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Ilulissat Greenland, midnight sun. Photo: Cody Whitelaw

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Authors:

Laurie Menviel, Post-doctoral Research Fellow, Climate Change Research Centre, UNSW Sydney and Gabriel Pontes, Post-doctoral Research Fellow, Climate Change Research Centre, UNSW Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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A vast network of ocean currents nicknamed the “great global ocean conveyor belt” is slowing down. That’s a problem because this vital system redistributes heat around the world, influencing both temperatures and rainfall.

The Atlantic Meridional Overturning Circulation funnels heat northwards through the Atlantic Ocean and is crucial for controlling climate and marine ecosystems. It’s weaker now than at any other time in the past 1,000 years, and global warming could be to blame. But climate models have struggled to replicate the changes observed to date – until now.

Our modelling suggests the recent weakening of the oceanic circulation can potentially be explained if meltwater from the Greenland ice sheet and Canadian glaciers is taken into account.

Our results show the Atlantic overturning circulation is likely to become a third weaker than it was 70 years ago at 2°C of global warming. This would bring big changes to the climate and ecosystems, including faster warming in the southern hemisphere, harsher winters in Europe, and weakening of the northern hemisphere’s tropical monsoons. Our simulations also show such changes are likely to occur much sooner than others had suspected.

The Atlantic Meridional Overturning Circulation (AMOC): what is it and why is it so important? (National Oceanography Centre)

Changes in the Atlantic Meridional Overturning Circulation

The Atlantic ocean circulation has been monitored continuously since 2004. But a longer-term view is necessary to assess potential changes and their causes.

There are various ways to work out what was going before these measurements began. One technique is based on sediment analyses. These estimates suggest the Atlantic meridional circulation is the weakest it has been for the past millennium, and about 20% weaker since the middle of the 20th century.

Evidence suggests the Earth has already warmed 1.5ºC since the industrial revolution.

The rate of warming has been nearly four times faster over the Arctic in recent decades.

Meltwater weakens oceanic circulation patterns

High temperatures are melting Arctic sea ice, glaciers and the Greenland ice sheet.

Since 2002, Greenland lost 5,900 billion tonnes (gigatonnes) of ice. To put that into perspective, imagine if the whole state of New South Wales was covered in ice 8 metres thick.

This fresh meltwater flowing into the subarctic ocean is lighter than salty seawater. So less water descends to the ocean depths. This reduces the southward flow of deep and cold waters from the Atlantic. It also weakens the Gulf Stream, which is the main pathway of the northward return flow of warm waters at the surface.

The Gulf Stream is what gives Britain mild winters compared to other places at the same distance from the north pole such as Saint-Pierre and Miquelon in Canada.

Our new research shows meltwater from the Greenland ice sheet and Arctic glaciers in Canada is the missing piece in the climate puzzle.

When we factor this into simulations, using an Earth system model and a high-resolution ocean model, slowing of the oceanic circulation reflects reality.

Our research confirms the Atlantic overturning circulation has been slowing down since the middle of the 20th century. It also offers a glimpse of the future.

Simplified schematic of the Atlantic Meridional Overturning Circulation in the North Atlantic
As they travel northwards, the Gulf Stream and North Atlantic Current cool as they lose heat to the atmosphere. The waters then become dense enough to sink to depth and form North Atlantic deep water, which travels southward at depth and feeds the other ocean basins. Modified from Nature Reviews Earth & Environment (2020)

Connectivity in the Atlantic Ocean

Our new research also shows the North and South Atlantic oceans are more connected than previously thought.

The weakening of the overturning circulation over the past few decades has obscured the warming effect in the North Atlantic, leading to what’s been termed a “warming hole”.

When oceanic circulation is strong, there is a large transfer of heat to the North Atlantic. But weakening of the oceanic circulation means the surface of the ocean south of Greenland has warmed much less than the rest.

Reduced heat and salt transfer to the North Atlantic has meant more heat and salt accumulated in the South Atlantic. As a result, the temperature and salinity in the South Atlantic increased faster.

Our simulations show changes in the far North Atlantic are felt in the South Atlantic Ocean in less than two decades. This provides new observational evidence of the past century slow-down of the Atlantic overturning circulation.

Infographic showing how the North Atlantic is connected to the South Atlantic by the overturning circulation
The addition of meltwater in the North Atlantic leads to localised cooling in the subpolar North Atlantic and warming in the South Atlantic. https://www.nature.com/articles/s41561-024-01568-1


What does the future hold?

