Global warming is an average increase in the temperature of the atmosphere near the Earth’s surface and in the troposphere, which can contribute to changes in global climate patterns. Global warming can occur from a variety of causes, both natural and human induced. In common usage, “global warming” often refers to the warming that can occur as a result of increased emissions of greenhouse gases from human activities.
It is the phenomenon of increasing average air temperatures near the surface of earth over the past one to two centuries. Since the mid-20th century, climate scientists have gathered detailed observations of various weather phenomena (such as temperature, precipitation, and storms) and of related influences on climate (such as ocean currents and the atmosphere’s chemical composition). These data indicate that Earth’s climate has changed over almost every conceivable timescale since the beginning of geologic time and that, since at least the beginning of the Industrial Revolution, the influence of human activities has been deeply woven into the very fabric of climate change.
Giving voice to a growing conviction of most of the scientific community, the Intergovernmental Panel on Climate Change (IPCC) reported that the 20th century saw an increase in global average surface temperature of approximately 0.6 °C (1.1 °F). The IPCC went on to state that most of the warming observed over the second half of the 20th century could be attributed to human activities, and it predicted that by the end of the 21st century the average surface temperature would increase by another 1.8 to 4.0 °C (3.2 to 7.2 °F), depending on a range of possible scenarios. Many climate scientists agree that significant economic and ecological damage would result if global average temperatures rose by more than 2 °C [3.6 °F] in such a short time. Such damage might include increased extinction of many plant and animal species, shifts in patterns of agriculture, and rising sea levels.
The scenarios referred to above depend mainly on future concentrations of certain trace gases, called greenhouse gases that have been injected into the lower atmosphere in increasing amounts through the burning of fossil fuels for industry, transportation, and residential uses. Modern global warming is the result of an increase in magnitude of the called greenhouse effect, a warming of Earth’s surface and lower atmosphere caused by the presence of water vapour, carbon dioxide, methane, and other greenhouse gases. Of all these gases, carbon dioxide is the most important, both for its role in the greenhouse effect and for its role in the human economy.
Causes of global warming
The greenhouse effect
It is the warming effect that results from short wave (infrared, visible, and ultraviolet) solar radiation being largely able to pass unhindered to the surface of the Earth, where it is re-radiated as longer wave (infrared) radiation (as happens in a greenhouse). This outgoing long wave radiation is partially absorbed by Green House Gases (especially water vapor and carbon dioxide) in the atmosphere. The failure of most of the radiant energy to escape means that it is ‘recycled’ and retained as heat in the lowest part of the atmosphere, an important component in maintaining the Earth’s surface temperature. Without greenhouse warming the Earth’s average surface temperature would be around –18°C (0°F) and unable to support life. The natural effect of the recycling of radiant energy is, however, being enhanced by increases in the concentration of greenhouse gases particularly carbon dioxide (CO2), owing to human activity, notably since the beginning of the Industrial Revolution (around 1700 AD). The burning of fossil fuels (oil, coal, and natural gas) and the clearing of land and burning of vegetation, in particular, contribute to the rise of carbon dioxide concentrations. It is predicted that the enhanced greenhouse effect resulting from the increased concentrations of greenhouse gases will generate increased Global Warming and contribute to global climate change. Computer models have been used to attempt to predict the amount of warming that may result from further increases in greenhouse gas levels
The influences of human activity on climate
Human activity has influenced global surface temperatures by changing the radiative balance governing the Earth on various timescales and at varying spatial scales. The most profound and well-known anthropogenic influence is the elevation of concentrations of greenhouse gases in the atmosphere. Humans also influence climate by changing the concentrations of aerosols and ozone and by modifying the land cover of Earth’s surface.
Some of the human activities which change the climate are:
- Industrial activities, which emit a variety of atmospheric pollutants including SO2, particulate matter, photochemically reactive hydrocarbons, chlorofluorocarbons, and inorganic substances (such as toxic heavy metals)
- Burning of large quantities of fossil fuel, which can introduce CO2, CO, SO2, NOx, hydrocarbons (including CH4), and particulate soot, polycyclic aromatic hydrocarbons, and fly ash into the atmosphere
- Transportation practices, which emit CO2, CO, NOx, photochemically reactive (smog forming) hydrocarbons, and polycyclic aromatic hydrocarbons
- Alteration of land surfaces, including deforestation
- Burning of biomass and vegetation, including tropical and subtropical forests and savanna grasses, which produces atmospheric CO2, CO, NOx, and particulate soot and polycyclic aromatic hydrocarbons
- Agricultural practices, which produce methane (from the digestive tracts of domestic animals and from the cultivation of rice in waterlogged anaerobic soils) and N2O from bacterial denitrification of nitrate-fertilized soils.
