Globally, electricity and heat generation remain the largest sources of carbon emissions, because burning coal, gas and oil to power homes and factories releases huge amounts of CO2. In a growing number of individual countries, however, transport has overtaken them. Cars, trucks, aircraft and ships burn fossil fuels continuously, and their emissions have not fallen as quickly as those from power plants in places where the electricity grid has shifted toward renewables. The shift is a sign of how unevenly decarbonisation is progressing. Countries that have cleaned up their power sector find that road transport becomes the dominant source of emissions, so further progress depends on electric vehicles, public transit and cleaner fuels. The change is a useful reminder that emissions problems move around as societies succeed in tackling one sector.

In 1909 D. W. Griffith made a 14-minute film about a speculator who corners the wheat market and doubles the price of bread. A farmer sows his field while a 'Wheat King' gambles in the Chicago wheat pit; bakers raise the loaf from five cents to ten and the poor line up for bread. Griffith cut back and forth between the farm, the feast and the breadline, a technique called parallel editing, so audiences saw both ends of the economy at once. The film drew on Frank Norris's stories and on a real scheme: months earlier Chicago financier James Patten had cornered wheat, sent bread prices up and made about $5 million, roughly $185 million today.