The greenhouse effect: natural and enhanced
The ‘natural’ greenhouse effect makes life as we know it possible on Earth. Without this effect, the average temperature would be about – 18°C (well below the freezing point of water), rather than its current 14°C.
Earth’s surface temperature is determined by the radiative balance, the net difference between the energy gained from incoming sunlight and the amount lost into space as infrared radiation. The Earth’s atmosphere acts like a transparent blanket, letting in light but trapping some of the heat it generates. Without an atmosphere, all of this energy would be lost to space.
This natural effect relies on ‘greenhouse’ gases in our atmosphere allowing sunlight to pass through, and trapping some of the resulting heat energy that radiates back up from the Earth’s surface.
|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 (UNEP/GRID-Arendal).|
The human-induced build-up of greenhouse gases in the atmosphere is known as the 'enhanced' greenhouse effect or 'anthropogenic climate change'.
Since the start of the Industrial Revolution in about 1750, human activities such as the burning of fossil fuels, including coal and oil, have dramatically increased the concentration of greenhouse gases in our atmosphere. As a result, the rate of heat-loss from the Earth has slowed, creating a warming effect. 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 exist only under certain climatic conditions, 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 varies considerably in space and time because it has a short ‘lifetime’ in the atmosphere. Because of this variation, it is difficult to measure globally averaged water vapour concentration. Carbon dioxide has a much longer lifetime and is well mixed throughout the atmosphere. The current concentration is about 0.04 per cent. 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. 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; it tends to act as part of a feedback loop rather than being a direct cause of climate change. (Read more about feedback in Climate systems).
Carbon dioxide is the largest single contributor to human-induced climate change. NASA describes it as 'the principal control knob that governs the temperature of Earth'. 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 that 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 have almost no greenhouse effects.
Carbon dioxide and the carbon cycle
All living organisms contain carbon, as do 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, the atmosphere, plants, soil and fossil fuels is known as the carbon cycle.
The carbon cycle includes the movement of carbon dioxide:
- into and out of our atmosphere
- between the atmosphere, plants and other living organisms through photosynthesis, respiration and decay
- between the atmosphere and the top of the oceans.
|The carbon cycle, showing the movement of carbon between land, the atmosphere and the oceans. Yellow numbers are natural fluxes and red numbers are human contributions in gigatonnes of carbon per year. White numbers indicate stored carbon (The Carbon Cycle , NASA).|
On longer time scales, chemical weathering and limestone and fossil fuel formation decrease atmospheric carbon dioxide levels, whereas volcanoes return carbon to the atmosphere. This is the dominant mechanism of control of carbon dioxide on timescales of millions of years.
Because the carbon cycle is essentially a closed system, any decrease in one reservoir of carbon 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. ‘Sinks’ are oceans, plants and soils, which absorb more carbon dioxide than they emit (in contrast, carbon sources emit more than they absorb).
Increases in greenhouse gases due to human activities
Carbon dioxide is being added to the atmosphere faster than it can be removed by other parts of the carbon cycle.
Since the Industrial Revolution there has been a large increase in human activities such as fossil fuel burning, land clearing and agriculture, which affect the release and uptake of carbon dioxide.
According to the most recent Emissions Overview, carbon dioxide and other greenhouse gases are produced in NSW by the following activities or sources:
- stationary energy sources, such as coal-fired power stations (47 per cent)
- transport (18 per cent)
- coal mines (12 per cent)
- agriculture (11 per cent)
- land use (7 per cent)
- land change (3 per cent)
- waste (2 per cent).
Carbon dioxide released into the atmosphere from burning fossil fuels carries a different chemical fingerprint from that released by natural sources such as respiration and volcanoes. This makes it possible to identify the contribution of human activity to greenhouse gas production.
Data collected by CSIRO show that the concentration of carbon dioxide in our atmosphere in 2013 was approximately 395 parts per million. The level of carbon dioxide in the Earth’s atmosphere is now higher than at any time over the past 800,000—and possibly 20 million—years.
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.
- Our climate system: how it works and changes includes information about feedback loops, radiative forcing and climate sensitivity
- Climate Is Always Changing. How Do We Determine the Causes of Observed Changes? by the Intergovernmental Panel on Climate Change’s (IPCC). Includes details on how we identify the sources of atmospheric greenhouse gases.
- The Science of Climate Change: Questions and Answers
- Recent greenhouse gas levels by the US Carbon Dioxide Information Analysis Center
- State of the Climate 2014 by the Bureau of Meteorology and CSIRO
- Climate Change: Science and Solutions for Australia by CSIRO
- Carbon Cycle by NASA Earth Observatory
- Climate change information for kids
- NSW Emissions Overview – an inventory of greenhouse gas emissions in NSW
- How we account for greenhouse gases in NSW