Citizen science shows systematic changes in the temperature difference between air and inland waters with global warming.

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-T) as a proxy for sensible heat flux (Q). If Q is directed upward, corresponding to positive T-T, it can enhance CO and CH emissions from inland waters, thereby contributing to increased greenhouse gas concentrations in the atmosphere. The students found mostly negative T-T across small ponds, lakes, streams/rivers and the sea shore (i.e. downward Q), with T-T becoming increasingly negative with increasing T. Further examination of T-T using high-frequency temperature data from inland waters across the globe confirmed that T-T is linearly related to T. Using the longest available high-frequency temperature time series from Lake Erken, Sweden, we found a rapid increase in the occasions of negative T-T with increasing annual mean T since 1989. From these results, we can expect that ongoing and projected global warming will result in increasingly negative T-T, thereby reducing CO and CH transfer velocities from inland waters into the atmosphere.