It’s official: Australia’s ocean surface was the hottest on record in 2024

Moninya Roughan, UNSW Sydney

Australia’s sea surface temperatures were the warmest on record last year, according to a snapshot of the nation’s climate which underscores the perilous state of the world’s oceans.

The Bureau of Meteorology on Thursday released its annual climate statement for 2024 – the official record of temperature, rainfall, water resources, oceans, atmosphere and notable weather.

Among its many alarming findings were that sea surface temperatures were hotter than ever around the continent last year: a whopping 0.89°C above average.

Oceans cover more than 70% of Earth’s surface, and their warming is gravely concerning. It causes sea levels to rise, coral to bleach and Earth’s ice sheets to melt faster. Hotter oceans also makes weather on land more extreme and damages the marine life which underpins vital ocean ecosystems.

What the snapshot showed

Australia’s climate varies from year to year. That’s due to natural phenomena such as the El Niño and La Niña climate drivers, as well as human-induced climate change.

The bureau confirmed 2024 was Australia’s second-warmest year since national records began in 1910. The national annual average temperature was 1.46°C warmer than the long-term average (1961–90). Heatwaves struck large parts of Australia early in the year, and from September to December.

Average rainfall in Australia was 596 millimetres, 28% above the 30-year average, making last year the eighth-wettest since records began.

And annual sea surface temperatures for the Australian region were the warmest on record. Global sea surface temperatures in 2024 were also the warmest on record.

According to the bureau, Antarctic sea-ice extent was far below average, or close to record-lows, for much of the year but returned to average in December.

What caused the hot oceans?

It’s too early to officially attribute the ocean warming to climate change. But we do know greenhouse gas emissions are heating the Earth’s atmosphere, and oceans absorb 90% of this heat.

So we can expect human-induced climate change played a big role in warming the oceans last year. But shorter-term forces are at play, too.

The rare triple-dip La Niña Australia experienced from 2020 to 2023 brought cooler water from deep in the ocean up to the surface. It was like turning on the ocean’s air-conditioner.

But that pattern ended and Australia entered an El Niño in September 2023. It lasted about seven months, when the oscillation between El Niño and La Niña entered a neutral phase.

The absence of a La Niña meant cool water was no longer being churned up from the deep. Once that masking effect disappeared, the long-term warming trend of the oceans became apparent once more.

Water can store a lot more heat than air. In fact, just the top few metres of the ocean store as much heat as Earth’s entire atmosphere. Oceans take a long time to heat up and a long time to cool.

Heat at the ocean’s surface eventually gets pushed deeper into the water column and spreads across Earth’s surface in currents. The below chart shows how the world’s oceans have heated over the past 70 years. Changes in the world’s ocean heat content since 1955. NOAA/NCEI World Ocean Database

Why should we care about ocean warming?

Rapid warming of Earth’s oceans is setting off a raft of worrying changes.

It can lead to less nutrients in surface waters, which in turn leads to fewer fish. Warmer water can also cause species to move elsewhere. This threatens the food security and livelihoods of millions of people around the world.

Just last week, it was reported that tens of thousands of fish died off northwestern Australia due to a large and prolonged marine heatwave.

Warm water causes coral bleaching, as experienced on the Great Barrier Reef in recent decades. It also makes oceans more acidic, reducing the amount of calcium carbonate available for organisms to build shells and skeletons.

Warming oceans trigger sea level rise – both due to melt water from glaciers and ice sheets, and the fact seawater expands as it warms.

Hotter oceans are also linked to weather extremes, such as more intense cyclones and heavier rainfall. It’s likely the high annual rainfall Australia experienced in 2024 was in part due to warmer ocean temperatures.

What now?

As long as humans keep burning fossil fuels and pumping greenhouse gases into the atmosphere, the oceans will keep warming.

Unfortunately, the world is not doing a good job of shifting its emissions trajectory. As the bureau pointed out in its statement, concentrations of all major long-lived greenhouse gases in the atmosphere increased last year, including carbon dioxide and methane.

Prolonged ocean warming is driving changes in weather patterns and more frequent and intense marine heatwaves. This threatens ecosystems and human livelihoods. To protect our oceans and our way of life, we must transition to clean energy sources and cut carbon emissions.

At the same time, we must urgently expand ocean observing below the ocean’s surface, especially in under-studied regions, to establish crucial baseline data for measuring climate change impacts.

The time to act is now: to reduce emissions, support ocean research and help safeguard the future of our blue planet.

