Warren Buffett has dismissed the possibility that climate change could prove a big risk to Berkshire Hathaway’s insurance business, in a combative annual letter to shareholders. In the letter, released on Saturday, Mr Buffett took on a dissident shareholder who plans to raise global warming at Berkshire’s annual meeting next month, as well as critics of the company’s Clayton Homes mobile homes subsidiary and politicians who paint a gloomy picture of the US economy.