Unprecedented warming in Lake Tanganyika and its impact on humanity

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"Unprecedented warming in Lake Tanganyika and its impact on humanity"

Lake Tanganyika lake surface temperature[

This post by physicist John Cook was first published in Skeptical Science. The figure is surface temperature from Lake Tanganyika paleorecord for the past 1,500 years. Orange shading is 95% error bars.

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.

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.

— John Cook

JR:  In the comments, Cook adds this in response to a query:

The paper does compare the Lake Surface Temperature (LST) to the Northern Hemisphere temperature reconstruction. I left that part of the paper out of my blog post for reasons of brevity…. Here’s the graph with the black area showing the LST and the coloured range showing the Northern Hemisphere ‘hockey stick':

Cook notes:

Note that while certain regions during the Medieval Warm Period were hotter than current condition, the global average was cooler than today. Here’s a temperature map of the Medieval Warm Period. Temperatures are relative to the 1961 to 1990 period. So if a region is yellow, orange or red, it’s warmer than the 1961 to 1990 period. There are various regions that were warmer than the late 20th Century. But there were other regions that were cooler also, denoted by the blue regions.

JR:   At Cook’s Medieval Warm Period page you’ll find:

Figure 3: Surface temperature anomaly for period 1999 to 2008, relative to the 1961- 1990 reference period. Gray areas indicates regions where adequate temperature data are unavailable (NOAA).

Cook notes:

The Medieval Warm Period was not a global phenomenon. Warmer conditions were concentrated in certain regions. Some regions were even colder than during the Little Ice Age. To claim the Medieval Warm Period was warmer than today is to narrowly focus on a few regions that showed unusual warmth. However, when we look at the broader picture, we see that the Medieval Warm Period was a regional phenomenon with other regions showing strong cooling. Globally, temperatures during the Medieval Period were less than today.

Related Post:

A Hockey Stick in Melting Ice

figure

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23 Responses to Unprecedented warming in Lake Tanganyika and its impact on humanity

  1. BillD says:

    Lake Tanganyika’s reduced productivity involves the same mechanism as the recent analysis in Nature of a substantial decline in global oceanic phytoplankton production over the last century. For more studies on lakes and climate, check out the special issue of Limnology and Oceanography published in December 2009 on Lakes and climate change. About two thirds of the articles are available as free downloads, courtesy of fees paid by the authors.

    http://www.aslo.org/lo/toc/vol_54/issue_6_part_2/

  2. Prokaryotes says:

    Water levels in Lake Baikal, the world’s deepest and most voluminous freshwater lake as well as the natural laboratory for researchers, have dropped by more than one-third in July.

    The ongoing heat is blamed for the drying up of the lake, said a report from Natural Resources Ministry of the Russian Federation released Wednesday. http://news.xinhuanet.com/english2010/world/2010-08/01/c_13424899.htm

    One-third seems a bit off, but considering the lake lacks river waters the effect could be profound.

    Satellite spies vast algal bloom in Baltic Sea They added that a lack of wind and prolonged high temperatures had triggered the largest bloom since 2005. http://www.bbc.co.uk/news/science-environment-10740097

  3. Rob says:

    Interesting article. I would need to understand more about how lake surface temperatures are measured over the last 1500 years to fully comprehend the article. Has sun spots been taken into account.

  4. catman306 says:

    Inventors take note! And billionaire philanthropists.

    The world desperately needs a simple, effective, low cost, probably solar powered, combination aerator / deep water pump. We humans need to artificially pump oxygen to deeper levels of these bodies of water and bring up the cold, nutrient water from the bottom. These pumps might just keep areas of the ocean and fresh water lakes from becoming dead zones.

    When the oxygen in the plume of the Gulf oil spill dispersant mix is depleted by oil eating and methane eating bacteria, that area will become still another dead zone. Air/cold water pumps might make a difference..

    Some engineering wizard somewhere will work it out..

  5. Gord says:

    I was thinking that if the oceans become O2 starved because of a phytoplankton crash we could depend on the freshwater lakes for at least some protein.

    Looks like it will be a similar story in the large lakes.

  6. James Newberry says:

    Gord,

    There will be little relief for alternate food due to a cascade of climate impacts, only some of which we presently discuss. We should ramp down to zero emissions in one generation. This means controlled or uncontrolled bankruptcy, since $trillions are in hydrocarbon mining and combustion businesses. A first step would be de-subsidizing these while also providing feed-in tariffs for renewables.

  7. Bob Wright says:

    All this damage (also Greenland…) already! Its becoming more clear that 450 ppm CO2 is too dangerous a cap to aim for. Its already too dangerous. Mauna Loa recorded CO2 as high as 393 ppm this year. If the 2 ppm/year trend holds up, a child born today will be living in a 430 ppm world as a young adult. The pre-industrial equilibrium was about 280. Its getting really hard to have any optimism at all.

  8. Omega Centauri says:

    John, how do you get a 2C lake temperature rise, from a less than 1C rise of surface temperatures? I suspect other mechanisms are involved. I can make a few guesses:
    (1) Change in the amount of cloudiness. Is they area becoming sunnier as well as hotter?
    (2) Reduction of average wind speed, which would reduce evaporative cooling of the water.
    (3) Changes in the amount and temperature of inlet rivers. Is a lot of water being diverted for irrigation.

