Through our use of fossil fuels, namely coal, oil and natural gas, humans have changed the balance of the carbon cycle. Naturally, the carbon contained in these fossil fuels would be stored for millions of years. However, through combustion we’ve released this stored carbon as CO₂ into the atmosphere. The effect of this has been exacerbated through deforestation. Trees absorb a lot of CO₂, acting as a carbon sink. By cutting large numbers of them down this ability is lost and they become instead a carbon source. Plus, any carbon stored in the tree is released back into the carbon cycle.

Methane is also a product of burning fossil fuels. Another major source is cows and so the increase in methane has also been linked to more intensive cattle farming.

The major concern over the release of these gases is their effect on global warming. CO₂ and methane are a greenhouse gases. Their name derives from the fact that they’re able to stop heat energy from leaving the Earth’s atmosphere into space. As the heat is held in the Earth heats up, like a greenhouse. If the Earth holds in more heat than it would normally it has a global warming effect with disastrous outcomes. Already the polar ice caps have started to melt and changes to different environments are endangering plant and animal species.

Other evidence of global warming includes:

• Crop plant yields: an increase in temperature and a decrease in precipitation will make it harder to grow certain crops.
• Life-cycles and numbers of insect pests: it’s predicted that as rainfall patterns change and humidity increases that they’ll be an increase in pests, especially those that like to live in warmer climes and breed in damp areas.
• Wild animal and plant distributions: changes in the seasonal cycles will have major effects on plant and animal distributions, what with warmer winters and longer dry seasons. Already changes in the distribution of species and in their development and life cycles have been noticed. When abiotic factors change so will the species that live in those environments.