It is now almost certain that 2023 will be the warmest year on record, likely the warmest in at least 125,000 years. Multiple temperature records have been broken, with global average temperatures well above 1.5°C at times. The loss of Antarctic sea ice is accelerating at an alarming rate, along with many other indicators of rapid climate change. Does this mean that 2023 is the year that parts of the climate are in a much more dangerous state? Most people expect that when a system, such as someone’s body, an ecosystem, or part of the climate system, is put under stress, it will respond in a fairly predictable way—double the pressure, double the impact, and so on. This is true in many cases, but not always. Sometimes a system under stress changes steadily (or “linearly”) up to a point, but much larger or abrupt changes may be locked in. (By James Dyke, University of Exeter and David Armstrong McKay, Stockholm University)