We need to replace our existing coal, oil and gas sources of energy partly because they are beginning to run out and partly because their use involves emissions of gases which are producing serious, irreversible changes in our climate.
Estimates of future fossil fuel resources vary but some estimates, as in the graph below, suggest that we shall soon reach maximum production.
Even if peak production is a few years away it is clear that the costs of extraction of fossil fuels are going to increase significantly and their use will also incur increasingly unacceptable environmental costs. Even if it is economically and environmentally possible to extract fossil fuels it has now been established that we will have to leave a significant amount of the fuels in the ground in order not to cause unacceptable changes to our climate
It is now established that average global temperature have been significantly increasing over the last 100 years as shown in the graph below.
The Intergovernmental Panel on Climate Change (IPCC) in its latest report points out that the each of the last three decades has been successively warmer than any preceding decade since 1850 (http://www.ipcc.ch/report/ar5/wg1/). In the Northern Hemisphere, 1983–2012 was probably the warmest 30-year period of the last 1400 years. In addition, changes in many extreme weather and climate events have been observed since about 1950. For example, the frequency of heat waves has increased in large parts of Europe, Asia and Australia and the frequency and intensity of heavy precipitation events has increased in North America and Europe.
The warming of the earth has also resulted in an increase in sea level, partly due to the melting of glaciers and partly due to the thermal expansion of the oceans. The rate of sea level rise since the mid-19th century has been larger than during the previous 2000 years. Between 1901 and 2010, global mean sea level rose by about 0.2m and the current rate of increase is around 2-3mm per year and by the end of the century the sea level could be 400-600mm higher than it is today.
There are various possible factors causing climate change but it is now accepted by the overwhelming majority of scientists that a major cause is the increase in CO2, CO and other similar ‘greenhouse gases’, which have the effect of trapping long wave solar energy in the atmosphere and are largely the result of the increasing use of fossil fuels.
One way to reduce the emissions of greenhouse gases is through the use of renewable energy which have practically no emissions. Renewable energy devices such as solar thermal panels and solar photovoltaics (pv) uses the direct flow of solar energy to the earth. Other renewable energy systems (except tidal energy) are also powered by the flow of solar energy, although sometimes indirectly. For example, wind results from the differential heating of the atmosphere by the sun and wave energy uses the energy transferred from the wind to the sea while hydro power uses the energy of water obtained as part of the hydrological cycle, which is powered by the sun. Tidal energy is slightly different in that the tides are generated by the gravitational attraction of the moon. It should be noted that the burning of biomass (e.g. wood chips) is counted as renewable on the basis that the emissions resulting from the burning would have occurred over roughly the time scale by natural decomposition of the wood, in contrast to burning of fossil fuels which releases hydrocarbons which have been stored for millions of years. Although there are no net emissions of greenhouse gases associated with the operation of renewable energy devices, there are some small greenhouse gas emissions associated with the construction of renewable energy devices. The graph below compares the greenhouse gas emissions for renewable energy with conventional energy sources.
Ref Benjamin K. Sovacool. Valuing the greenhouse gas emissions from nuclear power: A critical survey. Energy Policy, Vol. 36, 2008, p. 2950
The UK emissions is committed to reducing carbon emissions by 80% by 2050 and overall emissions have reduced in recent year as shown in the graph below. This may be partly due to increased use of renewable energy but it is also due to other factors such as the economic recession, the change from coal to gas for electricity generation and heating and to the outsourcing of some manufacturing to other countries and it is by no means clear that we will be able to meet the 2050 target.