Adjusting the historical climate record

One of the favourite claims by the denialists is that the temperature record has been adjusted up in recent times and reduced in the past, to give the effect of rising temperatures over the last 6 or 7 decades, and that without these adjustments the records would show that global temperatures have not actually risen.

In fact, the long term rise in the un-adjusted record is greater than the rise in the adjusted record.  In the chart below, from Carbon Brief, the blue line is calculated using the "raw" (un-adjusted) data, the red line is the adjusted data.

Most of the adjustment upwards prior to 1940 is due to adjustments to the sea temperature anomaly. If you just look at the temperature anomaly of land alone, it has risen faster since the 1880s than the un-adjusted data, though not by much, and has been practically identical since the 1960s.

The article in Carbon Brief by Zeke Hausfather is long but instructive.  It's worth reading.

One final point.

It’s also worth noting that adjustments to temperature records are not decided by one single group of scientists. Rather, multiple different research teams have independently created their own land and ocean temperature records. 

While much of the underlying raw data is the same, each takes a somewhat different approach to adjustments and how to deal with areas of the earth with missing data. The resulting global temperature records from five different groups, along with the raw data, are shown in the figure below.

The raw data are easily available.  The denialists are welcome to do their own calculations.  Strangely enough they don't want to.

(All charts and quotes from Carbon Brief: Explainer: How data adjustments affect global temperature records)