The causes of global warming

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Global warming

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Updated 3/11/2011, published online 11/29/2010

The greenhouse effect is vital to life on earth. It is caused by a number of gases that occur naturally in the earth’s atmosphere. Since the 20th century, this effect has been exacerbated by additional gas emissions linked to human activity, such as farming and the use of fossil fuels. According to many scientists, this phenomenon is contributing to the current global warming. This rise in the earth’s temperature is increased or reduced by complex chemical and climate interactions.

The greenhouse effect
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The Greenhouse Effect, a Natural Phenomenon

Like the stars and other planets, the earth is surrounded by an atmosphere made up of various gases. When the sun’s rays pass through this atmosphere, they are partially absorbed by the ground. Part of this solar energy reflects off the ground and back into space in the form of infrared radiation – invisible electromagnetic radiation beyond the red part of the solar spectrum.

The earth’s atmosphere acts like a greenhouse whereby the sun’s heat is trapped by water vapor.

This reflected radiation is absorbed by a number of atmospheric gases, including water vapor (H₂O), carbon dioxide (CO₂ ), methane (CH₄), nitrous oxide (N₂O) and ozone (O₃). These gases store this heat and then release it into space and the earth's surface. As the heat is again absorbed and then reflected back by the ground and the gases, it becomes trapped, heating up the earth and its atmosphere.

Thus, the earth’s atmosphere acts like a greenhouse where the sun’s energy, trapped by water vapor, raises the temperature within. This is why the gases that cause this phenomenon are called greenhouse gases .

The greenhouse effect was discovered in 1824 by the French mathematician and physicist Joseph Fourier. In the 1870s, greenhouse gases were studied by the Irish physicist John Tyndall. In 1896, the Swedish chemist Svante August Arrhenius established that rising concentrations of atmospheric CO₂ must lead to increases in the earth’s temperature.

The greenhouse effect is vital to life.

True. Without the greenhouse effect, all the radiation emitted by the earth would dissipate back into space and the planet would be an inhospitable ball of ice with average surface temperatures of no more than -18°C! However, the intensification of this phenomenon, due in particular to human activity, is causing an unusual rise in temperatures which is having significant impact on the environment.

Greenhouse Gases and Human Activity

Since the 1990s, reports by the IPCC (Intergovernmental Panel on Climate Change) have emphasized the role played by human activity on the global warming observed in the 20th century. Around thirty major scientific organizations, including NASA and the World Meteorological Organization, have drawn similar conclusions.

Taking account of the time it takes the climate system to react to any change affecting it, the rise in the earth’s temperature coincides with the Industrial Revolution, which dates back to the 18th century in Europe. From this time on, societies developed thanks to massive use of fossil fuels (oil, natural gas and coal) which provided the energy necessary to develop industry and transport. However, burning these energy sources releases carbon dioxide, the most common greenhouse gas in the atmosphere: it causes 75 % of human-related carbon emissions (the rest results from agriculture or deforestation). Between 1750 and 2002, the concentration of CO₂ in the earth’s atmosphere rose 31 %¹.

Since the end of the 20^th century, global energy demand and consumption have risen considerably. Consequently, annual carbon dioxide emissions rose 80 % between 1970 and 2004² and a further 36.5 % between 1990 and 2007.

Crop and livestock farming cause significant amounts of greenhouse gas emissions:

• Forestry generates 17.4 %³ of greenhouse gas emissions on a global scale, while farming causes 13.5 % of these emissions. In particular, deforestation (where trees are cut down and not replaced) reduces the capacity of the ecosystem to absorb CO₂. Instead of being absorbed by the growing trees and the forest floors, this gas remains present in the atmosphere and intensifies the greenhouse effect.

• The use of nitrogen fertilizers on farms also increases the concentration of nitrous oxide (N₂O) in the atmosphere. This gas intensifies the greenhouse effect through its capacity to absorb infrared radiation, which is 310 times higher than that of CO₂, and through its long life span in the atmosphere (120 years).

• In the wet terrain of rice fields, fermentation caused by a number of microorganisms releases methane (CH₄). This gas, which occurs in very small amounts in the atmosphere, plays a key role in the exacerbation of the greenhouse effect: its capacity to absorb infrared radiation is 20-50 times higher than that of CO₂. However, its life span in the atmosphere is only 12 years, compared to a century for CO₂.

