Two weeks after the Paris agreement on a new international effort to fight climate change, extreme weather across the world illustrates the impact that global warming is having already, in combination with the natural climatic variability. Record December warmth is affecting large areas of the northern hemisphere, including most of Europe and the eastern US, while severe flooding hits places from Paraguay to the north of England. Climate scientists pin responsibility for the exceptional weather on man-made warming, combined with random variability and El Niño, the natural heating of the tropical Pacific Ocean that occurs every few years. David Rooke, an expert on flooding and deputy chief executive of the UK Environment Agency, said on Monday that flood preparations required a “complete rethink” as a result of climate change. “We are in a period of known extremes and we are moving into a period of unknown extremes,” he told BBC Radio. The World Meteorological Organisation expects the global average temperature this year to hit a record high — “the symbolic and significant milestone of 1C above the pre-industrial era” — as carbon dioxide and other greenhouse gases resulting from human activities trap more heat in the atmosphere. The Paris accord called on governments to contain global warming to 1.5C, with 2C as an outside limit.