The heat is being caused by the ridiculous amount of carbon dioxide that we're producing. Over the past thousands of years carbon dioxide levels have NEVER been so high, and the heat-trapping nature of it is really screwing our planet over.
A graph by NASA illustrating the terribly high level of carbon dioxide we suddenly have now a'days.
Thanks to the burning of fossil fuels for our electricity and transportation, deforestation, and many other industry-related processes our carbon dioxide production is way higher than the amount of absorption the earth can keep up with.
It's only getting worse, as well. In the last few years the increase of carbon dioxide production isn't exactly coming to a halt.
As we continue to outpace the Earth's absorption rate of carbon dioxide, we continue to raise temperatures a melt a bunch of ice that really doesn't need to be melted.
Fortunately, many environmentalists are working on solutions to cut down on the use of fossil fuels to slow down our production of carbon dioxide, but a lot of damage has already been done.