Communicating the Links between Climate Change and Heat Waves with the Climate Shift Index

Extreme weather, including heat waves, poses a significant fi cant threat to ecosystems and human health. As global temperatures continue to rise, the frequency and severity of heat waves will increase. Because of this, communicating heat-related risks to the public is increasingly important. One commonly used communication tool is the Climate Shift Index (CSI), which establishes how much more likely an extreme weather event, such as a heat wave, has been made by climate change. To test the impact of the CSI on people's understanding of the links between climate change and extreme weather, we conducted an experiment informing 3902 American adults that climate change made the July 2023 heat wave in the United States at least 5 times more likely. In addition to this standard CSI wording and two control messages, we also explored the effectiveness of reframing magnitude as a percentage and whether mechanistic and attribution explanations of the relationship between climate change and heat waves further increase understanding. All treatments increased the belief that climate change made the July 2023 heat wave more likely and is making heat waves in general more likely as well. Additionally, we found that expressing the magnitude as a percentage was more effective than the standard CSI framing. We also found that just talking about the heat wave, without mentioning climate change, was enough to change beliefs.