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Unprecedented Warming in Lake Tanganyika and its impact on humanity

Posted on 20 May 2010 by John Cook

Lake Tanganyika, in East Africa, is the second largest lake in the world (by volume). The lake supports a prodigious sardine fishery which provides a major source of animal protein for the region as well as employment for around 1 million people. Direct observations over past 90 years find that Lake Tanganyika has warmed significantly. At the same time, there's been a drop in primary productivity in the lake impacting sardine populations. To further explore this matter, geologists took lake cores to determine the lake's surface temperature back to 500 AD (Tierney 2010). They found that warming in the last century is unprecedented over the last 1500 years.

Lake Tanganyika lake surface temperature
Figure 1: Lake Surface Temperature from Lake Tanganyika palaeorecord for the past 1,500 years, measured in core KH1 (red line) and MC1 (dark red line). Orange shading is 95% error bars.

What effect does temperature have on the lake's sardine population? To answer this question, a proxy for primary productivity was also reconstructed from the lake cores. Primary productivity was determined from the percentage of biogenic silica in the sediment. They found that over the last 1500 years, when temperature rose, primary productivity fell. In the last 150 years, productivity plummeted from relatively high levels during the early 1800s to some of the lowest sustained values during the past 1,500 years.

How does temperature affect primary productivity? When the surface of the lake warms, the waters become more stratified. This makes it harder for cold currents to rise from the bottom. These currents carry nutrients from the depths toward the surface as food for algae. Sardine then feed off the algae. A less productive lake means fewer fish and therefore less food and income for those living in the region.

The stratification is confirmed by deep-water instrumental measurements which find less warming at deeper layers, revealing an increased temperature gradient. Nevertheless, another possible cause in changing rainfall is explored. Higher rates of precipitation may increase primary productivity. Charcoal levels in the lake cores were used as a proxy for humidity (eg - low humidity leads to drought which corresponds with more bushfires). However, they found a weak correlation between charcoal levels and productivity. The stronger relationship between temperature and productivity led the authors to conclude that it's temperature, not rainfall, that is largely controlling primary productivity.

There's also a strong match between Northern Hemisphere temperature reconstructions and the Lake Surface Temperature reconstruction. Temperatures on Lake Tanganyika have largely followed global trends over the past 1500 years as well as the past half-century. From this, the authors infer that surface temperatures in this region vary in concert with the global average and that the recent anomalous warming is a response to anthropogenic greenhouse-gas forcing. As lake temperature and primary productivity are closely related, this is evidence of another impact of man-made global warming on humanity - in this case, the communities and regional economy around Lake Tanganyika.

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Comments 151 to 160 out of 160:

