Detecting land cover change over semi-arid rangeland is important for monitoring vegetation responses to drought, population expansion, and changing agricultural practices. Such change can be detected using vegetation indices, but these do not represent non-green vegetation and are dominated by seasonal changes. An alternative is to observe spatial changes in thermal emissivities, a measure that responds to soil surface composition and vegetation cover. Because soil emissivities are usually stable, temporal emissivity changes could be due to vegetation cover changes. Using ASTER thermal infrared observations, the technique is applied to observations over the Jornada Experimental Range in New Mexico between 2001 and 2003. The study showed spatially coherent regions where broadband emissivities decreased as much as 3%. These coherent regions may correspond to decreased vegetation densities, suggesting that the technique could be helpful for monitoring rangeland cover