Greenhouse effect and gases

An EPA Factsheet

The Issue

Since the onset of the Industrial Revolution, which began in the late 1700s, mankind has increased its use and dependence on fossil fuels (oil, coal and gas) primarily to power transportation and industrial processes and to generate electricity. When burned, fossil fuels emit carbon dioxide and other gases. The use of fossil fuels, combined with rapid deforestation and land management practices has dramatically added to natural greenhouse gas levels in the Earth's atmosphere. There is overwhelming scientific evidence that this extra greenhouse pollution is causing irreversible changes to the world's climate. Without urgent action to reduce our dependence on fossil fuels and develop clean energy alternatives, humans risk catastrophic climate effects.

The Earth's Atmosphere

Air surrounds the Earth and extends as far as 1,600 kilometres above the surface. This air is called the atmosphere. Nitrogen makes up about 78 per cent of the atmosphere, and oxygen makes up about 21 per cent. The remaining 1 per cent consists mainly of argon and small amounts of other gases such as carbon dioxide and trace amounts of hydrogen, ozone, methane, carbon monoxide, helium, neon, krypton and zenon. Air also contains water vapour and particles of dust. The air gets thinner the farther it is from the Earth. About 1,600 kilometres above the Earth, the atmosphere gradually fades into space.

The atmosphere is made up of three layers:

The troposphere is the lowest layer and extends to about 17km above the Earth's surface. It contains most of the planet's air and is where the Greenhouse Effect takes place. Wind, storms, most clouds and other weather features all take place in the troposphere. Other parts of the atmosphere are above the troposphere.

The stratosphere extends from about 17km to about 50km above the Earth's surface. The upper-most part of this layer contains the Ozone Layer.

The thermosphere is the upper-most layer of the Earth's atmosphere and extends from about 50km to 640km above the Earth' s surface.

What is the Greenhouse Effect?

Energy from the sun, mainly in the form of visible light, drives the earth's weather and climate, and heats the earth's surface. In turn, the earth radiates energy in the form of infrared radiation (thermal radiation), back into space. "Greenhouse gases" in the troposphere such as water vapour, carbon dioxide, ozone, methane and nitrous oxide block some of the infrared radiation from escaping into space. This heat-trapping ability of the atmosphere is called the "greenhouse effect". Without this natural greenhouse effect, temperatures would be much lower than they are now, and life on Earth as we know it today would not be possible. Without the greenhouse effect, the average temperature of the Earth would be about -18°C instead of its hospitable 15°C.

What is the Enhanced Greenhouse Effect?

Some greenhouse gases occur naturally in the atmosphere, while others result from human activities. Naturally occurring greenhouse gases include water vapour, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3). However, certain human activities add to the levels of most of these naturally occurring gases. These added amounts of greenhouse gases, plus emissions of non-naturally occurring greenhouse gases are changing how the atmosphere absorbs energy. This is all happening at an unprecedented speed. The result is known as the "enhanced greenhouse effect".

Greenhouse gases

Human-induced emissions of greenhouse gases are called anthropogenic emissions. Below are some greenhouse gases and their anthropogenic sources:

Carbon dioxide is released into the atmosphere when solid waste, fossil fuels (oil, natural gas, and coal) and wood and wood products are burned. Carbon dioxide is also released into the atmosphere when trees are cut down during land clearing and deforestation.

Methane is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from the decomposition of organic wastes in municipal solid waste landfills, and the raising of livestock.

Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of solid waste and fossil fuels.

Greenhouse gases that are not naturally occurring include byproducts of foam production, refrigeration, and air conditioning called chlorofluorocarbons (CFCs), as well as hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) generated by industrial processes.

Each greenhouse gas differs in its ability to absorb heat in the atmosphere. HFCs and PFCs are the most heat-absorbent. Methane traps over 21 times more heat than carbon dioxide, and nitrous oxide absorbs 270 times more heat than carbon dioxide.

Why are greenhouse gas concentrations increasing?