The latest climate projections suggest the Atlantic overturning circulation will weaken by about 30% by 2060. But these estimates do not take into account the meltwater that runs into the subarctic ocean.

The Greenland ice sheet will continue melting over the coming century, possibly raising global sea level by about 10 cm. If this additional meltwater is included in climate projections, the overturning circulation will weaken faster. It could be 30% weaker by 2040. That’s 20 years earlier than initially projected.

Such a rapid decrease in the overturning circulation over coming decades will disrupt climate and ecosystems. Expect harsher winters in Europe, and drier conditions in the northern tropics. The southern hemisphere, including Australia and southern South America, may face warmer and wetter summers.

Our climate has changed dramatically over the past 20 years. More rapid melting of the ice sheets will accelerate further disruption of the climate system.

This means we have even less time to stabilise the climate. So it is imperative that humanity acts to reduce emissions as fast as possible.The Conversation

Living planktonic foraminifera photos by Haruka Takagi, Katsunori Kimoto, Tetsuichi Fujiki, Hiroaki Saito, Christiane Schmidt4 , Michal Kucera and Kazuyoshi Moriya5, CC BY-SA 4.0 via Wikimedia Commons

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1) Orbulina universa, (2) Sphaeroidinella dehiscens, (3) Globigerinoides sacculifer, (4) Globigerinoides conglobatus, (5) Globigerinoides ruber (white), (6) Globigerinoides ruber (pink), (7) Globoturborotalita rubescens, (8) Globoturborotalita tenella, (9) Globigerinella calida, (10) Globigerinella siphonifera Type I, (11) Globigerinella siphonifera Type II, (12) Globigerinella adamsi, (13) Globigerina bulloides, (14) Turborotalita quinqueloba, (15) Turborotalita humilis, (16) Hastigerina pelagica, (17) Hastigerinella digitata, (18) Neogloboquadrina incompta, (19) Neogloboquadrina pachyderma, (20) Neogloboquadrina dutertrei, (21) Pulleniatina obliquiloculata, (22) Globorotalia inflata, (23) Globorotalia menardii, (24) Globorotalia scitula, (25) Globorotalia crassaformis, (26) Globorotalia truncatulinoides, (27) Candeina nitida, (28) Globigerinita glutinata, (29) Globigerinita uvula, and (30) Tenuitella fleisheri. Images by Haruka Takagi, Katsunori Kimoto, Tetsuichi Fujiki, Hiroaki Saito, Christiane Schmidt , Michal Kucera and Kazuyoshi Moriya CC BY-SA 4.0 via Wikimedia Commons

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This article is republished from The Conversation under a Creative Commons license. Read the original article.
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Global temperature records are expected to exceed the 1.5 °C threshold for the first time this year. This has happened much sooner than predicted. So can life on the planet adapt quickly enough?

In our new research, published today in Nature, we explored the ability of tiny marine organisms called plankton to adapt to global warming. Our conclusion: some plankton are less able to adapt now than they were in the past.

Plankton live in the top few metres of ocean. These algae (phytoplankton) and animals (zooplankton) are transported by ocean currents as they do not actively swim.

Climate change is increasing the frequency of heatwaves in the sea. But predicting the future effects of climate change is difficult because some projections depend on ocean physics and chemistry, while others consider the effects on ecosystems and their services.

Some data suggest that current climate change have already altered the marine plankton dramatically. Models project a shift of plankton towards both poles (where ocean temperatures are cooler), and losses to zooplankton in the tropics but might not predict the patterns we see in data. Satellite data for plankton biomass are still too short term to determine trends through time.

To overcome these problems, we have compared how plankton responded to past environmental change and modelled how they could respond to future climate changes. As the scientist Charles Lyell said, “the past is the key to the present”.

We explored one of the best fossil records from a group of marine plankton with hard shells called Foraminifera. This comprehensive database of current and past distributions, compiled by researchers at the University of Bremen, has been collected by hundreds of scientists from the seafloor across the globe since the 1960s. We compared data from the last ice age, around 21,000 years ago, and modern records to see what happened when the world has previously warmed.

We used computational models, which combine climate trends with traits of marine plankton and their effect on marine plankton, to simulate the oceanic ecosystems from the last ice age to the pre-industrial age. Comparing the model with the data from the fossil record is giving us support that the model simulated the rules determining plankton growth and distribution.