We know that the earth is surrounded by a mixture of gases. The Earth’s atmosphere consists of roughly 79.1% nitrogen, 20.9% oxygen, 0.03% carbon dioxide, and trace amounts of other gases Greenhouse gases are a natural part of the atmosphere. Greenhouse gases include water vapour, carbon dioxide, methane, nitrous oxide, halogenated fluorocarbons, ozone, perfluorinated carbons, and hydrofluorocarbons. Water vapor is the most important greenhouse gas, but human activity doesn’t have much direct impact on its amount in the atmosphere.
Global warming is caused by an increase in the levels of these gases brought about by human activity. The greatest impact on the greenhouse effect has come from industrialization and increases in the amounts of carbon dioxide, methane, and nitrous oxide. The clearing of land and burning of fossil fuels, for example, have raised atmospheric gas concentrations of soot and other aerosols (fine particles in the air). Manufactured greenhouse gases and particles, rather than the occasional volcanic eruption, now account for higher gas concentrations. The planet has begun to warm at a steep rate, and future temperature increases are predicted by climatic models programmed with the volumes of gases released yearly into the atmosphere. Some scientists are already seeing the consequences of global warming, such as the melting of the polar ice sheets and rising sea levels. Greenhouse gases warm Earth’s surface by increasing the net downward longwave radiation reaching the surface. The relationship between atmospheric concentration of greenhouse gases and the associated positive radiative forcing of the surface is different for each gas. A complicated relationship exists between the chemical properties of each greenhouse gas and the relative amount of longwave radiation that each can absorb. What follows is a discussion of the radiative behaviour of each major greenhouse gas.
Radiative forcing is a measure of how the energy balance of the Earth-atmosphere system is influenced when factors that affect climate are altered.
Some Green House Gases
Water vapour is the most potent of the greenhouse gases in Earth’s atmosphere, but its behaviour is fundamentally different from that of the other greenhouse gases. The primary role of water vapour is not as a direct agent of radiative forcing but rather as a climate feedback—that is, as a response within the climate system that influences the system’s continued activity. This distinction arises from the fact that the amount of water vapour in the atmosphere cannot, in general, be directly modified by human behaviour but is instead set by air temperatures. The warmer the surface, the greater the evaporation rate of water from the surface. As a result, increased evaporation leads to a greater concentration of water vapour in the lower atmosphere capable of absorbing longwave radiation and emitting it downward.
Carbon dioxide (CO2) is one of the most significant Green house gases. Natural sources of atmospheric CO2 include outgassing from volcanoes, the combustion and natural decay of organic matter, and respiration by aerobic (oxygen-using) organisms. These sources are balanced, on average, by a set of physical, chemical, or biological processes, called “sinks,” that tend to remove CO2 from the atmosphere. Significant natural sinks include terrestrial vegetation, which takes up CO2 during the process of photosynthesis. A long-term balance between these natural sources and sinks leads to the background, or natural, level of CO2 in the atmosphere.
Methane (CH4) is the second most important greenhouse gas. CH4 is more potent than CO2 because the radiative forcing produced per molecule is greater. In addition, the infrared window is less saturated in the range of wavelengths of radiation absorbed by CH4, so more molecules may fill in the region. However, CH4 exists in far lower concentrations than CO2 in the atmosphere, and its concentrations by volume in the atmosphere are generally measured in parts per billion (ppb) rather than ppm. CH4 also has a considerably shorter residence time in the atmosphere than CO2 (the residence time for CH4 is roughly 10 years, compared with hundreds of years for CO2).
Natural sources of methane include tropical and northern wetlands, methane-oxidizing bacteria that feed on organic material consumed by termites, volcanoes, seepage vents of the seafloor in regions rich with organic sediment, and methane hydrates trapped along the continental shelves of the oceans and in polar permafrost. The primary natural sink for methane is the atmosphere itself, as methane reacts readily with the hydroxyl radical (OH-) within the troposphere to form CO2 and water vapour (H2O). When CH4 reaches the stratosphere, it is destroyed. Another natural sink is soil, where methane is oxidized by bacteria.
Surface-level ozone and other compounds
The next most significant greenhouse gas is surface, or low-level, ozone (O3). Surface O3 is a result of air pollution; it must be distinguished from naturally occurring stratospheric O3, which has a very different role in the planetary radiation balance. The primary natural source of surface O3 is the subsidence of stratospheric O3 from the upper atmosphere (see below Stratospheric ozone depletion). In contrast, the primary anthropogenic source of surface O3 is photochemical reactions involving the atmospheric pollutant carbon monoxide (CO). The best estimates of the concentration of surface O3 are 50 ppb, and the net radiative forcing due to anthropogenic emissions of surface O3 is approximately 0.35 watt per square metre.
