Life on Earth depends on the presence of greenhouse gases in the atmosphere to insulate our planet’s surface against the chill of space; without them the Earth’s average climate would be about 33 °C cooler.
The atmosphere is largely transparent to the Sun’s energy, most of which arrives in the form of light. At the Earth’s surface, this energy is partly reflected back out to space, and partly absorbed and re-radiated as heat. The greenhouse gases in the atmosphere can bothmuch of the outgoing heat energy.
The atmospheric concentrations of some greenhouse gases are being affected directly by human activities namely carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3), and synthetic gases, such as chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs). Water vapour is also a major greenhouse gas, but its concentration in the atmosphere is not influenced directly by human activities.
Change in greenhouse gas concentrations since 1975 SOURCE: State of the Climate 2014 (Bureau of Meteorology and CSIRO)
Since the industrial revolution around 1750, human activities have added significant quantities of greenhouse gases into the atmosphere. CO2 levels are rising mainly because of the burning of fossil fuels and deforestation. The global mean CO2 level in 2013 was 395 parts per million. This concentration represents a 43 per cent increase from pre-industrial levels; it is likely to be at the highest concentration in at least 2 million years.
Methane and nitrous oxide concentrations, mostly from agriculture, have increased by 150% and 20% respectively since 1750.
The additional greenhouse gases are contributing to global warming and to associated climatic changes.
The global average concentration of water vapour quickly rises in response to an increase in global temperature, due to the increased water-holding capacity of a warmer atmosphere. Because water vapour is a greenhouse gas, the original warming is amplified. This amplification is known as a positive feedback.
Half of the CO2 released to the atmosphere is absorbed by natural CO2 sinks, on the land and in the ocean, helping to mitigate emissions from human activities. One consequence of the additional CO2 in the oceans is a 26% increase in acidification since 1750.