The shrinking of Lake Chad cannot be blamed on anthropogenic CO2

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Gore shows a series of four images of Lake Chad on page 116 of his book. The pictures show the lake shrinking from about 25,000 square kilometers in 1963 to about 1,500 square kilometers in 2001. Very dramatic indeed, this drought was abrupt and catastrophic for the people in this region south of the Sahara desert. Gore implies the following sequence of events: Anthropogenic CO2 causes Lake Chad to dry up, then the stresses caused by this depleted resource cause, or exacerbate, regional violence, famine and genocide.

Gore is wrong for two reasons

1. Lake Chad was shrinking long before anthropogenic increases of CO2. It is known that Lake Chad was vastly bigger several thousand years ago than it was in 1963, when Al Gore starts his tale about the lake’s demise. Paleological evidence makes it clear that abrupt changes have been common and the lake has shrunk to its current size or smaller multiple times in the last thousand years.

2. The current low level of Lake Chad is due, at least in part, to greater demands on the water of the in-flowing rivers. The population of the region has more than doubled since the most recent drought began and a large fraction of the water that would normally flow into the lake is being dammed and diverted for irrigation long before it reaches the lake.

Lake Chad was already shrinking

While he is almost certainly right about water shortages aggravating social problems, he is certainly wrong about anthropogenic CO2 being the cause. His aim goes wildly off the mark when he makes an important, but wrong, point on the page 117 where he says "When it was full, Lake Chad was the sixth largest lake in the world…” This gives the impression that prior to 1963, “when it was full,” the Eden that was Lake Chad had existed in a 25,000 square kilometer steady state perpetually from the deep past.

The truth is that Lake Chad has undergone extraordinary changes in the relatively recent past. The Lake Chad of 1963 was just a tiny remnant of what is known to paleontologists as “Lake Megachad.” Just a relatively short six to seven thousand years ago, when Badarian culture was populating upper Egypt and the Yangshao and Longshan cultures of prehistoric China were cultivating grains, domesticating animals and making villages, Lake Megachad was the biggest lake in the world! It was 400,000 square kilometers – five times bigger than Lake Superior is today! 7000 years ago its surface area was 16 times bigger than it was in 1963, and its volume was dozens of times greater. In the Journal The Holocene, Drake and Bristow (2006) point out that Lake Megachad may have been as large as 800,000 square kilometers further back in time.

According to Section 3.1 of the United Nations Environmental Programme (UNEP) report, Africa’s Lakes – Atlas of Our Changing Environment, Lake Chad “levels regressed until, between 5,000 and 2,500 years ago, the lake assumed its current level with periodic oscillations. By 1908 the lake levels were so low that the lake resembled a vast swamp with small northern and southern pools. During the 1950s, levels again increased, joining the southern and northern pools, so by 1963 the lake covered 22,902 km2 (8,842 square miles).” Note that 1963 is the year the Gore conveniently starts his tale about Lake Chad. Somehow he forgets to tell his readers and viewers that 1963 Lake Chad was at its highest level for the entire 20th century. (See figure 1.) In fact, the lake size increased dramatically during the 50 years previous to 1963. Gore’s narrative would not be quite so compelling if he had started it in 1908, because by 2001 the lake level was almost the same as it was back in 1908.

Butzer (1983) did a comprehensive paleo-environmental review of Lake Chad and the entire Sahel to put the recent drought into historical perspective. He took into account stream deposits, fossil sands, lake beds, abandoned shorelines, paleosols, fossil pollen and lake microorganisms. He says the primary record has been “well fixed in temporal terms by an unusually large number of radiocarbon dates.” The paper gives numerous examples of rapid rises and falls for Lake Chad over the last 20,000 years. Comparison of the Lake Chad data to data from the rest of the Sahel “shows that the history of Lake Chad is fully representative of hydrological changes across the Sahel.” He concludes with “The lake records discussed here are among the most detailed available for the Late Quaternary of Africa.” He points out that droughts like the most recent one “are verified on at least 6 occasions since 1400 AD, and may have a recurrence frequency of three times per century.” This drought, he says, “falls well within the range of short and medium variability directly documented for the last few centuries and indirectly shown for the last 12 millenia.”

