Why is the Earth's temperature increasing?

The Earth's temperatures have been slowly increasing due to the Greenhouse effect. When solar radiation reaches the Earth, it is able to pass through the atmosphere because of it is high frequency radiation. Some of this radiation is then reflected back towards space at a lower frequency. Greenhouse gases, such as carbon dioxide and methane, in the atmosphere trap the low frequency emissions, and release them in random directions. As more Greenhouse gases are put into the atmosphere, less of the reflected heat from the sun is able to get to space, so the Earth gets warmer and warmer.
The ability of a greenhouse gas to trap heat over a certain amount of time is measured as it is compared to carbon dioxide. This is known as the Global Warming Potential of the gas. For example, methane has a 20 year GWP of 72. This means that if equal weights of carbon dioxide and methane were put into the atmosphere, over 20 years the methane would trap 72 times the amount of heat as carbon dioxide. Although carbon dioxide does not warm the earth as much, it is the largest percentage of the atmosphere. Carbon dioxide also is the gas with the most emissions from human activities such as the burning of fossil fuels. This is why when talking about global warming carbon dioxide is the gas most talked about.