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This year, for many Australians, it feels like summer never left. The sunny days and warm nights have continued well into autumn. Even now, in May, it’s still unusually warm.

Much of the southern half of the continent is experiencing both unseasonable warmth and dry conditions. This is linked to persistent high atmospheric pressure (called “blocking”) to the south and southeast of Australia.

While temperatures will fall across southern Australia as we approach the winter solstice, early indications are that this winter will be a warm one. Rainfall predictions are less certain.

The extra warmth we’ve experienced raises obvious questions about the influence of human-caused climate change. The warming signal is clear and it’s a sign of things to come.

A warm and dry autumn for many

March and April brought unseasonal heat to much of Australia.

March was widely hot, with temperatures several degrees above normal across much of the country. But April’s heat was largely restricted to the southeast.

Victoria had its warmest April on record, and parts of the state experienced temperatures more than 3°C above normal across both March and April.

Temperatures normally fall quite quickly over the southeast of Australia during April and May as the days shorten and the continent’s interior cools. But this year, southern Australia was unusually warm at the start of May. Some locations experienced days with maximum temperatures more than 10°C above normal for the time of year.

Records were broken in Hobart and parts of Melbourne, which had their warmest May nights since observations began.

While Queensland and the New South Wales coast have had very wet spells, including downpours from Tropical Cyclone Alfred at the start of March, other parts of Australia have been quite dry.

The area between Adelaide and Melbourne has been exceptionally dry. A drought is unfolding in the region after a severe lack of rainfall, with deficits stretching back over the past year or so. Western Tasmania is also suffering from a severe lack of rainfall since the start of autumn, although welcome rain fell in the past week.

And it’s not just on land that unusual heat has been observed. The seas around Australia have been warmer than normal, causing severe coral bleaching to the west and east of the continent, harmful algal blooms and other ecosystem disruptions.

Blocking highs largely to blame

A high pressure system has dominated over the south and southeast of Australia over the past few months.

High pressure in the Tasman Sea can sometimes get stuck there for a few days. This leads to what’s known as “blocking”, when the usual passage of weather systems moving from west to east is obstructed. This can lock in weather patterns for several days or even a week.

Repeated blocking occurred this autumn. As winds move anticlockwise around high pressure systems in the Southern Hemisphere, blocking highs in the Tasman Sea can bring moist, onshore winds to the New South Wales and Queensland coasts, increasing rainfall. But such high pressure systems also bring drier conditions for the interior of the southeast and much of Victoria and South Australia.

Often, these high pressure systems also bring northerly winds to Victoria, and this can cause warmer conditions across much of the state.

High pressure systems also tend to bring more clear and sunny conditions, which increases daytime temperatures in particular. Air in high pressure systems moves down towards the surface and this process causes warming, too.

Australia sits between the Pacific and Indian Oceans and is subject to their variability, so we often look there to help explain what’s happening with Australia’s climate. In autumn though, our climate influences, such as the El Niño-Southern Oscillation and the Indian Ocean Dipole, are less active and have weaker relationships with Australian climate than at other times of year. Neither of these climate influences is in a strong phase at the moment.

A warm winter on the cards

One big question is how long the heat will last. In parts of southeast Australia, including Melbourne, average temperatures drop quickly at this time of year as we approach the winter solstice.

However, the seasonal outlook from the Bureau of Meteorology points to a high likelihood of a relatively warm winter.

Australians rarely escape having a winter without any significant cold spells, but the long-range forecast suggests we should anticipate above-normal temperatures on average. Both daytime maximum temperatures and nighttime minimum temperatures are expected to be above average generally this winter.

Global warming is here

The elephant in the room is climate change. Human-caused climate change is increasing autumn temperatures and the frequency of late season heat events. As greenhouse gas emissions continue at a record pace, expect continued warming and a greater chance of autumn heatwaves in future.

The effect of climate change on rainfall is less clear though. For the vast majority of Australia, there is high uncertainty as to whether autumn will become wetter or drier as the world warms.

Andrew King, Associate Professor in Climate Science, ARC Centre of Excellence for 21st Century Weather, The University of Melbourne

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

Over the last decade, humanity’s emissions of carbon dioxide (CO₂) have stabilised after a period of huge growth. Average growth is now down to just 0.6% per year, compared to 2% per year in the previous decade. But levelling off isn’t the same as declining – and we’ve levelled off at a very high rate of emissions. The Global Carbon Project estimates human activities released a record high of 10.2 gigatonnes of carbon (GtC) in 2024.

