EdGCM: Climate Modeling for Research and Education

4) Climate Modeling and Global Warming
Introduction

At the current rate of increase, by the time today's high school students become grandparents the amount of carbon dioxide in the Earth’s atmosphere will be higher than it has been at anytime during the previous 50 million years of Earth history. Most, if not all, of this increase will be due to human influences – primarily the burning of fossil fuels in automobiles and power generating plants. Why do scientists care so much about this invisible gas that makes up only a fraction of a percent of the molecules in the Earth's atmosphere? The answer is that carbon dioxide is the most prevalent greenhouse gas on our planet. Its presence in our atmosphere helps make the surface of the Earth warm enough to inhabit, but its rapid increase is likely to lead to globally averaged temperatures that are warmer than any experienced on Earth since before the evolution of the human species. Many people are concerned about how this "global warming" could impact the environment, the economy, and the lives of people all over the world. Scientists therefore, need to learn more about the details of this real-world experiment that we are conducting within our atmosphere. How much warming will occur during the next several decades? Will it be evenly distributed, or will some locations warm more than others? Will the temperature increase gradually, or will there be abrupt shifts in the climate? And, how will warming temperatures influence other climate phenomena, such as rain, snow and winds?

In order to explore these questions, and hopefully answer them with a high degree of accuracy, climate scientists are increasingly relying on large computer models of the earth’s climate system. The most complex, and most relied upon climate models are collectively known as Global Climate Models.

Global climate models are computer programs that simulate the Earth’s climate system in three-dimensions and are commonly referred to by their acronym "GCM". Computer global climate models, or GCMs, are used extensively in the effort to predict future climate changes, such as the impacts of increasing greenhouse gases, and they are employed by geoscientists to explore the average state and variability of past climates, from the climate of the 20th century to that of the ancient Earth. In addition to their use in forecasting realistic, whole-earth climate changes of the past, present and future, global climate simulations help scientists examine the sensitivity of the climate system to altered internal and external forcings. Examples of sensitivity tests that scientists might try include examining how the climate reacts to random changes in the Sun’s energy output or calculating how much global temperature increases if atmospheric carbon dioxide is doubled. A computer climate simulation allows scientists to isolate individual factors, and then analyze their impact on discrete components of the climate system. For example, climate simulations can help determine the effect of increased CO2 on things that are important in our society such as the average start date of the growing season, the impact of solar variability on tropical precipitation, or the role of ocean heat transport in the intensity of mid-latitude storms. Of course, GCMs are not perfect crystal balls. After all, no matter how complex their representations of climate, the climate system itself is many times more complex. However, they are deeply dependent on fundamental laws of nature (as opposed to statistical correlations) and modern GCMs do an amazing job at simulating observed and historical climate change. Perhaps most important is that they provide us with an opportunity to explore and comprehend the processes of the Earth’s climate system in four dimensions - three in space and one in time. Understanding the relationship between Earth observations and the processes that led to them gives us an improved capacity for prediction, which can then be used to aid policymakers, ecologists, public health officials, natural resource departments, industries or populations effected by climate change, and many others.

Weather and Climate: Important Distinctions

Before delving more deeply into the subject of climate science it is important to understand the differences between weather and climate. Simply stated, weather describes the changing state of the atmosphere at a particular location over time intervals of short duration - minutes to perhaps no longer than a few weeks. It consists of atmospheric elements such as temperature, humidity, air pressure, wind speed and wind direction, precipitation, cloud cover and cloud types, and visibility. Hurricanes, thunderstorms, tornadoes, blizzards, rainbows, and fog also are a part of everyday weather.

Climate, on the other hand, is time-averaged weather and is characterized by statistics that help describe the mean weather conditions and variations at a single place, region, or worldwide for a particular period of time – a month, a year or over much longer intervals.

Despite continuing marvelous advances weather remains difficult to predict beyond a one-week period. Climate, on the other hand, tends to be far more “behaved” and thus more predictable. For instance, most of us could fairly easily predict that the next 5 Augusts in New York will be warmer on average than the average temperature of the next 5 Mays, or that next 5 Mays will be wetter than next 5 Augusts. The fact that climate uses longer averages to describe phenomena and that it deals with large-scale patterns which tend to repeat themselves from year to year make it more predictable. However, climate becomes more difficult to predict when forces beyond the normal range of variability alter the characteristic physical responses of the climate system itself. This type of situation is essentially what climate scientists now face, in light of the significant increase in carbon dioxide and other greenhouse gases in the atmosphere contributed by industrial growth and associated fossil fuel burning over the past century.

