Thermal Gradients Part 1 : Starting on Land with the Familiar

One 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.

ree Line      
In 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 10°C.

Mount Kilimanjaro's 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.

Mt. Kilimanjaro  

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

 Global Tree Line
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.   

Solar AngleIt 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.  

First - 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?

Second - 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.