“A change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods.”
(UNFCCC, 2006)
This results in global sea-level rise through the melting of ice caps and glaciers and seawater expansion. According the IPCC, sea levels have risen by 1 to 2 millimetres per annum during the 20th century. Furthermore positive radiative forcing has attributed to global surface temperature increases (Watson, R et. Al, 2001). The effects cause extreme weather events to occur, for instance flooding, drought and hurricanes. Not only does the event frequency increase but also the magnitude. According to Manahan, 1991 the earth’s global temperature is expected to increase by 3 degrees over the next few decades. This would be result in a reduction in precipitation and have adverse effects on water demanding processes such as industry and farming.
With all these statistics of how the global temperature has increased and how much is subject to anthropogenic sources, the question is raised about how this information is obtained and why it has been associated with human activity. The behaviour and interactions of the climatic systems are portrayed using Climate Models. Figure B, shows how global temperature has increased since 1861, when accurate recording began. (Houghton, J, 2004) Past measurements of sea, air and land surface temperatures have formulated the basis for the forecasting of future climate change. However,
“It is important to be aware that predictions from climate models are always subject to uncertainty because of limitations on our knowledge of how the climate system works and on the computing resources available.”
(Met Office, 2006a)
Figure B Global Temperature Increase (1861-2003)
(Houghton, J, 2004:57)
There is clear evidence to suggest that the global temperature is increasing. However, difficulties arise when it comes to assessing the level of anthropogenic influence in this global temperature rise. Furthermore, this notion has made future predictions of anthropogenic climate change a difficult concept, due to the fact that there may be other factors that contribute to overall climate change, for instance natural variation. Some of these include:
- The carbon cycle - which is responsible for variations in atmospheric CO2 content, for example releases from the ocean and weathering. This consequently leads to a change the climatic system
- Changes in topography - which have given rise to altered rainfall patterns and surface winds which have affected, observed patterns of weather and climate.
-
Volcanic activity – which releases sulphur dioxide and is converted into sulphate aerosols resulting in a cooling effect, a negative feedback. Importantly, “Major volcanic eruptions during the past 140 years caused a global mean cooling of 0.1 – 0.2 ۫C” (Harvey, L, 2000:29)
- Changes in the earth’s orbit – which are caused by the gravitational pull of other planets resulting in the oscillations in climate patterns, otherwise known as the Milankovitch cycles.
(Harvey, L, 2000)
As mention earlier, positive and negative feedbacks may occur in the climate system, adding to the complexity of forecasting. It is essential to understand feedbacks when it comes to accurate forecasting. Firstly is the aspect of water vapour feedback. As atmospheric temperatures increase so will evaporation and consequently lead to a higher water vapour content, resulting in a positive feedback. According to the IPCC,
``Feedback from the redistribution of water vapour remains a substantial uncertainty in climate models...”
(IPCC, 1995, cited in Horack, J and Spencer, R, 1997)
Under an increased global temperature ice caps and shelves may begin to melt resulting in reduced reflected radiation into space and an increase in global average temperatures by 20% under double the amount of current CO2 levels. (Houghton, J, 2004) The effect of increased CO2 levels may create a ‘fertilisation effect’. Plants utilise CO2 during the process of photosynthesis, thus as carbon dioxide increases, plant growth rates also increase. An experiment was undertaken to highlight the fertilisation capabilities of enhanced levels of CO2, Figure C.
Figure C Pine Tree Growth in relation to increased CO2
(Robinson, A et. Al., 1998)
This fertilisation effect may create a negative feedback as more CO2 would be absorbed resulting in an equalisation or possible reduction in global average temperatures.
“Probably the greatest uncertainty in future projections of climate arises from clouds and their interactions with radiation.” (IPCC, 2001) Clouds can have a negative and positive feedback. In one instance they can reflect the sun’s radiation similar to an albedo effect, creating a negative feedback. In the other instance they can reflect emitted infra-red radiation from the earth’s surface resulting in a positive feedback. However, the complexities of this feedback surround the issue that net feedback depends on a wide variety of variables, making climatic predictions very uncertain. (Harvey, L, 2000)
There are various climate models that are created using past and current data, each showing a different picture and each with their different purposes. Atmosphere general circulation model (AGCMs) are used to study atmospheric processes and variability in the climate. Ocean general circulation models (OGCMs) highlight ocean currents. Atmosphere-ocean general circulation models (AOGCMs) are used to predict future Carbon Dioxide levels and atmospheric responses to changes in atmospheric chemistry. (Met Office, 2006b) Regional climate models (RCMs) are more specialised and account for local conditions that could alter patterns of climate change such as Mountains. However, ‘more natural variability is found in local climates which often make this model more uncertain.’ (Houghton, J, 2004:134). The results of the models lead to different forecasts being made. Due to the complex nature of variability, anthropogenic climate change cannot be ‘pin-pointed’, but it can be predicted using different scenarios.
Figure D shows several different scenarios with different results. With so many models and predictions, it is clear that there is a high level of ambiguity concerning the extent of forecasting and anthropogenic inputs. Figure D shows how different scenarios show different forecasts.
Figure D Forecast of sea level rise and temperature change using several scenarios
(Watson, R et. Al, 2001:11)
Indeed, it is evident that limits in knowledge, data collection accuracy and data analysis have contributed to anomalies in predictions of anthropogenic climate change. Figure E highlights these discrepancies between modelled and observed data. This is a factor that could question the validity of the current models and future predictions and highlights the difficulties with forecasting.
Figure E Comparison between modelled and observations of temperature rise since 1960
(Watson, R et. Al, 2001:7)
In retrospect, using the variety of models the IPCC predicts that sea level rise and temperature will increase by 0.32 metres and 2.6 degrees respectively. (Watson, R, et. Al., 2001:8, 9)
In conclusion, it has been proclaimed that changes in the accuracy of data collection have made forecasting more accurate in relation to observed values. However, if future climate change has been based on past measurements, during a time where technology and data accuracy was less, does this question the reliability of current climate models and forecasted models? Furthermore, because there are so many models and scenarios each with a different forecast, it is difficult to decide which one bests reflects the anthropogenically altered climate of 2050, if any.
It is important to bare in mind that there maybe political influence in forecasting. In light of economic successes, a damning future may result in over precautionary measures being adopted to reduce current levels of greenhouse gases, which could impact the economy of a particular country. Is it possible that subtlety may be reflected in certain forecasts in order to prevent this over precaution. Indeed, it is this level of uncertainty surrounding anthropogenic climate change that the United States has not signed the Kyoto Protocol. In hindsight, political stance may be important when forecasting future climate change.
Finally, evidence suggests that anthropogenic emissions contribute to climate change but the percentage as yet is not know and may never be known. Natural variations have questioned this level of impact that humans play in climate change, if any at all, which continues to be very subjective. The complexities of the climate system and the lack of human knowledge and technology formulate a difficult task of forecasting future climate change. The scenarios can be very subjective and open to interpretation. At such an early stage in our understanding of the climate system and the earth’s responses to climatic change, it is left to the future of technological advancements and better knowledge to enable us to forecast more accurately, and to adopts measures in response to these forecasts.
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Andrews, J et. Al. 1996, An Introduction to Environmental Chemistry, Blackwell Science
Harvey, L, 2000, Climate and Global Environmental Change, Prentice Hall
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MetOffice, 2006b, Climate Models, MetOffice
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URL:
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