What leads to an increase in average global temperatures?

Enhanced Greenhouse Effect

Gradually over time more and more greenhouse gases of carbon dioxide mostly from the burning of fossil fuels (transport and electricity) and methane from cattle and melting tundra builds up in the atmosphere and traps more of the sun's rays and heat around the earth.

This is carbon and methane that has been locked away for millions of years. It has not been part of the normal carbon cycle, but now it is all being released. It is all this extra gas that is the problem.

A further cause has been the deforestation of the planet. Trees absorb carbon dioxide, but they can't if they're cut down.

The earth's temperature has been relatively standard for the past 1000 years until 1860 when the Industrial Revolution began, factories sprang up and we began burning coal. Since then the temperature has been rising rapidly.

See the graph links below.