A group of scientists think they can geoengineer the Arctic’s melting ice, thickening it by pumping freezing seawater onto the surface during the winter.    Geoengineering is the large-scale – and often experimental – manipulation of the environment in an attempt to slow down climate change. The question is, is it mad science? Or our last hope?      Science correspondent Thomas Moore has visited the Climate Repair Unit in Cambridge, where they're attempting to thicken Arctic ice. He joins Niall to explain the research and the controversies surrounding it.    Producer: Soila Apparicio  Editor: Mike Bovill