If you have noticed beaches looking thinner and rockier of late, you’re observing a phenomenon that’s being felt across the Pacific Rim.

Sydney scientists have shown that beaches along Australia’s south-east coast slim down by up to 20 per cent during cooler La Nina climate cycles and beef up again when the warmer El Nino cycles begin.

On the other side of the Pacific, beaches in the Americas see the opposite effect, expanding during La Nina and shrinking in El Nino times.

“For the first time, we’ve been able to measure how things are changing on a massive scale,” said Dr Mitchell Harley, a senior lecturer from the Water Research Laboratory at the University of NSW.

“Previously we’ve only been able to get precise measurements from a handful of beaches on any timescale to look at the patterns of change brought about by El Nino and La Nina.”

El Nino and La Nina are climate cycles driven by water temperature in the Pacific. Australia has just emerged from an unusual triple-dip La Nina, which fuelled storms and extensive flooding in eastern states last year.

Using a treasury of images sourced from NASA’s Landsat and the European Space Agency’s Sentinel-2 satellites, Harley and colleague Dr Kilian Vos built a software tool that could scan decades of change over 8300 kilometres of coastline around the Pacific Ocean.

“From that incredible imagery, we’ve developed new techniques to detect the shoreline changes using algorithms that are powered by machine learning,” Harley said.

The team found a strong correlation between La Nina cycles and increased coastal erosion at many NSW beaches as storms and strong waves gnawed away at the coast. El Nino cycles tend to be warmer and calmer, allowing beaches to gradually replenish lost sand.