Citizen science projects have a long history in ecological studies. The research usefulness of such projects is dependent on applying simple and standardized methods. Here, we conducted a citizen science project that involved more than 3500 Swedish high school students to examine the temperature difference between surface water and the overlying air (T_w-T_a) as a proxy for sensible heat flux (Q_H). If Q_H is directed upward, corresponding to positive T_w-T_a, it can enhance CO₂ and CH₄ emissions from inland waters, thereby contributing to increased greenhouse gas concentrations in the atmosphere. The students found mostly negative T_w-T_a across small ponds, lakes, streams/rivers and the sea shore (i.e. downward Q_H), with T_w-T_a becoming increasingly negative with increasing T_a. Further examination of T_w-T_a using high-frequency temperature data from inland waters across the globe confirmed that T_w-T_a is linearly related to T_a. Using the longest available high-frequency temperature time series from Lake Erken, Sweden, we found a rapid increase in the occasions of negative T_w-T_a with increasing annual mean T_a since 1989. From these results, we can expect that ongoing and projected global warming will result in increasingly negative T_w-T_a, thereby reducing CO₂ and CH₄ transfer velocities from inland waters into the atmosphere.