Sports Utility Vehicles, the big cars blamed for causing huge damage to the planet, now make up two-thirds of all new car sales.  More commonly known as SUVs, many people are choosing them for their increased comfort and a feeling of safety.  They were originally designed for off-roading in the countryside, but now they are often more of a status symbol.   Their larger size and weight mean they're big polluters, so their growing number is undoing years of progress towards cleaner air.  On this week's ClimateCast, Tom Heap speaks to the Tyre Extinguishers, climate activists who are notorious in cities around the world for deflating the tyres of SUVs to protest against their pollution.  Plus, hear from AutoTrader's Erin Baker about why so many of us want these bigger cars. Producers: Alex Edden and Gemma Watson Assistant producer: Iona Brunker Editors: Wendy Parker