With respect to anthropogenic climate change, the question of who knew what when is of growing legal significance, as jurisdictions take up questions of climate legal liability. Yet despite an enormous volume of scientific literature and recurring expert assessments, no concise account traces what we know about anthropogenic climate change, how we know it, and when the evidence of it became mature. By "mature" we mean an evidence base that is methodologically standardised, institutionally sustained, and independently cross-validated across multiple instruments and groups. Existing syntheses, whether multi-volume assessment reports, detailed monographs, or consensus analyses, do not provide this information in a form accessible to non-specialist professional audiences. This review assembles such a maturation timeline. For each of the four major indicators of climate change – temperature, sea level, greenhouse gases, and cryosphere (ice sheets, glaciers, snow cover, and permafrost) – we trace the progression of expert knowledge. Typically, it starts with isolated, sometimes idiosyncratic observations, and then goes through methodological standardisation and national institutionalisation, eventually turning into coordinated international observing systems. By presenting these developments side by side, we show that the principal observational pillars of climate science developed into internationally standardised, independently cross-validated evidence bases between the mid-twentieth and the early twenty-first centuries. These observations, combined with climate model simulations of what the world would have been like without human intervention, constitute the empirical foundation of the modern scientific case for anthropogenic climate change, as well as the potential ethical and legal case for climate liability. This latter consideration has gained relevance since the International Court of Justice – in its 2025 advisory on the obligations of states in respect of climate change – affirmed the scientific record as central to establishing breach of international environmental law.