Climate change – It’s decidedly non-linear. There will be surprises.

NOAA’s ‘Coral Reef Watch’ page shows sea surface temperatures in early June 2026, not as actual temperatures, but as anomalies – the number of degrees Celsius above or below usual for that place and time of year. The image shows the rapidly developing el Niño now under way as a streak of red heading out into the Pacific from Peru. Image courtesy NOAA.

The great majority of people now accept that climate is changing. What too many do not appreciate is that the change in store for us is not just a general warming across the globe, and it is not going to be a slow, linear type of change. Its effects on the environment and on our lives will not be simple and there will be surprises. Hopefully, many surprises will be good ones, but some bad ones will doubtless occur.

Recently, the news media have carried stories concerning two global, oceanic phenomena that have large effects on the world’s climate – AMOC and el Niño. One, AMOC, is a relatively stable global current system that appears to be changing because of the warming caused by our emissions of greenhouse gases. The other, el Niño, is a more-or-less predictable, global-scale climate oscillation that seems likely to become more extreme as warming progresses. Our knowledge of both phenomena is growing, but far from complete, and both create uncertainty when we try to anticipate or project future weather and climate.

Hold that thought, because I cannot resist this aside: Recent advances in our understanding of both phenomena illustrate a quintessential feature of science – in contrast to other ways of knowing, science is always unfinished. That feature of science is often used by those who would prefer to reject scientific consensus and rely on other types of knowledge – religious, cultural, esthetic – in which understanding is generally assumed to be complete. I think that in matters of climate, we need to be relying on science, while recognizing that as knowledge grows our understanding may change.

El Niño

Consider the el Niño oscillation. Hundreds of years ago, Peruvian fishermen observed that many years were characterized by strong offshore winds, cool temperatures, and good fishing, while others were characterized by weak offshore, or even onshore winds, warmer temperatures and very poor fishing. Since the poor fishing conditions generally first appeared during winter months and then extended into summer or even through a couple of years, they termed this unfavorable wind system el Niño, or little boy, referencing the Christ child who was born in winter. The more typical, and favorable wind system was la Nina or little girl – the opposite of little boy. Scientists now know that fishing fluctuated with the el Niño oscillation because strong trade winds, blowing offshore, moved surface waters towards the central Pacific, and colder, nutrient-rich waters upwelled to replace that water. Those nutrients fed the anchovies on which the fishing community depended. During the el Niño phase, with weaker trade winds, little to no upwelling occurred and with fewer nutrients in the water, the anchovies were less abundant.

That warm water piling up in the eastern tropical Pacific can be seen in the image above, from NOAA’s Coral Reef Watch site. A long thin tongue of warmer than usual water extends out from Peru and Ecuador two thirds of the way across the Pacific. It will become even more obvious towards the end of this year as the el Niño develops further. Indeed, back in March, NOAA had provided a projection of what we can expect during August to October. But I had to find it reprinted in Down to Earth, because it appears to have been scrubbed from NOAA’s site as the Trump administration continues to work to make America greatly ignorant again.

Projection, based on trends and conditions at March 2026, of likely distribution of sea surface temperature anomalies during August to October. The thin plume of warmer than usual water evident in the image of SST anomalies in early June, at the top of this post, is projected to become stronger and to reach much further west. Image from A. Sangomla, in Down to Earth, 8 April 2026.

What those fishermen, and the first ocean scientists studying the phenomenon did not know was that the el Niño – la Nina fluctuation was not just something about Peru. It was part of a much larger, cross-Pacific oscillation involving wind and current systems. Meanwhile, climate scientists, in Australia were studying an ocean-wide phenomenon in patterns of air pressure in the south Pacific, in which the usual strong differential between high east Pacific pressure and low west Pacific pressure enhances the trade winds flowing from east to west. Every now and then, this system switched as air pressure in the west became higher and air pressure in the east fell, causing the trade winds to weaken or even reverse. Lacking the poetry of Peruvian fishermen, they named this switcheroo the Southern Oscillation.

As oceanography and meteorology became progressively broader in scale, and better integrated, the unity and truly global scale of this ocean current and wind system cycle was recognized, and it was named ENSO – el Niño Southern Oscillation. While ENSO is primarily a Pacific basin phenomenon, its weather effects extend far from Peru, encompassing essentially the entire world.

While scientists have been studying ENSO for decades, there remains considerable uncertainty about the causes of this oscillation. It is not regular, so el Niños occur every 2 to 7 years or so, and whether the primary initiator or driver is a change in air pressure, a change in surface temperatures, or a change in water currents remains unclear. A classic chicken and egg problem, yet the combined pattern of air pressure, temperature and wind force has been monitored since the 1950s.

Graph showing fluctuations in RONI from 1950 to the present. Data and graph courtesy NOAA Climate Prediction Center.

