Answer: Carbon dioxide levels today are higher than at any point in at least the past 800,000 years. Global atmospheric carbon dioxide concentrations (CO2) in parts per million (ppm) for the past 800,000 years. ... Carbon dioxide concentrations are rising mostly because of the fossil fuels that people are burning for energy. The large numbers of land animals raised to feed the Earth's growing population results in increased carbon dioxide levels in the atmosphere due to farming practices and respiration and methane production. This is another example of how human activity indirectly affects biogeochemical cycles in a significant way. 8. Gases in the atmosphere, such as carbon dioxide, trap heat similar to the glass roof of a greenhouse. These heat-trapping gases are called greenhouse gases. During the day, the Sun shines through the atmosphere. Earth's surface warms up in the sunlight. Greenhouse gases let the sun's light shine onto the Earth's surface, but they trap the heat that reflects back up into the atmosphere. In this way, they act like the insulating glass walls of a greenhouse.
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