Energy entering the Earth’s atmosphere coming from the Sun is in the ultraviolet, visible and infrared regions of the electromagnetic spectrum. As a result of the Earth’s temperature being lower than that of the Sun, the radiation being re-emitted from the Earth has a lower amount of energy and is in the infrared region of the spectrum. Water vapour, carbon dioxide and methane molecules in the atmosphere absorb some of this radiation. These are some of the ‘greenhouse gases’.
Greenhouse gases are gases which allow most of the Sun’s ultraviolet and visible radiation to enter but prevent some of the Earth’s infrared radiation from leaving. The O-H bonds in the water, the C=O bonds in the carbon dioxide and the C-H bonds in the methane all vibrate at different frequencies in the infrared range, and this is why they absorb the infrared radiation. After absorbing the energy from the infrared radiation, the bonds then re-emit it. This maintains the Earth’s average surface temperature, and without it the surface temperature of the Earth would be between -20 and -40 degrees Celsius. This is called the greenhouse effect.
The greenhouse effect forms a steady system of equilibrium, where the rate at which energy is being absorbed by the Earth is equal to the rate at which energy is being radiated back into space. The increase in emissions of greenhouse gases to the atmosphere is disrupting the balance of the greenhouse effect. Increased levels of carbon dioxide and other greenhouse gases in the atmosphere, due to various reasons such as deforestation and the burning of fossil fuels, have led to more infrared radiation being absorbed and re-emitted back to Earth than the amount of energy being radiated back into space, increasing the temperature of the Earth. This is called Global Warming.
The extent to which molecules absorb infrared radiation is dependant on the way their bonds vibrate. The more modes of bond vibration possible, the higher the absorption of infrared radiation. Due to the asymmetric vibrations that affect the dipole moment of the molecule, molecules which have 3 or more atoms absorb strongly in the infrared region. Other than carbon dioxide (CO2), water (H2O) and methane (CH4), there are three other main greenhouse gases: dinitrogen oxide (N2O), CFCs, and tropospheric ozone (O3).
The actual greenhouse effect of a given gas depends on both its atmospheric concentration AND its ability to absorb infrared radiation (its ‘greenhouse factor’). The greenhouse factor is the extent to which an atmospheric gas absorbs infrared radiation relative to the same amount of carbon dioxide. Carbon dioxide has a value of 1. The table on the next slide shows the contribution of five of the greenhouse gases that we have mentioned (water is not included) to the greenhouse effect. It demonstrates how the greenhouse effect depends on both atmospheric concentration of the gases AND greenhouse factor.
The build-up of greenhouse gases in the atmosphere will lead to an increase in the Earth’s surface temperature (global warming). In 2000 the Earth’s average temperature was 1°C warmer than the value in 1850. Between 1970 and 2000 the rise was 0.4°C. The average yearly rise in temperature between 1850 and 1970 was 0.005°C, and between 1970 and 2000 it was 0.013°C, more than two times the previous value. These statistics indicate the rate of acceleration of global warming. Computer predictions suggest that the temperature is rising a lot faster than previously predicted.
The accelerated rise in temperature would be so fast that natural systems would not be able to adapt and world food productions would be seriously affected. Originally it had been believed that the greater amount of CO2 in the atmosphere would increase forest growth due to more photosynthesis, and that this would remove some carbon dioxide from the atmosphere. More recent predictions however provide evidence that the increased temperatures will increase respiration within the forests and that this will cause die-back of healthy plants. The model of higher temperatures causing die-back predicts that the majority of the Amazon rainforest will disappear due to die-back. As a result of this the carbon in the wood of the trees would be released as carbon dioxide, increasing the amount of CO2 in the atmosphere. Due to the lack of trees, there will also less carbon dioxide being absorbed for photosynthesis.
The higher temperatures would also increase microbial activity, and the hotter soil would cause more releasing of carbon dioxide from microbes. Extreme weather events are also already occurring more often due to global warming. In 1989, it was predicted that the greenhouse effect would cause a rise in the Earth’s average temperature of between 1.5 and 2.5°C by 2100, however more recent predictions suggest that this rise may reach 8°C above the 1850 level by 2100 for land masses. The cuts to CO2 emissions proposed at the Climate Change Convention will not stop the predicted changes.
It is now predicted that the level of carbon dioxide in the atmosphere in 2100 will be three times the current level, taking into account die-back of rainforests and increased soil microbial activity, and assuming that there is no reduction in CO2 emissions. These predictions indicate how necessary it is to tackle global warming urgently. If no changes are made, the effects of global warming will badly affect most areas of life, including agriculture, sea levels, ecosystems, water resources, human health and the weather.
The statistics shown on the last few slides have been passed on to the government by chemists, as proof of the devastating effects of climate change and as persuasive material to convince the government to help to fight against global warming. There are a few potential solutions which have to environmental problems which have been thought of which could reduce the amount of carbon dioxide being released into the atmosphere. These are discussed on the next slide.
1. Using sources of energy which avoid the burning of fossil fuels. For example renewable sources such as solar, wind, wave, tidal and geothermal energy. There is also the possibility of more nuclear power stations . These would reduce CO2 emissions but there are many other environmental concerns associated with them.2. Conserving energy to reduce the burning of fossil fuels.3. CCS-carbon capture and storage. This method involves capturing carbon dioxide released from power stations or industrial processes, and potentially storing it underground or into the deep oceans. Storing the CO2 in the deep oceans is seen as ideal because it can take centuries for the deep waters to reach the surface. However, there are fears that this will eventually acidify the oceans, causing damage to the ecosystem. Pumping the CO2 underground would help us to get the few remaining supplies of oil and gas to the surface, and the CO2 can react with metal oxides in the rock to form solid carbonates or can dissolve in the salty water trapped underground. Many locations in the UK are suitable for this type of CCS.
Some international treaties have been held targeting the reduction of carbon dioxide emissions in order to combat climate change. In December 2006, the Kyoto Protocol was signed by 169 countries, stating that they would reduce CO2 emissions. This was a big step, the USA and Australia, some of the largest producers of carbon dioxide, refused to sign it. Also countries which were seen as still developing industrially, such as China and India, did not have to reduce their emissions. Chemists, using techniques such as infrared spectroscopy, with infrared spectrometers based on satellites in the sky, have a significant role in monitoring agreements such as the Kyoto Protocol, as they can measure the levels of carbon dioxide emissions coming from different areas of the planet.