We found that some subtropical and tropical species’ optimum temperature for peak growth and reproduction could deal with seawater warming in the past, supported by both fossil data and model. Colder water species of plankton managed to drift to flourish under more favourable water temperatures.

Our analysis shows that Foraminifera could handle the natural climate change, even without the need to adapt via evolution. But could they deal with the current warming and future changes in ocean conditions, such as temperature?

Future of the food chain

We used this model to predict the future under four different degrees of warming from 1.5 to 4 °C. Unfortunately, this type of plankton’s ability to deal with climate change is much more limited than it was during past warming. Our study highlights the difference between faster human-induced and slower-paced geological warming for marine plankton. Current climate change is too rapid and is reducing food supply due to ocean stratification, both making plankton difficult to adapt to this time.

Phytoplankton produce around 50% of the world’s oxygen. So every second breath we take comes from marine algae, while the rest comes from plants on land. Some plankton eat other plankton. That in turn gets eaten by fish and then marine mammals, so energy transfers further up the food chain. As it photosynthesises, phytoplankton is also a natural carbon fixation machine, storing 45 times more carbon than the atmosphere.

Around the world, many people depend heavily on food from the ocean as their primary protein sources. When climate change threatens marine plankton, this has huge knock-on effects throughout the rest of the marine food web. Plankton-eating marine mammals like whales won’t have enough food to prey on and there’ll be fewer fish to eat for predators (and people). Reducing warming magnitude and slowing down the warming rate are necessary to protect ocean health.

Global warming will continue to rise: image Peggy Anke

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Authors: Andrew King, Senior Lecturer in Climate Science, The University of Melbourne and Tilo Ziehn, Principal Research Scientist, CSIRO

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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The world is striving to reach net-zero emissions as we try to ward off dangerous global warming. But will getting to net-zero actually avert climate instability, as many assume?

Our new study examined that question. Alarmingly, we found reaching net-zero in the next few decades will not bring an immediate end to the global heating problem. Earth’s climate will change for many centuries to come.

And this continuing climate change will not be evenly spread. Australia would keep warming more than almost any other land area. For example if net-zero emissions are reached by 2060, the Australian city of Melbourne is still predicted to warm by 1°C after that point.

But that’s not to say the world shouldn’t push to reach net-zero emissions as quickly as possible. The sooner we get there, the less damaging change the planet will experience in the long run.

Reaching net-zero is vital

Global greenhouse gas emissions hit record highs in 2023. At the same time, Earth experienced its hottest year.

Analysis suggests emissions may peak in the next couple of years then start to fall. But as long as emissions remain substantial, the planet will keep warming.

Most of the world’s nations, including Australia, have signed up to the Paris climate agreement. The deal aims to keep global warming well below 2°C, and requires major emitters to reach net-zero as soon as possible. Australia, along with many other nations, is aiming to reach the goal by 2050.

Getting to net-zero essentially means nations must reduce human-caused greenhouse gas emissions as much as possible, and compensate for remaining emissions by removing greenhouse gases from the atmosphere elsewhere. Methods for doing this include planting additional vegetation to draw down and store carbon, or using technology to suck carbon out of the air.

Getting to net-zero is widely considered the point at which global warming will stop. But is that assumption correct? And does it mean warming would stop everywhere across the planet? Our research sought to find out.

Centuries of change

Computer models simulating Earth’s climate under different scenarios are an important tool for climate scientists. Our research used a model known as the Australian Community Climate and Earth System Simulator.

Such models are like lab experiments for climate scientists to test ideas. Models are fed with information about greenhouse gas emissions. They then use equations to predict how those emissions would affect the movement of air and the ocean, and the transfer of carbon and heat, across Earth over time.

We wanted to see what would happen once the world hit net-zero carbon dioxide at various points in time, and maintained it for 1,000 years.

We ran seven simulations from different start points in the 21st century, at five-year increments from 2030 to 2060. These staggered simulations allowed us to measure the effect of various delays in reaching net-zero.

We found Earth’s climate would continue to evolve under all simulations, even if net-zero emissions was maintained for 1,000 years. But importantly, the later net-zero is reached, the larger the climate changes Earth would experience.

Warming oceans and melting ice

Earth’s average temperature across land and sea is the main indicator of climate change. So we looked at that first.

