Carbon dioxide and methane are called greenhouse gases. The property of greenhouse gases is they they heat energy in them and does not let it escape.
Both Carbon dioxide and methane when present in atmosphere, absorb heat from sun and heat which would is reflected from the Earth's surface. With increase in fuel consumption release of these gases has grown tremandously. Thus, with increase in Carbon dioxide and methane more and more heat is being trapped in the atmosphere and therefore, the Earth's average is rising decade after decade.
