COVER Whether summer extreme weather events—floods, wildfires, droughts and heat waves—will increase in future years depends on competition between the warming effect of increasing greenhouse gases and the opposing effect of regional cooling from atmospheric pollutants called aerosols (which reflect sunlight back to space), a new study finds. Presently, assumptions about the nature and concentrations of aerosols in climate models differ, largely due to complex underlying chemistry and physics. This leads to different estimations of future summer weather events. Seeking a better grasp on the likelihood of an increase in extreme weather events in years ahead, Mann et al. modeled how summer events look in future as conditions favorable to quasi-resonant amplification (QRA)—a phenomenon whereby jetstream patterns showcase large undulations that lock in place, trapping weather systems between them—increase. In some simulations, these events actually decrease, largely because of aerosol's cooling effect, which offsets the warming from greenhouse gases. Ultimately, say the authors, how summer event intensity and frequency will play out in future is critically dependent on human behaviors related to fossils fuels and aerosols. [CREDIT: MARK THIESSEN/NATIONAL GEOGRAPHIC IMAGE COLLECTION]