Figure 1. 130 year history of Lake Chad elevation. Note that the 20th century maximum occurred in 1963. The rapid elevation drop at the end of the 19th century occur ed before significant increased in anthropogenic CO2. The most recent elevation data has the lake level almost as high as it was 100 years previous. Double click the image to see a larger version.

And there’s more. There were other megalakes that existed in the Sahara. Lake Megafezzan was north of Lake Megachad in present day Libya, the Chotts Megalake was in present day northern Algeria and the Ahnot-Moyer Megalake in central Algeria. Figure 2 shows a map of North Africa with the now missing megalakes. The inset of the North American Great Lakes (from Google Earth) is at the same scale and is included for comparison purposes. These lakes did not dry up millions of years ago. They still existed as bodies of fresh water during in the mid-holocene just thousands of years ago, when human beings had already spread over the entire globe. It clearly was not anthropogenic CO2 that caused the mega-lakes to dry up or Lake Chad to shrink and oscillate so widely in the previous several millenia. How can “An Inconvenient Truth” make the claim that anthropogenic CO2 is responsible for Lake Chad’s recent shrinking and oscillations when this is a continuation of a long existing pattern.

Figure 2. The Megalakes of the Sahara region. These gigantic lakes have disappeared in recent millenia. Lake Chad is the only remaining trace of water from these giants. The inset shows the North American Great Lakes at the same scale (from Google Earth) for comparison purposes. The megalake size and shape data is from the Megalakes Project. Double click on the image to see a larger version.

Figure 3. The site of Lake Megafezzan as it appears today. All that remains is sand. (Image from Google Earth)

Larger and larger fractions of the in-flowing water is being diverted before it reaches the lake.

A major contributing factor to Lake Chad’s reduced size in the 21st century is the human manipulation of the water system through damming and irrigation. Multiple dams and irrigation systems have been built upstream on the rivers that feed the lake. The Global International Waters Assessment (GIWA) report by the University of Kalamar on behalf of the United Nations Environmental Programme states that

“The GIWA Assessment considered anthropogenic stream flow modification as having a severe impact on freshwater availability. Although the Lake has already dried out several times in the past and therefore recent shrinkage is not a new phenomenon, the trend has been severely exacerbated by human stream flow modification….In areas of the Lake Chad Basin, despite the relative abundance of water at times, the flow of rivers has been constantly diminishing (Nami 2002) partly due to decreasing rainfall in the hydrologically active upstream basins but also as a consequence of the increased abstraction for human consumption. This abstraction has dramatically modified stream flow through the construction of dams upstream of the catchment, that have not taken sufficient account of the people and ecosystems downstream of the development.”

A profound example of the impact of human diversion of water from Lake Chad is illustrated in the (UNEP) report mentioned above, where it is pointed out that “Since the 1960’s human demands for water near Lake Chad have grown rapidly. Between 1960 and 1990, the number of people living in the lake’s catchment area has doubled from 13 million to 26 million.” This growing need for water has resulted in huge irrigation projects and dams along the rivers that feed Lake Chad. Of the Komadougou-Yube river system the report states “The upper basin used to contribute approximately 7 km3/yr to Lake Chad. Today, the bulk of this water is impounded in reservoirs within Kano province in northern Nigeria, and the system provides just 0.45 km3/yr.” By my calculations that is enough impounded water each year to double the current volume of the Lake.

