Two-thirds of the United States is expected to bake under what could be record high temperatures heading into the weekend. As a result, government agencies have issued warnings that can feel ominous.

An “oppressive and dangerous heat,” warned the National Weather Service. “Excessive heat, a ‘silent killer’,” echoed a news release by the National Oceanic and Atmospheric Administration. “Extreme heat is hazardous,” tweeted the NYC Emergency Management Department.

But people with health issues, older people and young children are especially susceptible to the effects of extreme heat. It’s a threat that grows as climate change continues.

Climate change makes heat waves more frequent

To understand how climate change increases the frequency of heat waves, it helps to think of the Earth’s temperature as a bell curve said Michael Mann, the director of the Penn State Earth System Science Center.