Anomaly Baselines Do They Matter?
February 8, 2010Posted by on
Scientifically and to people in the know of how Anomaly maps and graphs work, no they don’t because they know the “zero point” is an arbitrarily selected average and that is all. To these people they know the amount of the temperatures above or below this picked zero point has no real meaning, the only thing of importance is the trend. Is the trend rising or falling and by how much.
To someone that has not looked into this nor knows the science of it all yes the Baseline does matter and you can see this by the way people in the climate community call that Baseline that is arbitrarily picked “the normals”. When NASA GISS says that the baseline of 1951-80 is the “normals” to Joe Sixpack he hears something different. What he hears is that the average of the temperatures of that period is The Normal Temperature of The Planet. So from that when he looks at that graph any temperature above the “normal temperature of the planet” is hot, any below is cold. The more above the line the worse off we are, so we must get rid of the CO2. In the first Graph I’m going to show is from GISS based on their 1951-80 Baseline from between the years 1980 – 2000:
So here in Figure 1 we see a graph that has the entire temperature above what GISS has determined the “normal” period, thus leaving an impression that the world is hotter then normal and as the trend line shows getting worse.
Now lets see what happens when I switch to a different Baseline, the Baseline of 1979 – 98 which has been used by the satellites. First I have to convert the Anomalies from the 1951-80 Baseline back into temperatures. When you visit the GISS site you will find they state that 14 deg C is the number to use. So I just add 14 to all the anomalies and get temps. Then I just take the temps from between the years 1979 to 1998 add them together and divide by 20. This now gives me the average for that Baseline. I now subtract that from the temps and I get the Anomalies for that baseline.
Here in Figure 2 when Joe Sixpack looks at it he will get a different impression. What he see’s is that back at the start of the graph the temps were a little low, then they got to “normal and fluctuated around before jumping up. However at the end of the graph he sees the temps falling back to almost reach the “normal” temp. Yes the trend shows warming but to Joe it doesn’t look that bad since we are not that much above the normal temp.
Now ask yourself this question which one of these two graphs is going to persuade not just Joe Sixpack but Paul the Politician that there is a problem that needs to be addressed. The Graph that fluctuates around zero or the one with the temperature completely above zero.
Now some might want to argue the point that this is all conjecture and just my opinion, to which I would respond that it is not mine alone it is also the opinion of Dr. Phil Jones and some of the other “Climategate” scientists. In the Emails one of the reasons Dr. Jones did not want to switch the CRU’s normals from 1961-90 to the updated ones was that it would reduce the “impression” of warming.
First up David Parker from the UK Met Office:
There is a preference in the atmospheric observations chapter of IPCC
AR4 to stay with the 1961-1990 normals. This is partly because a change
of normals confuses users, e.g. anomalies will seem less positive than
before if we change to newer normals, so the impression of global
warming will be muted. Also we may wish to wait till there are 30 years
of satellite data, i.e until we can compute 1981-2010 normals, which
will then be globally complete for some parameters like sea surface
Followed up by remarks from Phil Jones:
Just to reiterate David’s points, I’m hoping that IPCC will stick with 1961-90.
The issue of confusing users/media with new anomalies from a
different base period is the key one in my mind. Arguments about
the 1990s being better observed than the 1960s don’t hold too much
water with me.
There is some discussion of going to 1981-2000 to help the modelling
chapters. If we do this it will be a bit of a bodge as it will be hard to do things properly for the surface temp and precip as we’d lose loads of stations with long records that would then have incomplete normals.
If we do we will likely achieve it by rezeroing series and maps in an ad hoc way.
There won’t be any move by IPCC to go for 1971-2000, as it won’t help with satellite series or the models. 1981-2000 helps with MSU series and the much better Reanalyses and also globally-complete SST.
20 years (1981-2000) isn’t 30 years, but the rationale for 30 years isn’t that compelling. The original argument was for 35 years around 1900 because Bruckner found 35 cycles in some west Russian lakes (hence periods like 1881-1915). This went to 30 as it easier to compute.
Personally I don’t want to change the base period till after I retire !
Now I bolded some key sections. David Parker points out that warming will be muted if they switched to a more modern “normal”. Dr. Jones points out two very important pieces of information here also. The first being the whole “Anything less then 30 years is not climate” meme is crap, nothing but utter crap. When they started all this they selected 35 years because someone found 35 year cycles and they only went to 30 years because it was easier to compute. So the next time you see “It’s not 30 years” refer them to Dr. Jones.