Greenhouse Gases: Sources, Sinks, and Destructive Mechanisms

The gases in the atmosphere which absorb outgoing emissions of the Earth’s infra-red (long-wave) radiation and then reemit radiation are known as greenhouse gases and include carbon dioxide (CO2), water vapor (H2O), nitrous oxide (N2O), methane (CH4) and ozone (O3) and chlorofluorocarbons (CFCs). What makes these gases act as "greenhouse" gases is that they absorb frequencies that lie in the infrared part of the spectrum, the same range in which the Earth radiates energy to space, allowing them to capture much of this energy. {mostpagebreak} {atmospheric composition table} Carbon dioxide is released from the interior of the Earth through volcanic eruptions, and is produced by animal respiration, soil processes, combustion of carbon compounds and oceanic evaporation. Concentrations of CO2 are continuing to rise because of anthropogenic (human produced) emissions from the burning of fossil fuels and tropical deforestation. Conversely, carbon dioxide is dissolved in the oceans and absorbed by vegetation during photosynthesis.

Global Carbon Cycle (from Terrestrial Ecosystem Ecology Laboratory).

Water vapor, which has the greatest greenhouse effect in the atmosphere, is released into the atmosphere from open water surfaces such as oceans and lakes through the process of evaporation and to a much lesser extent from land surfaces and vegetation by means of evaporation and evapo-transpiration. Water vapor experiences a change of state, from gas to liquid, through the process of condensation. Most water vapor is found in the troposphere. Its concentration is established through internal mechanisms of the climate system and is not appreciably affected in a direct way by human actions. However, if greenhouse gases added by man cause the planet to warm, additional evaporation from the oceans would lead to increased water vapor in the atmosphere, amplifying the greenhouse effect.

Global Water Cycle (from NASA ESE).

Nitrous oxide is formed by numerous reactions of microorganisms in the oceans and soils. It is also produced by various anthropogenic actions that include industrial combustion, vehicle exhausts, the burning of biomass and the use of chemical fertilizers. N2O is destroyed in the stratosphere by photochemical reactions driven by sunlight.

Methane is a very significant greenhouse gas. Current emissions of CH4 are primarily due to the growing of rice, the digestive processes of grazing cattle, the mining of coal and drilling of oil and natural gas deposits, and decomposition of natural wastes in landfills. Methane is destroyed in the troposphere through its reaction with free hydroxyl (OH) radicals:

CH4 + OH -> CH3 + H2O

Ozone (O3), which is produced naturally in the stratosphere, is mixed into the lower atmosphere in small quantities. When some oxygen molecules (O2) absorb solar ultraviolet radiation they split to yield two oxygen atoms. These freed atoms may then combine with remaining O2 molecules to form ozone:

O2 + O -> O3

During the last century, some additional ozone has been produced near the surface by the action of sunlight on polluted air that results from emissions of motor vehicles, the burning of fossil fuels in power plants, and biomass burning.

The main cause of ozone depletion in the stratosphere is reaction of the O3 with man-made chemicals like chlorofluorocarbons (CFCs, which are further discussed below). CFCs rise into the stratosphere from the surface and chlorine is released through reaction with ultraviolet radiation. Chlorine, in turn, reacts with the highly unstable ozone, causing its destruction.

Chlorofluorocarbons (CFCs) have been used in refrigerants and aerosol propellants and have also been found in industrial pollution. They are composed of carbon, chlorine and fluorine molecules. CFC concentration steadily rose from the time of its development in the late 1920’s until production ended in January 1996. CFCs have the potential to significantly affect climate because of their considerable radiative forcing effects (hundreds to thousands of times greater than CO2) and century-long atmospheric lifetimes. Replacing CFCs are hydrofluorocarbons or HFCs, which also have a potent greenhouse effect, but do not have the atmospheric longevity of CFCs.

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