the rapid melting of the polar caps and glaciers in the late 21st century had caused enough of a reduction in salinity to stop cold water from sinking and had shut down the great oceanic conveyor belt , and the only offset which saved the world from becoming totally ice free for the first time in 34 million years or so was the fact that the gulf stream had shifted south and that the dust that had been generated from the middle east war in 2036 , and a brief exchange between north korea and china , had been the start of a new cooling cycle .