Surface temperature is not so different from models

There's a new paper out in Geophysical Research Letters, which looks at global surface temperature. This time the purpose is to compare the observations with climate models. What's interesting is that the authors picked up something that I didn't know about, though scientists probably did. The surface temperature reported from climate models isn't the same as the surface temperature reported as observations. Almost, but not quite.

When an apples to apples comparison is made between climate models and observations, then one third of the discrepancy disappears. Here is a chart from Kevin Cowtan that illustrates this. (Read on below for further explanation).

Figure 4: Global mean temperature calculated from an ensemble of climate models using the simplest method (red), or using the same method we use for the observations (blue), and some intermediate methods. (Note that all but the blue line are global coverage.) Source: Kevin Cowtan

Kevin Cowtan is the lead author of an all star team, many of whom you'll probably recognise: Zeke Hausfather, Ed Hawkins, Peter Jacobs, Michael E. Mann, Sonya K. Miller, Byron A. Steinman, Martin B. Stolpe, and Robert G. Way.

Kevin Cowtan wrote about this new paper on his blog. He explained how the results of models aren't reported the same way as observations. I've put the differences in the table below. In the models, the ocean surface temperature is measured by the air above the ocean surface. In the observations, it's the sea water temperature, not the air temperature, that's recorded: 

	Land	Oceans
Models	Air above the land surface	Air above the sea surface
Observations	Air above the land surface	Water at the sea surface

As Kevin Cowtan wrote:

While the pattern of warming in the 20th century is similar between the models and the observations, over recent decades the models show a little more warming than the observations. We don't expect individual model runs to match one another, because they have different weather. Similarly, the real world has its own weather, so we don't expect it to match the mean of the models. However it is legitimate to ask if the difference between the models and observations can be explained by weather alone.

Part of the explanation lies with recently addressed problems in the observations (Cowtan & Way 2014, Karl et al 2015) and a temporary slowdown in warming associated with changes in ocean circulation and other natural factors (see this lecture and commentary). However we have identified another factor not included in the IPCC graph and similar comparisons: the graph is not comparing the same things. The results from the models use air temperatures over the whole globe, whereas the observations use a mix of air temperature over land and water temperature for the oceans - Figure 2.

One of the first questions that occurred to me was why this difference is only showing up now. Why were past observations so close to the models? Well, the answer lies in the Arctic, where summer sea ice has been declining rapidly. Kevin put up a diagram to illustrate:

Figure 3: As sea ice retreats, some grid cells change from taking air temperatures to taking water temperatures. If the two are not on the same scale, this introduces a bias. Source: Kevin Cowtan

The bias is introduced because now that the ice is melting, sea surface temperature observations are being recorded in areas that were previously covered in ice. This has the effect of lowering the observations compared to the models. The discrepancy doesn't start to show up until around 2005.

Kevin Cowtan's blog article gives a very clear explanation of the work - you can read it here. Another co-author, Ed Hawkins has an article about it too.

Reaction from conspiracy theorising science deniers

As expected, there is the usual huffery puffery nonsense from science deniers. Anthony Watts must have been in a hurry because he just posted the press release without his usual "claim" headline. He did try to mislead his readers though (without much success), by posting one of John Christy's wrong charts instead of a chart from the paper itself.

From the WUWT comments

Taylor Pohlman doesn't understand that a large part of the discrepancy is because of the recent melting of Arctic sea ice, so there is no major discrepancy in earlier years:

July 30, 2015 at 9:31 am
Because the models are ‘cooked to hind cast this period. The thing is, if the correction he’s talking about gets made to the 1975-1995 peri on, it will then likely be to low to match actual a – like the preverbial thread, this thing just keeps unraveling…

Doug Sorensen cries "squirrel", pointing to the troposphere and away from the surface:

July 30, 2015 at 9:24 am
I think Dr. Cowtan may be juggling his apples a bit. GISS and HADCRUT use “a combination of air and sea surface temperature readings.” However, satellite and balloon data do not. They measure as much of the troposphere as they can, which makes them very much like the model output.

BeefArt asks a useful question, which gets some unhelpful answers:

July 30, 2015 at 9:38 am
Curious why you didn’t use any of the actual graphics from Cowtan et al. They show something dramatically different from Christy’s graphic that you used. Seems a little misleading, don’t you think? 

markstoval wrongly thinks this research was funded by a grant. It wasn't. (Why don't fake sceptics publish their findings if they are so sure of themselves?)

July 30, 2015 at 10:02 am
Dr Cowtan said: “When comparing models with observations, you need to compare apples with apples.”
You do need to compare apples to apples which is why Dr. Cowtan knew to compare the apples to orangutans. (and please send more grant money)

Most of the other comments are equally dismissive and many include conspiratorial accusations of fakery and fudgery. (Anthony Watts blog is a haven for conspiracy theorising science deniers. That's his target audience. Don't expect to ever read anything rational there.)

References and further reading

Kevin Cowtan, Zeke Hausfather, Ed Hawkins, Peter Jacobs, Michael E. Mann, Sonya K. Miller, Byron A. Steinman, Martin B. Stolpe, Robert G. Way (2015), "Robust comparison of climate models with observations using blended land air and ocean sea surface temperatures", Geophys. Res. Lett., 42, doi:10.1002/2015GL064888. (pdf here)

• New study narrows the gap between climate models and reality - press release at ScienceDaily.com
• Blog article by the lead author, Kevin Cowtan
• An apples to apples comparison of global temperatures - Blog article by co-author, Ed Hawkins
• Getting model-data comparison right - blog article by Tamino