September global temperatures – why is the HadCRUT series lagging behind GISTemp?

I was surprised to see the record high global temperature anomaly of +1.0°C for September 2020 in the GISTemp series, especially when September 2020 was only the third warmest in the HadCRUT series. It doesn’t help that the Americans use the 1951-1980 LTA whilst the British have moved on to use the much later 1981-2010 LTA. Not only is there a marked difference in September anomalies compared with other years, but the linear trend for the last thirty years are also widely different as well, with the GISTemp series for September showing a rise of 0.249°C per decade (that’s a whopping 2.49°C per century), whilst the HadCRUT series is 0.053°C lower with a rise of o.196°C per decade. Those different rates of change are quite large and are not just limited to the month of September as I have remarked upon before. These may seem very small almost trivial differences to most people but that’s what rises in global temperatures are gauged in these days. I have never read anyone on social media who has remarked about these differences in what are the two leading global temperature data series which I find rather strange.

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