How might a large oil spill unfold if the Houthis were to sink two or three tankers?

Are the Houthis sabotaging themselves by trying to sink tankers or bulk carriers off their coasts? Let's say two or three of these ships sink south of the Red Sea, not far from the Yemeni coast. The chances of these spills reaching the Yemeni coast are high because the country has about 2,000 km of coastline, with about 450 km in the Red Sea and about 1,550 km in the Gulf of Aden and the Arabian Sea.

Any massive environmental damage south of the Red Sea would be an incredible environmental disaster for Yemen and its immediate neighbours: Djibouti and Eritrea, and probably Sudan, Saudi Arabia and Egypt (and Somaliland and Somalia, if it were to happen in the Gulf of Aden). Saudi Arabia will have the resources to clean up its coast, but will the others?

The chances of these spills reaching the Yemeni coast are high because the country has about 2,000 km of coastline, with about 450 km in the Red Sea and about 1,550 km in the Gulf of Aden and the Arabian Sea.

I looked at about twenty scientific papers on physical oceanography, a discipline that studies ocean currents by looking at factors such as temperature, salinity and wind patterns because I wanted to see how an oil spill in or near the Bab el-Mandeb Strait would spread into the Red Sea. But because there is a water exchange with the Gulf of Aden, the 'arm' of the Indian Ocean that touches the Red Sea, I had to go a step further into a sub-discipline of physical oceanography called water budget analysis, which (like any budget) looks at the inputs (e.g. rainfall, rivers) and outputs (e.g. evaporation) of water into the Red Sea, as well as the inflows or outflows to or from the Indian Ocean. I even had to deal with bathymetry, another sub-discipline of physical oceanography that studies and measures the depth and shape of underwater features, in this case the seabed features of the Red Sea, mainly near the Bab el-Mandeb Strait. Finally, of course, I had to study the main characteristics of oil spills, in particular what is known as oil watering, when water emulsifies with oil (an emulsion consists of two or more immiscible substances, usually characterised by one liquid being dispersed in another liquid in the form of tiny droplets), which is more complex in seawater than in fresh water due to the presence of salt and other dissolved substances such as magnesium chloride, calcium chloride and potassium chloride.

I found the following facts:

1) While it is estimated that the entire volume of the Mediterranean Sea is replaced approximately every 70 to 100 years, the replacement in the Red Sea is faster, but it will still take many years or even decades for the water in the Red Sea to be replaced by inflowing water from the Indian Ocean. You can imagine what this means in terms of major oil spills.

2) The evaporation rate in the Red Sea exceeds the precipitation rate by more than 2 m.a-1. In plain English, this means that the water level in the Red Sea is falling by more than 2 metres a year because the amount of water evaporating is significantly greater than the amount of water added by rainfall or rivers.

The evaporation rate in the Red Sea exceeds the precipitation rate by more than 2 m.a-1. In plain English, this means that the water level in the Red Sea is falling by more than 2 metres a year because the amount of water evaporating is significantly greater than the amount of water added by rainfall or rivers.

3) Such a high rate of evaporation occurs because the average annual evaporative heat flux is about 180 W.m-2. This is more difficult to explain, but the following comparison will do: It is as if every square metre of the ocean surface had a 180 watt light bulb switched on all the time.

4) The surface salinity of the Red Sea is between 40 ppt - 41 ppt (40 to 41 parts of salt per 1000 parts of water), while near the Strait of Bab el-Mandeb, it is between 36 ppt - 37 ppt. The salinity is very high, considering that the average salinity of the world's oceans is about 35 ppt. Two main reasons for the high salinity are the evaporation rate, as water evaporates but the salt remains, and the almost total absence of rivers bringing fresh water into the Red Sea.

5) The southern Red Sea experiences seasonally reversing wind patterns, while the northern Red Sea faces predominantly unidirectional winds throughout the year. The wind pattern in the northern Red Sea is predominantly NNW throughout the year, while the southern Red Sea experiences SSE winds in summer and reverses to NNW in winter.

6) Because water is constantly evaporating from the Red Sea, the main flow is from the Indian Ocean into the Red Sea, not the other way around. This incoming water flows at the top because it is generally less salty and less dense than the water in the Red Sea; conversely, the water flowing towards the Indian Ocean, more specifically the Gulf of Aden, flows at the bottom because it is more dense.

Because water is constantly evaporating from the Red Sea, the main flow is from the Indian Ocean into the Red Sea, not the other way around. This incoming water flows at the top because it is generally less salty and less dense than the water in the Red Sea; conversely, the water flowing towards the Indian Ocean, more specifically the Gulf of Aden, flows at the bottom because it is more dense.

