Carbon dioxide levels are rising globally, and it is important to understand how this rise will affect plants over long time periods. Volcanoes such as Mammoth Mountain, California, have been releasing CO₂ from their flanks for decades, and this provides a test environment in order to study the way plants respond to long-term CO₂ exposure. We combined several airborne measurements to show that plants may have fewer, more productive leaves in areas with increasing CO₂.