    I think the big news, is that the Lake temperature is more sensitive than average global temperature.

    Prokaryotes:
    I’ve saw that claim somewhere else. But it is useless without being attached to some units. Dropped by a third of what? Obviously a 5000 foot deep lake has not lost a third of its water or its depth. Perhaps it was a third of a meter (13inches), that would at least be believable?

  9. command & control advantage says:

    Omega, take a look at the map over Russia and how it has warmed from 1961-1990 to 1998-2008. Looks about +1C. Presumably the other degree is I.R. to 1960 warming. After this year they’ll have to extend the legend on the map to ultraviolet colours..

  10. Clive in Oz says:

    Omega Centauri,(9)
    I suspect the mechanism you have overlooked is less mixing as noted in the article – the cooler deeper waters are not mixing with the surface waters. And the stronger the gradient in temperature that develops, the greater this effect will become.

  11. Francine Smith says:

    The temp records for this lake area are false. Dishonest people make up proxies. They can’t even estimate fires in the area by looking for charcoal ash.

  12. Paulm says:

    Unfortunate way to put. This…i think he meant the next decade not next few…..

    Robert Bindschadler, a research scientist at the University of Maryland, told the briefing: “While we don’t believe it is possible to lose an ice sheet within a decade, we do believe it is possible to reach a tipping point in a few decades in which we would lose the ice sheet in a century.”

    http://www.guardian.co.uk/environment/2010/aug/10/greenland-ice-sheet-tipping-point

  13. Lewis Cleverdon says:

    At 12, the brazen slander of scientists’ integrity, that offers not even a scrap of evidence, demonstrates three points clearly –

    – the shills’ own utter lack of personal integrity in resorting to the standard fascist technique of “guilt by denunciation” –

    – and the fact that massively damaging climate destabilization is now evidently under way is making them increasingly desperate –

    – and that they can’t even comprehend a plainly written report of climate research – the proxy in question was charcoal dust, not charcoal ash.

    They’re right to be getting desperate – their funding is going to be cut off abruptly as the corporations realize the public relations disaster they’ve assembled, and the shills patently lack both the intelligence and the integrity to have much chance when they’re thrown back into the ranks of the unemployed.

    Regards,

    Lewis

  14. BillD says:

    I agree with Omega above. The idea of Lake Baikal losing a third of its water in a short period of time lacks credibility. The lake contains one third of the earth’s fresh water. A third of a meter seems more likely.

  15. Prokaryotes says:

    Omega, here are some more realistic numbers “7cm a year”.

    The temperature in many parts of eastern Russia drought, has led the world’s deepest freshwater lake – Lake Baikal reduced. Russian water sector data indicate that the current Lake Baikal overall reduction of 37% water, lake dropped 7 cm a year. http://www.520web.info/?p=337

    Air temperatures reached 40 degrees Celsius in the Siberian region located alongside Lake Baikal, sending soil temperatures as high as 61 degrees in some areas, the local agriculture ministry said. http://www.themoscowtimes.com/news/article/state-aid-pledged-as-heat-hits-farmers/410281.html

    However the temperature now is about 22C

  16. Prokaryotes says:

    Forcast

    Monday
    Chance of Rain. Partly Cloudy. High: 23 °C . Wind WNW 14 km/h . 30% chance of precipitation (water equivalent of 0.32 mm).

    Tuesday Night
    Chance of Snow. Overcast. Low: -1 °C . Wind SW 10 km/h . 20% chance of precipitation (trace amounts). http://www.wunderground.com/global/stations/30710.html

  17. Dubai says:

    I agree with Clive in Oz that the deeper layers are not mixing and it distorts the surface temperature. There would be some reason for the surface temperature changing quickly.

  18. Speedy says:

    Regarding Baikal, I think another optimon might be that the amount of water flowing into the lake has dropped by a third.

  19. Dan says:

    Don’t the laws of heat transfer dictate that heat transfer (and the currents that move heat) increase with the size of the heat gradient?

    This statement, from above, confuses me, then: “When the surface of the lake warms, the waters become more stratified. This makes it harder for cold currents to rise from the bottom.”

    What am I missing?

  20. Leif says:

    You are confusing heat transfer with stratification, Dan, @ 20. Mixing of the ocean strata is primarily cause by wind intensity, direction, bottom topography and currents, (also often caused by prevailing winds), not the heat transfer of the adjacent surfaces. The larger the thermal difference between the strata the more energy needed to get the warm to sink or the cold to rise and thus mix. Strata can be hundreds or even thousands of feet thick. The surface heat transfer is minimal. Ever been swimming and moved from warm to cold water in a stroke or two. How about adding cold to hot water in a bathtub? In still water the change can be agonizingly slow, prompting thrashing.

  21. Omega Centauri says:

    Decreased stratification leading to greater warming doesn’t make sense physically. And tropical lakes only have mildly cooler deep water, there is no supply of cold water as in common seasonally at high latitudes. Again surface water warming substantially greater than land warming is a surprising result. Something fairly strong must be driving it.

  22. Dubai says:

    Bob we desparately need to plant more forests on arid farm land to absorb the increased CO2 levels. There needs to be a limit on the number of population per forested area of the world.