Tanganyika Revisited

Guest Post by Willis Eschenbach

The new Nature Magazine article on Lake Tanganyika, “Late-twentieth-century warming in Lake Tanganyika unprecedented since AD 500”, discussed a couple days ago by Anthony Watts here, was quite interesting to me. In 2003 I had contributed a “Communications Arising” to Nature Magazine regarding earlier claims that AGW was causing productivity loss in the Lake. As a result, I am very familiar with the available records for the lake.

Figure 1. Rainbow over Lake Tanganyika

I was puzzled by the claims in the new article regarding the changes in Lake Tanganyika surface temperatures, because I knew that there was almost no historical data on lake surface temperature. I wondered how they determined the surface temperature of the lake over the past 1,500 years. So I sprung the $18 to purchase the Nature paper and find out …

It turns out that they used a proxy called TEX86, which has been used in other studies. But how did they calibrate the proxy to the lake surface temperature (which they call “LST”)?

Well … they didn’t calibrate it. In their theory, no calibration is needed. However, that seems like a very problematic assumption, as there are always confounding factors for proxies that mean that they need to be calibrated to the instrumental record. Some of these factors are listed in their Supplementary Information.

How well does their reconstruction correspond with air temperatures? Well … rather than compare the reconstruction to local temperatures over the last 50 years, and despite the fact that Lake Tanganyika is in the Southern Hemisphere, they compare the reconstruction to a famous Northern Hemisphere reconstruction …

Figure 2. A most ingenious way to hide the differences between two graphs, by redacting the front information so you can’t see the back information. Note that part (a) uses the discredited Hockeystick and various Hockeystick clones (the so-called “independent reconstructions”) as its basis for comparison.

Commenting on this figure, they say (emphasis mine):

Our LST reconstruction is qualitatively similar to Northern Hemisphere temperature reconstructions (Fig. 3a), implying that Tanganyika LST largely followed global trends in temperature during the past 1,500 years, much as it has in the past half-century. As LST closely tracks air temperatures over the instrumental period, we can also infer that air temperatures in this region of East Africa varied in concert with the global average and thus were controlled primarily by the major forcings influencing temperatures over this timescale, both natural (solar radiation, volcanism) and anthropogenic (greenhouse-gas emissions; refs 19, 20). The temporal resolution of our dataset precludes comparison between Tanganyika LST and volcanic events of the past, but we can compare our record with changes in solar irradiance (total solar irradiance (TSI) anomaly, estimated from 10Be in ice cores21; Fig. 3b). TSI and Tanganyika LST share some similar centennialscale features, including maxima near 1350 and minima at 1450, 1250 and 1000. However, TSI variability clearly does not explain the dramatic twentieth-century increase in LST, which, as with global temperatures, is probably a response to greenhouse-gas forcing.

Unfortunately, in their paper they neglected to show how the Lake Tanganyika LST “closely tracks air temperatures over the instrumental period” of the “past half-century”. To remedy this lacuna, I have plotted the only two longer-term temperature stations on the lake along with the MSU data and the proxy-derived LST:

Figure 3. Ground station temperatures, UAH MSU, and proxy lake surface temperature (LST), 1950-1996

As you can see, while their proxy LST generally agrees with the air temperature over the last half of the record, it does very poorly during the first half. So no, the LST proxy reconstruction does not “closely track air temperatures over the instrumental period.”

Finally, Tierney with some other co-authors have published previously in Science Magazine (subscription required) on the Tanganyika LST. In the current (2010) paper, they say (emphasis mine):

Before the twentieth century, LST varied between 22.5 C and 24.3 C (Fig. 2a). LSTs were relatively warm between ad 500 and 700, followed by an interval of cool LSTs that lasted until ad 1100. Lake Tanganyika then experienced a period of extended warmth between 1100 and 1400, followed by a return to cooler LSTs between 1400 and 1500 and more variable temperatures until 1900. Beginning around 1900, LSTs trend upwards, rising about 2 C in 100 years (see Fig. 2 inset). Our uppermost sample from core MC1 (identified using 210Pb dating as about ad 1996), calibrates to 25.7 C.

OK, so the current paper says that in the last 1,500 years the LST has varied between a low of 22.5 C to a high of 25.7 C. During the last 50 years of the record, their proxy LST value rises by 1.6 C.

And in the current paper, they also say:

Our records indicate that changes in the temperature of Lake Tanganyika in the past few decades exceed previous natural variability.

But in their previous (2008) paper, which used the same TEX86 proxy, they had said:

Holocene lake [Tanganyika] surface temperature (LST) fluctuated between 27° and 29°C …

And during the Holocene, their 2008 paper shows a change of 1.65 C in 50 years, which is larger than the recent change shown in the 2010 paper.

Despite citing the earlier paper in their current paper, they don’t mention these discrepancies … which does make me wonder just how good their proxy is. It also make me curious about what they mean by “previous natural variability”. During the Holocene, by their own figures, the Lake Tanganyika LST was 3 C warmer, and changed temperature faster, than in the last fifty years of their more recent proxy record.

[UPDATE] You know how sometimes you have this nagging feeling that you’ve left something out, and you can’t think of what it was? When I woke up this morning, I realized what I had wanted to say.

This is truly a watershed paper in that it purports to be a study of the changes in lake surface temperature (LST) over time, but they present no measurements of the changes in the LST over time. The only actual surface temperatures mentioned in the paper are the following, all from 2003:

Our uppermost sample from core MC1 (identified using 210Pb dating as about ad 1996), calibrates to 25.7 C. This is within the range of 2003 measurements of seasonal LST for the Kalya Slope area (25.5-26.3 C; see Fig. 2 inset) and is also similar to the annual average LST measured near Mpulungu, at the southern end of the lake (26.1 C; ref. 16).

Unfortunately, reference 16 is very vague. It is:

Descy, J-P. et al. Scientific Support Plan for a Sustainable Development Policy (SPSD II), Part II: Global Change, Ecosystems and Biodiversity Atmosphere and Climate (Belgian Science Policy, 2003).

Research showed this is the Belgian CLIMLAKE project, which I had studied before, and which had some interesting results. Here’s one of them:

Figure 4. Satellite derived lake temperatures. SOURCE – CLIMLAKE FINAL REPORT.

As you can see, on a single day the surface temperature of the lake varies by 4° C from coldest to warmest. I couldn’t find their “2003 measurements of seasonal LST” or their “annual average LST”, although Figure 29 of that CLIMLAKE report does show a three year temperature record for two places on the lake, so I suppose they might have used those.

(As an aside, my high school science teacher would never have allowed such a vague citation as reference 16 above, I’d have gotten a “D” on the paper if not an “F”. “Make it easy to find”, he’d say, “point me right at it. Cite me chapter and verse.” But I digress …)

My point is, the Tierney 2010 report is a study of the change in Lake Tanganyika surface temperature over time, which contains no measurements of the change in LST over time, and which has exactly three actual surface temperature measurements, which are poorly cited, are from different parts of the lake, and are all from 2003 …