Why humanity's 'greatest ally against climate change' may be about to switch sides

So while the most noticeable signs of climate change – like out-of-season wildfires, devastating floods and deadly heatwaves – are all on land, there is clearly a lot going on beneath the waves that most of us do not see.

The rise in sea levels – fuelled by melting ice sheets and expansion as the water warms – is the most obvious one to humans, particularly those who inhabit low-lying Pacific Islands and places like Indonesia’s capital Jakarta, which has the misfortune of being on the coast, low-lying and the fastest-sinking city in the world.

There are other serious concerns, such as the acidification caused by extra carbon in the water, a threat to shellfish and coral reefs, and the disruption of ocean currents that have a huge influence on our climate.

A sudden depature from the norm

Sea surface temperatures tell a similar story to those on land – they are rising – and this is documented in near-real time by the Copernicus Climate Change Service. So on Sunday, the average global ocean temperature, between 60 degrees north to 60 degrees south, was 20.88 degrees Celsius, which, for someone who has occasionally dipped a timid toe into the North Sea, sounds lovely.

Until recently, every year, the average temperature would peak in about March or April, as the southern hemisphere neared the end of its summer, then fall back, with a second, smaller peak towards the end of the north’s summer.

READ MORE: Why climate change tipping points that could lead to runaway global warming keep me awake at night – Professor Tim Lenton

However, in May 2023, something strange happened. After a brief dip, temperatures plateaued when they should have been falling and then rose dramatically to set a new record high temperature in August, the wrong time of year. If the graph had been data from a machine, you might think that, after a long build-up, something had snapped.

Part of the explanation is that the world was experiencing the natural ‘El Nino’ weather phenomenon, which has a warming effect. However this is a regular event and previous iterations did not result in temperatures breaking free from the usual, seasonal pattern.

Watch a live climate event unfold

Fortunately, this pattern returned last year, although temperatures have remained at a higher level than before 2023. The mercury had been fairly steadily rising, but the Copernicus graph showing sea surface temperature anomalies has a pronounced warm spike from 2023 onwards.

In an interview, Dr Samantha Burgess, Copernicus Climate Service’s deputy director, told me: “We've seen, in the last two years, a stronger departure from average [temperatures]. We've not seen that type of signal before, that strength of signal, and scientists are still trying to understand why we saw such a departure with an El Nino event that wasn't particularly strong.”

Over the next few weeks, we will discover whether or not there will be another abrupt departure from the historic pattern and we can all watch it play out on the near-live Copernicus graph. The service also has a graph for air temperatures and an interactive globe that allows you to see which parts were hotter or cooler than normal on a given day all over the world.

Tipping points

To me, the strange and record-breaking sea surface temperatures of 2023, which were surpassed in 2024, are an early warning sign that something fundamental is changing in our oceans, that they are moving closer to the “tipping points” feared by climate scientists.

With global warming recently rising above 1.5C for a full year for the first time, Dr Burgess pointed to some of the potential consequences of allowing this process to continue.

“One of the tipping points in the ocean is for ecosystems. So if we get up to two degrees of warming, it's predicted that we'll lose coral reefs and other calcifying organisms,” she said. “If we get up to two degrees of warming, it's predicted to significantly disrupt our ocean circulation as well and, in the UK, in particular, we're much warmer than we would be due to warm ocean currents keeping our temperature quite mild.”

Net zero better than geo-engineering

Concern about climate change has prompted research into “geo-engineering”, with scientists investigating ways to artificially cool the Earth. One such project involves removing carbon from the sea, storing it, and returning the decarbonised water, which is then better able to absorb carbon from the air.

Such research may strike some as hopeful, but as far as I’m concerned, it’s mildly terrifying. If we have reached a point where we must start interfering with the climate on a global scale, we really are in trouble. The potential for unintended consequences when dealing with such a complex system seems obvious and immense.

The drive to reduce our carbon emissions to net zero by 2050 may be difficult – or even, as Conservative leader Kemi Badenoch has claimed, “impossible”. However, to my mind, it still provides the safest, surest way out of the growing climate crisis, for all the practical problems that completing such an endeavour within 25 years will inevitably create. The task is vast, but so is the threat.

The oceans have been our climate friend for decades but, as on land, the effects of rising temperatures are becoming increasingly hard to miss – or dismiss. If we fail to act as scientists advise, humanity may find that we have angered a sleeping giant, a kraken in the deep, and that our ally has become our enemy.