A growing body of evidence suggests that most of the world's lakes are supersaturated with CO2 and export significant amounts of CO2 to the atmosphere. Still, the temperature dependence of the partial pressure of CO2 (pCO2) in lakes, which is the main driver of carbon flux across the air-water interface, has not yet been assessed. Analyzing a global-scale database of 4902 lakes, we show that temperature is not an important regulator of pCO2 in lakes. Instead, the concentration of dissolved organic carbon (DOC), a substrate for microbial respiration, explains significant variation in lake pCO2. Contrary to what may be expected from the physiological constraints of temperature, effects of climate change on the carbon balance of lakes may not be due to rising temperature per se, but rather to climatically induced changes in the export of DOC from terrestrial soils to aquatic habitats.