A global climate model tries to represent the Earth's climate.
Climate is generically speaking the average weather, or the average
meteorological conditions (temperature, humidity, wind speed, rainfall, etc).
It is also the variation in the meteorological conditions: for example, daily
maximum and minimum temperature, the annual cycle, etc. Climate can be defined
by the statistics at a particular location or for a region or even globally.
Similar climate characteristics of various regions across the globe have
been categorized by the Russian geographer Wladimir Köppen. A simplified map of the K&omul;ppen
classification system can be seen here. His classification system defined climate regions
based on the annual cycle of temperature and precipitation.
Radiation: What Drives the Climate
Global climate is generally only defined by the average surface temperature.
That is determined by how much energy is received by the sun and how much
energy is released by the Earth into space. Both the energy from the sun and
from the Earth are in the form of radiation. Radiation travels
through space in the form of waves. These waves are electromagnetic, meaning
that they are composed of alternating electric and magnetic fields.
The waves can be characterized by the length between the wave crests, or
wavelength. The wavelengths of some radiation are incredibly
microscopic—a few millionth of a meter (micrometers or μm), a few
billionth of a meter (nanometers or nm), or a few tenths of a nanometer
(Angstrom or Å)—while others are of very conventional units like
centimeters (cm), millimeters (mm), or meters (m). For some wavelengths, it is
more convenient to speak of how many wave crests would pass a fixed location
per second (Hertz or Hz), or frequency. Thus, scientists have
characterized radiation based upon their wavelength or
frequency. Types of radiation include the X-rays used in medical imagery,
ultraviolet radiation which is harmful to life on Earth, visible light which
we are able to see, infrared radiation, microwaves used to cook food, and
radio and television waves.
All objects emit radiation, including you and me. The amount of total
energy emitted by an object as radiation is dependent on the temperature of the
object. The warmer an object is the more radiation it emits. All objects emit
all types of radiation, but they don't emit them all equally. The wavelength
that is emitted the most is also dependent on temperature. The warmer an
object is the smaller the wavelength of maximum emission. The temperature of
the sun's outermost layer, the photosphere, where we get most of the sunlight
that we receive on Earth is over 20 times hotter than the temperature at the
Earth's surface. So, the Sun not only emits much more radiation than the Earth
but also emits most of its radiation at a lower wavelength. Most of the Sun's
radiation is visible light which is fortunate, because that's what we use to
see the world. The Earth however emits most of its energy at lower wavelengths
in the infrared.
Climate on Earth is maintained through a balance between the incoming solar
radiation and the terrestrial infrared emission. In other words, the amount of
energy emitted by the Earth must equal the amount of energy received by the Sun.
So, what happens if there is a slight radiative imbalance? The climate has to
adjust until it reaches equilibrium again. The change in the global mean
surface temperature due to such a radiative imbalance is called climate
sensitivity.
The Components of the Earth System
How the Earth responds to such a radiative imbalance is based upon the
interactions between the components of the Earth system pictured here. The component that responds the quickest to a
radiative forcing is the atmosphere, the shell of air that envelops
the Earth. It is about 10 km deep composed of predominately molecular oxygen
(O2) and nitrogen (N2). Oxygen obviously is important to
the respiration of animals, but it is some of the trace gases that are very
important climatologically. Some of these trace gases, called greenhouse
gases, absorb infrared radiation but do not absorb much solar radiation.
Therefore, most of the sunlight is allowed to reach the surface which absorbs
almost all of it, thus warming it up. It emits infrared radiation which is
partially absorbed by the greenhouse gases. The atmosphere emits radiation at
its local temperature which is cooler than the surface, so it emits less
radiation than the surface. The greenhouse gases essentially redirect some of
the Earth's emission back down to the surface, making it warmer than it would
be without an atmosphere. This is what is called called the greenhouse
effect. Without it, the average surface temperature would be 20°C
below freezing.
The most effective greenhouse gas is actually water vapor. Some other
greenhouse gases include methane and manmade chlorofluorocarbons (CFCs). Of
recent concern is carbon dioxide, because its concentration is steadily
increasing due to human activities including industrial practices, fossil fuel
combustion, and deforestation. Climate studies have long been used to study
how sensitive the global climate is to carbon dioxide concentrations.
The oceans respond the slowest to a radiative imbalance. Part of
that is due to the fact that it takes more energy to heat up 1 kg of water by
1°C than it does for 1 kg of air. Another reason is that the oceans are so
immense. They cover a little over 70% of the Earth's surface. With an average
depth of 11 km (a little more than the depth of the atmosphere) and a density
that is about 1000 times more than that of air near the surface, the mass of
the oceans are about 250 times greater than that of the atmosphere (Wallace and
Hobbs 2006).