  1. Subsequent to the study being published, Jessica Tierney did a telephone interview with Reuters which is reported here. Interesting the final note from Reuters about how the paper apparently admits that other factors, like overfishing, may be doing more harm than any warming. If the study considers that "other" factors may be more significant in falling productivity than any warming, then logically the study must have had to quantify those other factors before being able to calibrate the proxy chosen for productivity, namely BSi, and the temperature data from the relevant instrumental records available. Does anyone have any information on the proportions allocated to the various factors to allow the calibration? 'INTENSE WARMING' MOST climate change studies have focused on the atmosphere, but increasingly scientists are studying the effects on the oceans, seas and lakes, which all absorb a huge amount of heat. The paper argues that recent rises in temperature are correlated with a loss of biological productivity in the lake, suggesting higher temperatures may be killing life. 'Lake Tanganyika has become warmer, increasingly stratified and less productive over the past 90 years,' the paper says. 'Unprecedented temperatures and a ... decrease in productivity can be attributed to (human) ... global warming.' The rise in temperature over the past 90 years was about 0.9 degrees Celsius and was accompanied by a drop in algae volumes. 'We're showing that the trend of warming that we've seen is also affecting these remote places in the tropics in a very severe way,' Tierney said by telephone from the United States. 'We've seen intense warming in recent times ... not down to natural variations in climate.' She said the lake life had been harmed because in a lake as deep as Tanganyika, the nutrients form at the bottom but the algae needed to make use of them live at the top. Higher surface temperatures mean less mixing of waters at the top and bottom.' That's why a warmer lake means less life.' But the paper admits that other factors, like overfishing, may be doing more harm than any warming. -- REUTERS
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  2. I wonder what mechanism of error would produce Tierney's graph, what the coincidence of problems would be that would yield such an image that resembles others we've seen? Stuck at this point, it seems, until somebody extends the path of research. Thanks for the link, johnd.
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  3. johnd at 02:59 AM on 25 May, 2010 Two seperate issues are being conflated, johnd. 1. Primary productivity "Primary productivity" relates to the generation of biomass at the bottom of the foodchain largely by fixing of CO2. So in marine/lake environments this is normally measured as algal load or density. This is the measure that Tierney (and others) report on both as contemporary measures and as a proxy. So "primary productivity" has a particular and specific meaning. 2. Productivity in relation to fish yields. This is a different issue, and is the one that is subject to "other factors". This has got nothing to do with Tierney et al's proxy for primary productivity which is something else entirely (see 1.). As is very clear from the paper, the reduction in fish catch in recent decades may have some relation to the reduction in primary productivity (see 1.), but is more likely due to over-fishing: so Tierney et al. (2010) state:
    Specifically, from AD 1913 to 2000 the lower metalimnion (~110 m depth, just below the thermocline) of the lake warmed by 0.9 °C, the hypolimnion (below ~300 m depth) warmed by 0.2 °C and phytoplankton biomass between 1975 and 2000 decreased by 70% (ref. 2). The late-twentieth-century drop in primary production in Tanganyika may have contributed1 to declines in catch per unit effort during the late twentieth century (absolute catch increased between the 1950s and 1990s; refs 4,6), although most of this short-term change is probably the result of changing fishing intensity and technologies4.
    This is quite clear from reading the paper. I don't think anyone is "admitting" anything. They're simply describing what the evidence indicates.... Incidentally, the Straits Times headline: "Lake Tanganyika's life is dying" is rather overwrought! And both the Straits Times and Reuters accounts fail to seperate the two meanings of "productivity" clearly. So they also give the impression that there is a causal chain from warming to reduced fish catch, whereas the causal chain as described in the paper is from warming to reduced primary productivity (fixing of carbon into biomass at the bottom of the food chain). This (loss of PP) may have impacted fish catch, but the main contribution in the short term is likely over-fishing.
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  4. chris at 04:46 AM, the abstract for the article gives the impression that "productivity" and "primary productivity" are somewhat more closely related rather than separate, and perhaps like both Reuters and The Straits Times they should have made it clearer also. Thats the trouble, all put their own spin on everything. Just to be precise, the study measured biogenic silica BSi, the production of which is controlled by the availability of silica and other nutrients amongst other factors, as the proxy for primary productivity.
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  5. BSI is being used as a proxy for productivity. Generally using BSI requires that diatoms comprise the majority of the primary producers. A recent paper by Stenuite et al in the J. of Plankton Research "Photosynthetic picoplankton in Lake Tanganyika: biomass distribution patterns with depth, season and basin" found that picocyanobacteria ranged from 44 to 99% of total phytoplankton biomass. On the limnology of Lake Tanganyika- Victor Theodorus Langenberg thesis-rejects Verburgs (used by Tierney as suport for BSI proxy) contention that changes in phytoplankton biomass (biovolume), in dissolved silica and in transparency support the idea of declining productivity.
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  6. Pat Moffitt at 11:29 AM, given the definitions that Chris has applied above, can you clarify whether it is productivity or primary productivity that you note as BSi being a proxy for. The abstract for the Nature article states "We conclude that these unprecedented temperatures and a corresponding decrease in productivity can be attributed to anthropogenic global warming, with potentially important implications for the Lake Tanganyika fishery." Unless they are confusing the terminology, the inferred direct link between productivity and AGW is at odds with the definitions above. One other point I would like to pick up on from the study before this thread, like Lake Tanganyika, dies, is the assumed relationship between humidity and charcoal levels, the assumption being that low humidity leads to drought which corresponds with more bushfires, and more bushfires means higher charcoal levels. Given that the correlation between charcoal levels and productivity was weak, that infers that at times higher charcoal levels instead of correlating with low productivity, correlated with higher productivity. I feel that this is perhaps the more correct assumption to make. Whilst fires are needed to produce the charcoal that is then washed into the lake to settle in the sediments, fuel for the fires is required first, and it is a well established fact that fuel loads increase most with the prolific growth resulting from wetter, not drier conditions. In addition, the majority of charcoal that results from fires anywhere in the catchment areas will require heavy runoff in order to be washed into the lake, either from the areas surrounding the lakes or via any of the rivers that feed into the lake during periods of high flow rates. Therefore higher charcoal levels may not necessarily indicate more fires, but quite likely higher fuel loads, and if higher charcoal levels correlate with periods of higher productivity, then the proposition of "Higher rates of precipitation may increase primary productivity" may well be a factor that has to be included rather than being dismissed as it was.
