Well well, the same mechanisms that warm Mars could be warming Earth. Say, ohh,...THE SUN!
Nah, cant be. Variable energy output from the sun over thousands of years?
Why thats...thats.. just crazy talk!
Of course, there's the (I think) wild idea from one Richard C. Hoagland suggesting that because our Solar System is moving through a certain point in hyperdimensional space that all of the planets are warming up....