Something strange is happening in the Pacific and we must find out why

Unexpectedly, the eastern Pacific Ocean is cooling. If this “cold tongue” continues, it could reduce greenhouse gas warming by 30 per cent – but also bring megadrought to the US.

FOR years, climate models have predicted that as greenhouse gas emissions rise, ocean waters will warm. For the most part, they have been correct. Yet in a patch of the Pacific Ocean, the opposite is happening. Stretching west from the coast of Ecuador for thousands of kilometres lies a tentacle of water that has been cooling for the past 30 years. Why is this swathe of the eastern Pacific defying our predictions? Welcome to the mystery of the cold tongue.

The Pacific “cold tongue”, an area of ocean that stretches West from Ecuador is cooler than expected

This isn’t just an academic puzzle. Pedro DiNezio at the University of Colorado Boulder calls it “the most important unanswered question in climate science”. The trouble is that not knowing why this cooling is happening means we also don’t know when it will stop, or whether it will suddenly flip over into warming. This has global implications. The future of the cold tongue could determine whether California is gripped by permanent drought or Australia by ever-deadlier wildfires. It influences the intensity of monsoon season in India and the chances of famine in the Horn of Africa. It could even alter the extent of climate change globally by tweaking how sensitive Earth’s atmosphere is to rising greenhouse gas emissions.

Given all this, it isn’t surprising that climate scientists are trying to find out what is going on with increasing urgency. Like any good mystery, this is a tale of intrigue, confusion and competing theories. We haven’t quite solved it yet. But just acknowledging its existence will help us plan for possible shifts. And by cracking the case, we will unlock the final details of our climate future.

The Pacific Ocean is the perfect place to harbour secrets. It is the largest, deepest ocean on Earth – so vast that it covers more surface area than all land combined. Getting a handle on how it will respond to increasing levels of greenhouse gases in the atmosphere is a mammoth challenge. That is in large part because there are big natural variations in the climate of the tropical Pacific, which influence weather around the world.

Pacific cooling

For example, every three to five years or so, the Pacific flips from La Niña conditions, where surface water temperatures in its equatorial region are relatively cool, to El Niño conditions, where these waters become warmer than average. This cycle, known as the El Niño-Southern Oscillation, or ENSO, is caused by changes in winds blowing across the ocean and movement of water between the cooler depths and the warmer surface.

Then there is the Pacific Decadal Oscillation. Its exact cause is unclear, but its effects mirror those of ENSO, although usually over a timescale of between 20 and 30 years. These changes make it hard to tease out long-term trends. So when researchers first spotted the cold tongue in the 1990s, they put it down to the region’s extreme – but natural – variability.

Richard Seager at Columbia University in New York was one of the first to challenge this view. In 1997, he co-authored a paper warning that the equatorial Pacific was cooling, a trend not seen in climate models. Since then, data on sea surface temperatures has confirmed Seager’s suspicions. The eastern Pacific (near the Americas) has always been cooler on average than the western part of the ocean (near Asia) by 5°C or 6°C, but between 1980 and 2019, this temperature difference widened by about 0.5°C.

Misleading climate models

Today, growing numbers of scientists have come to share Seager’s doubts that the cold tongue can be explained by natural variability. If climate models fail to reflect the cold tongue, they could be seriously misleading. “We have something to be concerned about and we need to figure out what it is,” says Isla Simpson at the US National Center for Atmospheric Research.

This really matters because there are huge implications for our climate’s future. Warming waters in the western Pacific and cooling waters in the east lead to more low-lying cloud over vast stretches of the eastern Pacific. “More intense clouds mean more reflected sunlight,” says David Battisti at the University of Washington in Seattle. This, in turn, means less warmth entering Earth’s atmosphere and getting trapped as a result of greenhouse gases. In other words, a cooling eastern Pacific slows the rate of global warming.

The future of the Pacific Ocean’s “cold tongue” will influence the intensity of monsoons in India

If the current trend continues, the cold tongue could reduce the level of projected global warming by a whopping 30 per cent compared with the predictions we get from climate models. That is the difference between global average temperatures rising by a further 1.3°C by the end of the century, or by a further 1.9°C. Crucially, the same volume of greenhouse gas emissions enters the atmosphere in both scenarios. However, it also means the base state of the climate would be more similar to La Niña, increasing the risk of droughts in the Horn of Africa and the US Southwest.

On the other hand, if climate models have got it right and the eastern Pacific warms, the rate of global heating will be higher and there will be different regional impacts. The base state of the climate will be more like El Niño, increasing coral reef bleaching, making the Amazon hotter and drier, bringing more droughts to Australia and Indonesia and leading to fatal heatwaves in India as monsoons fail. Meanwhile, parts of the Americas will experience more storms, with heavy rains leading to devastating floods and landslides in countries such as Peru and Ecuador.