7) The Red Sea has a narrow axial trough, i.e. a deep central channel running along its length. This trough is 1000 to 2000 metres deep in the north, but especially in the centre of the Red Sea. However, this deep trough does not reach the southern part near Bab el-Mandeb, which is a shallow shelf area of the landmass connecting the Arabian Peninsula to the African continent, with depths of less than 200 metres.

8) Although shallow, the bathymetry of the southern part of the Red Sea near the Bab el-Mandeb Strait is topographically complex, including channels, sills ('mountains' on the seabed) and islands. The strait itself, for example, is divided by the island of Perim into two main channels: the eastern channel (Bab Iskender) and the western channel (Bab el-Mandeb proper). The western channel is deeper and wider, making it the preferred route for shipping (even before the Houthi attacks on shipping). Check out MarineTraffic, VesselTracker, VesselFinder, ShipLocation or DataLastic to see how ships are using the western channel.

Given these eight key oceanographic scientific facts, how might a large oil spill unfold if the Houthis were to sink two or three tankers in the southern Red Sea??

Before I go any further, there is an important caveat: I am barely a self-taught oceanographer because of my passion for climate change and shipping. Physical oceanographers would have to confirm or reject my conclusions. But in the absence of a scientific paper on this scenario, I am taking the first step and inviting the experts to give us robust predictions.

Before I go any further, there is an important caveat: I am barely a self-taught oceanographer because of my passion for climate change and shipping. Physical oceanographers would have to confirm or reject my conclusions. But in the absence of a scientific paper on this scenario, I am taking the first step and inviting the experts to give us robust predictions.

Besides, like many things in life, complex phenomena require general equilibrium approaches, as scientists do with climate predictions due to global warming: It is almost like "mixing all the ingredients in a deep pan and letting it boil" until the final result is found. In this case, the eight key features I have identified, and indeed a few more, should be included in sophisticated models to find the results. Without such a complex model, my approach is piecemeal: I look at each of the features individually or at most two or three of them and explore the implications.

Here are my conclusions:

The high evaporation rates would leave a viscous, sticky oil that would be difficult to handle. Because of the high evaporation rates in the Red Sea, and because the lighter and more volatile components of the oil would evaporate faster than usual, the composition of the oil would change rapidly over a short period of time. As the lighter components evaporate, the oil's viscosity and adhesiveness (its tendency of oil to stick or adhere to surfaces) would increase, altering its interaction with marine life and coastlines.

Natural biodegradation would take much longer. With more viscous oil residues, the biodegradation process by which microorganisms break down the oil into less harmful substances would be much slower. In other words, the changed chemical composition and physical form of the weathered oil may inhibit the efficiency of naturally occurring oil-degrading bacteria and fungi (weathering is not the same as watering, as the spilled oil undergoes physical and chemical changes due to exposure to environmental factors such as sunlight, wind, waves and temperature changes, in addition to the watering that occurs when it mixes with water).

The viscosity of the oil would also reduce the dispersion of the spilled oil. A thick spill is less effective at natural dispersion, a process by which the oil breaks up into smaller droplets that mix with the water column. The adhesive nature of the oil would concentrate it near the surface or along coastlines rather than being biodegraded or diluted in the water.

The high salinity would exacerbate the spill. Due to the high salinity of the Red Sea, the mixing of oil and water would produce an atypical emulsion, even compared to other seawater environments. This process would form stable, viscous mixtures with a denser composition than either oil or seawater alone.

The high salinity would exacerbate the spill. Due to the high salinity of the Red Sea, the mixing of oil and water would produce an atypical emulsion, even compared to other seawater environments. This process would form stable, viscous mixtures with a denser composition than either oil or seawater alone.

To summarise the previous conclusions, the sticky oil would resist natural dispersal (as the high rate of evaporation would make it almost impossible for non-emulsified oil to persist) and would hinder the breakdown into smaller droplets that are more easily diluted or broken down by microorganisms. Moving on from evaporation and salinity:

Incoming water from the Gulf of Aden would push the spilled oil deeper into the Red Sea. Incoming surface currents from the Gulf of Aden would spread the oil spillage further into the North Sea. While the high evaporation would tend to localise the oil in the southern part of the Red Sea, the current coming from the south would counteract this. In other words, this influx of less salty, less dense water would reverse some of my previous conclusions. Whether this influx of water would help or not is hard to say. The coasts of Eritrea, Sudan, Saudi Arabia and Egypt, which would not be much affected if the spill were confined to Yemen, Djibouti and southern Eritrea, could suffer.

The spilled oil would remain in the Red Sea and not be diluted in the Indian Ocean waters of the Gulf of Aden. The distinct water stratification in the Red Sea, characterised by denser, saltier water beneath lighter, less salty surface water, acts as a barrier, preventing the easy transport of surface oil into the Indian Ocean. This stratification increases the likelihood that an oil spill would persist in the Red Sea, prolonging the potential risks to its ecosystems. Such containment of the spill within the Red Sea increases the potential for lasting environmental damage in this partially enclosed marine region. With extensive coral reefs, many of which are already under pressure from overfishing, pollution and coral bleaching caused by climate change, an oil spill confined to the Red Sea could exacerbate these pressures.