Moninya Roughan, Professor in Oceanography, UNSW Sydney

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

Read More........→February 13, 2025

What’s the difference between climate and weather models? It all comes down to chaos

Nadia Piet/AIxDESIGN & Archival Images of AI / Better Images of AI , CC BY-SA Andy Hogg, Australian National University; Aidan Heerdegen, Australian National University, and Kelsey Druken, Australian National University

Weather forecasts help you decide whether to go for a picnic, hang out your washing or ride your bike to work. They also provide warnings for extreme events, and predictions to optimise our power grid.

To achieve this, services such as the Australian Bureau of Meteorology use complex mathematical representations of Earth and its atmosphere – weather and climate models.

The same software is also used by scientists to predict our future climate in the coming decades or even centuries. These predictions allow us to plan for, or avoid, the impacts of future climate change.

Weather and climate models are highly complex. The Australian Community Climate and Earth System Simulator, for example, is comprised of millions of lines of computer code.

Without climate and weather models we would be flying blind, both for short-term weather events and for our long-term future. But how do they work – and how are they different?

The same physical principles

Weather is the short-term behaviour of the atmosphere – the temperature on a given day, the wind, whether it’s raining and how much. Climate is about long-term statistics of weather events – the typical temperature in summer, or how often thunderstorms or floods happen each decade.

The reason we can use the same modelling tools for both weather and climate is because they are both based on the same physical principles.

These models compile a range of factors – the Sun’s radiation, air and water flow, land surface, clouds – into mathematical equations. These equations are solved on a bunch of tiny three-dimensional grid boxes and pieced together to predict the future state.

These boxes are sort of like pixels that come together to make the big picture.

These solutions are calculated on a computer – where using more grid boxes (finer resolution) gives better answers, but takes more computing resources. This is why the best predictions need a supercomputer, such as the National Computational Infrastructure’s Gadi, located in Canberra.

Because weather and climate are governed by the same physical processes, we can use the same software to predict the behaviour of both.

But there most of the similarities end.

The starting point

The main differences between weather and climate come down to a single concept: “initialisation”, or the starting point of a model.

In many cases, the simplest prediction for tomorrow’s weather is the “persistence” forecast: tomorrow’s weather will be similar to today. It means that, irrespective of how good your model is, if you start from the wrong conditions for today, you have no hope of predicting tomorrow.

Persistence forecasts are often quite good for temperature, but they’re less effective for other aspects of weather such as rainfall or wind. Since these are often the most important aspects of weather to predict, meteorologists need more sophisticated methods.

So, weather models use complex mathematics to create models that include weather information (from yesterday and today) and then make a good prediction of tomorrow. These predictions are a big improvement on persistence forecasts, but they won’t be perfect.

In addition, the further ahead you try to predict, the more information you forget about the initial state and the worse your forecast performs. So you need to regularly update and rerun (or, to use modelling parlance, “initialise”) the model to get the best prediction.

Weather services today can reliably predict three to seven days ahead, depending on the region, the season and the type of weather systems involved.

Chaos reigns

If we can only accurately predict weather systems about a week ahead before chaos takes over, climate models have no hope of predicting a specific storm next century.

Instead, climate models use a completely different philosophy. They aim to produce the right type and frequency of weather events, but not a specific forecast of the actual weather.

The cumulative effect of these weather events produces the climate state. This includes factors such as the average temperature and the likelihood of extreme weather events.

So, a climate model doesn’t give us an answer based on weather information from yesterday or today – it is run for centuries to produce its own equilibrium for a simulated Earth.

Because it is run for so long, a climate (also known as Earth system) model will need to account for additional, longer-term processes not factored into weather models, such as ocean circulation, the cryosphere (the frozen portions of the planet), the natural carbon cycle and carbon emissions from human activities.

The additional complexity of these extra processes, combined with the need for century-long simulations, means these models use a lot of computing power. Constraints on computing means that we often include fewer grid boxes (that is, lower resolution) in climate models than weather models.

A machine learning revolution?

Is there a faster way?

Enormous strides have been made in the past couple of years to predict the weather with machine learning. In fact, machine learning-based models can now outperform physics-based models.

But these models need to be trained. And right now, we have insufficient weather observations to train them. This means their training still needs to be supplemented by the output of traditional models.

And despite some encouraging recent attempts, it’s not clear that machine learning models will be able to simulate future climate change. The reason again comes down to training – in particular, global warming will shift the climate system to a different state for which we have no observational data whatsoever to train or verify a predictive machine learning model.

Now more than ever, climate and weather models are crucial digital infrastructure. They are powerful tools for decision makers, as well as research scientists. They provide essential support for agriculture, resource management and disaster response, so understanding how they work is vital.

Andy Hogg, Professor and Director of ACCESS-NRI, Australian National University; Aidan Heerdegen, Leader, ACCESS-NRI Model Release Team, Australian National University, and Kelsey Druken, Associate Director (Release Management), ACCESS-NRI, Australian National University

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

Read More........→February 11, 2025