The greatest inflow to Lake Chad comes from the south via the Chari-Logone River. However, since the 1970s the Chari-Logone stream flow has been drastically modified. The construction of the 30 kilometer wide Maga Dam for the creation of Maga Lake, and 80 kilometers of dykes along the Lagone downstream from the dam have had a profound impact in Lake Chad. This construction was part of the well intentioned SEMRY irrigation project to open up more agricultural land (mostly rice) and fish farming. According to GIWA, "This diversion of water from the Chari River for agricultural purposes has contributed to the decreasing stream flows and the discharges into, and extent of, the Lake Chad....According to expert opinion the most significant GIWA assessed immediate cause [for low stream flow into Lake Chad] is the increased diversion of rivers and the associated unsustainable use of water resources."

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References
Drake and Bristow, Shorelines in the Sahara: geomorphological evidence for an enhanced monsoon from palaeolake Megachad, The Holocene, Vol. 16, No. 6, 901-911 (2006)
DOI: 10.1191/0959683606hol981rr. Get copy here

United Nations Environmental Programme (UNEP) report, Africa’s Lakes – Atlas of Our Changing Environment, section 3.1 Get copy here

Butzer, K.W., Paleo-Environmental Perspectives on the Sahel drought of 68-73, GeoJournal 7.4, 369-374 (1983) Get copy here

Global International Water Assessement (GIWA) , Lake Chad Basin – GIWA Regional assessment 43 - assessment, Get copy here


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Criticisms of Al Gore's "An Inconvenient Truth"

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Al Gore's An Inconvenient Truth became a big part of the British secondary educational landscape when it was made part of the official curriculum. This was big victory for those who feel the world is in imminent danger because of anthropogenic global warming. However, the High Court of London recently said "not so fast." Steward Dimmock sued under Section 406(1)(b) of the Education Act 1996, claiming that the movie is a form or political indoctrination. That law says that school governing bodies and head teachers "shall forbid... the promotion of partisan political views in the teaching of any subject in the school".

The court ruled that 1.) in order to show this movie to the children teachers must make clear that the film is a political work and promotes only one side of the argument. 2.) If teachers present the film without making this plain they may be in breach of section 406 of the Education Act 1996 and guilty of political indoctrination. 3.) Nine inaccuracies have to be specifically drawn to the attention of school children.

I will use these nine cited inaccuracies as a jumping off point for my criticisms of An Inconvenient Truth, both the book and film. These nine inaccuracies are:

• The film claims that melting snows on Mount Kilimanjaro evidence global warming. The Government’s expert was forced to concede that this is not correct. more...


• The film suggests that evidence from ice cores proves that rising CO2 causes temperature increases over 650,000 years. The Court found that the film was misleading: over that period the rises in CO2 lagged behind the temperature rises by 800-2000 years. more...


• The film uses emotive images of Hurricane Katrina and suggests that this has been caused by global warming. The Government’s expert had to accept that it was “not possible” to attribute one-off events to global warming. more...


• The film shows the drying up of Lake Chad and claims that this was caused by global warming. The Government’s expert had to accept that this was not the case. more...


• The film claims that a study showed that polar bears had drowned due to disappearing arctic ice. It turned out that Mr Gore had misread the study: in fact four polar bears drowned and this was because of a particularly violent storm. more...


• The film threatens that global warming could stop the Gulf Stream throwing Europe into an ice age: the Claimant’s evidence was that this was a scientific impossibility.


• The film blames global warming for species losses including coral reef bleaching. The Government could not find any evidence to support this claim.


• The film suggests that sea levels could rise by 7m causing the displacement of millions of people. In fact the evidence is that sea levels are expected to rise by about 40cm over the next hundred years and that there is no such threat of massive migration.


• The film claims that rising sea levels has caused the evacuation of certain Pacific islands to New Zealand. The Government are unable to substantiate this and the Court observed that this appears to be a false claim.

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Rahmstorf extrapolates out more than five times the measured temperature domain

This is part of a series of posts concerning Problems with the Rahmstorf (2007) paper.

Critique #3. Rahmstorf extrapolates out more than five times the measured temperature domain.