The spilled oil would remain in the Red Sea and not be diluted in the Indian Ocean waters of the Gulf of Aden. The distinct water stratification in the Red Sea, characterised by denser, saltier water beneath lighter, less salty surface water, acts as a barrier, preventing the easy transport of surface oil into the Indian Ocean.

If an oil spill were to occur during the winter, the spilled oil could eventually migrate out of the Red Sea. The NNW winds would exert a directional force on the sea surface, pushing it towards the Bab el-Mandeb Strait, the narrow passage between the Red Sea and the Gulf of Aden. In this case, oil carried south by NNW winds could potentially enter the Gulf of Aden. This scenario could extend the environmental impact of the spill from the confined Red Sea to the vast waters of the Indian Ocean through the Gulf of Aden.

If an oil spill were to occur during the summer, the thick emulsion would continue to spread across the Red Sea. SSE winds exert a force on surface waters, including any oil spill, that directs them northwards away from the Bab el-Mandeb Strait. This directional effect plays a crucial role in determining the initial movement of the spilled oil. As the oil is driven north, it would spread over a larger area of the sea. The extent of this dispersion would depend on the speed and consistency of the winds and the ocean currents they influence.

An oil spill near the Bab el-Mandeb would have a great impact on the ecosystem. As the area near the strait has shallow shelves, with depths of less than 200 metres, the entire 'basin' would be vulnerable to the immediate and concentrated effects of an oil spill. Oil can sink rapidly to the seabed, affecting benthic organisms (animals and plants that live on or near the seabed) such as corals, sponges, starfish, shellfish, etc.

An oil spill near the Bab el-Mandeb would have a great impact on the ecosystem. As the area near the strait has shallow shelves, with depths of less than 200 metres, the entire 'basin' would be vulnerable to the immediate and concentrated effects of an oil spill. Oil can sink rapidly to the seabed, affecting benthic organisms (animals and plants that live on or near the seabed) such as corals, sponges, starfish, shellfish, etc.

Of course, as well as oil, there could be spills of natural gas, chemicals and fertiliser, depending on the ships involved. And of course the spill could also occur in the Gulf of Aden, near the Bab el-Mandeb Strait, where the Houthis are also attacking ships. I have made my contribution and leave these additional scenarios to other analysts.

I haven't gone into the challenges of cleaning up a major oil spill. Let's just say that the rapid weathering of the oil due to high evaporation rates and high salinity, resulting in thicker, stickier residues, would pose significant challenges to conventional clean-up approaches. These changes would require more laborious, time-consuming and sometimes less effective response tactics. Responding to such spills would be more complex and costly, highlighting the need for preparedness and specialised equipment to effectively mitigate environmental damage.

Would the West (US and Europe) rush into such a clean-up operation? No. Why invest millions of dollars/euros in a disaster caused by the same Yemenis? And why risk the lives of Americans or Europeans to clean up such a mess when the Houthis could also attack those oil response vessels? And why clean up such a mess when the Houthis could sink other tankers even before the clean-up is complete?

Would the West (US and Europe) rush into such a clean-up operation? No. Why invest millions of dollars/euros in a disaster caused by the same Yemenis? And why risk the lives of Americans or Europeans to clean up such a mess when the Houthis could also attack those oil response vessels? And why clean up such a mess when the Houthis could sink other tankers even before the clean-up is complete?

Saudi Arabia would certainly clean up any oil spill that reached its shores because it has the money to do so, but what about three impoverished countries? Yemen, Djibouti and Eritrea? The latter two could get some help, but why risk sending people and resources to Yemen when the Houthis could repeat such an environmental disaster over and over again? Incidentally, fishing is quite important to the Yemeni economy, not only for domestic consumption but also for exports of molluscs, frozen and fresh fish and crustaceans, which account for almost 9% of exports.

So, I end this article as I began it:

Are the Houthis sabotaging themselves by trying to sink tankers or bulk carriers off their coasts?

So, I end this article as I began it: Are the Houthis sabotaging themselves by trying to sink tankers or bulk carriers off their coasts?

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*Two days before each article, I am publishing a shorter piece (a post). Subscribe to my newsletter (on my profile), which I have renamed "Geopolitics around the Red Sea," to be notified of these articles. In future articles I will return to the geopolitical analysis of the Red Sea crisis, which I find much easier to write than this week's more technical topic!

Pablo Rodas-Martini holds a PhD and an MSc from Queen Mary and Westfield College (renamed Queen Mary University), University of London.