The climate is the average weather, usually over a 30-year time period,for a particular region and time; it reflects prevailing conditions and trends. The earth's atmosphere is a highly variable and dynamic system and can be influenced by both natural forces and human activities. Natural changes in climate result from processess internal to the climate system, such as the complex transfer of heat, moisture and momentum between the atmosphere and the ocean. Natural changes can also stem from external "forcings" of climate, such as a change in the Sun's energy output or the amount of volcanically-ejected gases and particles in the atmosphere.
Climate change, in recent years, has been a topic of increasing concern in the scientific community. Climate change historically has occured as a result of natural forces but there has been increasing evidence that human activities in the last 200 years have had a significant effect upon the atmospheric system. These activities, particularly fossil fuel combustion, have been resulting in significant emissions of anthropogenic greenhouse gases, primarily carbon dioxide. These have already altered the chemical composition of the atmosphere, thus creating an 'enhanced greenhouse effect'. These gases include carbon dioxide(CO2), methane (CH4), nitrous oxide (N20), and ozone depleting substances(including chlorofluorocarbons and hydrochlorofluorocarbons) and their replacements(including hydrofluorocarbons and perfluorocarbons). They are generated by a variety of human activities, including fossil fuel combustion, waste disposal in landfils, use of refrigeration, agricultural and industrial practises, and deforestation. During pre-industrial times, the concentration of atmospheric carbon dioxide was approximately 280 parts per million by volume (ppmv). Today, the carbon dioxide concentration is a little more than 360 ppmv, nearly a 30% increase.
The greenhouse effect - which allows incoming solar radiation to pass through the earth's atmosphere but prevents much of the out-going infra-red radiation from escaping into outer space - is a natural process. The natural greenhouse effect has kept the Earth's average surface temperature approximately 33 degrees Celsius warmer than it would be were there no atmosphere.
'Global warming' refers to a long-term rise in the average temperature of the earth. Observations show that the global average surface temperaturehas increased by about 1 degree Fahrenheit over the past century. The surface temperature this century is as warm or warmer than any other century since at least 1400AD and the last few decades have been the warmest this century. Analysis indicates that this is an unusually large, rapid and prolonged warming trend, and suggest that the warming is due to human influences. Perhaps of most concern is the rate at which this change appears to be happening. Most natural climatic changes occured far less rapidly and have given communities chance to adapt. In the present day any rapid changes like sea-level rise or drought could spell disaster for poorer nations unable to cover the cost.
| COUNTRY | % | AFFECTED CROP |
|---|---|---|
| CANADA | +8 | SPRING WHEAT |
| USA | -1 | SPRING WHEAT |
| USSR | +7 | SPRING/WINTER WHEAT |
| AUSTRALIA | -4 | CEREALS |
| ARGENTINA | -3 | CEREALS |
| INDIA | -4 | CEREALS |
Countries like Australia have the wealth to support the nation should such a decrease in productivity occur, however India cannot afford to have its cereal cultivation damaged as most of its population depends upon subsistance farming. Social adjustment is least possible for the poor majority and often the result is mass migration of people in search of food and space to live.
The Uncertainty of Climate Change
The complexity of the earth's climate system creates uncertainty over exactly what will happen. Climate scientists have indictated that there may be an increase in the number of storms and their severity, increasingly severe droughts, sea-level rise, greater occurance of wild/bush fires and more frequent flash flooding. There could also be secondary consequences, such as the spread of pests and diseases into new regions as climate changes to their advantage. However with regard to the actual extent of climate change there is still great scientific uncertainty. It is now accepted that some change will occur but there have been significant differences in opinion as to the exact causes and the consequences of the change. To overcome this uncertainty it is neccessary to be able to:-
1. Understanding the Climate SystemThe climate system is very complex and at present there is no complete understanding of all the processess involved. The major problem with predicting the extent of global climate change is how will the earth system respond to changes in the atmospheric composition. Weather stations and satellite data are being used to measure and record present weather conditions and these observations help to explain atmospheric circulation. However there is substantial room for error in this method of obtaining data. Weather stations are not evenly distributed across the earth, there are high concentrations in Europe, China, USA and Australia but there are very few in Africa, Asia and the oceans - vast areas have been missed. Satellite derived data has a lower level of accuracy than do the ground measurements. Many measurements have to be taken throughout the day rather than all at once to build up a picture and this leads to a decrease in accuracy. Many environmental systems such as the atmospheric one change over timescales longer than those which can be observed directly, and it is therefore necessary to turn to human and natural archives (e.g. ice-cores) to study these.
Changes in mean global surface temperatures may be offset or enhanced by feedback mechanisms from other areas. More heat energy in the atmosphere could lead to increased evaporation from the surfaces of water bodies and therefore increased water vapour in the atmosphere. This may increase cloud cover, especially if there is an increase in the number of dust particles in the troposphere, which will reduce the amount of solar radiation entering the lower layers of the atmosphere thus balancing out temperature change. On the other hand at any given temperature there is a limit to the amount of moisture that the air can hold. When this limit is reached the air is said to be saturated. An increase in air temperature would allow more water vapour to be held but in areas where water is not plentiful the air might not reach saturation level and clouds will not form.
2. Identifying Climate ChangeAccording to British scientists the earth has experienced an overall increase in global mean surface temperature, however USA results have shown no increase at all. Which prediction is correct - if either? The difference in results may be due to differing experimental conditions, it is likely that the USA weather stations are located in rural areas and the majority of British weather stations may be located in urban areas where it is possible that the urban heat island may be having an effect and causing the apparent rise in temperature. The global mean temperature may not be the best method of assessing climate change as there is less change annually worldwide as the dry periods cancel out the wet periods. Regional studies in marginal areas may produce more reliable results. By the time we are able to unequivocally measure a change in climate due to greenhouse warming complex feedback processes may already be underway that we can do nothing about.
3. Predicting Climate ChangeIn an attempt to predict and quantify climate change there has been increasing use of GCMs or General Circulation Models which attempt to simulate numerically a segment of the atmospheric system. These provide a basis for the IPCC predictions about the response of climate to greenhouse-gas forcing. All GCMs predict some warming, but they differ in its magnitude, with estimates between 1.9 and 5.2 degrees celsius for a doubling of CO2-equivalent. They also exhibit little coherance in the simulated patterns of climate change over the earth's surface under a 2 x CO2 scenario. The models agree that land areas would warm more rapidly than the oceans, and that northern latitudes would experience a temperature rise greater than the global mean, at least in winter. For other factors, such as effective soil moisture, there is no such agreement with individual regions predicted to become wetter in some GCM experiments but drier in others. This divergence is caused by the fact that GCMs, like all numerical simulations, are simplifications of the real world. GCMs simulate the atmosphere in a three-dimensional grid with a horizontal spacing of between 250 to 800 km. In reality, many atmospheric processess take place at much smaller scales than can be resolved by the model grid, and have to be represented in other, simplified ways.
Models, which may be incorrect, are used by different groups to make decisions about the future. Companies are biased so will use the data best for them, how should the policy-maker decide which data to use?
There is much room for improvement in our present understanding of the situation we have put ourselves in, answers need to found, and quickly so that we are better equipped to deal with and alleviate the effects of a rapid change in climate.
Some Useful LinksAbout The AuthorMy name is Rachel Stebles and I am a first year student at Bristol University. I am studying Geography/Geology Joint honours with Computer Methods - far too much work for any normal student, why didn't I do pyschology? I hope you have found this web page understandable, accessable and completely free from all that f***ing advertising I've had to plough through to get the info.
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