In the process of photosynthesis, plants take in CO2, water and sunlight and give off O2 keeping the carbon as part of the carbohydrates they need for growth and repair. , This energy is used in respiration to break down the complex carbon compounds to yield CO2 and water. Plants and animals break up the food molecules and obtain the energy they need for their life processes. The carbon is stored in their tissues until they die., Coal and oil have been the basic fuels used by the developed nations to build their businesses and industries and a high standard of living. All of the carbon stored in them is released during burning, forming CO2 with the O2 in the air. Some goes back into the Earth's carbon reservoirs; the excess enters the atmosphere. , CO2 is removed from the air by several natural processes. The fastest uptake is into plants and the surface layer of the ocean. As a result of human activities, the carbon cycle is no longer balanced and gases such as CO2 and CH4 are building up in the atmosphere. , Along with the CO2, methane, CH4 is given off to the air during decomposition. Its concentration has risen as a result of human activities such as agriculture and waste disposal. CH4 levels have increased by 145% over the past 150 years. .

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