Observed Emergence of the Climate Change Signal: From the Familiar to the Unknown

Abstract

Changes in climate are usually considered in terms of trends or differences over time. However, for many impacts requiring adaptation, it is the amplitude of the change relative to the local amplitude of climate variability which is more relevant. Here, we develop the concept of “signal‐to‐noise” in observations of local temperature, highlighting that many regions are already experiencing a climate which would be “unknown” by late 19th century standards. The emergence of observed temperature changes over both land and ocean is clearest in tropical regions, in contrast to the regions of largest change which are in the northern extratropics—broadly consistent with climate model simulations. Significant increases and decreases in rainfall have also already emerged in different regions with the United Kingdom experiencing a shift toward more extreme rainfall events, a signal which is emerging more clearly in some places than the changes in mean rainfall.

Emerging new climate extremes over Europe

“…As in Hawkins et al (2020), we estimate the S/N by regressing seasonal local climate variations onto seasonal global mean surface temperature (GMST) change, that is, where L(t) is the observed local anomaly of some climate metric, G(t) is a smoothed version of GMSTs for the period 1951-2016, and α and β are the parameters of the linear model. We limit the analysis to the period 1951-2016 since this is the period available for the impact-related indices (see description of the indices below).…”

“…In this context, Frame et al (2017) describe how the climate changes with respect to past experience using the terms' unusual' (S/N > 1), 'unfamiliar' (S/N > 2) and 'unknown' (S/N > 3). Using these categories, Hawkins et al (2020) calculate the S/N of changes in temperature and precipitation observations and show that many regions are already experiencing a climate that would be unknown by late nineteenth century standards. Over Europe, changes in temperature and temperature extremes have been detected and attributed partially to anthropogenic influences both in models (e.g., King et al 2015) and observations (Mahlstein et al 2012).…”

Emerging New Climate Extremes Over Europe

“…As in Hawkins et al (2020), we estimate the S/N by regressing local climate variations onto GMST (global mean surface temperature) change…”

“…In this context, Frame et al (2017) describe how the climate changes with respect to past experience using the terms' unusual' (S/N>1), 'unfamiliar' (S/N > 2) and 'unknown' (S/N > 3). Using these categories, Hawkins et al (2020) calculate the S/N of changes in temperature and precipitation observations and show that many regions are already experiencing a climate that would be unknown by late 19th century standards. Over Europe, changes in temperature and temperature extremes have been detected and attributed partially to anthropogenic influences both in models (e.g., King et al 2015) and observations (Mahlstein et al 2012).…”

Reducing uncertainty in local climate projections

“…There is now clear evidence that the recent increase of the average Earth's temperature is mostly due to human activities [1]. Concurrently, the anthropogenic influence is not detected everywhere at the local scale [2,3]. Natural climate variability can blur the emergence of the anthropogenic signal for the next few years at high latitudes, while a significant warming is already reported in several tropical regions [4,5].…”