Nitrous oxides and fluorinated gases
Additional trace gases produced by industrial activity that have greenhouse properties include nitrous oxide (N2O) and fluorinated gases (halocarbons), the latter including sulfur hexafluoride, hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs). Nitrous oxide is responsible for 0.16 watt per square metre radiative forcing, while fluorinated gases are collectively responsible for 0.34 watt per square metre. Nitrous oxides have small background concentrations due to natural biological reactions in soil and water, whereas the fluorinated gases owe their existence almost entirely to industrial sources.
The production of aerosols represents an important anthropogenic radiative forcing of climate. Collectively, aerosols block—that is, reflect and absorb—a portion of incoming solar radiation, and this creates a negative radiative forcing. Aerosols are second only to greenhouse gases in relative importance in their impact on near-surface air temperatures. Aerosols have the ability to influence climate directly by absorbing or reflecting incoming solar radiation, but they can also produce indirect effects on climate by modifying cloud formation or cloud properties.
Perhaps the most important type of anthropogenic aerosol in radiative forcing is sulfate aerosol. It is produced from sulfur dioxide (SO2) emissions associated with the burning of coal and oil. Since the late 1980s, global emissions of SO2 have decreased from about 73 million tons to about 54 million tons of sulfur per year.
Nitrate aerosol is not as important as sulfate aerosol, but it has the potential to become a significant source of negative forcing. One major source of nitrate aerosol is smog (the combination of ozone with oxides of nitrogen in the lower atmosphere) released from the incomplete burning of fuel in internal-combustion engines. Another source is ammonia (NH3), which is often used in fertilizers or released by the burning of plants and other organic materials. If greater amounts of atmospheric nitrogen are converted to ammonia and agricultural ammonia emissions continue to increase as projected, the influence of nitrate aerosols on radiative forcing is expected to grow.
There are a number of ways in which changes in land use can influence climate. The most direct influence is through the alteration of Earth’s albedo, or surface reflectance. Land-use changes can also influence climate through their influence on the exchange of heat between Earth’s surface and the atmosphere.
Stratospheric ozone depletion
Since the 1970s the loss of ozone (O3) from the stratosphere has led to a small amount of negative radiative forcing of the surface. This negative forcing represents a competition between two distinct effects caused by the fact that ozone absorbs solar radiation. In the first case, as ozone levels in the stratosphere are depleted, more solar radiation reaches Earth’s surface. In the absence of any other influence, this rise in insolation would represent a positive radiative forcing of the surface. However, there is a second effect of ozone depletion that is related to its greenhouse properties. As the amount of ozone in the stratosphere is decreased, there is also less ozone to absorb longwave radiation emitted by Earth’s surface. With less absorption of radiation by ozone, there is a corresponding decrease in the downward re-emission of radiation. This second effect overwhelms the first and results in a modest negative radiative forcing of Earth’s surface and a modest cooling of the lower stratosphere by approximately 0.5 °C (0.9 °F) per decade since the 1970s.
Natural influences on climate
There are a number of natural factors that influence Earth’s climate. These factors include external influences such as explosive volcanic eruptions, natural variations in the output of the Sun, and slow changes in the configuration of Earth’s orbit relative to the Sun. In addition, there are natural oscillations in Earth’s climate that alter global patterns of wind circulation, precipitation, and surface temperatures. One such phenomenon is the El Niño/Southern Oscillation (ENSO), a coupled atmospheric and oceanic event that occurs in the Pacific Ocean every three to seven years.
Effects of Global Warming
There are two major predicted effects of global warming:
- Increase of temperature on the earth by about 3° to 5° C (5.4° to 9° Fahrenheit) by the year 2100.
- Rise of sea levels by at least 25 meters (82 feet) by the year 2100.
Increasing global temperatures are causing a broad range of changes. Sea levels are rising due to thermal expansion of the ocean, in addition to melting of land ice. Amounts and patterns of precipitation are changing. The total annual power of hurricanes has already increased markedly since 1975 because their average intensity and average duration have increased (in addition, there has been a high correlation of hurricane power with tropical sea-surface temperature).
Changes in temperature and precipitation patterns increase the frequency, duration, and intensity of other extreme weather events, such as floods, droughts, heat waves, and tornadoes. Other effects of global warming include higher or lower agricultural yields, further glacial retreat, reduced summer stream flows, species extinctions. As a further effect of global warming, diseases like malaria are returning into areas where they have been extinguished earlier.
Manahan, Stanley, 2000, Environmental Chemistry, Lewis Publishers, London