An oversight in historical weather records means we have underestimated how much the climate warmed last听century. The finding means we are 0.1掳C closer to passing the internationally-agreed limit of 2掳C than we thought.
鈥淕lobal warming has been stronger than we think,鈥� says Rasmus Benestad of the Norwegian Meteorological Institute in Oslo, who led the analysis.
The problem stems from around the world. 鈥淎t the start of the temperature records, you have most measurements from Europe and North America, and also along the trade routes,鈥� says Benestad. 鈥淭he area that was covered was about 20 per cent of the Earth鈥檚 area.鈥�
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This has been a favourite theme of climate deniers, some of whom have argued that the temperature record is too incomplete to be reliable. But in fact temperatures tend to be similar across regions.
鈥淚f you have a warm winter in Britain, you tend to have a warm winter in most of Europe,鈥� says Benestad. As a result, climatologists have largely been able to fill in the gaps in the early record, and there is a high degree of confidence that the Earth has warmed due to greenhouse gas emissions.
However, Benestad and his colleagues found a subtler problem. The early weather stations were all in regions where the temperature does not vary too much from month to month. Only in later decades were stations built in places like Siberia, where month-to-month changes are larger.
To find out if this was a problem, Benestad鈥檚 team ran computer models of the global climate for 1861 to 2017, and noted how the simulated global average temperature changed. Then they calculated the global average again, this time limiting the models to only use data corresponding to the weather stations that were present in each year.
They found that choices of early weather stations did create a problem. The older temperature records came out slightly too warm, while more recent ones were slightly too cold. The net result was that the warming between 1881-1910 and 1986-2015 had been underestimated by 0.1掳C.
The upshot is that the 鈥渟afe space鈥� in which we can keep burning fossil fuels is smaller than we thought.
Journal reference:听Geophysical Research Letters,听