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  7. johnd at 05:47 AM on 26 May, 2010 My comments were with respect to primary productivity. To simplify what is being said by Tierney--Most of the nutrients required to grow the food that the fish eat (directly or indirectly) are recycled from the deeper portions of the lake. (95% internal lake recycling cited) The amount of nutrients entering the lake from runoff etc is not significant. The fish population fluctuates with the amount of food that is produced. Water has different densities at different temperatures. The larger the difference in densities the more stable the water column becomes. A stable or stratified water column makes it difficult to mix the water which brings the nutrient rich lower water mass to the surface. The mixing is provided by the wind and the more stable the water column the more wind energy that is required. Increasing temperatures tend to make water columns more stable and thus theoretically could diminish lake mixing and thus primary productivity. A reduction in primary productivity may lead to a reduction in the catchable fish supply. Tierney assumes by using the charcoal that the lake was less productive than in periods when the lake was arid, and cooler and as such it is temperature that is controlling the primary productivity of the lake. (The BSI or biogenic silica index measures the amounts of biological derived silica in the sediment. Diatoms, which have a high silica content, can be used as a n estimate of primary productivity when the dominant fraction of primary producers are diatoms.) But we need to know much more information than just the temperature differential. We need wind speed, sheer, depth of mixing etc to understand if the temperature gradient problem from surface heating is significant. The Lake Tanganyika Regional Fisheries Programme (TREFIP) ENVIRONMENTAL IMPACT ASSESSMENT REPORT -GCP/INT/648/NOR and its modeling of wind and lake mixing found that temperature was only of secondary importance. So temperature alone cannot tell us the amount of mixing and nutrient recycling. My previous comment makes note that diatoms and thus BSI may not be a good proxy for Lake Tanganyika as at times it sees most of its primary production coming from cyanobacteria- not diatoms. A presentation by Hecky and Verburg…/Physical%20and%20Ecological%20Responses%20of%20the%20Great%20 showed the switch from cyanobacteria in the wet season (warm) to diatoms in the dry season (cool) on an annual basis. The cyanobacteria do not appear as biogenic silica in the cores and as such will not be measured as productivity. Tierney’s correlation of BSI with LST may be nothing more than diatoms being relatively more plentiful in periods of lake upwelling (aridity and low T) and cyanobacteria during periods of low upwelling (wet, high T and stable stratification). Tierney’s BSI as a result may say nothing about the overall changes in productivity of the lake. (The BSI simply reflecting the Lake’s primary productivity mode switching between cyanobacteria and diatoms.) Without a reliable proxy for total productivity the assumed correlations to temperature and fishery catch becomes less grounded. Another marker for primary production is chlorophylla. Langenberg’s 2008 thesis on the Lake cites evidence that chlorophylla analysis shows no decline in productivity in the period of the 1970 to 1990s. Nor have secchi measurements shown any increase I lake clarity. Tierney has acknowledged a “potentially” large role for overfishing in her Nature paper- however the media interviews have tended to diminish the relative threat of overfishing. TREFIP has demonstrated the reality of overfishing. There have been over two decades work trying to get the multiple interests and nations involved in the Lake fishery to agree to an enforceable/workable fishery harvest plan. Pointing a finger of blame at global warming may very well undo these vital efforts. Tierney cautions that increasing temperature may imperil the percentage of the food supply derived from fish. One can gain no understanding on the relative risk to food security in this region until one presents the information in the context of overall food production and risks -which is not given. She acknowledges t increasing temperatures have been accompanied by increased rainfall. She states that as many as 1million people are “employed” in the fishery however 2/3rds of the surrounding population survive by eking out a subsistence agriculture existence. I don’t have the answer yet (but am checking) but it seems answering the impact of increased rainfall on agricultural productivity may be an even more important question.
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  8. Pat Moffitt at 08:20 AM, I concur mostly with what you are saying, and feel that this is at last focusing on what I took to be the primary objective of the study. As you noted, within the the media the focus has been drawn to other factors which then snowballs as it drags in others whose perception is programmed by the headlines they read. With regards to nutrient recharge from run off, looking at it from the Primary ProductION perspective, whilst it is a very large body of water, any foodstuffs that are harvested from the water permanently strip nutrients out that ultimately must be replaced. Run off would replace some of these nutrients directly into the immediate zone where they can be recycled rather than taking them into storage at lower levels, but that is only one side of the equation.
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  9. johnd at 08:53 AM on 26 May, 2010--If we are looking at the total amount of nutrients within the entire lake water column- then harvest will have a minimal effect on the total nutrient store. (The residence times are enormous). Internal recycling is critical to this lake however the production of biomass is complex and terrestrial sources do have a role. I don't really see the loss of nutrients to the fish catch as a vital link here. Overfishing may have deleterious and multiple effects that can cascade through the system but I don't think it is related to the loss of nutrients within the harvested fish. Langenberg's thesis posits that the changes in fish production catch data are the result of fishing practices not climate. In fact he showed that increasing temperatures that lead to a shallower mixed layer does not necessarily result in decreased production. However the ability to blame climate may perversely lead to a collapse in the fishery by undermining the need of he involved parties to create and enforce a workable fishery management plan.
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  10. Pat Moffitt at 14:01 PM, I agree totally with you on the dangers of wrongly assigning blame, particularly in this instance, it is something that also applies to many other issues globally. I cannot help but wonder how much influence respected journals have when they perhaps unconsciously create a preconceived notion in their readers minds when studies such as this one with overlapping implications are titled for publication. Getting back to the matter of nutrients, I do acknowledge that it is a large body of water with an expected large reserve of nutrients. But even in oceans, rivers through the creation of suitable habitats such as deltas, and the delivery of nutrients, sustain rich fishing grounds that have become dependent on the inputs from the river and thus are subject to the conditions that control the inputs into the river system in areas remote from the fishery itself. These are only general observations and I don't have any knowledge specific to this lake such as the distribution and output of any fishings grounds or even if there are such areas within the lake, but I agree totally that the focus should not be allowed to be taken off the work that needs to be done, not only to further understand what is happening and why, but as you noted, to create some workable plan to sustain the lakes output into the future.
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