In most of Europe, fitting a heat pump is one of the most powerful actions a person can take to reduce their carbon footprint. But in Norway, where clean-yet-inefficient electrical resistance heaters have long been common, upgrading to a heat pump is often a purely financial decision. Two-thirds of households in this Nordic country of 5 million people have a heat pump, more than anywhere else in the world. For many years, Norwegians and their neighbours heated their homes with fossil fuels. But during the 1973 oil crisis, when prices shot up, the country's political leaders made a conscious choice to promote alternatives. "Norway ensured early on that fossil-fuel heating was the most expensive option, making heat pumps cost competitive," said Dr Jan Rosenow from the Regulatory Assistance Project, a thinktank that works to decarbonise buildings. "They did this by taxing carbon emissions from fossil heating fuels. That's been the key to incentivise heat pump adoption." Norway also trained up a workforce to install them. Heat pumps' efficiency has been increased over decades, partly because of the early adopters in Nordic countries who tinkered away to the point where a modern version can deliver three to five units of heat for every unit of electricity used to power it. An efficient gas boiler, on the other hand, can only produce as much heat as the energy contained in the fuel being burned. A heat pump will have a smaller carbon footprint than a gas boiler even when plugged into an electricity grid.