This introduction outlines the conceptual framework of climate witnessing, a new approach to memory, testimony and cultural engagement with environmental crisis in the Anthropocene. Building on foundational theories of trauma and testimony developed in Holocaust and memory studies, we trace how witnessing has evolved in response to planetary-scale transformations. Expanding the category of witness to include nonhuman entities, ecological assemblages and deep temporalities, we nonetheless emphasise the continuing importance of human mediation and ethical responsibility. Situating this special issue within fourth-wave memory studies, we engage with current debates in ecocriticism, posthumanism and environmental justice, while also reflecting on the risks of overly diffusing agency. The introduction explores the representational challenges of narrating slow and distributed forms of environmental harm, and considers how aesthetic practices and prosthetic memory technologies can mobilise grief and solidarity across species and generations. We propose climate witnessing as both a critical lens and a cultural practice attuned to the urgency and incompleteness of remembering environmental loss amid planetary crisis.