Global warming

Global warming

1. Greenhouse gases

   • Some gases in the Earth’s atmosphere absorb infrared radiation. One of these is carbon dioxide. Even though carbon dioxide is only about 0.04 per cent of the atmosphere, it is a very important greenhouse gas because it absorbs infrared well.
   • The Sun’s rays enter the Earth’s atmosphere
     • Heat is emitted back from the Earth’s surface at a lower principal frequency than that emitted by the Sun
       • Some heat passes back out into space
         • But some heat is absorbed by carbon dioxide, a greenhouse gas, and becomes trapped within the Earth’s atmosphere. The Earth becomes hotter as a result
   • Water vapour and methane
     • Other greenhouse gases are water vapour, and also methane. Even though methane is present in trace (tiny) amounts only, it is a very efficient absorber of infrared.

2. The carbon cycle

   • The amount of carbon dioxide in the atmosphere is controlled by the carbon cycle.
   • Processes that remove carbon dioxide from the air:
     • photosynthesis by plants
     • dissolving in the oceans.
   • Processes that return carbon dioxide from the air:
     • respiration by plants, animals and microbes
     • combustion ie burning wood and fossil fuels such as coal, oil and gas
     • thermal decomposition of limestone, for example, in the manufacture of iron, steel and cement.
   • Cellulose
     • All cells contain carbon, because they all contain proteins, fats and carbohydrates. For example, plant cell walls are made of cellulose, a carbohydrate.
   • Decomposers
     • Decomposers, such as microbes and fungi, play an important role in the carbon cycle. They break down the remains of dead plants and animals and, in doing so, release carbon dioxide through respiration.
   • For thousands of years, the processes in the carbon cycle were constant, so the percentage of carbon dioxide in the atmosphere did not change. Over the past 200 years, the percentage of carbon dioxide in the atmosphere has increased steadily because humans are:
     • burning more and more fossil fuels as energy sources
     • burning large areas of forests to clear land, which means that there is less photosynthesis removing carbon dioxide from the air.

3. Global warming

   • Although the changes have been gradual, most - but not all - scientists agree that the climate is getting gradually warmer. This is called global warming.
   • Most - but not all - scientists lay the blame for this on human activities increasing the amount of carbon dioxide in the atmosphere.
   • Global warming could cause:
     • climate change
     • extreme weather conditions in some areas.
   • Climate change may make it impossible to grow certain food crops in some regions. Melting polar ice, and the thermal expansion of sea water, could cause rising sea levels and the flooding of low-lying land. Extreme weather events become more likely due to increased convection accompanied by more water vapour being present in the hotter atmosphere.
   • Computer climate models - Higher tier
     • One piece of evidence that supports the view of scientists who blame human activities for global warming has been provided by 'supercomputers'. Computer generated climate models, based on different amounts of carbon dioxide in the atmosphere, produce the same changes as have been observed in the real world.