Warming up to climate change: Why do global sea surface temperatures matter?

Credit: Climate Reanalyzer

So why are the oceans getting warmer? And why are rising sea surface temperatures a cause of worry? We take a look at these questions in this week’s edition of Warming up to Climate Change — a series of explainers, in which we answer some of the most fundamental questions about climate change, the science behind it, and its impact. You can scroll down to the end of this article for the first nine parts of the series.

Why are the oceans getting warmer?

Since the Industrial Revolution kicked off in the 19th Century, human activities such as burning fossil fuels have released high levels of greenhouse gases (GHGs) in the atmosphere. Carbon dioxide, methane, ozone, and nitrous oxide are some of the notable GHGs, which essentially trap heat in the atmosphere and contribute to global warming. As a result, the average global temperature has risen at least 1.2 degree Celsius above pre-industrial times.

Notably, almost 90 per cent of the extra heat trapped by GHGs has been absorbed by the oceans, making them steadily warmer over the decades. Speaking to the Wired magazine, biological oceanographer Francisco Chavez of the Monterey Bay Aquarium Research Institute in California, said: “The oceans are our saviours, in a way…Things might be a lot worse in terms of climate impacts, because a lot of that heat is not only kept at the surface, it’s taken to depths.”

Apart from global warming, there are other factors which have led to a spike in ocean temperatures. For instance, El Niño — a weather pattern that refers to an abnormal warming of surface waters in the equatorial Pacific Ocean — has contributed to both ocean warming and rising global surface temperatures. However, the global daily average SST began to soar well before El Niño fully developed and remains unusually high as the weather pattern is now waning.

There is also less dust blowing off the Sahara Desert recently due to weaker-than-average winds. Typically, the dust forms a “giant umbrella that shades” the Atlantic water and reduces ocean temperatures, according to the Wired magazine report. But now, the umbrella has partially folded and more of the Sun is beating down on the ocean, the report added.

Why are rising sea surface temperatures a cause of worry?

Higher ocean temperatures can have irreversible consequences for marine ecosystems. For instance, warmer oceans lead to an increase in ocean stratification — the natural separation of an ocean’s water into horizontal layers by density, with warmer, lighter, less salty, and nutrient-poor water layering on top of heavier, colder, saltier, nutrient-rich water. Usually, ocean ecosystems, currents, wind, and tides mix these layers.

The rise in temperatures, however, has made it harder for water layers to mix with each other. Due to this, oceans are able to absorb less carbon dioxide from the atmosphere and the oxygen absorbed isn’t able to mix properly with cooler ocean waters below, threatening the survival of marine life.

Nutrients are also not able to travel up to the surface of the oceans from below. This could threaten the population of phytoplankton — single-celled plants that thrive on the ocean surface and are the base of several marine food webs. Phytoplankton are eaten by zooplankton, which are consumed by other marine animals such as crabs, fish, and sea stars. Therefore, if the phytoplankton population plummets, there could be a collapse of marine ecosystems.

Warmer oceans cause marine heat waves (MHWs), which occur when the surface temperature of a particular region of the sea rises to 3 or 4 degree Celsius above the average temperature for at least five days. Between 1982 and 2016, such heatwaves have doubled in frequency and have become longer and more intense, according to a 2021 study by the UN’s Intergovernmental Panel on Climate Change (IPCC).

MHWs are devastating for marine ecosystems as they contribute to coral bleaching, which reduces the reproductivity of corals and makes them more vulnerable to life-threatening diseases. They also impact the migration pattern of aquatic animals.

According to several studies, higher ocean temperatures may also result in more frequent and more intense storms like hurricanes and cyclones. Warmer temperatures lead to a higher rate of evaporation as well as the transfer of heat from the oceans to the air. That’s why, when storms travel across hot oceans, they gather more water vapour and heat. This results in more powerful winds, heavier rainfall, and more flooding when storms reach the land — meaning heightened devastation for humans.

The only way to avoid or blunt the aforementioned consequences is to reduce GHG emissions. The world, however, is nowhere close to doing that. In 2023, the concentration of GHG reached the highest levels ever recorded in the atmosphere, according to C3S.

Here are the previous instalments of the series: part 1, part 2, part 3, part 4, part 5,  part 6, part 7, part 8, and part 9.