Tiny particles of desert dust in the atmosphere can affect Earth’s climate by interacting with radiation. While it is well known that dust reflects sunlight and can cool the climate, it also has a warming effect by trapping heat—much like greenhouse gases do. This warming effect, which comes from dust interacting with longwave (heat) radiation, has increased over time as desert dust levels have risen since the preindustrial era. Our study uses observations and a simple model to estimate how strong this warming is. We find that the effect is roughly twice as large as what current climate models predict, mainly because the models overlook very large dust particles and how dust scatters heat radiation. We also show that this warming peaked in the 1980s and has slightly boosted global warming. Including this overlooked effect in climate models would help make future climate projections more accurate.