Last year, the atmosphere’s concentration of CO₂ rose at the fastest rate on record. Over the last decade, atmospheric CO₂ increased an average of 2.4 parts per million (ppm) a year. But last year, concentrations jumped by 3.5 ppm, reaching 424 ppm in the atmosphere. These concentrations are more than 50% higher than the pre-industrial period.

While we’re burning more fossil fuels than ever, recent emissions growth has been offset by falling rates of deforestation and other land use emissions.

Why are CO₂ concentrations still rapidly increasing? We’re still pumping massive amounts of long-buried CO₂ into our atmosphere. The only way for this carbon to leave the atmosphere is through natural carbon sinks – and they’re struggling to keep up.

How do we know the amount of CO₂ in the atmosphere?

Perched on a remote and windy clifftop on Tasmania’s northwest tip lies the Kennaook/Cape Grim Baseline Air Pollution Station. This station has an important job: monitoring baseline changes in atmospheric gases. The location was chosen because air here has travelled hundreds of kilometres over the ocean in an area unaffected by local pollution.

For decades, Australian scientists have directly measured the changes to the atmosphere here. Alongside other monitoring stations worldwide, this gives us an accurate and precise record of changes in greenhouse gases and ozone depleting chemicals in the atmosphere.

Filling the bathtub

Carbon dioxide is very good at trapping heat. Over the Earth’s 4.5 billion years, pulses of CO₂ have created hothouse worlds, very different to the pleasant climate humans have enjoyed since the last ice age, about 11,000 years ago. The last time CO₂ went past 400 ppm was likely more than two million years ago.

It’s easy to confuse CO₂ emissions and concentrations of CO₂ in the atmosphere. Emissions influence atmospheric concentrations, but they are not the same.

Releasing long-buried carbon back into the atmosphere by burning fossil fuels and producing CO₂ emissions is like turning on the tap in a bathtub and the amount of water in the tub is the atmospheric concentration.

The Earth has natural ways of dealing with carbon dioxide. Plants, soils and oceans are carbon “sinks” – they all draw down carbon from the atmosphere and store it. Think of them as the bath’s plughole.

The problem is, we’re filling up the tub with CO₂ much faster than the Earth’s carbon sinks can pull them out. As a result, CO₂ concentration in the atmosphere rises. Atmospheric CO₂ matters because it is what actually influences climate.

If we apply current global emissions and scenarios where emissions decrease either steadily or rapidly to the CSIRO Simple Carbon-Climate Model, we can estimate how much our bathtub is likely to fill. These graphs show emissions must be significantly cut before we can start to see a fall in atmospheric concentration.

Why did CO₂ concentration jump last year?

The single largest influence in last year’s spike in CO₂ concentration is likely to be changes to carbon sinks.

Every year, oceans, forests and soils absorb about half the emissions humans produce. But this figure isn’t set – it changes as the Earth’s systems change.

For instance, plants grow more in wetter years and store more carbon in their structures through photosynthesis and growth.

But climate change is making fires more intense and more frequent. As trees burn, they release stored carbon back to the atmosphere. Emissions from enormous wildfires in Canada in 2023 and South America in 2024 likely contributed to the atmospheric CO₂ jump.

Recent research suggests a weakened biosphere has strongly contributed. Severe droughts across the northern hemisphere in 2024 cut the ability of the planet’s soils and plant life to soak up and store CO₂.

The speed at which carbon sinks soak up CO₂ depends on environmental conditions, which are largely out of our control. As climate change worsens, the capacity of natural carbon sinks to draw down our emissions will likely reduce.

In the bathtub analogy, water leaves the tub through the plughole. If the plughole narrows, less water can escape and our tub will fill up even faster.

The main lever we can control is the tap on the bathtub – the emissions we produce. Many nations are now cutting their emissions, but not enough to begin the sharp decline in concentration we need.

In the 1980s, the Earth’s thin, protective layer of ozone – just 10 parts per million – was being eaten away by chlorofluorocarbons (CFCs) and other chemicals in fridges, air conditioners and aerosol cans. Nations replaced these chemicals and the ozone hole began to close. Fossil fuels are far more important to our current way of life than CFCs were. But we now have good options to replace them across many industries.

This is a crucial moment. Our current rate of emissions will only cause CO₂ concentrations and global temperatures to rise. Natural carbon sinks will not pull out enough carbon to stabilise our climate on a time frame meaningful to humans. The earlier the action and decrease in emissions, the better our future.

Issy Borley, Research Technician, CSIRO; Cathy Trudinger, Senior Research Scientist, CSIRO, and Ray Langenfelds, Scientist, Atmospheric Research, CSIRO

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