(An article contributed by the British Prime Minister David Cameron to the Observer on the climate change talks currently taking place in Cancun.)Over the past twelve months we’ve seen the devastation that unchecked climate change could bring – floods in Pakistan, forest fires in Russia, mudslides in China. And yet over the same twelve months we’ve seen a growing despondency about international efforts to protect our planet. Copenhagen was a disappointment for everyone who cares about climate change. Though some important steps were taken, simply not enough progress was made. But today, as the world looks to Cancun, I want to argue that everyone who cares about climate change should take heart – because I believe there are three clear reasons to have hope for the future.