The jet stream, a narrow band of westerly winds circling the Northern Hemisphere, is stagnating, causing extreme heat across much of the globe, and climate change could make it worse, a new study has found. “The jet stream is currently in a stationary position, meaning that weather systems are stuck in a sustained pattern that is causing heat to build up in some regions of the world, such as southern Europe, southern North America and eastern China,” said Liz Stephens, a climate scientist at the University of Reading who was not associated with the new research.