Sunday, October 1

Extreme marine heatwaves and ‘worlds’ so near yet so far apart


The Wallace Line marked in red. Note that it follows the 100 fathom line of deep water. Taken from ‘The Malay Archipelago’, which was first published in 1869.

JULY 2023 was confirmed as the global hottest air temperature month at 16.95 degrees Celsius breaking the previous record of July 2019 by 0.33 degrees Celsius. This was not a surprising record with heatwaves covering large areas of China, the USA, southern Europe, northern Africa and even South America. These figures were released by Copernicus (the European Union’s Climate Change Service) using computer models absorbing millions of temperature data from satellites, aircraft, ships, and weather stations around the world.

As amazingly, it was on Aug 23, 2023 that the ocean temperatures reached a high of 20.97 degrees Celsius between latitudes 60 degrees north and 60 degrees south. This record came at an unseasonal time of the year for globally ocean temperatures normally peak in March coinciding when sea waters reach their warmest in the southern hemisphere.

August’s record temperatures will inevitably rise over the next few months as the equatorial waters in the eastern Pacific Ocean are affected by the El Nino episode thus growing much warmer. We can expect global ocean temperatures to be pushed up into hitherto ‘uncharted territories’!

Why has this rise in marine temperatures occurred?

Our oceans, which normally absorb 25 per cent off carbon dioxide and capture almost 90 per cent of the excess heat from such emissions, have changed as they have become warmer meaning a reduction in their ability to absorb this gas. The atmosphere and the oceans work in tandem for what affects one, affects the other. Not only do our oceans absorb heat but they radiate heat leading to more water evaporation thus adding more moisture into the atmosphere resulting in more intense and prolonged rainfall.

We have already experienced the consequences of this in late July and August as vicious tropical cyclones have hit China and Japan, and in the UK, where, in some parts, more than the normal July and August average monthly rainfall fell within a few hours causing extensive flooding.

The southern Atlantic Ocean has seen unusually warm sea surface temperatures in its present mid-winter months and nor surprisingly when land temperatures were broken by 5 degrees Celsius in Buenos Aires and in Chile recording record high temperatures of 37 degrees Celsius.

Heatwave effects on marine life

Such atmospheric heatwaves result in less mixing of deep ocean waters with nearer surface waters and thus fewer nutrients are dragged up from below to sustain surface nutrients. Higher sea temperatures also reduce the amount of oxygen in the water causing immense problems to fish and other species of marine life. These two factors can affect the migratory patterns of fish. The traditional turn over of the seas by an El Nino episode along the shores of Chile and Peru has, in past years, seen a fertile fishing ground for local fishermen with record tuna catches. We will have to wait and see for details of fish catches there this year.

Elsewhere in the world there is another picture as whales migrate even further north in search of cooler waters, upsetting the food chain by affecting the fish stocks and livelihoods of local fisherfolk. It is thought that this year’s 37 serious shark attacks on humans have occurred as the sharks have become confused by hotter waters. British waters have seen a rapid increase in shark numbers and Portuguese men of war, large venomous jellyfish, with colder water species of fish, cod and herring, migrating even further northwards.

Climate change has resulted in these heatwaves and wherever we are in the world we have felt its effects. Clearly, our oceans are under the greatest stress now than at any point in our short history. No wonder fish prices have soared! As I munch through my traditional Friday night tasty fish pie, I wonder from what oceans in the world the fish I am eating have been caught, and whether I can afford to eat fish in the future.

Sea Temperature Anomalies on Aug 21, 2023. – Photo from NASA’s Earth Observatory

Worlds so near yet so far apart – Wallace Line

It was during Alfred Russel Wallace’s extensive and detailed recorded journeys in the Malay Archipelago, between 1854 and 1862, that he observed at first hand the differences in the fauna between the Indo-Malaysian realm and the Austro-Malaysian region separated by the Wallace Line. Wallace’s attention to this biogeographical division was stimulated by a pamphlet that George Windsor Earl wrote in 1845 entitled, ‘The Physical Geography of South Eastern Asia and Australia’. Wallace, in his 1869 book, ‘The Malay Archipelago’, recognised Earl by praising his, and I quote: “merit of first indicating the division of the Archipelago into an Australian and an Asiatic region, which it has been my good fortune to establish by more detailed observations”.

The demarcation of the Wallace Line is best observed between the islands of Bali and Lombok – only 40km apart across the Lombok Strait – where in Bali the fauna is Asian to include civets, woodpeckers, and, previously, tigers, and in Lombok the fauna is Australian composed of porcupines, zebra finches, and white cockatoos.

Why this difference in the species?

Not only does the line run between Bali and Lombok but also along the Makassar Strait between Borneo and the Philippines and is the result of the merging of four major tectonic plates and several micro-plates and changes in the world’s sea levels during the Ice Ages. The sea water depth between these islands is on average 250 metres and it is this depth that has kept animals on either side of the Lombok Strait apart.

During the Pleistocene glaciations (1.4 to 12,000 years ago) the islands of Bali, Java, and Sumatra were connected to mainland Asia as a result of a fall in sea-level, but the deepwater channel kept Lombok and the Lesser Sunda archipelago isolated from the Asia mainland and thus were colonised by Australian fauna. Hence two separate biomes emerged.

Wallace realised that the sea depth on either side of his hypothetical line was much deeper than elsewhere in the region and thus mitigated against any animal that could fly or swim over long distances when migrating. Those who have travelled from Bali to Lombok may have spotted differences in the Balinese and Sasak languages and, indeed, cultural differences with Hindu temples in Bali and Muslim mosques in Lombok. There are food differences from suckling pig in Bali and beef satay in Lombok.

Wallace’s Line runs much deeper than the bed of the seas and has recently been used to demonstrate changes in the anthropology, human genetics, and the linguistic properties and even propensities of each side of his line.

Geologically, this region of our world is one of the most complex areas where tectonic plates meet and shift from time to time leading to outbursts of magma in the form of lava from its many volcanoes and fissures. It is interesting to note that in the first chapter of his book Wallace indirectly mentions the concept of ‘continental drift’, which 46 years later was postulated by the German meteorologist Alfred Wegener.

One just needs to read the first chapter of ‘The Malay Archipelago’ to appreciate the significance of his findings to biography and biology. This book should be a must for all school libraries in Sarawak, Sabah, Brunei, and Kalimantan.