Archive for February, 2010
The 1990s station dropout does not have a warming effect
Posted by drj | Filed under announcement
Tamino gives his results for his GHCN based temperature reconstruction. It is well worth reading. He also gives a comparison between stations that are reporting after 1992, and those that “dropped out” before 1992. He concludes that there is no significant difference in the overall trend. In other words refuting the claim that the 1990s station dropout has a warming effect. His results are preliminary and for the Northern Hemisphere only.
Tamino’s analysis use only the land stations; in order to write this blog post I tweaked ccc-gistemp so that we can produce a land index (python tool/run.py -s 1-3,5 now skips step 4, avoids merging in the ocean data, and effectively produces a global average based only on land data).
It is very easy to subset the input to ccc-gistemp and run it with smaller input datasets. So in this case I can split the input data into stations reporting since 1992, and those that have no records since 1992, and run ccc-gistemp separately on each input. I created tool/v2split.py to split the input data. Specifically I ran step 0 (which merges USHCN, Antarctic, and Hohenpeissenberg data into the GHCN data) to create work/v2.mean_comb then split that file into those stations reporting in 1992 and after, and those not reporting after the cutoff. Then I ran steps 1,2,3, and 5 of ccc-gistemp to create a land index:
It is certainly not the case that the warming trend is stronger in the data from the post-cutoff stations.
The differences between these results and Tamino’s are interesting. Both show good agreement for most of the 20th century. These data show more divergence than Tamino’s in the 1800’s. Is that because we’re using Southern Hemisphere data as well, or is it because of the difference in station combining? Further investigation is merited.
We hope to make “experiments” of this sort easier to perform using ccc-gistemp and encourage anyone interested to download the code and play with it.
Update: Nick B obliges with a graph of the differences:
On integers, floating-point numbers, and rounding
Posted by Nick.Barnes | Filed under status
Progress continues on the ccc-gistemp project. Anyone interested is welcome to go on over to the source code browse page and peruse it.
- Paul Ollis has done excellent work separating all the I/O code from the main algorithm, and refactoring it so that data can flow through the entire program without passing through several intermediate data files.
- David Jones has made a tool for indexing plain-text data files for random access, and has been working SVG-based visualisation tools. Together, one day these will let us provide a snappy graphical interface for answering questions like “how did the peri-urban adjustment on this station work?”
- I have been working on removing rounding from the whole system. Until now we have often found ourselves having to round values in order to maintain exact equivalence with GISS results (which may have been rounded for output to an intermediate data file which is read by a later phase). For example, rounding temperatures to the nearest tenth degree Celsius, or latitude and longitude values to the nearest tenth degree. I mentioned this in email with Dr Reto Ruedy of GISS, and he assured me that all such rounding is incidental to the algorithm – an accident of history. So we are removing it from our version, to help clarify the algorithm. We will end up with the only explicit rounding in the system being done in order to write the final result files.
- Next I am hoping we will extract the main numerical parameters of the algorithm – for instance, the 1200km station radius for gridding, the 4 rural stations required for peri-urban adjustment – to a separate module, where they can be easily modified by anyone interested in experimenting with different values.
We are aiming for a release 0.4.0 of ccc-gistemp to happen around the end of February or in early March, time permitting. The specification of this version is something like “no I/O, no rounding, and explicit parameters”, and we’re pretty close to that now.
Rounding in GISTEMP has prompted a lot of discussion in the blogosphere, and since I have been working in that area in ccc-gistemp, I thought I could write a few words here to clarify it. There is a lot of general misunderstanding of computer arithmetic, even among professional programmers. I have dealt with the nitty-gritty of it in various capacities in the past, and hopefully can convey some of my expertise.
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NASA GISS wants to use our code
Posted by Nick.Barnes | Filed under Uncategorized
After the release of ccc-gistemp 0.3.0, I contacted Dr Reto Ruedy of NASA GISS to ask him to try out the release and have a look through it.
Dr Ruedy responded, thanking us for our effort, and saying “I hope to switch to your version of that program”. After some further discussion, he clarified this:
When GISS has the resources:
Ideally, we would like to replace our whole code
.
They are busy with other things, and won’t have the resources for quite some time. Also, we will need to do some more work, to interface our code with various GISS tools (such as the station data web page). Nonetheless this is very much to the credit of the whole ccc-gistemp team. Well done, everybody.