Observed emergence of the climate change signal: from the familiar to the unknown

Ed Hawkins, Luke Harrington, Dave Frame, Manoj Joshi, A. King, M. Rojas, R. Sutton

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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.

Original languageEnglish
Article numbere2019GL086259
Number of pages10
JournalGeophysical Research Letters
Issue number6
Early online date10 Mar 2020
Publication statusPublished - 28 Mar 2020


  • emergence
  • extremes
  • observations
  • rainfall
  • signal-to-noise
  • temperature

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