Population-based emergence of unfamiliar climates

Time of emergence, which characterizes when significant signals of climate change will emerge from existing variability, is a useful and increasingly common metric. However, a more useful metric for understanding future climate change in the context of past experience may be the ratio of climate signal to noise (S/N)—a measure of the amplitude of change expressed in terms of units of existing variability. Here, we present S/N projections in the context of emergent climates (termed ‘unusual’, ‘unfamiliar’ and ‘unknown’ by reference to an individual’s lifetime), highlighting sensitivity to future emissions scenarios and geographical and human groupings. We show how for large sections of the world’s population, and for several geopolitical international groupings, mitigation can delay the onset of ‘unknown’ or ‘unfamiliar’ climates by decades, and perhaps even beyond 2100. Our results demonstrate that the benefits of mitigation accumulate over several decades, a key metric of which is reducing S/N, or keeping climate as familiar as possible. A relationship is also identified between cumulative emissions and patterns of emergent climate signals. Timely mitigation will therefore provide the greatest benefits to those facing the earliest impacts, many of whom are alive now