The greenhouse effect and causes of climate change
The greenhouse effect: natural vs enhanced
The ‘natural’ greenhouse effect makes life as we know it possible on Earth. Without this natural process, the average temperature at the Earth’s surface would be about minus 18°C – well below the freezing point of water – rather than its current 14°C.
Earth’s surface temperature is determined by the balance between the energy gained from incoming sunlight and the heat energy lost into space from the warmed planet. This is called Earth’s radiative balance. When the Sun shines on the Earth, it warms the surface. Like any warm object in a cold place, the warmed Earth radiates some of the energy it receives from the Sun back into space as invisible infrared radiation. Without an atmosphere, all of this energy would be lost to space. However, the Earth’s atmosphere acts like a clear blanket, letting light in but trapping some of the heat on the way out.
The greenhouse effect is possible because of the properties of certain gases in our atmosphere, commonly called ‘greenhouse’ gases. These gases let sunlight pass through but absorb and re-emit heat energy that radiates back from the Earth’s surface. Just like a blanket, these gases slow the rate of heat loss from the planet into space as shown.
The greenhouse effect describes how certain gases in our atmosphere increase the temperature on Earth’s surface by preventing some of the energy radiating from the planet’s surface from being lost into space. Source: Philippe Rekacewicz, UNEP/GRID Arendal Maps and Graphics Library.
Human-induced climate change related to the build-up in the atmosphere of greenhouse gases is also known as 'anthropogenic climate change' or the 'enhanced' greenhouse effect. Since the start of the Industrial Revolution around 1750, human activities (including the burning of fossil fuels such as coal and oil) have dramatically increased greenhouse gas concentrations in our atmosphere. This has had a warming effect on our planet because increasing amounts of greenhouse gases in the atmosphere slow the rate of heat loss from the Earth by trapping it in the atmosphere. More than 85 per cent of the additional heat in our atmosphere is absorbed by the oceans.
The enhanced greenhouse effect is expected to change many of the basic weather patterns that make up our climate, including wind and rainfall patterns, and the incidence and intensity of storms. Every aspect of our lives is in some way influenced by the climate. For example, we depend on water supplies that only exist in a certain kind of climate and our agriculture requires particular ranges of temperature and rainfall.
The most important greenhouse gases are water vapour and carbon dioxide (CO2). Both are present at very small concentrations in the atmosphere. Water vapour can vary from less than 0.01 per cent by volume to more than 3 per cent, while carbon dioxide is around 0.04 per cent (Blasing 2011). Other greenhouse gases in our atmosphere include methane, nitrous oxide and chlorofluorocarbons.
Water vapour accounts for about half the present-day greenhouse effect but its concentration in the atmosphere is not influenced directly by human activities. The amount of water in the atmosphere is related mainly to changes in the Earth’s temperature (CSIRO 2011). For example, as the atmosphere warms it is able to hold more water. Although water vapour absorbs heat, it does not accumulate in the atmosphere in the same way as other greenhouse gases but tends to act as part of a feedback loop rather than being a direct cause of climate change (read more about feedback in Our climate system: how it works and changes).
Carbon dioxide is the largest single contributor to human-induced climate change (CSIRO 2011). NASA describes it as 'the principal control knob that governs the temperature of Earth' (NASA 2010). Although other factors (such as other long-lived greenhouse gases, water vapour and clouds) contribute to Earth's greenhouse effect, carbon dioxide is the dominant greenhouse gas which humans can control in the atmosphere.
The two most abundant gases in the atmosphere are nitrogen (comprising 78 per cent of the dry atmosphere) and oxygen (21 per cent) but they exert almost no greenhouse effect (IPCC 2007).
Carbon dioxide and the carbon cycle
All living organisms contain carbon. Carbon also exists in gases (such as carbon dioxide) and minerals (such as diamond, peat and coal).
The movement of carbon between large natural reservoirs in rocks, the ocean, atmosphere, plants, soil and fossil fuels is known as the carbon cycle.
The carbon cycle includes the movement of carbon dioxide:
On longer time scales, chemical weathering, and limestone and fossil fuel formation decrease atmospheric carbon dioxide levels, while volcanoes return carbon to the atmosphere. This is the dominant control on carbon dioxide on timescales of millions of years.
Because the carbon cycle is essentially a closed system, any decrease in one reservoir of the element leads to an increase in others. For at least the last several hundred thousand years, up until the Industrial Revolution, natural sources of carbon dioxide were in approximate balance with natural ‘sinks’, producing relatively stable levels of atmospheric carbon dioxide. The oceans, plants and soils are carbon dioxide sinks because they absorb more carbon than they emit. Carbon sources emit more carbon dioxide than they absorb.
Increased greenhouse gases due to human activities
Since the Industrial Revolution there has been a large increase in human activities, such as the burning of fossil fuels, and land clearing and agriculture. These affect the release and uptake of carbon dioxide. An increase in greenhouse gas levels in the atmosphere is causing an enhanced greenhouse effect on Earth.
In NSW, a range of activities lead to the emission of carbon dioxide and other greenhouse gases with:
Read more about this in the emissions overview in NSW.
Carbon dioxide released into the atmosphere by human activities from burning fossil fuels carries a trademark signature that differs from that released by natural sources such as respiration and volcanoes. This makes it possible to identify the relative contribution of human activities towards greenhouse gas levels.
Carbon dioxide is being added to the atmosphere faster than it can be removed by other parts of the carbon cycle. In 2012, the concentration of carbon dioxide in our atmosphere was about 390 parts per million (CSIRO 2012). The amount of carbon dioxide in the Earth’s atmosphere is now higher than the levels over the past 800,000 years and possibly as long ago as 20 million years.
Read more about the impact of increasing greenhouse gases on global temperatures in climate change mitigation.
Global atmospheric concentrations of the other greenhouse gases, methane and nitrous oxide, also now exceed pre-industrial values. For the latest measurements, visit CSIRO’s Cape Grim Greenhouse Gas Data.
More information
Page last updated: 23 November 2012
