There have been several episodes over the past 3.5 million years when climate conditions were warmer than during the late Holocene. These episodes can provide insights into potential future climate impacts and ecosystem feedbacks, especially on timescales that are rarely explored with climate models. I will present the main outcomes of PAGES’ Integrative Activity “The Earth System in a Warmer World”. Our observation-based synthesis of the understanding of past intervals with temperatures within the guardrails of the Paris Agreement suggests that there is a low risk of runaway greenhouse gas feedbacks for global warming of no more than 2°C. However, substantial regional environmental impacts can occur. A global average warming of 1–2°C with strong polar amplification has, in the past, been accompanied by significant shifts in climate zones and the spatial distribution of land and ocean ecosystems. Sustained warming at this level has also led to substantial reductions of the Greenland and Antarctic ice sheets, with sea-level increases of at least several metres on millennial timescales. Comparison of paleo observations with climate model results suggests that, due to the lack of certain feedback processes, model-based climate projections may underestimate long-term warming in response to future radiative forcing by as much as a factor of two, and thus may also underestimate centennial-to-millennial-scale sea-level rise.