A global agreement to implement a substantial carbon tax would be the most efficient way of tackling climate change, according to a new study by the International Monetary Fund.To limit global warming to 2°C or less – the level deemed safe by science – large emitting countries need to take ambitious action. For example, they should introduce a carbon tax set to rise quickly to $75 a tonne in 2030, the study says.This would be a “quantum leap” from today’s average of $2 per tonne, IMF deputy director of fiscal affairs Paolo Mauro said.