Scientists generally believe that the combustion of fossil fuels and other human activities are the primary reason for the increased concentration of carbon dioxide in the atmosphere. Increases in carbon dioxide emissions account for about 70% of the enhanced greenhouse effect to date. Plant respiration and the decomposition of organic matter release more than 10 times the amount of carbon dioxide released by human activities; but these releases have always been in balance with the carbon dioxide absorbed by plant photosynthesis.

What has changed in the last few hundred years is the additional release of carbon dioxide by human activities. Energy burned to run cars and trucks, heat homes and businesses, and power factories is responsible for about 80% of society's carbon dioxide emissions, about 25% of methane emissions, and about 20% of global nitrous oxide emissions. Increased agriculture, deforestation, landfills, industrial production, and mining also contribute a significant share of emissions.

Since the beginning of the industrial revolution, atmospheric concentrations of carbon dioxide have increased nearly 30%, methane concentrations have more than doubled, and nitrous oxide concentrations have risen by about 15%. The increases in these gases have enhanced the heat-trapping capability of the earth's atmosphere. Sulfate aerosols, which are a common air pollutant, cool the atmosphere by reflecting light back into space, however, sulfates are short-lived in the atmosphere, vary from region to region and cause acid rain so therefore should not be relied upon to negate the effects of the heat-trapping gases.

Estimating future emissions is difficult because it depends on demographic, economic, technological, policy and institutional developments. Several emissions scenarios have been developed based on differing projections of these underlying factors. For example, by 2100, in the absence of emissions control policies, carbon dioxide concentrations are projected to be 30-150% higher than they are today.

For further information on the enhanced greenhouse effect see the CSIRO's Greenhouse Information Sheet

What is Global Warming?

The climate system must adjust to rising greenhouse gas levels in the atmosphere in order to keep the global "energy budget" in balance. In the long term the Earth must get rid of energy at the same rate at which it receives energy from the sun. Since a thicker blanket of greenhouse gases helps to reduce energy loss to space, the climate system must change somehow to restore the balance between incoming and outgoing energy.

This adjustment will include a "global warming" (often referred to as "climate change") of the Earth's surface and lower atmosphere. However, this is only part of the reaction. Warming is the simplest way for the climate to get rid of the extra energy. But even a small rise in temperature will be accompanied by many other changes: in cloud cover and wind patterns, for example. Some of these changes may act to enhance the warming (positive feedbacks), others to counteract it (negative feedbacks).

What are the signs that Global Warming is already occurring?

Since the late 19th century, the average surface temperature of the earth has increased by as much as 0.4 degrees Celsius.

The year 1998 was the warmest ever recorded, and the ten warmest years on record have all occurred in the last 15 years.

1998 was the 20th consecutive year with above average normal global temperatures.

Australian temperatures have been increasing since the 1950s.

Atmospheric concentration of carbon dioxide is the highest it has been at any time in the past 160,000 years, and is still rising.

Globally, glaciers are melting and sea ice is declining.

Rising ocean temperatures are causing the corals of the Great Barrier Reef to bleach and die.

What are the threats from Global Warming?

As the earth gets warmer, there's a serious risk the climate will change in ways that will seriously disrupt life on Earth. Among the severest impacts: a rise in sea level; more heat waves and droughts; more extreme weather events, producing floods and property destruction; and tropical diseases spreading to areas where they've never been known before. If we don't take action, global warming will threaten our health, our cities, our farms, and our forests, wetlands, and other natural habitats.

Specifically, if we continue to burn fossil fuels at the current rate some effects may be:

• Global temperatures may increase up to 3.5°C by 2100.
• Sea levels may rise by as much as 50cm by 2100.
• Entire South Pacific island nations could be swamped by rising sea levels and storm surges.
• Millions of people living in low lying coastal nations, such as Bangladesh, could be displaced by rising sea levels.
• Conditions favourable to diseases like malaria and dengue fever will likely spread to new areas.

For more information about Global Warming and Climate Change see Greenpeace Australia's Greenhouse and Climate Change Archive

This factsheet has been adapted from U.S. EPA
Last updated September 2002 

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