Joint post with Roger Andrews
In my recent post titled The Diverging Surface Thermometer and Satellite Temperature Records, Roger posted 4 charts in the comments that I felt were both interesting and important. For those not up to speed with the importance of comparing surface thermometer with satellite data it boils down to understanding the rate of global warming and whether or not the lower troposphere is still warming as all the climate models predict should be happening. The satellite data (UAH and RSS) shows little to no warming since 1997 – the famous pause – while all the surface thermometer records now show continued warming (e.g. GISS LOTI and HadCRUT4). One of the data sets must be faulty and Roger’s charts cast some further light on this issue.
Here we deal with the charts in the reverse order from Roger’s comment. Figure 1 compares the UAH satellite (over sea only) and Hadley sea surface temperature (HadSST3) records. HadSST3 makes up 71% of HadCRUT4, the combined land-ocean record commonly used to define the Earth’s ‘surface temperature’. The grey trace shows the difference between these two data sets and shows a near flat line. There is literally no difference between surface temperatures and temperatures measured by satellites over the oceans. This suggests that both these data sets are reliable.
Figure 1 Satellite over-sea temperature (UAH) compared with sea surface temperature (HadSST) data. The difference between the two is the grey trace at bottom which shows the gradient of satellite and surface data over oceans are the same.
This points to the faulty data lying in the land based data. Why should the satellites work reliably over the oceans and not over land? We can’t think of a reason. But there is a good reason for suspecting the land based thermometers since these measure temperatures in a totally different way to the SSTs. Roger has long argued that SST and air temperature data over land should not be combined into a single index since they are measuring different things.
If we do the same comparison over land we see there is a difference (Figure 2) with the land based Crutem4 index showing about 0.3˚C more warming than the UAH over-land data since 1980. One needs to recall that the difference between the gross satellite and surface data is tiny, of the order +0.15˚C since 1980. Since the land based data only accounts for 29% of the total, this difference between the land based data sets may account for about 0.09˚C of the gross difference, i.e. most of it.
Figure 2 Comparing the satellite data over land with the land surface thermometer data shows a small difference with the surface thermometers running about 0.3˚C warmer. Enough to explain most of the difference between the global satellite and surface indexes.
We can take this a step further. Comparing the UAH over-land and over-sea we find that the air over land does indeed appear to be warming marginally more rapidly than the oceans. I would feel inclined to put that down to non GHG related human activity such as urban sprawl, deforestation and irrigation.
Figure 3 Comparing satellite data over sea and over land suggests that the land may be warming slightly faster.
If we compare the surface records we see that the land is warming much faster than the oceans, of the order +0.5˚C since 1980. Does this mean that CO2 is a more potent GHG over land? We don’t think so. The different behaviour can be explained by either adjustments made to land surface thermometer records or by land surface thermometers being more sensitive to growing population and land surface changes than satellites. The thermometers are after all normally located close to human habitation.
Figure 4 Comparing the surface thermometer data over land and over sea suggests that the air over land is warming much faster than the oceans.
Finally, in the comments Luis pointed out that HadCRUT3 was largely in agreement with UAH since 1997. So I have checked this out.
Figure 5 Comparison of HadCRUT3, 4 and UAH since 1997. HadCRUT4 re-writes the record books. Note the offset between HadCRUT and UAH is down to different datum / base periods used.
It is indeed the case that since 1997, UAH has been on a flat, slowly declining trend (-0.13˚C per century). HadCRUT3 was on a flat, slowly rising trend (+0.18˚C per century). HadCRUT4 is on a much more steeply rising trend of +0.59˚C per century. With a stroke of the brush, the pause was written out of history.
Satellite and surface thermometer data agree over the oceans. They used to agree better over land until HadCRUT4 supplanted HadCRUT3, ending the pause and causing land surface thermometers to diverge from the satellite data sets.