We found this temperature would continue to rise slowly under net-zero emissions – albeit at a much slower rate than we see today. Most warming would take place on the ocean surface; average temperature on land would only change a little.

We also looked at temperatures below the ocean surface. There, the ocean would warm strongly even under net-zero emissions – and this continues for many centuries. This is because seawater absorbs a lot of energy before warming up, which means some ocean warming is inevitable even after emissions fall.

Over the last few decades of high greenhouse gas emissions, sea ice extent fell in the Arctic – and more recently, around Antarctica. Under net-zero emissions, we anticipate Arctic sea ice extent would stabilise but not recover.

In contrast, Antarctic sea ice extent is projected to fall under net-zero emissions for many centuries. This is associated with continued slow warming of the Southern Ocean around Antarctica.

Importantly, we found long-term impacts on the climate worsen the later we reach net-zero emissions. Even just a five-year delay would affect on the projected climate 1,000 years later.

Delaying net-zero by five years results in a higher global average surface temperature, a much warmer ocean and reduced sea ice extent for many centuries.

Australia’s evolving climate

The effect on the climate of reaching net-zero emissions differs across the world.

For example, Australia is close to the Southern Ocean, which is projected to continue warming for many centuries even under net-zero emissions. This warming to Australia’s south means even under a net-zero emissions pathway, we expect the continent to continue to warm more than almost all other land areas on Earth.

For example, the models predict Melbourne would experience 1°C of warming over centuries if net-zero was reached in 2060.

Net-zero would also lead to changes in rainfall in Australia. Winter rainfall across the continent would increase – a trend in contrast to drying currently underway in parts of Australia, particularly in the southwest and southeast.

Knowns and unknowns

There is much more to discover about how the climate might behave under net-zero.

But our analysis provides some clues about what climate changes to expect if humanity struggles to achieve large-scale “net-negative” emissions – that is, removing carbon from the atmosphere at a greater rate than it is emitted.

Experiments with more models will help improve scientists’ understanding of climate change beyond net-zero emissions. These simulations may include scenarios in which carbon removal methods are so successful, Earth actually cools and some climate changes are reversed.

Despite the unknowns, one thing is very clear: there is a pressing need to push for net-zero emissions as fast as possible.The Conversation

Extreme weather in a changing climate

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Authors:

Simon H. Lee, Lecturer in Atmospheric Science, University of St Andrews; Hayley J. Fowler, Professor of Climate Change Impacts, Newcastle University, and Paul Davies, Chief Meteorologist, Met Office and Visiting Professor, Newcastle University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Extreme weather is by definition rare on our planet. Ferocious storms, searing heatwaves and biting cold snaps illustrate what the climate is capable of at its worst. However, since Earth’s climate is rapidly warming, predominantly due to fossil fuel burning, the range of possible weather conditions, including extremes, is changing.

Scientists define “climate” as the distribution of possible weather events observed over a length of time, such as the range of temperatures, rainfall totals or hours of sunshine. From this they construct statistical measures, such as the average (or normal) temperature. Weather varies on several timescales – from seconds to decades – so the longer the period over which the climate is analysed, the more accurately these analyses capture the infinite range of possible configurations of the atmosphere.

Typically, meteorologists and climate scientists use a 30-year period to represent the climate, which is updated every ten years. The most recent climate period is 1991-2020. The difference between each successive 30-year climate period serves as a very literal record of climate change.

This way of thinking about the climate falls short when the climate itself is rapidly changing. Global average temperatures have increased at around 0.2°C per decade over the past 30 years, meaning that the global climate of 1991 was around 0.6°C cooler than that in 2020 (when accounting for other year-to-year fluctuations), and even more so than the present day.

A moving target for climate modellers

If the climate is a range of possible weather events, then this rapid change has two implications. First, it means that part of the distribution of weather events comprising a 30-year climate period occurred in a very different background global climate: for example, northerly winds in the 1990s were much colder than those in the 2020s in north-west Europe, thanks to the Arctic warming nearly four times faster than the global average. Statistics from three decades ago no longer represent what is possible in the present day.

Second, the rapidly changing climate means we have not necessarily experienced the extremes that modern-day atmospheric and oceanic warmth can produce. In a stable climate, scientists would have multiple decades for the atmosphere to get into its various configurations and drive extreme events, such as heatwaves, floods or droughts. We could then use these observations to build up an understanding of what the climate is capable of. But in our rapidly changing climate, we effectively have only a few years – not enough to experience everything the climate has to offer.