Extrapolation is risky business. Even when the fitted model accurately describes the real data over its domain, extrapolation beyond that domain can lead to very poor predictions. When the fitted model does not accurately describe the measured data (Rahmstorf's unbinned sea level rise rate vs. temperature, see figure 3, here, for example) the result can be truly bizarre. The NIST Engineering Handbook states:

Modeling and prediction allows us to go beyond the data to gain additional insights, but they must be done with great caution. Interpolation is generally safer than extrapolation, but mis-prediction, error, and misinterpretation are liable to occur in either case...The best attitude, and especially for extrapolation, is that the derived conclusions must be viewed with extra caution. !

Rahmstorf's projection for future sea level (figure 4 in his paper), is reproduced in part in figure 1, below, and makes it look as if his measurement domain is 120 years and that he has extrapolated out another 100 years. But in reality, his measurement domain was in decrees C of temperature anomaly, and his range was in sea level rise rate. Extrapolating out 100 years based on 120 years of data would be bad enough, but he actually extrapolates out more that 5
°C based on 0.8 °C of data. See figure 2! This is an extrapolation of poorly fit data to over six time the measured data domain!!!

Figure 1. Reproduction of Rahmstorf's figure 4, showing "sea-level projections
from 1990 to 2100." This image gives the impression that it shows an extrapolation from measured sea level data spanning 120 years out for an additional 100 years.

Figure 2. But the real extrapolation is from the sea level rise vs temperature plot. First he fits a straight line to a twisted piece of spaghetti, then extends that line way, way out.

This type extreme form of extrapolation is best summed up by Mark Twain in Life on the Mississippi (1883). Twain writes about effect of cutting across "horseshoe curves" in the river over the years in order to shorten it.

" The Mississippi between Cairo and New Orleans was twelve hundred and fifteen miles long one hundred and seventy-six years ago. It was eleven hundred and eighty after the cut-off of 1722. It was one thousand and forty after the American Bend cut-off. It has lost sixty-seven miles since. Consequently its length is only nine hundred and seventy-three miles at present.

Now, if I wanted to be one of those ponderous scientific people, and `let on' to prove what had occurred in the remote past by what had occurred in a given time in the recent past, or what will occur in the far future by what has occurred in late years, what an opportunity is here! Geology never had such a chance, nor such exact data to argue from! Nor `development of species', either! Glacial epochs are great things, but they are vague--vague. Please observe. In the space of one hundred and seventy-six years the Lower Mississippi has shortened itself two hundred and forty two miles. This is an average of a trifle over one mile and a third per year. Therefore, any calm person, who is not blind or idiotic, can see that in the Old Oolitic Silurian Period, just a million years ago next November, the Lower Mississippi River was upward of one million three hundred thousand miles long, and stuck out over the Gulf of Mexico like a fishing-rod. And by the same token any person can see that seven hundred and forty-two years from now the Lower Mississippi will be only a mile and three-quarters long, and Cairo and New Orleans will have joined their streets together, and be plodding comfortably along under a single mayor and a mutual board of aldermen. There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact."

Back to series of posts concerning Problems with the Rahmstorf (2007) paper.

1. Rahmstorf, A Semi-Empirical Approach to Projecting Sea Level Rise, Science 315, 368 (2007)

Back to series of posts concerning Problems with the Rahmstorf (2007) paper.

"Warmer Oceans, Stronger Hurricanes," Trenberth, Scientific American, July 2007

Hyperbole comes in many forms. This article in Scientific American came with a full page artist's rendering of a "future hurricane." I have shown a very small (to avoid copyright lawyers) copy of the picture below, with a blow up of one section. The caption for the picture in the magazine says "Future hurricanes could be more severe thanks to global warming." The blow up shows a giant hurricane bearing down on the Mediterranean and the East coast of the United States

Figure 1. The small picture at the left is a miniature version of the 8 inch by 11 inch full page artist's rendering of a "future hurricane" form page 44 of the July 2007 Scientific American. The right side shows a blow up of part of the picture.

