Greenhouse effect
The greenhouse effect is a natural phenomenon arising from heat absorbing components – called greenhouse gases (GHGs) – in our atmosphere. It leads to an increase in the Earth’s surface temperature above what would be expected in the absence of these GHGs. The most important GHGs are water vapour (H2O) and carbon dioxide (CO2). Other greenhouse gases include chlorofluorocarbons (CFCs), methane (CH4), nitrous oxide (N2O) and ozone (O3). While all GHGs are only present as trace amounts in the atmosphere, their effect is enormous: without the natural greenhouse effect, the Earth’s average surface temperature would be only -18 ˚C in stead of the current 15 ˚C.
Although the phenomenon is named after the effect of solar radiation passing through glass and warming a greenhouse, the underlying heat retaining mechanism is fundamentally different, and it is better compared to the effect of a blanket that traps the body heat and prevents it from escaping into the room, thus keeping the body warm on cold nights.
The greenhouse effect is best explained by a schematic figure showing the balance of incoming and outgoing radiation:
Although the phenomenon is named after the effect of solar radiation passing through glass and warming a greenhouse, the underlying heat retaining mechanism is fundamentally different, and it is better compared to the effect of a blanket that traps the body heat and prevents it from escaping into the room, thus keeping the body warm on cold nights.
The greenhouse effect is best explained by a schematic figure showing the balance of incoming and outgoing radiation:
The Earth’s atmosphere is nearly transparent to sunlight (which has light frequencies in the range of UV and visible light), and much of the sunlight reaches the Earth’s surface where it is partly reflected and partly absorbed. The absorption leads to warming of the Earth’s surface and emission of this energy at lower infrared (IR) frequencies (thermal radiation, heat). This IR radiation cannot escape freely into space because of the presence of atmospheric GHGs which absorb IR radiation. As these GHGs absorb IR radiation, they, along with the surrounding air, will warm up, and will in turn emit IR radiation – partly upward into space, and partly downward back towards the Earth’s surface. This effectively traps part of the IR radiation between the Earth’s surface and the lower 10 km of the atmosphere. As a result, the earth’s capacity to lose heat is reduced and the average surface temperature rises above what would be expected without this heat-trapping effect.
The mass burning of fossil fuels such as oil and coal has lead to increasing concentrations of GHGs, intensifying the natural greenhouse effect and thus leading to a global temperature increase: global warming.
Sources:
Kushnir, J. (2000). Solar Radiation and the Earth's Energy Balance. Lecture from ‘The climate system’, Columbia University, New York. Available on http://eesc.columbia.edu/courses/ees/climate/lectures/radiation/index.html
Le Treut, H., R. Somerville, U. Cubasch, Y. Ding, C. Mauritzen, A. Mokssit, T. Peterson and M. Prather, 2007: Historical Overview of Climate Change. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