The Relative Oceanic Nino Index (RONI) depicts the alternation between el Niño and la Nina conditions. Values above 0.5 signal el Niño while values less than -0.5 signal la Nina conditions, and the more extreme the value is, the stronger the impact is, in terms of wind strength, pressure differential and ocean current speed. Values between +0.5 and -0.5 are intermediate. The fluctuation is far from regular and each el Niño phase has its own distinctive form. El Niños in 1997-8 and 2015-16 were particularly strong.

And in 2026, as a weak la Nina wanes and we enter intermediate conditions again, NOAA is predicting a strong el Niño commencing this month or in June and likely peaking during the northern winter. World Meteorological Service is also predicting a strong el Niño, and the media are talking about a super el Niño this winter and in 2027.

The effects of el Niño are global and affect both temperature and precipitation. In Canada, winters tend to be warmer than usual, and storms tend to be more severe on the west coast. While there is as yet no evidence that climate change is affecting ENSO behavior, that possibility remains and may become more evident as temperature rises in coming years. For now, however, the warming due to our release of greenhouse gases simply adds to the temperatures experienced at the peak of the el Niño cycle. For example, the 1997-8 el Niño is widely recognized as the reason 1998 was the hottest year ever until surpassed by 2005 and remained among the 10 hottest years on record until 2019.

As I write this in late May, we can be pretty certain an el Niño is coming, but doubt remains about how strong it will be. My money is on a strong one, meaning a milder winter in Ontario in 2026-27.

Atlantic Meridional Overturning Circulation

AMOC is also enormously important in setting climate around the world, but it is a much larger, and much more stately beast than is el Niño. While el Niño involves air pressure, winds, and largely wind-driven surface currents, AMOC involves large current systems in both the upper and deeper levels of ocean basins – currents that are driven primarily by what is called thermohaline circulation.

This is a much-simplified image of AMOC. Warm water in upper ocean layers travels north, losing heat as it goes. That heat warms the northern hemisphere, especially Europe. Eventually, this salty, southern water becomes cool enough to sink below the northern water around it, and it then travels back towards the south in the deep ocean. Image © Stephan Rahmstorf and Oceanography.

AMOC is an ocean current system within the Atlantic that is responsible for moving enormous quantities of warm, salty, surface waters north from the tropics, into the far north Atlantic where, cooling and becoming more dense, this water slowly sinks to the deep ocean and flows back south as a deep current until it reaches Antarctica. In the south, AMOC links up with other large ocean basin current systems to form what is called the global ocean conveyor belt.

This similarly simplified image portrays the Global Ocean Conveyor Belt. Red currents are in surface waters and blue in deep waters. If it were not for the ocean conveyor system, which moves well oxygenated surface waters to the depths, the deep ocean would be devoid of life. Image © S. Rahmstorf.

In North America, we are perhaps most familiar with the Gulf Stream, that portion of AMOC which flows through the Florida Strait and northeastward towards Newfoundland. Like most rivers, this stream picks up additional water from tributaries as it flows, so that the volume of water flowing north along the Florida coast (30 million m³ per second) becomes five times larger (150 million m³per second) as it continues eastward along the south of Newfoundland and across towards Europe. These volumes of water dwarf the flow from all the rivers into the north Atlantic (about 600,000 m³ per second), including the Mississippi and the Amazon.

Despite its size, the Gulf Stream is still a surface phenomenon. Off the US east coast, it is about 100 km wide, and about 1 km deep in an ocean that averages about 4 km deep. While the flow represents a vast quantity of water being transported, the size of the stream is such that the velocity is quite stately. Faster toward the surface, the water moves north at around 3.5 to 6 kilometers per hour.

The water in the Gulf Stream has come from the surface waters of the tropical ocean. It is warm, and with evaporation, over time it slowly becomes more salty. It carries all this heat northward, gradually shedding heat to surrounding water and the atmosphere as it goes. Without this transport of heat, the northern latitudes would be a lot colder than they are and icebergs from Greenland would drift further south before fully melting. Still, losing heat as it travels north and east, and becoming slowly saltier because of evaporation, this formerly tropical surface water becomes cold and more dense than the water surrounding it, which has been considerably diluted (less salty) by meltwater from glaciers and sea ice in the Arctic and North Atlantic. Gradually, the surface-flowing Gulf Stream begins to sink below less salty northern waters, eventually descending several kilometers into the deep Atlantic, where it flows slowly southward again. While winds play a role in the movement of the Gulf Stream, it is this sinking of surface waters into the deep ocean – the thermohaline pump – which is the primary driver of the circulation of the AMOC as a whole.

Note that, largely because of geography, there is not a south Atlantic equivalent of the Gulf Stream. Surface water from the temperate south Atlantic flow northward, into the tropics to join the Gulf Stream. This lack of a mirror image between the north and south Atlantic is why the northern hemisphere is approximately 1.4^oC warmer than the southern hemisphere. (Now there is an item to be recalled the next time the cocktail party conversation stalls!)

Unlike el Niño, climate scientists do understand the interactions between AMOC and climate change. Indeed, AMOC has always been built into global climate models because its role in heat transport is a significant factor in influencing weather.

