Much attention is rightly given to changes in global mean surface temperature – it is the key metric for assessing how our climate is changing and evaluating mitigation strategies. However, no-one directly experiences changes in global mean temperature – it is only through local variations that changes in climate are felt.
So, what have global temperatures ever done for us?
[with apologies to Monty Python’s Life of Brian]
It is perhaps surprising that the fingerprint of a warming climate is clearly emerging on very small spatial scales and our new paper examines this topic.
Consider central England – an area which occupies just 0.005% of the planet’s surface. For this region, the year-to-year variability in annual temperatures is much larger than for a global average (Figure 1) – this is obviously because such small areas are sensitive to fluctuations in the weather, and these fluctuations are spatially averaged in a global mean.
But, note that the familiar features of the global temperature record are clearly visible by eye in the central England record too, just overlaid by the large annual fluctuations. When averaging over multi-decadal timescales, the correlation between the global and local timeseries is r = 0.96. In this instance, averaging over time is similar to averaging over space (also see animation in Figure 2).
For summer precipitation in the same central England region, the signal of a changing climate is far less clear – there is a small decline in rainfall with increasing global temperatures, but the correlation on multi-decadal timescales is much lower than for temperature (r = –0.48).
Is this type of relationship true in other locations? Figure 3 shows the regression between local and global annual temperatures. In nearly every location, local temperatures are related to global changes, with generally larger regression parameters over land as expected. Global temperatures explain large fractions of the local variance in temperature in most locations.
But not everywhere. One obvious example is the North Atlantic, where local temperatures vary in a way that is apparently unrelated to global temperature (Figure 3). This is thought to be due to large fluctuations in the strength of the deep ocean overturning circulation which cause the observed variations in this region. The observations of this deep ocean circulation show that is slowing currently, although it is not yet clear why.
To summarise, if you had measured temperatures in your back garden for the last 150 years you would probably be able to see the signal of global temperature change emerging from the noise of local fluctuations. For rainfall, this would be far less clear. Conversely, global average temperature describes the overall trajectory of temperatures at virtually every location – which is one of the reasons why it is such an important metric of climate change. Read more
Paper: Sutton, Suckling & Hawkins, 2015, ‘What does global mean temperature tell us about local climate?’, Phil. Trans. A, 373, 20140426, doi: 10.1098/rsta.2014.0426