Extreme weather events require what meteorologists might call a “perfect storm”. For example, extreme heat in the UK typically requires the northward movement of an air mass from Africa combined with clear skies, dry soils and a stable atmosphere to prevent thunderstorms forming which tend to dissipate heat.

Such “perfect” conditions are intrinsically unlikely, and many years can pass without them occurring – all while the climate continues to change in the background. Based on an understanding of observations alone, this can leave us woefully underprepared for what the climate can now do, should the right weather conditions all come together at once.

Startling recent examples include the extreme heatwave in the Pacific north-west of North America in 2021, in which temperatures exceeded the previous Canadian record maximum by 4.6°C. Another is the occurrence of 40°C in the UK in summer 2022, which exceeded the previous UK record maximum set only three years earlier by 1.6°C. This is part of the reason why the true impact of a fixed amount of global warming is only evident after several decades, but of course – since the climate is changing rapidly – we cannot use this method anymore.

Playing with fire

To better understand these extremes, scientists can use ensembles: many runs of the same weather or climate model that each slightly differ to show a range of plausible outcomes. Ensembles are routinely used in weather prediction, but can also be used to assess extreme events which could happen even if they do not actually happen at the time.

When 40°C first appeared in ensemble forecasts for the UK before the July 2022 heatwave, it revealed the kind of extreme weather that is possible in the current climate. Even if it had not come to fruition, its mere appearance in the models showed that the previously unthinkable was now possible. In the event, several naturally occurring atmospheric factors combined with background climate warming to generate the record-shattering heat on July 19 that year.

The highest observed temperature each year in the UK, from 1900 to 2023

A graph showing the highest observed temperature in the UK between 1900 and 2023.
The hottest days are getting hotter in the UK. Met Office/Kendon et al. 2024

Later in summer 2022, after the first occurrence of 40°C, some ensemble weather forecasts for the UK showed a situation in which 40°C could be reached on multiple consecutive days. This would have posed an unprecedented threat to public health and infrastructure in the UK. Unlike the previous month, this event did not come to pass, and was quickly forgotten – but it shouldn’t have been.

It is not certain whether these model simulations correctly represent the processes involved in producing extreme heat. Even so, we must heed the warning signs.

Despite a record-warm planet, summer 2024 in the UK has been relatively cool so far. The past two years have seen global temperatures far above anything previously observed, and so potential extremes have probably shifted even further from what we have so far experienced.

Just as was the case in August 2022, we’ve got away with it for now – but we might not be so lucky next time.

Humidity forecasts over North America and the northeast Pacific Ocean. Kochov et al. / Nature

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Image: Humidity forecasts over North America and the northeast Pacific Ocean.
Kochov et al. / Nature Sanaa Hobeichi, UNSW Sydney




A new system for forecasting weather and predicting future climate uses artificial intelligence (AI) to achieve results comparable with the best existing models while using much less computer power, according to its creators.

In a paper published in Nature today, a team of researchers from Google, MIT, Harvard and the European Centre for Medium-Range Weather Forecasts say their model offers enormous “computational savings” and can “enhance the large-scale physical simulations that are essential for understanding and predicting the Earth system”.

The NeuralGCM model is the latest in a steady stream of research models that use advances in machine learning to make weather and climate predictions faster and cheaper.

What is NeuralGCM?

The NeuralGCM model aims to combine the best features of traditional models with a machine-learning approach.

At its core, NeuralGCM is what is called a “general circulation model”. It contains a mathematical description of the physical state of Earth’s atmosphere, and it solves complicated equations to predict what will happen in the future.

However, NeuralGCM also uses machine learning – a process of searching out patterns and regularities in vast troves of data – for some less well-understood physical processes, such as cloud formation. The hybrid approach makes sure that the output of the machine learning modules will be consistent with the laws of physics.

Google researchers explain the NeuralGCM model.

The resulting model can then be used for making forecasts of weather days and weeks in advance, as well as looking months and years ahead for climate predictions.

The researchers compared NeuralGCM against other models using a standardised set of forecasting tests called WeatherBench 2. For three- and five-day forecasts, NeuralGCM did about as well as other machine-learning weather models such as Pangu and GraphCast. For longer-range forecasts, over ten and 15 days, NeuralGCM was about as accurate as the best existing traditional models.

