What is the cheapest way to cut carbon?

IN THE TRENDIER parts of Berlin, cargo bikes are the rage. Locals use the bicycles, which have a wheelbarrow-sized box at the front, to do the weekly shop or ferry children around. Because they cut carbon-dioxide emissions, local authorities are subsidising the craze. But the well-intentioned schemes look pricey when you consider how much carbon is abated. One such scheme costs the city €370,000 ($450,000), but is expected to reduce emissions by only seven tonnes a year. That works out at over €50,000 per tonne abated. The equivalent figure for schemes that support the sale of low-carbon heating systems, by contrast, is €200 per tonne.More than 100 countries and 400 cities (including Berlin) have promised to get to net-zero emissions by 2050 or before. Investors and regulators are encouraging companies to do the same. To meet these goals policymakers and bosses will have to pick from a menu of policies, from building wind farms to subsidising low-carbon jet fuel. That raises an important question: what is the cheapest way to cut carbon?