Broadscale changes in land-use patterns, such as deforestation, can significantly alter the roughness and reflectivity of the surface for solar radiation, and hence the absorbed radiation, evaporation and evapotranspiration. In the process, changes in regional climate can occur. Broadscale changes in land use also impact on the global climate by enhancing the natural greenhouse effect, for example by reducing the land's capacity to absorb carbon dioxide (e.g. through deforestation) and by increasing the carbon emission from the land (e.g. through increased biomass decay), both of which lead to greater concentrations of greenhouse gases.
The Urban Heat Island (UHI) refers to the observation that towns and cities tend to be warmer than their rural surroundings due to physical differences between the urban and natural landscapes. The concrete and asphalt of the urban environment tend to reduce a city’s reflectivity compared with the natural environment. This increases the amount of solar radiation absorbed at the surface. Cities also tend to have fewer trees than the rural surroundings and hence the cooling effects of shade and evapotranspiration are reduced. The cooling effects of winds can also be reduced by city buildings.
The UHI is enhanced by human activities within the urban environment. Pollution has a warming effect on a city, in addition to the heat released by industrial processes, household heating and car use. As cities grow, the UHI effect becomes stronger, creating an artificial warming trend in the temperature record.
Depending on the weather conditions, overnight temperatures in the centre of a large city can be up to 10°C warmer than the rural surroundings. The urban landscape has other impacts on the local climate, such as reduced average wind speed due to the blocking effect of buildings and greater frequency of flash flooding owing to the higher proportion of ground sealed with concrete and asphalt and a corresponding reduction in natural drainage.
Tropospheric aerosols (i.e. microscopic airborne particles) influence the radiative balance of the trial processes and forest burning is largely at the regional level (Figure 28). With the pollutant load on the atmosphere generally continuing to increase, the impacts of aerosols on climate will continue to be significant.
Any changes in the relative mix and atmospheric concentration of greenhouse gases, whether natural or human-induced, will lead to changes in the radiative balance of the atmosphere, and hence the level of greenhouse warming.
- Greenhouse gases influenced by human activity -
Carbon Dioxide CO2
Sources – Fossil fuel burning, deforestation, biomass burning, gas flaring, cement production
Sinks – Photosynthesis, Ocean surface
Lifetime in atmosphere – 5 to 200 years
Proportional contribution to greenhouse warming – 60%
Methane CH4
Sources – Natural wetlands, rice paddies, ruminant animals, natural gas drilling, venting and transmission biomass burning, coal mining
Sinks - Reaction with tropospheric hydroxyl (OH), removal by soils
Lifetime in atmosphere – 12 years
Proportional contribution to greenhouse warming – 20%
Halocarbons (includes CFCs, HFCs, HCFCs, perfluorocarbons
Sources – Industrial production and consumer goods. E.g. Aerosol propellents, refrigerants, foam-blowing agents, fire retardants)
Sinks – Varies between the types. E.g. CFCs and HCFCs: removal by stratospheric photolysis…HCFC, HFC: reaction with tropospheric hydroxyl (OH)
Lifetime in atmosphere – 2 to 50,000 years
Proportional contribution to greenhouse warming – 14%
Nitrous Oxide N20
Sources – Biological sources in oceans and soils, combustion, biomass burning, fertiliser
Sinks – Removal by soils, stratospheric photolysis
Lifetime in atmosphere – 114 years
Proportional contribution to greenhouse warming – 6%
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Now that you've been told otherwise, feel free to tell me that this was too long and you didn't read it.
** Our ref:
http://www.bom.gov.au/info/GreenhouseEffectAndClimateChange.pdf
OR tinyurled...
http://tinyurl.com/y6p57s