NeuralGCM was also quite successful in forecasting less-common weather phenomena, such as tropical cyclones and atmospheric rivers.

Why machine learning?

Machine learning models are based on algorithms that learn patterns in the data they are fed with, then use this learning to make predictions. Because climate and weather systems are highly complex, machine learning models require vast amounts of historical observations and satellite data for training.

The training process is very expensive and requires a lot of computer power. However, after a model is trained, using it to make predictions is fast and cheap. This is a large part of their appeal for weather forecasting.

The high cost of training and low cost of use is similar to other kinds of machine learning models. GPT-4, for example, reportedly took several months to train at a cost of more than US$100 million, but can respond to a query in moments.

A comparison of how NeuralGCM compares with leading models (AMIP) and real data (ERA5) at capturing climate change between 1980 and 2020. Google Research

A weakness of machine learning models is that they often struggle in unfamiliar situations – or in this case, extreme or unprecedented weather conditions. To do this, a model needs to be able to generalise, or extrapolate beyond the data it was trained on.

NeuralGCM appears to be better at this than other machine learning models, because its physics-based core provides some grounding in reality. As Earth’s climate changes, unprecedented weather conditions will become more common, and we don’t know how well machine learning models will keep up.

Nobody is actually using machine learning-based weather models for day-to-day forecasting yet. However, it is a very active area of research – and one way or another, we can be confident that the forecasts of the future will involve machine learning.The Conversation

Sanaa Hobeichi, Research Associate, Climate Change Research Centre, UNSW Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.

January 2024 ocean temperatures

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New extreme records have already been set for ocean temperatures. Not looking forward to what this means for ecosystems and extreme weather events around Aotearoa:

ocean temperatures Janaury 08

Daily global temperatures 2023 showing they exceed 1.5C almost continuously since July, and exceeded 2C 17-18 November.

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From Copernicus European Space Agency Climate Change Service; read the full report here.

Global surface air temperature highlights: 

  • 2023 is confirmed as the warmest calendar year in global temperature data records going back to 1850
  • 2023 had a global average temperature of 14.98°C, 0.17°C higher than the previous highest annual value in 2016
  • 2023 was 0.60°C warmer than the 1991-2020 average and 1.48°C warmer than the 1850-1900 pre-industrial level
  • It is likely that a 12-month period ending in January or February 2024 will exceed 1.5°C above the pre-industrial level
  • 2023 marks the first time on record that every day within a year has exceeded 1°C above the 1850-1900 pre-industrial level. Close to 50% of days were more than 1.5°C warmer then the 1850-1900 level, and two days in November were, for the first time, more than 2°C warmer.
  • Annual average air temperatures were the warmest on record, or close to the warmest, over sizeable parts of all ocean basins and all continents except Australia
  • Each month from June to December in 2023 was warmer than the corresponding month in any previous year
  • July and August 2023 were the warmest two months on record. Boreal summer (June-August) was also the warmest season on record
  • September 2023 was the month with a temperature deviation above the 1991–2020 average larger than any month in the ERA5 dataset
  • December 2023 was the warmest December on record globally, with an average temperature of 13.51°C, 0.85°C above the 1991-2020 average and 1.78°C above the 1850-1900 level for the month. You can access information specific for December 2023 in our monthly bulletin
Polar stratospheric clouds: Night Lights Films - Adrien Mauduit

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This article is republished from The Conversation under a Creative Commons license. Read the original article.

Our planet has warmed by about 1.2°C since 1850. But this warming is not uniform. Warming at the poles, especially the Arctic, has been three to four times faster than the rest of the globe. It’s a phenomenon known as “polar amplification”.

Climate models simulate this effect, but when tested against the past 40 years of warming, these models fall short. The situation is even worse when it comes to modelling past climates with very high levels of greenhouse gases.

This is a problem because these are the same models used to project into the future and forecast how the climate will change. They are likely to underestimate what will happen later this century, including risks such as ice sheet melting or permafrost thawing.

In our new research published today in Nature Geoscience we used a high-resolution model of the atmosphere that includes the stratosphere. We found a special type of cloud appears over polar regions when greenhouse gas concentrations are very high. The role of this type of cloud has been overlooked so far. This is one of the reasons why our models are too cold at the poles.

