In Which I Go Spelunking …

Guest Post by Willis Eschenbach

In my usual peripatetic wandering around the web, I came across an interesting paper called “Millennial- and orbital-scale changes in the East Asian monsoon over the past 224,000 years”, in Nature Magazine (subscription required), 28 Feb. 2008 , with Supplementary Online Information.

The paper uses “speleothems” to estimate past climate conditions. Speleothems are  secondary mineral deposits formed in caves. Stalactites and stalgmites are speleothems, and they come in a wide variety of sizes and shapes. Here’s a photo of some speleothems:

Figure 1. Speleothems in a New Zealand Cave.

What can we learn from the speleothems?

The authors used the speleothem data from two caves in China to investigate the climate changes over the last two glacial periods, a quarter million years or so. Being more interested in the recent past, and noticing that one of the datasets extended up to the year 1490, I decided to see what speleothems could tell us about the temperature changes in more recent times. So I got a large group of speleothem records from the NOAA Paleoclimatology web site.

I wasn’t interested in what happened thousands and thousands of years ago, so I got all of the long records that covered all or part of the period from the end of the last ice age to the present. This gave me 20 records.

The speleothems give us a record of what is called the “delta oxygen 18” (∂18O) value. This value is related to the temperature. The paper does not give the associated temperature values, so I converted them using the relationship described here as:

This is based on the average d[delta]18O/dT relation in modern precipitation (~0.6‰ °C-1), and the water-calcite fractionation that accompanies speleothem deposition (~-0.24‰ °C-1).

Decoded, this means that the change in temperature is equal to the change in ∂18O divided by (0.6 – 0.24), or ∂18O/0.36. Using that relationship, I calculated the temperatures from the various speleothems, and graphed them all with no further adjustment.

Figure 2. Raw data from 20 speleothem records. All of them have been converted from ∂18O using the relationship Temperature = ∂18O/-0.36. Black line is a 200-point Gaussian average. Different records are different colors.

While this was interesting, it appeared to me that the various records were likely not vertically aligned quite properly. After all, there is no a priori reason to think that they would all fit together, since they were simple anomalies (data minus average of that data) over different time periods.

So how to adjust them? There are several methods that are used to make this kind of adjustment to temperature anomalies for the global temperature records. GISS takes an average of two records in the area where they overlap, and adjusts on that basis. That was possible here, but seemed inaccurate. GHCN, on the other hand, uses one type of “first difference” method. However, their method requires that all of the datasets be on the same basis (annual, monthly, etc.), where in this case the measurements are at various random times that differ between datasets.

After some thought, I realized that I could use “first differences” in another way. The “first difference” is a new dataset that is made by calculating the difference between successive datasets. For example, if the dataset is {1, 2, 4, 8, 10}, then the first difference of that dataset is {(2-1), (4-2), (8-4), (10-8)}, or {1, 2, 4, 2}. This represents the differences between the points in the original dataset.

I realized that the standard deviation of the first difference is a measure of how well the various datasets fit together. (Standard deviation, “SD”, is a measure of how scattered the data is.)

So to adjust them, I first combined all of the 20 speleothem datasets into one single large dataset. Then I took the first difference of that single dataset. I measured the SD of the first difference data.

Then I adjusted each of the individual speleothem records by moving it slightly upwards and downwards, and used the increase or decrease of the SD to indicate which way it should be moved. I repeated this until the match was not improved by further testing and moving of the individual datasets. The result is shown in Figure 3.

Figure 3. Adjusted data from the same 20 speleothem records. All of them have been adjusted vertically to give the best fit. Black line is a 200-point Gaussian average.

This has improved the accuracy of the reconstruction. This is shown by the greater vertical range of the Gaussian average line.

So, what does all this mean? Heck, I don’t know, I’m investigating, not drawing conclusions. A few comments, in no particular order:

•  As is shown in the Greenland ice core records, we are currently at the cold end of the Holocene (the current interglacial).

•  Recent phenomena (Roman Warm Period, Medieval Warm Period, Current Warm Period) are scarcely visible at this scale. So much for the “uprecedented” nature of the recent rise.

•  The polar bears are not in any danger from the recent rise.

•  What’s up with the big jump and drop about 12000 years ago? I have not seen that in the ice core records, but it is present in these speleothem records from around the planet. [Update] A number of people have pointed out that this is almost certainly the “Younger Dryas” event. I hadn’t noticed it in the Vostok record, but a closeup of that record shows it.

•  The amount of the temperature change depends on the coefficient used to translate from d18O to temperature. So the numbers are likely in the right range, but may be somewhat too large or too small.

Anyhow, that’s my thoughts about what I’ve found out, I welcome yours. I continue with the investigation. It strikes me that I may be able to adjust the conversion factor (d18O/T) to see if that improves the fit of the data … should be interesting. Onwards …

DATA:

The caves used in this study were:

Cave, Location

Borneo_sch01, Borneo

Borneo_sch02, Borneo

Buckeye, Central US

Chilibrillo, Panama

Cold_Air, South Africa

Crystal, Midwest USA

Dayu, Central China

Dongge, Eastern China

Dongge04, Eastern China

Dongge05a, Eastern China

Heshang, Central China

Liang_Luar, Indonesia

Lianhua, Southern China

Lynds, Tasmania

Mystery, Midwest USA

Sanbao08, Central China

Sanbao10, Central China

Soreq_Bar, Israel

Spannagel, Austria

Venado, Costa Rica

In two cases, where there were several speleothem records from the same cave analysed by the same investigators, I have combined them into a single longer record. Data from different studies of the same cave have a year (e.g. “08”,”10″) appended to the name.

I have posted the data I used, along with the R file that I wrote to analyze the data, as a zip file here. Enjoy!