Somewhere in the financial districts of London, a trader follows the fluctuations of carbon on the screen. Only a few pieces of information make it there; the bids, the dates, the volume and the price. Two straight horizontal lines (one for selling and one for buying) circumscribe a winding line and enable the trader to bet on short-term price movements. Carbon is here fully commodified, which in its financial sense means that it is a totally standardised and commensurable unit, an asset class as money, oil or gold. But how did we end up in the financial district of London where recently Barclays bought the Swedish carbon trader Tricorona for a cash offer of US$142 million? At the time of the acquisition in 2010, the global carbon emission market was worth $144 billion and Barclays attempted to capitalise on Tricorona’s pre-2012 carbon offset portfolio of 43.7 million tonnes. But what are we to make of the fact that Barclays in June 2012 sold Tricorona back to the company’s management and that the world’s carbon market’s value declined to 40 billion euros in 2013, the lowest since 2007? On the other hand, in 2013 nine new national or regional emissions trading schemes (five of them in China) started to operate and the share of global emissions covered by emissions trading is expected to increase by 70 per cent from a 2005 basis (ICAP 2014). Despite the volatility and the different trends in the carbon economy, it is appropriate to ask the fundamental question of what kind of commodity is carbon? And what enables emission reductions to travel from, say, a hydropower installation in Chile through the digital screen in the financial circuits of London to end up neutralising the emissions of the cement industry in Sweden?