Business Green: The government yesterday announced it was to tighten further the already controversial criteria it uses to buy carbon offset credits in a move designed to ensure it only uses the most robust offsets on the global market. Following Tony Blair’s pledge in 2005 to ensure that all the government’s air travel was offset, Whitehall departments have only bought carbon offset credits that adhere to its code of conduct and meet standards set out by the UN’s Clean Development Mechanism (CDM) …