The very first paragraph of the article reminds the reader of the 2005 hurricane season and, of course, Katrina. So, I use a pair of pictures below to compare Katrina to the imagined "future hurricane." The first is a satellite image of Katrina shortly before it made landfall near New Orleans. The second is a detail of the Scientific American picture. Note that the sizes of the images have been adjusted to give the same scale.

Figure 2. Detail of Scientific American picture of "future hurricane" with same scale as image of Hurricane Katrina in figure 3,below.

Figures 3. Satellite image of Hurricane Katrina just hours before making landfall at New Orleans. This image is on the same scale at the artist rendering of a "future hurricane" in figure 2, above.

Figure 4. Juxtaposition of the Scientific American "future hurricane" and the very real Katrina from the satellite image. I have removed land masses from both pictures. Both pictures are on the same scale, as in figures 2 & 3.

Scientific American's "future hurricane" is bigger than the continent of North America. It is so big that it stretches from northern Brazil to southern Canada. It is as large as the North Atlantic Ocean. This is clearly extreme visual hyperbole, but it is also a metaphor for much of the global warming debate, where preposterous exaggerations and extrapolations abound.

Those who are convinced that we are headed for a future of giant hurricanes due to increased CO2 might consider the following journal articles to mitigate the effects of the seemingly endless fear mongering so common in the global warming debate:

1. In Low Atlantic hurricane activity in the 1970s and1980s compared to the past 270 years, Nyberg, et. al., point out that "reliable observations of hurricane activity in the North Atlantic only cover the past few decades." It is not possible to say, based on this short set of data, if the variation that has been seen during these few decades is greater than should be expected over longer time scales. However, they developed a proxy for both sea surface temperature and vertical wind shear covering 270 years. (Vertical wind shear is inversely related to hurricane formation). The result shows that "the average frequency of major hurricanes decreased gradually from the 1760s until the early 1990s, reaching anomalously low values during the 1970s and 1980s." It seems clear that the upswing in hurricane activity seen from the beginning of the satellite era to the present is largely a consequence of the beginning of the satellite era being at the low point of hurricane activity for the last 270 years.

2. The article in Nature, Intense hurricane activity over the past 5,000 years controlled by El Nin˜o and the West African monsoon," by Donnelly and Woodruff of the Woods Hole Oceanographic Institution in Massachusetts echos the concern that "the instrumental record is too short and unreliable to reveal trends in intense tropical cyclone activity." To overcome these limitations they used sediment deposits in coastal lagoons of the Caribbean to gauge hurricane activity on the century and millennial time scales over a 5000 year period. They found the frequency of intense hurricanes varied widely on these time scales during the past 5,000 years and that the frequency appears to be governed by the El Nin˜o/Southern Oscillation and the strength of the West African monsoon." Additionally, " sea surface temperatures as high as at present are not necessary to support intervals of frequent intense hurricanes."

3. The short instrumental record of hurricane activity was a motivation for Miller, et. al. in their 2006 Proceedings of the National Academy of Sciences paper, "Tree-ring isotope records of tropical cyclone activity." As trees grow, the oxygen isotope ratios of the water at that place and time are locked into their rings. It is also known that the precipitation of tropical cyclones and hurricanes have oxygen isotope ratios that are greatly different that more common causes of precipitation. Miller, et. al., examined long leaf pines (pinus pulustris) in Georgia because they have shallow roots and a distinct early season growth and late season growth in their rings. these combine to give a precise temporal fix on isotope ratio variation. Their study covered 1770 to 1990. Their analysis of the tree ring oxygen isotope data shows very close agreement with the instrumental data for the southeastern United States after 1940, verifying the efficacy of their method for earlier times. The overall results indicate "systematic, decadal- to multidecadal-scale variations" in the isotope ratios, and consequently variations in the number of hurricanes. Hurricane activity appears to have peaked in the 1770s, 1800s to 1820s, 1840s and 1850s, 1865 to 1880, and the 1940s to 1950s. The quietest decades are the 1780s through 1790s, and the 1970s. The 1970s saw the beginning of satellite tracking of hurricanes. The fact that there has been an upswing in hurricanes in the satellite record is much less alarming when you consider that the 1970s was one of the least active decades (at least for the southeastern United States) in over 200 years.

