Scientists discover the Southern Ocean is “sweating” more as climate change intensifies

But scientists say the island is changing.

Large areas are becoming wetter and boggier, while native megaherbs such as Pleurophyllum and Stilbocarpa are shrinking back. Researchers have long suspected rising rainfall was behind these changes. Now, a new study published in Weather and Climate Dynamics confirms that rainfall is increasing sharply, and the findings could have implications far beyond this isolated UNESCO World Heritage site.

Why the Southern Ocean Matters

The Southern Ocean is one of the planet's most important climate regulators.

It absorbs a huge amount of the heat trapped by greenhouse gases, along with a significant share of the carbon dioxide produced by human activity. Storm systems moving through the region also shape weather patterns across Australia, New Zealand, and other parts of the world.

Despite its importance, the Southern Ocean remains one of the least monitored places on Earth.

There are very few land areas, only a small number of weather stations, and nearly constant cloud cover. Because of this, satellites and climate models often struggle to accurately capture what is happening there.

That is why the weather observations collected on Macquarie Island by the Bureau of Meteorology and the Australian Antarctic Division are so valuable. The island provides one of the only long-term "ground truth" climate records in the Southern Ocean.

These detailed records include more than 75 years of daily rainfall and meteorological observations. Scientists regularly use them to check the accuracy of satellite data and computer simulations.

Scientists Investigate Rising Rainfall

Earlier studies showed rainfall on Macquarie Island had increased significantly over recent decades. Ecologists also observed widespread waterlogging that was damaging native vegetation.

However, researchers still did not fully understand why rainfall was increasing or how the island's weather systems were changing over time.

To investigate, scientists analyzed 45 years (1979-2023) of daily rainfall observations and compared them with a widely used climate reconstruction called the ERA5 reanalysis.

The goal was to determine whether the increase came from more storms overall or from storms producing heavier rainfall.

Researchers sorted each day in the dataset into one of five weather patterns based on atmospheric pressure, humidity, wind, and temperature. These patterns included low pressure systems, cold-air outbreaks, and warm-air advection (the warm air that moves poleward ahead of a cold front).

Storms Are Becoming Wetter

The study found that annual rainfall on Macquarie Island has increased by 28% since 1979, equal to roughly 260 millimeters of additional rain per year.

By comparison, the ERA5 reanalysis detected only an 8% increase, missing most of the observed change.

Scientists say the Southern Ocean storm track has gradually shifted closer to Antarctica over time, and this larger shift is now influencing the island's weather.

But researchers discovered something important. The increase in rainfall is not mainly because the island is experiencing more storms. Instead, storms are delivering more rain when they happen.

One type of wet weather pattern was largely replacing another, meaning the total number of wet systems was not the key factor. The storms themselves are simply becoming more intense in terms of rainfall.

Global Consequences Beyond Macquarie Island

Researchers say the findings could have major implications if the same rainfall intensification is occurring across the broader Southern Ocean storm belt, which multiple lines of evidence suggest may be the case.

More rainfall means more freshwater entering the upper layers of the ocean. This can strengthen the separation between ocean layers and reduce mixing within the water. Changes like these can affect the strength and movement of ocean currents.

The researchers estimate that, by 2023, the extra rainfall added roughly 2,300 gigatonnes of freshwater each year across the high-latitude Southern Ocean. That is far greater than recent freshwater contributions from Antarctic meltwater, and the gap continues to grow.

Additional rainfall also changes the salinity of surface waters. Salinity plays a major role in how nutrients and carbon move through the ocean, meaning these changes could affect the productivity and chemistry of the Southern Ocean, one of Earth's most important carbon sinks.

The Southern Ocean May Be "Sweating" More

More rainfall also requires more evaporation.

Evaporation removes heat from the ocean in much the same way sweat cools the human body. In the cloudy Southern Ocean, evaporation is one of the main ways the ocean loses heat.

According to the study, the Southern Ocean may now be cooling itself by 10-15% more than it did in 1979 because of the added evaporation needed to fuel the increase in rainfall.

In effect, researchers say the Southern Ocean may be "sweating" more as the climate warms.

A Climate Signal Scientists Cannot Ignore

Macquarie Island is only a tiny piece of land surrounded by the world's stormiest ocean.

Yet its long-term rainfall data suggests the Southern Ocean, a critical driver of global heat and carbon absorption, may be changing faster and more dramatically than scientists previously realized.

Researchers say the next step is determining how widespread these changes are across the Southern Ocean storm belt and understanding what they could mean for the global climate system in the future.