• Microbial digestion of fodder in the rumen of ruminants (cows and sheep) also produces methane which is released by the animals. Rising demand for meat and dairy products due to global demographic growth has led to an increase in the number of large farms, which generate significant quantities of CH₄.

Up until the 1990s chlorofluorocarbons (CFCs) were synthetic gases used as propellants in deodorant or shaving foam aerosols. In the 20th century, the concentration of CFCs in the atmosphere rose 4 %. Their capacity to absorb infrared radiation is 16,000 times higher than that of CO₂, meaning that they are contributors to the greenhouse effect. As they also lead to destruction of the ozone layer, they have been banned since 1990.

In all, current concentration levels of greenhouse gases in the atmosphere are today much higher than they have ever been over the last 650,000 years (we know about the composition of the atmosphere at that time through the analysis of ice cores taken from the depths of the polar ice sheets). Thus, according to the IPCC, “most of the observed increase in average global temperatures since the mid-20^th century is very likely due to the observed increase in anthropogenic GHG concentrations⁴” – i.e. caused by humans.

A complex phenomenon that is difficult to analyze

To understand global warming a large number of natural and human parameters must be taken into account. This is why its interpretation does not always have unanimous support in the scientific community.

Thus, a minority of scientists have challenged the results of IPCC studies. These scientists are often referred to as climate skeptics. Their reservations are varied:

   • According to these researchers, the computer models used to predict climate change do not accurately reflect the complexity of the earth’s climate system and they do not take account of scientific uncertainty surrounding this topic;

   • These scientists express doubts about the significance of the role attributed by the IPCC to human activity in explaining global warming. For these researchers, the IPCC reports underestimate the significance of fluctuations in solar activity in the global warming process;

   • Finally, some of these scientists believe that temperature variations recorded on the ocean surface are not due solely to global warming but also to decadal climate oscillations in the oceans (temperature fluctuations occurring every ten years on the surface of some seas, due in particular to the movement of masses of cold and hot water).

The IPCC itself acknowledges that its work contains a margin of error, particularly in the drafting of its reports. However, in January 2010, with regard to a particular matter it officially acknowledged that its 2007 report contained a prediction that was not based on conclusive scientific evidence⁵. This forecast contained a mistaken estimate of the pace at which the Himalayan glaciers are melting, predicting that these would completely disappear by 2035⁶.

After this error came to light, the UN commissioned an independent report in March 2010 on the IPCC and its operations from the InterAcademy Council (IAC), which is composed of 18 members drawn from academies of science worldwide. The IAC’s report, published in August 2010, concluded that most of the IPCC's work was credible but made several recommendations to reform the IPCC's operational practices⁷:

   • The IAC recommended that the IPCC establish an executive committee, a sort of governing body to include individuals from outside the climate science community such as NGO representatives and private sector stakeholders.

   • The IAC report also recommended that IPCC report authors pay more attention to comments made by reviewers, particularly in the case of controversial scientific issues.

   • The IAC also recommended that the IPCC respond more rapidly and with more transparency to criticisms leveled at it and that its members should not express personal opinions on topics discussed by the group. These recommendations were approved by the United Nations and accepted by the IPCC in May 2011.

Multiple interactions affecting climate change

The rise in greenhouse gas concentration levels in the atmosphere is raising average temperatures. This temperature rise in turn has diverse effects that may eventually raise the initial temperature even further:

• Rising average temperatures increases the rate of evaporation of water on earth and the amount of water vapor in the atmosphere. However, water vapor is a greenhouse gas and when concentration levels increase in the atmosphere, this phenomenon is exacerbated even further;

• Rising temperatures cause polar ice caps to melt to be replaced by water or land. Water and land absorb more solar radiation because they are less reflective than ice, which has high albedo (reflective capacity). This intensifies global warming and makes the ice melt faster. In addition, when it melts, permafrost (or permanently frozen subsoil) releases methane, another greenhouse gas stored in the ice.

Other phenomena also affect temperature changes:

   • Seen from the earth, clouds reflect infrared radiation onto the earth’s surface and contribute to rising temperatures. However, these same clouds also reflect part of the sunlight back into space, contributing to the cooling of the atmosphere. We do not yet know if the end result of these two phenomena will lead to a warming or a cooling of the earth.

   • Emissions from air pollutants related to human activity such as sulfate may have a cooling effect on climate as they reflect the sunlight penetrating the atmosphere back into space.

   • Cyclical fluctuations in solar radiation and volcanic eruptions also contribute to climate warming or cooling.