We build on social identity models of environmental collective action by developing the Environmental Cognitive Alternatives Scale (ECAS), which measures the ability to imagine what a sustainable relationship between humans and the rest of nature might look like. In Study 1 (N = 386), we developed the initial scale, and found evidence for its construct validity. The ECAS was associated with other relevant social identity theory variables—perceived (il)legitimacy and (in)stability of the current environmental status quo and engagement in environmental collective action, as well as with other environmental variables including pro-environmental consumer behavior and beliefs about anthropogenic climate change. In Study 2 (N = 393), we confirmed the factor structure of the ECAS, found additional evidence for its construct validity, and found that it was a strong predictor of environmental activist identification, explaining variance beyond extensive control variables including identification with nature, perceived (in)stability and (il)legitimacy, and beliefs about anthropogenic climate change. It also explained additional variance in willingness to engage in environmental collective action behavior beyond even environmental activist identification. Our results suggest that the ability to imagine cognitive alternatives to the environmental status quo might have important implications for whether people engage in pro-environmental collective action to mitigate climate-change and other environmental problems.