Global warming should have a deep impact on thermoregulation by ectothermic animals such as lizards, because they rely on behavior to select the microhabitats that allow them to keep their body temperature within their preferred -and physiologically optimal- range of temperatures. However, there is a paucity of studies that use the same methods, after a long enough period of environmental warming, to evaluate how much the body temperatures and regulatory behaviors of a given population of lizards have been affected by climate change. We chose a population of Psammodromus algirus lizards inhabiting a well-conserved temperate open forest, and we compared several thermoregulation parameters between the summers of 1997 and 2017: Body temperatures (Tbs) of field-active lizards, Environmental operative temperatures, that are the temperatures ‘available’ to lizards and can be estimated using randomly distributed physical models that integrate environmental heat loads (air and substrate temperature, solar radiation, wind speed, etc.) in the same way as lizards do, The extent to which both Tbs and operative temperatures deviate from the summer ‘target’ range, which for this species is 32.8-36.9 ºC (the ‘target’ range, or Tsel, is measured in a laboratory thermal gradient where lizards can select their Tbs with no physical or biotic restrictions); and The selection (use as opposed to availability) of sunlit and shaded patches along the day, that can be estimated by comparing the distributions of lizards (use) and randomly positioned physical models (availability). Both Tbs and environmental operative temperatures increased over the 20-years period examined; in 2017, 52% of field Tbs (N = 65) were above the target range, in sharp contrast with the Tbs selected by lizards in a laboratory thermal gradient, where only 2% of 43 Tbs were above the target range. Also, the selection of shaded patches in the field (87% of lizards in full shade, against less than 1% in full sun) was more intense than in 1997. In spite of this, both Tbs and their average distance to the target range were larger in 2017 (when only 8% of all available operative temperatures were within or below Tsel) than in 1997 (when most shaded patches offered favourable thermal opportunities, with plenty of operative temperatures within or below Tsel). Thus, our data suggest that the low availability of operative temperatures within or below Tsel, at least in mid-summer days, is forcing lizards to reach Tbs above their preferred range. This implies that the effects of global warming are already hindering the ability of reptiles to buffer environmental change by behavioral means (although lizards could compensate these detrimental effects by being active earlier in the year and/or enlarging their daily activity time in early spring or late autumn).