The oceans are a powerful regulator of climate through the transport of heat
within the ocean and between the ocean and the atmosphere. One way that the
oceans transport heat around is through the wind-driven circulation. This circulation comprises
the surface currents which are forced by the large-scale winds in the
atmosphere. On the western side of each ocean is a warm poleward current with
a cold return flow on the eastern side of each ocean.
Another mechanism to transport heat within the ocean is the meridional
overturning circulation (MOC), also known as the thermohaline circulation. The MOC is a global conveyor
belt of heat and salinity, or salt, driven by deep water formation of cold,
salty water mainly in the North Atlantic but also in the South Atlantic near
Antarctica. The deep water travels along the bottom of the ocean to get slowly
ventilated to the surface throughout the rest of the global ocean in order to
return to the deep water formation regions. The timescale that this happens is
on the order of centuries.
The MOC and the wind-driven circulation are not completely separate. The
strong warm poleward current along the Eastern Seaboard of North America called
the Gulf Stream feeds warm, salty water into the deep water formation region of
the North Atlantic. Since temperature and salinity are important to deep water
formation, it is believed that the MOC can be shut down if there is a large
influx of freshwater into the North Atlantic. Such an event is believed to
have happened about 12,000 years ago. At this time, the Earth was entering a
warm period with ice sheet melting going on. The melting ice sheets formed
large inland lakes that discharged into the North Atlantic all of a sudden at
that time. The shutdown of the MOC due to the large sudden influx of
freshwater shut off the global conveyor belt of heat resulting in a return to a
cooler climate (Hartmann 1994).
The oceans are the main source of moisture for the atmosphere. The water
condenses to form clouds which falls out as precipitation. If the
precipitation falls over land, it flows back to the ocean through surface
runoff and groundwater. This is known as the water cycle. The oceans plus surface water on land
are also referred to as the hydrosphere.
Currently, about 80% of the freshwater on Earth exists in a frozen state
(Hartmann 1994). The cryosphere encompasses all of the frozen water
on the surface of the Earth including the Antarctic and Greenland ice sheets,
mountain glaciers, sea ice, snow cover, and permafrost or permanent ice within
soil in the polar regions. Ice is bright, because it reflects more sunlight.
So, less solar radiation is absorb by ice. Most of the Earth's ice lies in the
Antarctic and Greenland ice sheets with a combined total of 97% of the total
mass of ice (Hartmann 1994). If they would completely melt, global sea level
would rise by 81 m. Sea ice is important to deep water formation, because when
ice freezes, it rejects most of the salt from sea water.
Life also affects climate. The biosphere refers to the climate
component that is composed of all life on Earth. In the oceans, microscopic
organisms called phytoplankton contain chlorophyll to use sunlight to convert
CO2 into carbohydrates which are used for energy (http://en.wikipedia.org/wiki/Phytoplankton).
They are the most abundant in the euphotic zone, the topmost layer of
the ocean where solar radiation is absorbed (Wallace and Hobbs 2006). Also,
they are more prevalent in certain regions. They tend to be the most prevalent
in parts of the ocean where there is upwelling, upward motion from the
ocean bottom which brings up nutrients for the phytoplankton. This upwelling
occurs along the eastern edges of the oceans. Some marine organisms combine
CO2 with calcium to form calcium bicarbonate as part of their shells
and skeletons. When they die, their carcasses fall to the ocean floor and get
incorporated into limestone rock (Wallace and Hobbs 2006).
On land, plants, like phytoplankton, have chlorophyll to convert
CO2 into carbohydrates. On a local scale, the vegetation canopy
increases the surface roughness thus slowing down the wind, changes the surface
energy cycle through transpiration or the release of water vapor from
leaves and by decreasing the reflectivity or albedo of the surface
(Wallace and Hobbs 2006).
The last component of the Earth system is the Earth itself. The solid crust
of the Earth is made up of plates. These plates move around on top of the
molten mantle. With time then, the continents and ocean basins are
reconfigured. The movement of the plates also force mountains to rise and give
birth to volcanoes. Ocean crust spreads out from mid-oceanic ridges. When the
ocean crust reaches a continental plate, it sinks underneath the continent,
because it is denser than continental crust. The ocean crust in sinking melts
creating a pool of magma that is released through volcanoes. These volcanoes
release CO2 when they erupt, because the limestone deposits melt as
well releasing the CO2 gas.
Any truly global climate model has to model each of these components except
for geological processes. Unfortunately, those can only be prescribed. In the
next chapters, we will explore how these models were developed, how they are
constructed, how their results are validated, and what their future is.
References
Hartmann, D. L., 1994: Global Physical Climatology. Academic
Press: San Diego, Calif.
Wallace, J. W., and P. V. Hobbs, 2006: Atmospheric Science: An
Introductory Survey. Academic Press: San Diego, Calif.