Data Munching | We can’t turn a blind eye to climate change when sea surface temperatures rise

Much has been written about how 2023 was the warmest year in the world in recorded history. On January 11, data from Copernicus Climate Change Service (C3S) of the European Centre for Medium-Range Weather Forecasts (ECMWF) confirmed that 2023 was the warmest ever year on record and by a massive margin.

Other datasets confirm this trend, too. Ocean heat, meanwhile, is also soaring to an all-time high, data shows, with an alarming spike already visible in 2024. What is ocean heat, and why does this matter? Here are some charts that explain this in detail.

How bad are sea temperatures?

Sea surface temperatures (SST) — the temperature of the ocean’s surface layer — across the world averaged at around 20.89°C over 2023. By itself, this number may not describe much, but what stands out is that this was a deviation of 0.69°C over what the world saw between 1982 and 2011 — the long period average of 20.20°C as calculated by the University of Maine’s Climate Reanalyser, which compiles global sea surface temperature data, as observed by the NOAA Optimum Interpolation SST V2. In her column, Cause and Effect, my colleague Tannu Jain described it as a “5.3 sigma event, which roughly translates into a once-in-200-year occurrence”.

A key distinction to note here is that the base is recent (1982-2011), and should not be confused with the often-discussed average of the pre-Industrial period (1850-1900) which is data from more than a century old. While deviations over centuries (though alarming in itself) gives a basic understanding of the impact of human activities on the Earth’s climate, any deviation in temperatures over the past few decades is all the more alarming as it highlights how quickly temperatures are spiralling out of control.

That 2023 would appear to be among the biggest deviations was always a given, keeping in mind that in the second half of the year, nearly every month a new weather record was set. But what is even more alarming right now is the magnitude of the head start that 2024 has started with. More often than not, it is quite challenging to present to a layperson tangible evidence of how the world is changing. But the chart below gives an idea of how severe the crisis really is and should make it likely to grasp the real magnitude of the climate crisis.

Chart 1: How do sea temperatures compare to previous years?

The early data for 2024, however, averages at a whopping 21.02°C. A caveat: the 2024 data only looks at the first three weeks. But it does give us a clear idea of how far ahead the year already is from even 2023, which itself was a runaway year in terms of SSTs.

Chart 2: 10 hottest years by sea surface temperature, and where 2024 has started

Chart 3: Why is sea surface temperature critical?

An easier way to look at the role of oceans in global warming is to think of the vast amount of water on Earth as a massive planet-wide heat absorption mechanism. All the excess heat generated is absorbed by the oceans and eventually sent back to the atmosphere. But the warmer you make the water, the harder it gets for the oceans to absorb more heat — like trying to dry yourself with a towel that is already damp.

Furthermore, warmer water also loses its ability to absorb carbon dioxide, which means more greenhouse gases in the atmosphere can retain more heat.

Another obvious impact of a rise in SST is that it causes the melting of polar ice caps and glaciers, especially in the Arctic and Antarctic regions, leading to rising sea levels. Since warmer items also expand (think thermal expansion from Class 5 science), when this impact is extrapolated to the oceans of the world, it means that waters would expand further adding to sea level rise.

Then, there’s coral bleaching — the phenomenon where coral reefs expel their symbiotic algae living, causing them to turn white and become more susceptible to diseases, and eventually their deaths which severely impact marine life. Similarly, rising SST can exacerbate ocean acidification, a process where the ocean absorbs more carbon dioxide, leading to a decrease in pH — harming marine life. Since the plankton that floats on ocean surfaces produce half of the earth’s oxygen, ocean acidification can eventually hit our ability to breathe fresh air.

Thus sea surface temperatures have a cascading effect — but that’s what a crisis looks like, not a single catastrophe, but a series of very unfortunate events.