That was bad news for some marine creatures, said Dr Yan Hong, associate researcher with the Chinese Academy of Sciences' Institute of Earth Environment and lead scientist of the study, published in . Some fish species need a significant temperature drop to lay their eggs, as cooling waters helped nutrients to rise.

"Exceptionally warm winters might disrupt this … and upset the delicate balance of marine ecology. Many species, ranging from big fish to microorganisms, could suffer," Yan said.

Warmer winters could also threaten humans by depleting fisheries and worsening air pollution. "The impact of climate warming could be stronger and more profound than previously thought," he said.

Previous studies on ancient climate change used samples of tree rings, sediment or historical records, but these were less sensitive to subtle fluctuations than the giant clams, resulting in rougher figures that often produced a "smoothened" picture of climate change.

By contrast, Yan said, the giant clam was a diligent "bookkeeper" that gave a "high-resolution image" of temperature changes. Living in shallow water, it recorded even tiny changes of sea surface temperature with the growth of a new layer on its shell every day.

The scientists studied the change of chemical compositions in the shell to reconstruct the climate record in monthly periods going back 2,500 years.

"The monthly record is only the first step. We are improving the analytical tools … so we may produce a daily record of temperature change over the last few millennia," Yan said.

The scientists' sample base has grown quickly from a dozen to nearly a 1,000 molluscs, most of which were collected by Chinese fishermen. They are also studying giant clam samples from other parts of the world.