Kevin E. Trenberth, "Warmer Oceans, Stronger Hurricanes," Scientific American, July 2007, p44-51. (Get copy here.)

Johan Nyberg, et. al., "Low Atlantic hurricane activity in the 1970s and 1980s compared to the past 270 years," Science, Vol 447, 2007. (Get copy here.)

Jeffrey P. Donnelly & Jonathan D. Woodruff, "Intense hurricane activity over the past 5,000 years controlled by El Nin˜o and the West African monsoon," Nature, Vol 447, 24 May 2007 (Get copy here.)

Dana L. Miller, et. al., "Tree-ring isotope records of tropical cyclone activity," Proceedings of the National Academy of Sciences, PNAS, Vol. 103, no. 39, September 26, 2006 (Get copy here.)

Critique of "A Semi-Empirical Approach to Projecting Future Sea-Level Rise" by Rahmstorf

A recent article in Science by Stefan Rahmstorf (2007) predicted extreme sea level rise during the 21st century. Rahmstorf's predictions went as high as 140 cm (55 inches), far beyond even the high edge of the uncertainty of the IPCC's unlikely A1Fl scenario (see here, page 820). This high estimate by the IPCC was 59cm (23 inches), with other other scenarios yielding considerably lower estimates. Following is a critique of Rahmstorf's method and conclusions.

This post has a quick summary of Rahmstorf's approach to to projecting sea-level rise for this century. Following that summary is a quick list of problems that I have identified in his paper, each with a link to subsequent posts with more detailed information.


Rahmstorf's Simple Model


Rahmstorf's simple model of sea level rise consists of a system in equilibrium, where the sea level and the temperature start out as constants. Then an instantaneous step occurs in the temperature, causing the sea level to rise. Eventually the sea level will rise to a new equilibrium, as shown below.

It is very important to note that the time required to arrive at the new equilibrium is, according to Rahmstorf, "to be on the order of millennia." This long time scale provides the other important point of this simple model. That is, over a short enough time scale the rate of sea level rise can be considered a constant (as illustrated in the above graph during the time where dH/dT is proportional to delta T). Rahmstorf posits that "this linear approximation may be valid for a few centuries."

Therefore, in this model, a temperature jump in the 1920s, for example, would result in a sea level rising at a constant rate for several hundred years, even without any subsequent temperature increases. Of course, subsequent temperature rises would each result in a greater sea level rise rate, but there would never be any drop in the rise rate for several hundred years, assuming no significant drops in the temperature. The following section puts this model on a mathematical footing.

Rahmstorf's Mathematical Strategy

1) Assume that the rate of sea level rise rate at any given time is proportional to the deviation form some global equilibrium temperature at that time. He expresses this in the following formula...

where H is the sea level, dH/dt is the sea level rise rate, T is the temperature, To is the the equilibrium temperature, and a is the constant of proportionality.

2) To and a can be derived by simply plotting dH/dt vs T and fitting to a line.

3) Once To and a have been determined, then the sea level for any given time, H(t), can be calculated by integrating equation (I), above, with respect to time...

4) By applying various temperature rise scenarios for the 21st century to equation (II), Rahmstorf predicts the sea level for the hear 2100 (H(2100)).


Problems with this model

1) Sea level rise rate vs. temperature is displayed in a way that erroneously implies that it is well fit to a line, as expressed in equation I, above. More...

2) The assumption that the time required to arrive at the new equilibrium is "on the order or millennia" is not borne out by the data. More...

3)Rahmstorf extrapolates out more than five times the measured temperature domain. More...