 Welcome back to NewsHour. Using heavy fuel oil to power ships in the Arctic is now banned. Environmentalists have lobbied governments for years to support the ban because of the climate-damaging effects of the fuel, which are exacerbated in the icy Arctic waters. Earlier, I spoke to Dr Sian Pryor, who's a marine scientist and lead advisor to the Clean Arctic Alliance, a coalition of campaign groups. First, what exactly is heavy fuel oil?