The Earth's solar energy budget

Just over one quarter of the incoming short-wave radiation from the Sun is reflected back into space by clouds and the Earth's surface. The remainder is absorbed by the Earth and its atmosphere. The heat-tapping quality of the atmosphere, the so-called 'greenhouse effect', is caused by gases that absorb long-wave radiation emitted by the Earth.

Radiation and the atmosphere (Source : Environment Canada, 1993)

The carbon balance

Carbon is present in the atmosphere and oceans. Huge amounts of carbon are also stored within the Earth in fossil fuels and sedimentary rocks (e.g. limestone), and on the Earth's surface in vegetation and soils. Carbon occurs in the atmosphere mainly in the form of CO₂. Before the Industrial Revolution, additions of CO₂ and other greenhouse gases to the atmosphere were balanced by removals, thus atmospheric concentrations of these gases did not vary much.

Atmospheric carbon levels (Source : Pembina Institute)

Greehouse gases: the big three

The main energy-absorbing or greenhouse gases, aside from water vapour (H₂0), are carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). CH₄ and N₂O absorb far more long-wave radiation than CO₂ and are much more potent greenhouse gases. However, CO₂ has the greatest influence because it is much more abundant than the other two gases.

The great CO₂ buildup

CO₂ concentrations in the atmosphere have increased 30% since the start of the Industrial Revolution in the 1700s. Scientists predict that atmospheric CO₂ will double from pre-industrial levels in the next 40 to 60 years. The cause of the rapid buildup of CO₂ is human activities: burning fossil fuels (coal, oil, and natural gas), agricultural practices, and deforestation. There is widespread concern that the recent dramatic buildup of CO₂ and other greenhouse gases is changing our climate. The continuing buildup of these gases is expected to profoundly warm the planet.

Atmospheric CO2 concentrations (Sources: WhiteHouse Inititives on Global Change; IPCC)

Did you know?
Venus, our planetary neighbour, has an atmosphere that is 98% CO₂. Due to the extreme greenhouse effect, temperatures reach 430ºC. Similar conditions wouls exist on Earth if all the carbon stored in rock and vegetation were to be released as CO₂.