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Causes of Climate Change

As seen in the Understanding Climate Section of this webpage, there are many influences over the Earth’s climate, which can be distinguished into ‘natural’ and ‘anthropogenic’ (human-induced) factors. Since the beginning of the 20th century, scientists have been observing a change in the climate that can not be attributed to any of the ‘natural’ influences of the past only. This change in the climate, also known as global warming, has occurred faster than any other climate change recorded by humans and so is of great interest and importance to the human population. The following sections look at the main causes of anthropogenic (human caused) climate change (for the ‘natural’ influences on climate change, refer to the above mentioned Understanding Climate section).

Greenhouse gases

The Earth has a natural greenhouse effect where certain gases (known as greenhouse gases) in the atmosphere allow the sunlight to enter but absorb the heat radiation. Because these gases absorb the heat, they  keep the average surface temperature on Earth around 14°C. Without the natural greenhouse effect, the Earth’s average surface temperature would be around -19°C.

oncentration of greenhouse gases from 0 to 2005
Image IPCC

Since the industrial revolution, human activity has increased the amount of greenhouse gases in the atmosphere (shown in the graph to the right). The increased amount of gases which absorb heat, has directly lead to more heat being retained in the atmosphere and thus an increase in global average surface temperatures. This change in temperature is known as global warming. The increase in temperature is also leading to other effects on the climate system. Together these affects are known as anthropogenic (human caused) climate change.

 

The main greenhouse gases include:

Water vapour. The most abundant greenhouse gas (GHG), however because it spends just a short time in the atmosphere, and humans have a very impact on the amount of water in the atmosphere, it is not considered the most important GHG.

Carbon dioxide (CO2). Is actually only a small part of the atmosphere, but one of the most important GHGs. CO2 is released naturally into the atmosphere through volcanic eruptions and animal respiration but it is also released through human activities such as deforestation and the burning of fossil fuels for energy. CO2 also spends a long time in the atmosphere increasing its impact. Since the industrial revolution, humans have increased atmospheric CO2 concentration by 30%.

Methane. The second most important GHG, is produced both naturally and through human activities. The most significant sources of Methane come from the decomposition of organic matter e.g. in landfills and in agriculture. Another large source is from the digestion of ruminants (cows, goats etc). Methane is a stronger GHG than CO2 because it can absorb more heat, however it is much less abundant in the atmosphere.

Nitrous oxide. A very powerful greenhouse gas which is heavily produced in the agriculture sector, specifically in the production and use of organic fertilizers. It is also produced when burning fossil fuels.

Chlorofluorocarbons (CFCs). These man-made compounds were produced for industrial use, mainly in refrigerants and air conditioners. They are now regulated under the Montreal Protocol due to their adverse affect on the Ozone Layer.

Since the beginning of the 20th century industrial activity grew 40-fold, and the emissions of greenhouse gases grew 10-fold.

CO2 monthly mean at Mauna loa
Image NOAA

The amount of CO2 in the air increased from some 280 parts per million by volume (ppmv) at the beginning of the century to 389 ppmv at the end of 2010. The amount of CO2 varies throughout the year as the result of the annual cycles of photosynthesis and oxidation, illustrated in the graph. Similarly, methane (CH4) rose from a preindustrial atmospheric concentration of around 700 parts per billion by volume (ppbv) to about 1,789 ppbv by 2007.


The overall warming from 1850 to the end of the 20th century was equivalent to about 2.5 W/m²; CO2 contributed around 60 per cent of this figure and CH4 about 25 per cent, with N2O and halocarbons providing the remainder. This has resulted in Earth’s average temperature increasing from 15.5°C  to 16.2°C in the last 100 years. The warming effect that would result from a doubling of CO2 from pre-industrial levels is estimated to be 4 W/m².

[Further information] WMO’s Global Atmosphere watch program’s Greenhouse Gas Research

[More in depth information] WMO’s Greenhouse gas bulletin reports the latest global concentrations of greenhouse gases.

Aerosols in the Atmosphere

Atmospheric aerosols are able to alter climate in two important ways:

  1. They scatter and absorb solar and infrared radiation
  2. They may change the microphysical and chemical properties of clouds and possibly their lifetime and extent.

The scattering of solar radiation acts to cool the planet, while absorption of solar radiation by aerosols warms the air directly instead of allowing sunlight to be absorbed by the surface of the Earth.

  • Human activity contributes to the amount of aerosols in the atmosphere in several ways.
  • Dust is often  a bi-product of agricultural processes.
  • Biomass burning produces a combination of organic droplets and soot particles.
  • Industrial processes produce a wide variety of aerosols depending on what is being burned or produced in the manufacturing process.
  • Exhaust emissions from transport generate a rich cocktail of pollutants that are either aerosols from the outset, or are converted by chemical reactions in the atmosphere to form aerosols.

The concentrations of aerosols are about three times higher in the Northern Hemisphere than in the Southern Hemisphere. This higher concentration is estimated to result in radiation forcing that is about 50 per cent higher for the Northern Hemisphere.

[More in depth information] on Aerosols can be found at the WMO Aerosol Research programme webpage

Land use change

Land-use changes (e.g. cutting down forests to create farmland) have led to changes in the amount of sunlight reflected from the ground back into space (the surface albedo). The scale of these changes is estimated to be about one-fifth of the forcing on the global climate due to changes in emissions of greenhouse gases. About half of the land use changes are estimated to have occurred during the industrial era, much of it due to replacement of forests by agricultural cropping and grazing lands over Eurasia and North America. The largest effect of deforestation is estimated to be at high latitudes where the albedo of snow-covered land, previously forested, has increased. This is because snow on trees reflects only about half of the sunlight falling on it, whereas snow-covered open ground reflects about two-thirds.

Overall, the increased albedo over Eurasian and North American agricultural regions has had a cooling effect.

Other significant changes in the land surface resulting from human activities include tropical deforestation which changes evapotranspiration rates (the amount of water vapour put into the atmosphere through evaporation and transpiration from trees), desertification, which increases surface albedo, and the general effects of agriculture on soil moisture characteristics. All of these processes need to be included in climate models.

Except for climate change studies there are few reliable records of past changes in land use. One way to build up a better picture of the effects of past changes is to combine surface records of changing land use with satellite measurements of the properties of vegetation cover. Such analyses show that forest clearing for agriculture and irrigated farming in arid and semi-arid lands are two major sources of climatically important land cover changes. The two effects tend, however, to cancel out, because irrigated agriculture increases solar energy absorption and the amount of moisture evaporated into the atmosphere, whereas forest clearing decreases these two processes.

[Further information] on the Greenhouse effect and its causes can be found in the IPCC’s frequently asked questions section: What is the greenhouse effect?, How do Human Activities Contribute to Climate Change and How do They Compare with Natural Influences?

[Further information] can be found on Carbon Dioxide and its human origins in the IPCC’s frequently asked questions section: Are the Increases in Atmospheric Carbon Dioxide and Other Greenhouse Gases During the Industrial Era Caused by Human Activities?

[More in depth information] on expected impacts and elements of climate change can befound in the  IPCC Fourth Assessment Report, working group 1, specifically chapter 9 on understanding and attributing climate change.

 

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