Which future should we prepare for? If the cooling in the eastern Pacific persists, it will have “big implications for regional climate predictions”, says Malte Stuecker at the University of Hawai’i at Mānoa. Yet at present, policy-makers, city planners and industry chiefs tend to get climate change forecasts based on models that feature a warming eastern Pacific.

To predict what will happen next, we first need to understand what is happening now. That is why some of the best brains in climate science are racing to discover why the real-world observations are so at odds with their climate models. There is no shortage of competing ideas.

Melting glaciers

Some think the answer could lie in the cold seas of the Southern Ocean around Antarctica. These waters, as in the eastern Pacific, are among the few places where sea surface temperatures have declined in recent decades. One likely driver is the melting of Antarctic glaciers as global temperatures rise. Another possibility is that ozone depletion and rising greenhouse gas emissions are strengthening winds in the region, intensifying the movement of cold air from Antarctica to the surface waters of the Southern Ocean. Whatever is causing the cooling there, Battisti and others theorise that it is having a knock-on effect in the tropical Pacific. Climate models don’t always include Antarctic meltwater in their calculations and have trouble correctly reflecting changes to sea temperatures, winds and currents in the Southern Ocean. However, Yue Dong at Columbia University has shown that, when those errors are corrected, climate models do start to show cooling in the tropical Pacific. As a result, “the near-future warming projections by current global climate models may be overestimated”, the work suggests.

Another possible explanation for the cold tongue is that climate change is altering winds and ocean upwelling – the process of colder water from the deep ocean rising to the surface – in the Pacific. As already noted, the western Pacific is naturally warmer than the east. That is thanks, in part, to upwelling of cold water in the eastern Pacific, which keeps surface temperatures cooler there. As climate change advances, this is amplified, says Alexey Fedorov at Yale University. “When you increase carbon dioxide concentration, radiative effects [from sunshine] are more effective in the west because there is no moderating effect of cold water coming to the surface.” As this warmer air over the western Pacific expands, it rises and heavier, cooler air rushes in to replace it, so that the prevailing winds (known as trade winds) blow more strongly from east to west. This amplifies the upwelling of cool water in the eastern Pacific, promoting further regional cooling. But that effect is seldom factored in. “There are some persistent biases in climate models that prevent them from fully replicating this mechanism,” says Fedorov.

It is most likely that a combination of these mechanisms is at play – with natural variation also a contributing factor. To crack the mystery, we need smarter climate models that are better at simulating everything from cloud cover to ocean currents, winds and melting glaciers.

DiNezio is pioneering this new generation of models using supercomputers. Early experiments have yielded promising results, they say, generating temperature trends in the Pacific that more closely match observed ones. But running the models is expensive and energy intensive, while access to supercomputers is limited because they are in such high demand.

Finding where the models are going wrong is only the first step in solving the cold tongue mystery, though. Ultimately, what everyone wants to know is whether the cooling trend is temporary or permanent.

However, some researchers think climate models will be proved right in the long run – that the basic physics of pumping vast volumes of greenhouse gases into the atmosphere means the eastern Pacific will eventually “flip” back to a warming state. When this reversal might happen is another matter: some say it could be in 20 years, others put it nearer to 100. “There are more mechanisms pointing to the El Niño-like warming in the very far future,” says Sarah Kang at the Ulsan National Institute of Science and Technology in South Korea.

If it does flip back, we need to know how this will affect global warming as a whole. The current El Niño event, which is likely to peak later this year, offers an opportunity to explore this question. As the Pacific warms during this phase of ENSO, cloud cover above it will shift geographically – and climate scientists will be watching closely. “We can look at how the clouds respond to an El Niño event, and that might give us some idea of how the clouds will react under these long-term changes,” says Stuecker, who is part of an international working group formed this year to study the cold tongue. This, in turn, will tell researchers more about the link between Pacific cloud cover and expected levels of atmospheric warming.

The melting of Antarctic glaciers as global temperatures rise may have a knock on effect on the tropical Pacific
Sebnem Coskun/Anadolu Agency via Getty Images

Any transition to a warming trend in the eastern Pacific would be globally disruptive, particularly if it takes us by surprise. Climate resilience plans, covering everything from water security to agriculture, could be rendered obsolete almost overnight, leaving nations scrambling to rebuild infrastructure and economies. “If we don’t have the ability to predict when this is going to happen, then all of a sudden we are going to have these big changes in regional climate effects that we’re not ready for,” says Robert Wills at the Swiss Federal Institute of Technology in Zurich.

Despite all this, solving the mystery of the cold tongue isn’t about proving climate models wrong, per se. On the big issues, from global average surface temperatures to Arctic ice melt, they have been remarkably accurate. When they warn that we are in for a warmer, wilder future, we can trust that they are correct. Rather, the cold tongue is the last big piece of the puzzle. Fit that in and we can build a more accurate picture of how life will change in a warming world – and how best to prepare for that future.

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