Enhanced Greenhouse Effect

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Enhanced Greenhouse Effect


The enhanced greenhouse effect refers to human activities that are adding to the warming of the atmosphere due to the greenhouse effect—the presence of gases that increases the atmosphere's retention of the heat energy of the sun.

The burning of fossil fuels including coal, oil, and natural gas, along with the clearing of land for agricultural use and urban development, are increasing the amount of the heat-retaining greenhouse gases in the atmosphere.

Although the reality of the enhanced greenhouse effect was debated by nations including the United States even to the end of the twentieth century, the evidence that human-related activities since the mid-eighteenth century have enhanced atmospheric warming is now considerable. Organizations including the United Nations' Intergovern-mental Panel on Climate Change (IPCC) no longer debate the reality of the enhanced greenhouse effect. In May 2007, U.S. President George W. Bush acknowledged that human activities are affecting the atmosphere and called for a reduction in emissions of carbon dioxide (CO2). Despite this, the United States remains opposed to the greenhouse gas reduction targets of the Kyoto Protocol, which was adopted in 1997.

Historical Background and Scientific Foundations

The atmosphere naturally contains gases that retain some of the heat energy of the sun. This reduces the amount of heat escaping from the atmosphere to space, and it is vital for maintaining life on Earth. If the atmosphere did not retain heat, Earth would become frozen and lifeless.

The capacity of the atmosphere to retain heat is due to the presence of gases including water vapor, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). These greenhouse gases allow sunlight to pass through the atmosphere to the surface of Earth. Some of this radiation is reflected back toward space as infrared radiation (heat). But, infrared radiation can also be absorbed by the greenhouse gases, which retain the heat in the atmosphere.

Before the mid-eighteenth century, the amounts of greenhouse gases remained relatively constant and allowed the atmosphere to retain a suitable amount of solar energy to power life. But, as nations such as Britain and later the United States began to shift from being predominantly agricultural to industrial, the amount of fossil fuels that were burned increased. The burning of fuels including coal, oil, and natural gas releases greenhouse gases (in particular CO2) into the atmosphere.

Increasingly, as forests have been cleared to make way for agricultural land, factories, and suburban housing, the amount of CO2-trapping vegetation has decreased. Additionally, wood-burning, either for disposal or fuel, releases CO2 to the atmosphere.

As the quantities of CO2 and the other greenhouse gases in the atmosphere rose, the temperature of the atmosphere began to increase beginning around the 1850s and has accelerated since 1950. It is this human-caused contribution of greenhouse gases and resulting global warming that is the enhanced greenhouse effect.

Compared to 200 years ago, the 2007 levels of greenhouse gases are much higher. As two examples, the levels of CO2 and CH4 are about 30% and approximately 145% higher in 2007 than at the beginning of the nineteenth century, according to a 2006 report from the U.S. Department of Energy (DOE).

The United States' contribution to the enhanced greenhouse effect is mainly through energy use. In 1990, CO2 emissions from human sources were about 6,150 million metric tons. By 2006, the DOE report chronicled, this amount had risen to about 7,100 million metric tons, a 15% increase. Carbon dioxide consistently accounts for approximately 82% of greenhouse-gas emissions in the United States, and the country contributes approximately 22% of global CO2 emissions.

When the level of greenhouse-gas emissions are related to population, Canada is one of the world's largest polluters per capita, with emissions of 22 tons (20 tonnes) per person per year.

Impacts and Issues

In its 2007 Assessment Report, the IPCC stated that the atmospheric concentrations of CO2, CH4, and N2O have substantially grown “as a result of human activities since 1750.” The IPCC noted that the concentrations of these greenhouse gases is now far greater than the amounts of the gases that were present in the atmosphere prior to the Industrial Revolution, as determined from measurements taken from ice cores. Since the ice core measurements enable atmospheric data to be gathered going back thousands of years, the large increase in greenhouse gases since 1750 is compelling evidence for the enhanced greenhouse effect.

The need to lessen the enhanced greenhouse effect has been recognized by the global community. Lessening the amount of greenhouse gases generated by reducing the use of fossil fuels, recycling materials, and using alternate, greenhouse-gas-free sources of energy is one approach. Other alternative energy sources such as nuclear power are more controversial.

A second approach seeks to reduce greenhouse gases by directly adding compounds into the atmosphere or altering Earth's surface. Agroforestry is an example of a surface altering strategy. The planting of trees can generate forests that soak up CO2 from the air. As of 2007, research is ongoing concerning the feasibility of trapping CO2 released from coal-burning plants, converting the gas to a liquid, and transferring it to reservoirs underground or at the ocean floor.


AGROFORESTRY: The practice of sustainably combining forestry with agriculture by combining trees with shrubs, crops, or livestock. Two forms of agroforestry are alley cropping (strips of crops alternating with strips of woodland) and silvopasture (pasturing grazing livestock under widely spaced trees). Benefits include greater soil retention, biodiversity encouragement, higher monetary returns per acre, and more diverse product marketing.

CLIMATE MODELS: Mathematical representations of climate processes. Climate models are computer programs that describe the structure of Earth's land, ocean, atmospheric, and biological systems and the laws of nature that govern the behavior of those systems. Detail and accuracy of models are limited by scientific understanding of the climate system and by computer power. Climate models are essential to understanding paleocli-mate, present-day climate, and future climate.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

INDUSTRIAL REVOLUTION: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.

INFRARED RADIATION: Electromagnetic radiation of a wavelength shorter than radio waves but longer than visible light that takes the form of heat.

KYOTO PROTOCOL: Extension in 1997 of the 1992 United Nations Framework Convention on Climate Change (UNFCCC), an international treaty signed by almost all countries with the goal of mitigating climate change. The United States, as of early 2008, was the only industrialized country to have not ratified the Kyoto Protocol, which is due to be replaced by an improved and updated agreement starting in 2012.

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

Altering Earth's atmosphere to try to control warming, such as by pumping light-reflective material into the atmosphere, is still conceptual as of late 2007. This approach is also controversial. Some critics argue that the strategy does not address the problem—the contribution of human activities to atmospheric warming— but rather is a stop-gap measure that allows the environmental damage to continue.

Despite present efforts, more needs to be done if the enhanced greenhouse effect is to be halted or reversed, according to the IPCC. In December 2007, delegates from 191 countries and organizations attending the United Nations Climate Change Conference held in Bali, Indonesia, began negotiations on climate change efforts after 2012, when the Kyoto Protocol on the reduction of greenhouse gas emissions expires.

See Also Abrupt Climate Change; Aerosols; Bali Conference; Carbon Dioxide (CO2); Greenhouse Effect.



DiMento, Joseph F. C., and Pamela M. Doughman. Climate Change: What It Means for Us, Our Children, and Our Grandchildren. Boston: MITPress, 2007.

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. New York: Rodale Books, 2006.

Lovelock, James. The Revenge of Gaia: Earth's Climate Crisis and the Fate of Humanity. New York: Perseus Books, 2007.

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.