During the most recent ice age, much of North America disappeared beneath ice sheets several kilometres thick. These frozen giants covered most of present-day Canada and parts of the northern United States, while smaller ones spread across northern Europe.

They dominated the landscape for thousands of years, storing so much water that global sea levels fell dramatically.

Scientists have long known these enormous ice sheets transformed the climate of the northern hemisphere. They shifted North Atlantic storms southward and influenced rainfall across Europe.

But what if these mountains of ice influenced climate much farther away than anyone realised? Could they have altered climate on the other side of the world?

In our new study, published in Climate of the Past, we show they could, despite the ice never reaching Australia. The discovery reveals just how interconnected Earth’s climate really is. It also reminds us that understanding Australia’s past and future climate requires looking far beyond our shores.

A world that looked different

Our study focuses on a climate about 49,000 years ago during the last ice age when the world looked very different from today.

The giant ice sheets in North America were not yet as large as when they reached their peak around 21,000 years ago, but they were already large enough to influence climate far beyond their edges.

At the same time, atmospheric carbon dioxide levels were very low, and changes in Earth’s orbit altered how sunlight was distributed across the planet compared with today.

Building ice sheets of this size required enormous amounts of water. As more water from the oceans became locked away as ice on land, sea levels fell by around 60 metres, exposing underwater landscapes.

Coastlines stretched much farther out than they do today. Australia and New Guinea were joined into a single landmass called Sahul, while many islands across Southeast Asia merged into another large landmass called Sunda.

But the ice sheets did much more than reshape coastlines.

Their bright surfaces reflected more sunlight back into space, cooling the planet. They also stood several kilometres high, forming gigantic walls of ice that disrupted the normal paths of winds moving around the globe.

Imagine placing a mountain range thousands of kilometres wide in the path of the atmosphere. Air cannot simply flow straight through it. Instead, it is forced around the obstacle, creating ripple effects that spread far beyond it. The North American ice sheets behaved much like a continent-sized mountain range.

Looking beyond the North Atlantic

While the regional effects of the northern hemisphere ice sheets across Europe and the North Atlantic are now well established, much less was known about whether the influence of the ice sheets extended far beyond the North Atlantic.

Could ice sheets in North America also influence rainfall in Australia?

To answer this question, we used a series of climate model experiments. Instead of changing everything at once, we sequentially altered atmospheric greenhouse gas concentrations, Earth’s orbital configuration (the position and orientation of Earth relative to the Sun), ice-sheet albedo (how much sunlight they reflected), and ice-sheet elevation.

This approach allowed us to identify which factor was responsible for different climate changes.

The height of the ice sheets reshaped global winds

While orbital conditions helped create a wetter Australian climate, one result stood out. The bright white ice helped cool the climate by reflecting sunlight back into space.

But it was the height of the North American ice sheets that produced the biggest changes in atmospheric circulation.

Their towering walls of ice redirected winds across the globe. Those changes rippled through the atmosphere into the tropics, shifting the tropical rainfall belt southward and strengthening the circulation that drives Australia’s summer monsoon.

This southward shift also reduced rainfall in parts of Indonesia and Papua New Guinea, while increasing summer rainfall across much of Australia.

A connected system

The past provides one of the best natural experiments available for understanding how Earth’s climate system works. These results show that changes in one part of the world can trigger responses thousands of kilometres away through the atmosphere.

As atmospheric greenhouse gases continue to rise and major changes are projected to occur at high northern latitudes, understanding how these changes influence the southern hemisphere will become increasingly important.

The giant ice sheets that once covered North America never reached Australia. Yet their influence did. They altered Australia’s rainfall, helped shape the landscapes inhabited by the continent’s earliest people some 60,000 years ago, and reminded us that Earth’s climate has always worked as one connected system. What happens in one part of the world can echo far beyond its borders.