Recognizing how climate change will impact populations can aid in making decisions about approaches for conservation of endangered species. The Blunt-nosed Leopard Lizard (Gambelia sila) is a federally endangered species that, despite protection, remains in extremely arid, hot areas and may be at risk of extirpation due to climate change. We collected data on the field-active body temperatures, preferred body temperatures, and upper thermal tolerance of G. sila. We then described available thermal habitat using biophysical models, which allowed us to (1) describe patterns in lizard body temperatures, microhabitat temperatures, and lizard microhabitat use, (2) quantify the lizards’ thermoregulatory accuracy, (3) calculate the number of hours they are currently thermally restricted in microhabitat use, (4) project how the number of restricted hours will change in the future as ambient temperatures rise, and (5) assess the importance of Giant Kangaroo Rat burrows and shade-providing shrubs in the current and projected future thermal ecology of G. sila. Lizards maintained fairly consistent daytime body temperatures over the course of the active season, and use of burrows and shrubs increased as the season progressed and ambient temperatures rose. During the hottest part of the year, lizards shuttled among kangaroo rat burrows, shrubs, and open habitat to maintain body temperatures below their upper thermal tolerance, but occasionally, higher than their preferred body temperature range. Lizards are restricted from staying in the open habitat for 75% of daylight hours and are forced to seek refuge under shrubs or burrows to avoid surpassing their upper thermal threshold. After applying climatic projections of 1 and 2˚C increases to 2018 ambient temperatures, G. sila will lose additional hours of activity time that could compound stressors faced by this population, potentially leading to extirpation. Finally, temperature-based activity estimation (TBAE) is an automated method for predicting surface activity and microhabitat use based on the temperature of an organism and its habitat. In an attempt to lessen impacts on sensitive species and costs, we assessed continuously logged field active body temperatures as a tool to predict the surface activity and microhabitat use of an endangered lizard (Blunt-nosed Leopard Lizard, Gambelia sila). We found that TBAE accurately predicts whether a lizard is above or below ground 75.7% of the time when calculated using air temperature, and 60.5% of the time when calculated using biophysical models. While surface activity was correctly predicted about 93% of the time using either method, accuracy in predicting below ground (burrow) occupancy was 62% for air temperature and 51% for biophysical models. Using biophysical model data, TBAE accurately predicts microhabitat use in 79% of observations in which lizards are in the sun, 47% in the shade, and 51% in burrows. Heliotherms bask in the sun, and thus body temperatures can shift rapidly when the animal moves to a new microhabitat. This sensitivity, makes TBAE a promising means of remotely monitoring animal activity, particularly for specific variables like emergence time and surface activity.