As heat waves increase in India with the rise in global warming, cities are particularly vulnerable because temperatures at the heart of the urban jungles can be as many as seven degrees higher than the areas surrounding them, a leading expert has said. Speaking exclusively to NDTV on Thursday, Robert Muir-Wood, Chief Research Officer at Moody's RMS, a risk management company, attributed this rise to a phenomenon called the urban heat island effect which, he said, is not only detrimental to productivity but can also cause illness.