<p>Cold environments, including glaciers, ice sheets, permafrost soils and sea ice, are common across the surface of the Earth. Despite the challenges of life at subzero temperatures, the global cryosphere hosts diverse microbial communities that support biogeochemical cycling and ecosystem functioning in areas where few other organisms can survive. However, the composition and function of cryosphere microbial communities, and the continued existence of cryosphere habitats, are threatened by ongoing climate change, which has disproportionate impacts in polar regions. In this Review, we survey the breadth of cryosphere habitats and the composition, function and unique adaptations of the microbial communities that inhabit them. We outline how climate change can affect these communities and the ecosystem services they provide through short-term changes in substrate availability, enzyme activity and redox potentials as well as longer-term changes in community composition. We also explore the wide-ranging consequences these changes may have for local ecosystems, human communities and the global climate. Finally, we outline the knowledge gaps in cryosphere microbial ecology that contribute to uncertainties about the future of these ecosystems in a warming world.</p>