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

E Hawkins,M Rojas,R Sutton,L Harrington,D Frame,A King,M Joshi

doi:10.1029/2019gl086259

Abstract

AbstractChanges 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.

Highlights

It was first noted that surface air temperatures were increasing at both local and global scales more than 80 years ago (Callendar, 1938; Kincer, 1933)
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
Mahlstein et al (2011, 2012) subsequently demonstrated that the signal had emerged in the observations, especially in the tropics in boreal summer, and with a similar pattern to that expected from climate model simulations

Summary

Introduction

It was first noted that surface air temperatures were increasing at both local and global scales more than 80 years ago (Callendar, 1938; Kincer, 1933). At the time it was unclear whether the observed changes were part of a longer term trend or a natural fluctuation—the “signal” had not yet clearly emerged from the “noise” of variability— Callendar (1938) did suggest that the increase in atmospheric carbon dioxide concentrations was partly to blame. Precipitation changes are apparent in some regions (e.g., Zhang et al, 2007) including in extremes (e.g., Min et al, 2011)

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Conclusion