Now the other part from Dr. Jones I bolded is more important. In it he stated that switching to a more modern time period would cause them to lose a lot of stations with long time records. This ties back to what he said just one sentence earlier to how the argument that the 90’s were better observed then the 60’s doesn’t hold much water with him, and he has a very good point if you are looking at it from surface stations only. There was better observations in the 1960’s then in the 90’s in the datasets as shown by the drop out in GHCN starting in the 1990’s. The only way the 90’s are better is through satellites. Now why is Dr. Jones worried about the loss of those stations with long records? Simple the loss of them will change the trend line.
This assertion that having stations in the Baseline that are not in the latter years of the 2000’s and that changes the trend is put forth by EM Smith and as I will show in a upcomming post that yes changing the number of stations does change the trend line and I will do this using the same stations I used in my NIWA comparison for GHCN. It is a very simple way to check, take a small area such as New Zealand and show the trend with all stations that fit the Baseline period in. Then take out every station that does not also survive past 2005 and recompute. In the case of GHCN for NZ that took the number of stations down from 12 to just 4.
Now if you think that email was a one off think again. The one above is from 2005 but here is another one from 2004 from Dr. Jones:
I will be around tomorrow (so Dec 21) until Dec 23 inclusive. Then again from Jan 3.
I will be checking email during the break from Dec 28 onwards.
Are you in control of the glossary additions and modifications?
As to change of base period – this seems like a decision for the whole of WGI. To redo the global temperature average, I can just move the series up/down, but this isn’t the correct way to do it. I should talk out a new base period from all the individual stations and recalculate anomalies for the oceans. For the oceans this isn’t a problem, but the land it is a serious problem. Many stations have good (i.e. near complete base periods for 1961-90) but I’ll lose hundreds, maybe over a thousand, stations if I went to 1981-2000.
For both surface temperature and precipitation we don’t have spatially complete datasets (like models) so it will be quite difficult.
For the circulation indices (like SOI and NAO) based on station pairs there is a variance term (SD). Some of the character of the series will change. We could easily adjust all these series by simple offsetting but it isn’t doing it properly.
I’m in the throws of a project with the HC checking all the 61-90 normals we have for series that are incomplete, to ensure we don’t have any biases. This has taken quite a time and I don’t want to waste the effort.
The arguments of Albert and Dave make a lot of sense – continuity with the TAR etc. These sort of things can be explained, but then the FOD will not be compatible with
all the papers we are referring to. This will lead to lots of confusion. I would like to stick with 1961-90. I don’t want to change this until 1981-2010 is complete, for 3 reasons : 1) We need 30 years and 81-10 will get all the MSU in nicely, and 2) I will be near retirement !! 3) is one of perception. As climatologists we are often changing base periods and have done for years. I remember getting a number of comments when I changed from 1951-80 to 1961-90. If we go to a more recent one the anomalies will seem less warm – I know this makes no sense scientifically, but it gives the skeptics something to go on about ! If we do the simple way, they will say we aren’t doing it properly.
Best idea might be to show some maps of 1981-2000 minus 1961-90 to show spatially where it makes a difference for temp and precip. Showing it is quite small and likely within the intermodel differences for years which are only nominally 1981-2000. This might
keep both sides happy.
We also probably need to consider WGII. Also the paleo chapter will find 1981-2000 impossible. 1961-90 is difficult for them but not insurmountable.
Earlier in the email chain there is this:
It is obviously possible to use 1980-2000 though it would require some data-processing work. The main objection is that anomalies (of temperature)would appear to be reduced relative to previous
publications and readers/policymakers could become confused. A minor objection is that 1980-2000 is a bit short. Satellite data are of course
in its favour. In due course, 1981-2010 will be ideal!
Notice the main objection is that the perception of warming will go down and might confuse policymakers.
In conclusion scientifically it will make no difference changing Baselines on the trend from a computed Global average temperature set, but you can influence people that don’t know that by just changing the perception and isn’t that what Climate Alarmism is all about? That there is such dire consequences from “warming” caused by man that we need to take immediate action. The graph in Figure 1 is much better suited for that then the graph in Figure 2.