Climate change is a highly emotional issue. For example, many people experience negative emotions like climate anxiety and eco-depression. Recent work emphasizes that positive emotions also influence how individuals engage with sustainability and pro-environmental behavior. Behaving in an environmentally friendly way may elicit positive feelings (warm glow), which could drive a self-reinforcing positive feedback loop of great applied value. However, previous research has largely been limited to self-reported pro-environmental behavior, which can have low validity. Here, we investigated the relationship between warm glow and pro-environmental behavior using two consequential behavioral tasks. In Study 1 (n = 237), participants who made more pro-environmental decisions in the Carbon Emission Task experienced more warm glow. Pre-registered Studies 2a (n = 803), 2b (n = 953), and 3 (n = 849) used the tedious Work for Environmental Protection Task for a more severe test. Results again showed more warm glow after more pro-environmental behavior. In addition, anticipated warm glow before the task predicted pro-environmental behavior, and behavioral effort mediated the relationship between anticipated and experienced warm glow. Together, these results provide a strong demonstration that warm glow and pro-environmental behavior are mutually reinforcing.