Uncertainties in the rates of small-scale mixing of greenhouse gases in the upper troposphere–lower stratosphere, between about 10 and 20 kilometers above Earth's surface, may be affecting predictions of how warm the Earth's surface could get in the coming decades, a new study reports. Both small-scale mixing processes and large-scale transport of atmospheric gases by winds affect the concentration of gases and hence the composition of the upper atmosphere, a region that modulates heating at the surface of the Earth. Scientists have a good understanding of the uncertainties in the rate of emission of greenhouse gases as well as those in the large-scale transport of atmospheric gases and aerosols by winds. However, they lack a similar rigorous understanding of the uncertainties in the rate of small-scale mixing, the process that smoothes out gradients in the concentrations of gases across the troposphere-stratosphere and ultimately affects climate projections.