There has been much blog chatter (reviewed here) recently about homogenisation of temperature, and adjustment in general. A few individual stations have been picked out and pored over. But homogenisation is a general effort to reduce bias prior to computing a global average, and the logical place to look for its effect is in that average.
This was something some bloggers were interested in doing back in 2010. Zeke writes on that here. His own investigation was mainly US. At that time, I started the TempLS code, and I've been using it for monthly reporting for over three years. It uses by default unadjusted GHCN land data, with ERSST for ocean. There general experience, noted back in 2010, was that it made little difference. TempLS compares well with the major indices.
In this post, I'll try to quantify that more, using the current improved graphics. I can compare directly the variants of TempLS with and without adjustment, with an active map below. The results are a little surprising, but the end effect is still small. A typical result is for TempLS mesh, where the trend 1910-2014 is 0.711°C/Cen, or 0.759°C/Cen after adjustment. But that is actually a high point of adjustment effect, and over more recent periods, adjustment actually has a cooling effect. Claims that AGW is a creature of adjustments are way off.
My previous post reviewed the analysis of station trends. They tend to be dominated by relatively short periods, even with a cut-off. It is fairly easy for a short term to produce a high trend, but it's the longer terms that contribute most to the average.
Update. I should mention a 2012 benchmarking paper by Victor Venema and many others.
The map below is a variant of the one on the latest data page. I've restricted it to global surface measures, and included the adjusted TempLS, marked as TLS_m_a (mesh) and TLS_g_a (grid). The clearest showing of the effect is with "trendback" mode. This shows the trend from arbitrary start year to present. Start year is shown on the x-axis. I've set it up showing the TempLS mesh unadjusted and adjusted, but you can make other choices. The grid comparison is interesting. For any graph, you can press on Data and it will show a new tab with the numbers. The Trendback button toggles between Trendback and timeseries modes (common anomaly base 1981-2010). Operation details are here.
Looking just at the TempLS mesh curves, from about 1960 back to 1900, the adjusted trend is higher. That is the "cooling the past", but it isn't much. Max about 0.04°C/Cen. After about 1970, the unadjusted trend is higher. If you switch to timeseries most (click Trendback), it seems that adjustment has relatively cooled the global temp in just the last few years, which affects the short term trends. But again, it isn't much. It looks a lot in trend, but short term trends are volatile. The grid version of TempLS doesn't really show this. It could be an Arctic effect.
Another way of looking at it is that the difference between adjusted and unadjusted is about the same as that between GISS and HADCRUT.
Remember that you can drag the plot, change the scale etc. Also, of course, change datasets, regress and smooth.