A new study in the journal Nature Cities shows that as temperatures rise aboveground, the number of subway riders reporting uncomfortable heat belowground increases. Northwestern University researchers analyzed thousands of crowdsourced social posts for keywords related to being hot in underground metro systems in Boston, London and New York. This could be increasingly true as climate change, driven by the burning of fossil fuels, makes the planet hotter. The researchers hope that the work influences heat mitigation efforts for commuters using underground infrastructure.