Much of Northern Canada is covered by ice and permafrost; however, the future of the permafrost is uncertain because the Arctic has been warming at three times the global average as a result of climate change in Canada. Canada's annual average temperature over land has warmed by , with changes ranging from  in various regions, since 1948. The rate of warming has been higher across the North and in the Prairies. In the southern regions of Canada, air pollution from both Canada and the United States—caused by metal smelting, burning coal to power utilities, and vehicle emissions—has resulted in acid rain, which has severely impacted waterways, forest growth and agricultural productivity in Canada.