Polar Stratospheric Clouds over Norway (Night Lights Films – Adrien Mauduit)


Back to the future

Looking into past climates can give us glimpses of possible futures for a range of extreme conditions. For us, this means we can use Earth’s history to find out how well our climate models perform. We can test our models by simulating episodes in the past when Earth was much warmer. The advantage of this is that we have temperature reconstructions for these episodes to evaluate the models, as opposed to the future, for which measurements are not available.

If we go back 50 million years or so, our planet was very hot. Carbon dioxide (CO₂) concentrations ranged between 900 and 1,900 parts per million (ppm), compared with 415 ppm today. Methane (CH₄) concentrations were likely also much higher.

Canada’s arctic archipelago was covered in lush rainforests inhabited by alligators, turtles, lizards and mammals.

For these plants and animals to survive, conditions must have been warm and ice-free year-round. Indeed, surface ocean temperatures exceeded 20°C near the north pole (at about 87°N) and 25°C in the Southern Ocean (at about 67°S).

This period called the early Eocene is a perfect test bed for our models, because it was globally very warm, and the poles were even warmer, meaning it was a climate with extreme polar amplification. In addition, the Eocene is recent enough for temperature reconstructions to be available.

But as it turns out, the models fail again. They are much too cold at high latitudes. What are our models missing?

Polar stratospheric clouds

In 1992 American paleoclimatologist Lisa Sloan suggested polar stratospheric clouds might have caused extreme warming at high latitudes in the past.

These clouds are a rare and beautiful sight today. They are also called nacreous or mother-of-pearl clouds for their vivid and sometimes luminous colours.

They form at very high altitudes (in the stratosphere) and at very low temperatures (over the poles). In the present day climate, they appear mainly over Antarctica, but have also been observed during winter months over Scotland, Scandinavia and Alaska, at times when the stratosphere was particularly cold.

Just like greenhouse gases, they absorb infrared radiation emitted by the Earth’s surface and re-emit a portion of this energy back to the surface. This suggests polar stratospheric clouds could be one of the missing puzzle pieces.

They warm the surface. And their effect could be significant, especially in winter, when the sun does not rise. But they are difficult to simulate in a climate model, so most models ignore them. This omission could explain why climate models miss some of the polar warming, because they miss a process that warms the poles.

Three decades after Sloan’s paper, a few atmosphere models are finally complex enough to allow us to test her hypothesis. In our research we use one of them and find that under certain conditions, the additional warming due to these polar stratospheric clouds exceeds 7°C during the winter months. This significantly reduces the gap between climate models and temperature evidence from the early Eocene. Sloan was right.

Implications for future projections

Our research explains why climate models don’t work so well for past climates when greenhouse gas levels were much higher than they are today. But what about the future? Should we be concerned?

There is some good news. While polar stratospheric clouds do warm the poles, they won’t be as common in the future as they were in the distant past, even if both CO₂ and CH₄ reach very high levels.

This is due to another difference between the Eocene and today: the position of continents and mountains, which were different back then and which also influence the formation of polar stratospheric clouds. So even if we hit early Eocene levels of CH₄ and CO₂ in the future, we would expect less polar stratospheric cloud to be formed. This suggests the standard climate models are better at predicting the future than the past.

It’s therefore unlikely the Arctic and Antarctica will be covered by these beautiful clouds anytime soon. But our research shows evidence from past climates can reveal processes that only become important when greenhouse gas concentrations are high. Some of these processes are not included in our models because models are tested against present day observations and other processes simply seemed more important to include. Looking into the past is a way of broadening our horizon and learning for the future.

global temperatures exceed 1°5C

Animated image: NASA

Exceeding 1.5°C temporarily, and just fust for one month, does not mean the Paris Agreement goal of staying under 1.5°C is lost, as September is generally the hottest month on Earth. However, a temporary rise above 1.5° is likely to trigger some irreversible tipping points.

Current policies and actions make it very likely, at least temporarily, to overshoot the Paris climate targets of 1.5–<2.0 °C above pre-industrial levels. If this global warming range is exceeded, potential tipping elements such as the Greenland Ice Sheet and Amazon rainforest may be at increasing risk of crossing critical thresholds. This raises the question of how much this risk is amplified by increasing overshoot magnitude and duration. …Our model analysis reveals that temporary overshoots can increase tipping risks by up to 72% compared with non-overshoot scenarios, even when the long-term equilibrium temperature stabilizes within the Paris range. 

– Wunderling et el, Global warming overshoots increase risks of climate tipping cascades in a network model, Nature Climate Change 13,  22 December 2022.

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