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What is Climate?

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.

1. What is Climate?
Radiation: What Drives the Climate
The Components of the Earth System
2. What is a Climate Model?
A Short History of the Development of Climate Models
The Difference Between Climate Modeling and Numerical Weather Prediction
The Ensemble of Climate Models
3. The Components of a Climate Model
Atmospheric Models
Ocean and Sea Ice Models
Land Models
Offline Mode
4. What is Next for Global Climate Modeling?
Transitioning to Earth System Modeling
The Need for Increased Resolution