In the north Atlantic, AMOC is moving heat at the rate of one petawatt – that is one quadrillion watts or 10¹⁵ watts. Or a quadrillion joules of heat energy per second. It’s also about 50 times the use of all energy by all of humanity. It’s a lot. And it keeps northern Europe much warmer than it would otherwise be. Projections for what would happen if AMOC collapsed totally show the world a lot warmer than now, except for the North Atlantic and the northern part of Europe which is about 2.5^oC colder than now – a little ice age.

This rather red image is a simulation of what the likely outcome of a complete cessation of AMOC would lead to. The map plots sea surface temperature anomalies between those conditions and the present day temperatures. Most of the world is a couple of degrees warmer than at present. Except for the north Atlantic region which persists as a place significantly colder than today. Image © S Rahmstorf.

As well as warming Europe, AMOC prevents water piling up along the east coast of North America. If AMOC slows substantially, the Gulf Stream will move closer to shore, but because it will slow down the Coriolis force, which I have never really understood, will cause sea level to rise at the coast. If the current rate of slowdown continues, that Coriolis effect alone will cause a sea level rise of 15-20cm off the Carolinas by 2100. (Good thing that North Carolina legislated a maximum permitted sea level rise in 2012 – just like King Canute, they won’t have to worry about coastal erosion and other problems that sea level rise brings.)

And by the way, not that it would likely have huge impacts on people, AMOC also brings oxygenated water to the deep ocean. With AMOC stalled, the deep ocean would go anoxic and all life there would be lost. This has happened many times in the last 100,000 years. Ice cores reveal that on each occasion, CO₂ emissions (perhaps due to volcanism) led to warming, AMOC slowdown, and deep ocean anoxia in the north Atlantic – these are termed Dansgaard-Oeschger events (named after a Danish and a Swiss paleo-climate experts who studied ice cores). In each event, the north Atlantic warming is very rapid – 10 to 15^oC within 10-20 years. No reason why it might not happen again.

But I am getting ahead of myself. What does climate change do to AMOC? This question was originally answered quite well by Henry Stommel, an American oceanographer working in 1961. He recognized that the Gulf Stream is driven by the fact that its water is particularly salty (having spent plenty of time in the tropics where there is a lot of evaporation and not much precipitation). It becomes even saltier as it travels north and is far saltier than the water around it, which has mostly flowed out of the Arctic Ocean and contains plenty of meltwater from ice and glaciers. And because it is saltier, once it cools sufficiently it sinks down driving the AMOC pump.

But Stommel went further developing a simple analytical model to explore what would happen if climate warmed. That Gulf Stream water would lose less heat to its surroundings, would become less salty through intermixing with the even less saltier surrounding water (all that extra melting of glaciers) and there would be no pump to move water downward.

Further, Stommel predicted that there would be a classic tipping point – AMOC would slow gradually (on geological timeframes) and then suddenly come to a halt. And reversing climate change would not immediately start it up again.

Stommel could sleep at night because such happenings were all very hypothetical and in the distant future. But today? Well, oceanographers have measured the slowdown. And their projections now suggest it could happen during the second half of this century. Not tomorrow, but definitely not in the distant future. And when it reaches that tipping point we will be incapable of speeding AMOC back up again. Were AMOC to stall completely, the planet would become a lot hotter quite quickly, while the north Atlantic and Europe would cool into a mini-ice age. For a very clearly written review of what we know about AMOC, this 2024 article by Stephan Rahmstorf, University of Potsdam, a leading climate scientist is worth diving into.

What do we do about this news?

The fact that we know what an el Niño can bring in terms of severe weather, and we have a growing capacity to project likely severity and timing, means that emergency response systems can be ready when the storms, the wildfires and the high temperatures arrive. In some parts of the world, the severe weather will result in food shortages and famine. Given in how many parts of the world populations are now at severe risk of famine, the changes in monsoons and other major precipitation patterns are likely the major risks posed to people by el Niño. Here in North America, that is not the case, but we can anticipate an unusual kind of year: milder winters, more severe storms in the west, but fewer hurricanes in the Caribbean.

AMOC is a different situation altogether. We need to ensure that scientific study of this system continues so that projections of likely slowdown can become more precise than at present. We also need wider understanding of what this can mean in various parts of the world. The consequences would not be a colder than usual year or two, but a substantial generational shift in climate. With 8 billion of us on the planet the disruptions to agriculture would certainly take a significant toll. We have some time, but not a lot of time to become better prepared.

What we definitely should not be doing, for either of these issues, is to spend millions of dollars dismantling an extensive marine monitoring program that was installed a decade ago with a projected useful life of at least 25 years. Yes, King Trump, with his dislike of anything that might tell us about climate or the environment, has driven another monumentally stupid decision – I just hope the teams tasked with dismantling and removing the ocean monitoring infrastructure find ways to move as slowly as AMOC does. Delay, delay, procrastinate, and maybe most of the array of instruments will still be in place when Trump departs Washington.

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