The term "climate change" is often used interchangeably with "global warming." However, given the wide range of impacts beyond temperature variations, the former is generally the preferred in the scientific community.
The greenhouse effect is absolutely vital to allowing life as we know it to survive on earth. Without the greenhouse effect, Earth would be a cold planet, with a mean surface temperature well below freezing. The greenhouse effect insulates earth, resulting in the mild temperatures at the earth's surface that have allowed life to flourish.
Using a very simple model, we can predict the mean surface temperature of the earth in the absence of a greenhouse effect. We know that about 340 W/m2 of solar power per unit surface area insolates our planet. About 30 percent of this energy is reflected, leaving an average of 240 watts to be absorbed by each square meter of surface area on earth.
All objects with a temperature above absolute zero emit radiation and the earth is no exception. According to physics, the power emitted by a black body (which for our purposes we will assume the earth to be) is sT4, where T is the surface temperature of the earth and s the Stefan-Boltzmann constant.
If the earth and space are at radiative equilibrium, meaning there is no net gain or loss of heat by the earth, we can solve for the temperature of the earth as a function of the insolation and Stefan-Boltzmann constant. Our model yields an average surface temperature of earth of 255 K (–18 degrees Celsius, or 0 degrees Fahrenheit). Many parts of the earth would be even colder. Imagine a world where the planet is covered by conditions we associate only with polar or subpolar regions - clearly this planet would be inhospitable to most forms of life on earth today.
Fortunately, the mean surface temperature of our planet is a much more pleasant 288 K (15 degrees Celsius, or 59 degrees Fahrenheit), allowing for temperate conditions over most of the planet suitable for the forms of life we know today. The missing piece of our model is the greenhouse effect - gases that warm our planet to approximately 60 degrees Fahrenheit (33 degrees Celsius) and produce the climate we know today. The two principal greenhouse gases in our atmosphere are carbon dioxide and water vapor. Other greenhouse gases include methane and the chlorofluorocarbons. These substances absorb heat in the infrared, the band of wavelengths at which the earth emits energy. They then reradiate this energy, directing some of it back toward the earth's surface. This is the extra source of heat that warms the earth beyond the frigid temperatures expected from our non-greenhouse model.
The two gases contributing most significantly to the natural greenhouse warming of the earth are water vapor and carbon dioxide. Methane, nitrous oxide, ozone and sulfur hexafluoride are also greenhouse gases but make a smaller contribution to the greenhouse effect because their concentrations are much lower.
Since the beginning of the Industrial Revolution, human activities have caused an increase in several greenhouse gases, most notably carbon dioxide, a trend most scientists believe is causing anthropogenic greenhouse warming. Over the past two and one-half centuries the concentration of carbon dioxide in our atmosphere has increased about 40 percent, from a pre-industrial level of about 280 parts per million by volume to a current level of 392 parts per million by volume. Carbon dioxide concentrations in the atmosphere are already higher today than at any time in the past 150,000 years. And if the consumption of fossil fuels such as coal and oil continues into the next century at projected rates with no mitigation, the carbon dioxide concentrations in the atmosphere would reach over 900 ppmv by 2100.
Other greenhouse gas emissions have been rising as well. Methane concentrations in the atmosphere have doubled since pre-industrial times. Other greenhouse chemicals, such as chlorofluorocarbons, perfluorocarbons, and hydrofluorocarbons, are synthetic and have only appeared in the atmosphere since the Industrial Revolution.
Greenhouse gases are not the only chemical agent contributing to the warming of Earth’s surface. In recent years increasing attention has focused on the role of black carbon aerosol particles in contributing to Earth’s warming. Black carbon aerosols are “soot”, a byproduct of incomplete combustion of fuels. Black carbon particles strongly absorb solar radiation and then re-emit radiation, so like the greenhouse gases they too can have a warming effect on the surface. Black carbon can also warm the Earth by reducing the albedo of snow and ice when soot is deposited on those otherwise reflective surfaces. Unlike many greenhouse gases, however, black carbon aerosol particles have a very short lifetime in Earth’s atmosphere, typically only residing in the atmosphere for a few days to a few weeks.
Each compound has a distinct capacity for greenhouse warming and a distinct chemical half-life, that is, the time a typical molecule spends in the atmosphere before reacting and forming a new compound. Many greenhouse substances, including methane and the halogen-containing compounds, contribute many times more pound-for-pound to the greenhouse effect than carbon dioxide. However, the sheer volume of carbon dioxide in the atmosphere compared to these other trace gases means that carbon dioxide is still by far the largest contributor to anthropogenic greenhouse warming. Additionally, while some greenhouse gases have a half-life of several decades, the half-life of carbon dioxide is on the order of a century. Most of the carbon dioxide we release today will still linger in the atmosphere in 2100 and beyond.
The climate system is extremely complex, and many forces other than the greenhouse effect contribute to the swings in our climate patterns. However, evidence is building that human influence is changing the climate of this planet. Many of the world's leading scientists argue that the warming experienced since the beginning of the 20th Century is at least partially anthropogenic in origin. In its Fourth Assessment Report, the Intergovernmental Panel on Climate Change concluded that “most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”
There are complex interrelationships involving air pollution, stratospheric ozone depletion and climate change. Human industrial and agricultural activity has been a driving factor in contributing to each of these problems. In a number of instances actions to limit emissions to address one problem will have effects on others as well.
Chlorofluorocarbons (CFCs) that are the leading cause of stratospheric ozone depletion are also powerful greenhouse gases so actions to curtail their use will help in climate protection as well as in preserving the stratospheric ozone layer. Similarly actions to substitute renewable energy for fossil fuels or to increase energy efficiency in order to protect the climate are likely also to result in an improvement in air quality. Efforts to reduce black-carbon emissions are particularly fruitful environmentally since black carbon aerosol particles contribute to both the global greenhouse effect and local air pollution.
Sometimes, however, there are tradeoffs between these objectives as control measures are directed toward one objective. Scrubbers on coal-fired power plants to reduce air pollution may result in more energy consumption and an increase in greenhouse emissions. Both increases in global mean surface temperature and depletion of the stratospheric ozone layer are likely to affect the photochemical reactions that create ground level ozone or smog and in most cases aggravate the air pollution problem, to some extent negating the effectiveness of many air pollution control measures.
The rich complexities of the earth's climate mean we cannot be sure what changes will result from the increase in carbon dioxide concentrations on a local scale. More research and refinement of the global circulation models (GCMs - also known as general climate models) are needed to reduce the range of error in predictions about future climate.
The GCMs are computer programs that simulate the earth's climate, taking into account an extraordinary number of variables describing the physical and chemical properties of the atmosphere, oceans, and continents. Over the past few decades the quality of GCMs has improved dramatically as computers have become faster and more powerful. However several weaknesses remain to be corrected in order to improve the GCMs' accuracy. The physics of clouds continues to be an area of difficulty for GCM simulation, as clouds are much smaller than the grids used by GCMs. Simulation of the deep ocean and cryosphere are also currently key areas of uncertainty in GCM simulations.
With additional research fueled by the ever-growing power of computers, the resolution of models should continue to improve in the next few years, allowing scientists to pinpoint a more accurate range of warming and sea level rise and allowing for better evaluation of the regional effects of global warming.
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