The ‘signal’ of a warming climate is emerging against a background ‘noise’ of natural internal variability. Both the magnitude of the signal and the noise vary spatially and seasonally. As society and ecosystems tend to be somewhat adapted to natural variability, some of the impacts of any change will be felt when the signal becomes large relative to the noise. So, it is important to note where and when this might occur.
The concept of a ‘time of emergence’ (ToE) has been discussed by several authors and the IPCC AR5 includes a section (18.104.22.168.2) on when and where the temperature signal will emerge. Each model’s ToE is defined using the standard deviation from a control simulation as the noise (N), and the temperature change since the mean of 1986-2005 as the signal (S).
The figure below (Fig. 11.11) shows maps of the median year in which S/N becomes larger than 1. The darker red colours, indicating earlier ToE, are found in the tropics. Even though the signal of change is not largest in the tropics, the variability tends to be smaller than at higher latitudes, thus giving earlier emergence times. The histograms illustrate the uncertainty in ToE for particular area averages, which is due to uncertainty in both the signal and the noise in different models.
In summary, near-term increases in temperature, relative to background variability, are expected to occur more rapidly in the tropics and sub-tropics than in mid-latitudes. Note however that this ToE metric is likely to be very different for other climate variables such as precipitation, winds or extremes.