GIStemp: Goddard Institute for Space Studies, temperature Series.
If we would study global temperature change over time, we need a temperature record over time, and over the globe. GIStemp attempts to create a temperature history with full coverage over time and over space. Unfortunately, the (GHCN – Global Historical Climate Network) data start with one thermometer in Germany: Berlin Tempel in 1701
Over time, thermometers are added, and they slowly migrate south and to both the new, and old, worlds. Eventually, about 1900 A.D., there are sufficient thermometers on the globe to get a partial idea what is happening. But climate is subject to cyclical changes. Some, like the Pacific Decadal Oscillation, have about a 40 to 60 year full cycle length. Others, like solar cycles that run 178 years, and Bond Events – a 1500 year cycle, are a bit longer. A 100 year record is inadequate to allow for these events.
At its core, GIStemp tries to bridge this gap, both in time and in space, between the one thermometer and the globe, and between the 100 years and the 1500. This is a noble goal, but is just “A thermometer too far” to bridge.
First, it glues together some added data from Hohenpeissenburg, and from the Antarctic research stations. It squashes together the U.S. data from USHCN with the same U.S. data from GHCN. And to deal with the poor spacial coverage in the 1700’s, it deletes everything older than 1880. (While this gives a smoother spacial coverage, it does not handle the past quite as well; it now “starts history” at the bottom of the end of the Little Ice Age.)
In many cases, our thermometer record is made of fragments. A thermometer may appear in the record for a decade (sometimes less) then disappear just as quickly. W.W.II caused a great ‘drop out’ of Pacific Island thermometers, for example. The growth of The Jet Age added thermometers at vacation spots around the globe at Tropical Vacation Destinations, but not all “stuck”. And folks move to new homes. So we have a thermometer here, and it moves there. Two records from different places. One over grass near the woods, the next over tarmac at the Jet Airport. GIStemp tries to stitch these patchworks together into a smooth quilt of coverage. Some thermometers get stretched this way or that (over time and over space). Some get their temperatures adjusted higher or lower (via a thing called “The Reference Station Method”) to better join with their neighbours. Where needed, missing data are often fabricated to try and glue the bits together. If a piece, even after such a stretch, is shorter than 20 years, it gets thrown away.
In the end, we are still left with gaps. (The entire southern hemisphere ocean band has less than 1% of the thermometers, and those are at the airport on a few specific islands for the most part). So we have a patchwork quilt, but with some rather large holes, and some pieces are stretched out of all recognition. (A thermometer may be stretched to 1000 km away. Rather like saying that London is a good proxy for the beach in the south of France.)
Some places have changed over time. Cities grow, and get hotter, as they fill with cars, tarmac, heaters or A/C vents, airports and jet engines, and coal or nuclear power plants. To adjust for this, GIStemp looks at “nearby” stations up to 1000 km away and guesses who is rural and who is urban and “adjusts” for it. Unfortunately, like all guesses, this sometimes does not work well. Large airports are often marked as “rural” since they have few residents living there. The largest US Marine Air Station, Quantico Virginia, is classed as rural, for example. Pisa Italy takes a look at Hohenpeissenburg on the German approach to the Alps as a ‘nearby’ rural station and Pisa promptly has it’s past made colder (an odd way to adjust for the present being too warm… making it look even warmer in comparison).
So we’ve ironed out our quilt, even if some bits stuck to the iron and got scorched a bit and others were melted and smeared.
At this point, the globe is divided into a “grid” of “boxes”. The data that we do have (after the stitching and stretching and ironing and…) are now assumed to be pristine and pure and suitable for telling about even more places where we have no data. A station of the record may now fill in a set of boxes on the grid up to 1200 km away. This means, for example, that the airport on Diego Garcia can “fill in” the ocean covering an area roughly the size of Western Europe.
In the final steps, the grid of boxes is compared to the past for those grids of boxes (said past having been dutifully made up if need be) and an “anomaly map” is made which would then show that the ocean 1200 km out to sea (but reflecting the tarmac at the new military jet airport in Diego Garcia today) is now warmer than when a passing ship dunked a bucket in it during a passage of the 1950’s. (Or a Ship of the Line passing in the late 1800′s. Hadley CRU provides historical sea surface temperature anomalies that are merged at the very end, as an option).
Is an Anomaly an Odd Thing?
If you compare a temperature now with what it has typically been, the difference is the “anomaly”. If the average is 15 C, and today is 16 C; you have a 1 C “anomaly”.
It is important here to note that GIStemp creates an anomaly map. I have frequently run into folks who assert that “Since GIStemp uses anomalies and not temperatures, changes of thermometer locations will have no effect.”. But in reality, GIStemp uses averages of thermometer readings, sometimes dozens of them, to create an anomaly map. You can not use the nature of the product to protect you from the process…
One final note: There has been A Great Dying lately for thermometers. Since about 1990, there has been a reduction in thermometer counts globally. In the USA, the number has dropped from 1850 at peak (in the year 1968) to 136 now (in the year 2009). As you might guess, this has presented some “issues” for our thermal quilt. But do not fear, GIStemp will fill in what it needs, guessing as needed, stretching and fabricating until it has a result.
In Japan, no thermometers now record above 300 meters. Japan has no mountains now. For California, where we once had thermometers in the mountain snow and in the far north near Oregon; there are now 4 surviving thermometers near the beach and in the warm south. But GIStemp is sure we can use them as a fine proxy for Mount Shasta with it’s glaciers and for the snows and ice of Yosemite winters.
In the end, it will produce it’s quilt. Scorched in some spots? Sure. A few holes, some patched over with tropical airport tarmac? Well, yes. But a fine quilt all the same! Bright thermal reds sometimes reaching far out to sea and way up north. And even reaching from an Island near the Falklands (Base Orcadas) over to Antarctica for those years before we had thermometers on the continent.
A patchwork quilt I’m sure we can all trust to keep us comfortable.
After all, we only have this history, so we must make do with what we’ve got. Even if it isn’t enough and even if riddled with holes. And even if, in re-imagining it, some parts get melted, scorched and smeared. Otherwise we’d have to admit we just don’t have the data to describe the globe in such detail in the past; and that would not be very comforting at all.
For a more “terse” summary of the issues with AGW, you can see: