out through a larger volume, it is diluted and
temperature drops at less
at a rate of about 9.8°C/1000m of increased elevation.
If the rising air is saturated with water vapor the lapse rate is
slower at about 3.0°C/1000m. Rather than worrying about
details such as wet and dry air, the International Civil Aviation
Organization (ICAO) has defined an international standard atmosphere
with a lapse rate of 3.56°C/1000m from sea level up to 11km, higher
that Mt. Everest at 8,848m. With dry air the base of a tropical
5000m mountain might be a blasting hot 40° C but freezing on
top. An arctic or antarctic mountain the same height on a very
warm day could be 0°C at the base and a deeply cold -37°C on
top. We will discover as we examine the ocean, seawater shows
nowhere nere such great temerature extremes.
The Global Thermal Gradient and the Curvature of Earth
One of the earliest observation made by exploring biogeographers
was that the climate change found relatively short distances up
mountain slopes was similar to that found by moving many kilometers
over lowland towards the poles of the Earth. This effect of latitude is
very obvious when the treeline on mountains is observed. On
tropical mountains the treeline may be has high as 4000m. On a
boreal mountain (locateded between 50° and 60° latitude)
the treeline may be as low as 2000m. Progressing much north of
65° even the lowland remains cold enough all year for there to be a
permanently frozen layer of ground, permafrost.
The presence of permafrost combines with other harsh climate factors to
effectively prevent the growth of large trees. This zone, the tundra, marks the gobal treeline.
was explained above that mountain air cools with elevation due to the
fact that sunlight heats the ground first. Then the ground heats
the air making it less dense and causing it to rise. Cooling with
latitude is also related to the manner in which the Sun warms the
Earth, but the phyical cause is different. Fortunately it is
relatively simple. Look at the illustration. A band of
sunlight arriving near the equator illuminates and heats a concentrated
spot on the surface of Earth. An identical parallel band of
sunlight arriving at boreal latitudes contains exactly the same amount
of energy. But, when it illumunates and heats the Earth it is
spreadout over a much larger area. It is a matter of simple
geometry. A bean of light shining on a surface at an angle of
90° transfers energy to a concentrated area. That same light
shining at 60° or less transfers the same energy over a larger
area. The poles are cold becaues the Earth is a sphere receiving
mostly parallel energy from the Sun.
Before We Consider the Ocean
Since the purpose of this lecture is
to introduce the idea that environmental gradients play an especially
important role on the continental margins of the sea, it is not
appropriate to include much more about the more familiar gradients on
land. Terrestrial biogeographers must always consider two
questions about gradients.
- which of the many patterns in species distribution and biodiversity
that we observe going up mountains are caused to some degree by
temperature-related elevation gradients?
which of the many patterns in species distribution and biodiversity
that we observe between equator and the poles are casued to some degree
by tem[perature-related latitude gradients?
Ocean scientist can and should ask similar questions about marine life.
To what extent is the heterogeneity of ocean life explained by
the physical heterogeneity of the ocean itself? On land in
the sea, however, the question of cause must be answered cautiously.
Even when causes are physical many gradients may coincides in
such a manner as to confound the answer. Especially important,
species interact so intensively and distributions are so influenced by
historical events, that patterns with simple physical causes must be
the exception rather than the rule.