Guest Post by Willis Eschenbach
I love African names. I mean, could there be a more euphonious name than “Dikembe Mutumbo”? That’s just poetry. In any case, this post is about a place charmingly yclept “Kilwa Kisiwani Gereza”. It seems it’s a new poster child for the dire effects of “climate change”. This alleged victim of evil sea level rise is the ancient African trading center of Kilwa Kisiwani in Tanzania, located at 8.97°S, 39.50°E. The city was originally of Muslim origin. It was taken over briefly by the Portuguese explorer Vasco de Gama in 1502. There’s a World Heritage Historical Site there, crumbling into ruin. The Heritage folks say:
The ancient fortress of Gereza … is slowly collapsing into the Indian Ocean as sea level rises due to climate change.
Figure 1. The city of Kilwa Kisiwani (“Kilwa on the Island), from a Portuguese drawing done some time prior to 1572. Source.
So what is all of the sea level hyperventilation about? For that, we need to turn to the Global Heritage Network. Here’s what they have to say:
Figure 2. Information on the fortress of Gereza from the Global Heritage Network. Date of the lower photograph unknown. Photos from the GHN Briefing Paper (PDF)
Not a pretty picture …
The city has an interesting history. A Portuguese site says:
Currently only the turret of Gereza is of Portuguese origin, with the remaining structure undergoing significant changes after the Portuguese left the island in 1512, giving place to the Omani sultans, which have shared power for centuries. The main wooden gate, decorated with floral motifs mixed with verses from the Koran, is the original and clearly of Arab origin. A sign in front, already a good few decades old, cites an urgent conservation project implemented by the department of antiquities of Tanzania and UNESCO, with Norwegian funding. The state of the structure is pitiful, which well explains the fact that since 2004, this place has been on the prestigious list of World Heritage Sites in Danger. The disassembled, out-of-order spotlights in front of the fort testify to that very well.
Kilwa was an important city in the Muslim commercial empire, which extended from the East African coast to the Moluccas, passing through the Persian Gulf and India, though at the time the Portuguese arrived they were already in decline. In 1502, Vasco da Gama took over the city, making the king a vassal of of the Portuguese King. With the proceeds of the first tribute in gold from the new African possession the famous monarch ordered the making of the famous “Custodia of Bethlehem”, one of the masterpieces of Portuguese art. Simultaneously with Sofala, a fortress in Kilwa was erected in 1506, under the direct supervision of D. Francisco de Almeida, the first structure built of stone and mortar in the region – a typical medieval castle with four corner towers and one tower, not in the central square, but leaning against the main face of the wall. Its main function was to provide shelter to passengers of the boats on the India route. The Portuguese presence, however, was quite short. Due to high maintenance costs and questions of military strategy (there were already forts near the Mombasa and Mozambique), Kilwa was abandoned by the Portuguese in 1512, again assuming the status of one of the city-states of the Swahili world. It would then be occupied and reoccupied by successive sultans that would adapt it to their needs and desires. In 1843, the transformation of nearby Kivinje in seaport led to the abandonment of Kilwa, and the consequent destruction of their buildings.
There’s a detailed history of Kilwa Kisiwani here .
So is “climate change” in the form of sea level rise really a problem for the Heritage site, the fortress Gereza? The Global Heritage Network folks say it is. But there is no mention of that on their main information site.
Well, let’s do what the Global Heritage folks didn’t do … take a look at the observations to see what the sea level is doing in that area. The nearest (and only nearby) tide station with a somewhat long-term record is in Zanzibar, and is available from the web site of the Permanent Service for the Mean Sea Level (PSMSL). You can also download satellite data showing sea level changes for the area just offshore of Kilwa Kisiwani (9°S, 40°E) from the University of Colorado. I have overlaid them below.
Figure 3. Tide gauge (Zanzibar) and satellite (offshore from Kilwa Kisiwani) records of sea level height. Photo of the World Heritage Site (photo source).
As you can see, we have very good agreement between the satellite and tide gauge records, which increases the confidence in both. As you can also see, over the last quarter century the massive recent sea level rise has brought the local sea level about back to where it was 25 years before …
So we can throw out all of the nonsense about sea level rise. Since 1985, sea level dropped about two-four inches (50-100 mm) and rose back up again. Anyone who thinks that was the threat to the ancient fort isn’t following the story.
So if sea level rise is not the cause of the fort crumbling into the sea, what is? From an examination of the site, it seems obvious that the answer is plain old garden variety erosion. Here’s what the area looks like:
Figure 4. Overview of Kilwa Kisiwani.
This is a tidal laguna with surrounding marshlands. There are large areas of mangroves, and extensive sedimentation. Given the area of the laguna and the size of the outlets, with the local ~ four foot tides it gets fairly strong tidal currents. In other words, as a seaman I can assure you that we have no reason to expect that any of the islands, sandbars, and channels will have great permanence. In Figure 5 there is a closer look, this time an aerial view looking southeast along the coast to duplicate Figure 1, reveals more about the underwater geography.
This type of sedimentary, marshy land is never stable. Year after year the islands and the channels shift and change. Rather than being surprised that the 500-year-old fort is falling in the ocean, we should be surprised that it has lasted this long.
This kind of sedimentation and buildup is very sensitive to the immediate conditions. Consider the sand and sediment in the lower picture in Figure 2. You may not have noticed that the sand has built up on the left side of the ancient fortress. Contrast that to the way the sand has eaten away, leaving only stone, on the right side of the fortress. The cause of this is the mere presence of the stones in the foreground. You can see how they have changed the circulation, and are causing even further erosion at the base of the fort … and the Global Heritage folks want us to believe the culprit is the (non-existent) sea level rise?
This is another example of why I say that the problem in climate science is not communication. The problem is not that the scientists have not figured out how to get their message across.
The problem is bogus science and, as in this case and far too often, exaggerated or false claims. Once again, a serious climate claim from an internationally respected organization (this time the Global Heritage Network) has been shown to be a complete fantasy … and there’s no amount of better spin or framing or communication that will solve that.