Plants integrate environmental stimuli to optimize photosynthesis versus water loss by controlling stomatal apertures. However, stomatal responses to temperature elevation and the underlying molecular genetic mechanisms remain less studied. We developed an approach to clamp leaf-to-air vapor pressure difference (VPDleaf) at fixed values and recorded robust, reversible warming-induced stomatal opening in intact plants. We analyzed stomatal temperature responses of mutants impaired in guard cell signaling pathways for blue light, abscisic acid (ABA), CO2, and temperature-sensitive proteins such as Phytochrome B (phyB) and EARLY-FLOWERING-3 (ELF3). We confirmed that leaves of phot1-5/phot2-1 mutants lacking blue-light photoreceptors showed only partial reduction of warming-induced stomatal opening. Moreover, ABA biosynthesis, phyB, and ELF3 were not essential for the warming response. Strikingly, Arabidopsis (dicot) and Brachypodium distachyon (monocot) mutants lacking guard cell CO2 sensors and signaling components, including ht1, mpk12/mpk4-gc, and cbc1/cbc2, lost the stomatal warming response, indicating a conserved mechanism across different plant lineages. Additionally, warming rapidly increased photosynthesis, leading to a reduction in intercellular [CO2]. Interestingly, further heat stress caused stomatal opening independently of photosynthesis. New insights into brassinosteroid signaling pathways will also be presented. We provide genetic and physiological evidence that the stomatal warming response is triggered by increased CO2 assimilation and CO2 sensing in guard cells. Furthermore, increasing heat stress appears to activate a pathway for stomatal opening that is uncoupled from photosynthesis. These findings suggest that our current understanding of the molecular mechanisms mediating heat stress-induced stomatal opening needs revision, and a new model will be presented.