Climate change is one of the greatest challenges of our times. We humans must change our behavior in the short run to mitigate the effects of climate change in the long run. Policymakers develop environmental policies to motivate this behavioral change. However, people may nonetheless fail to change to more sustainable practices, defeating the effectiveness of environmental policies. Public communications, such as mass media campaigns, can enhance compliance with environmental policies, but they have not yet lived up to their potential. In this contribution, I propose that environmental communication would be more effective if public communicators took account of the (often counterintuitive) social and psychological processes that influence sustainable behaviors. I present research evidence of the impact of cognitive biases (discounting, control perceptions, optimism bias, denial, defensive avoidance, and reactance), emotions (fear and hope), and expectations (about the intentions of the communication source and other people's environmental behaviors) on the power of environmental public communication as a tool to promote sustainable behaviors. If social and psychological processes are neglected in the design of environmental public communications, messages can backfire and achieve the opposite of what was intended. To improve environmental communications, I propose three communication design suggestions: keep it simple, balance the message, and provide an action perspective.