WASHINGTON - Big volcanic eruptions cooled the tropics over the past 450 years, an effect masked by rising global temperatures, says a new study.

Scientists already agree that large eruptions have lowered temperatures at higher latitudes in recent centuries, because volcanic particles reflect sunlight back into space.

For instance, 1816, the year following the massive Tambora eruption in Indonesia, became known as ‘The Year Without a Summer’, after low temperatures caused crop failures in northern Europe and eastern North America.

More extensive evidence comes in part from tree rings, which tend to grow thinner in years when temperatures go down. This is one of the first such studies to show how the tropics have responded, said co-author Rosanne D’Arrigo, scientist at the Tree Ring Lab at Columbia University.

‘This is significant because it gives us more information about how tropical climate responds to forces that alter the effects solar radiation,’ said D’Arrigo. The other co-authors were Rob Wilson of Lamont and the University of St. Andrews, Scotland and Alexander Tudhope of University of Edinburgh.

Along with tree rings, the researchers analysed ice cores from alpine glaciers, and corals, taken from a wide area of the tropics.

When the earth cools, not only do trees tend to grow less, but isotopes of oxygen in corals and glacial ice may shift. All showed that low-latitude temperatures declined for several years after major tropical eruptions.

The samples, spanning 1546 to 1998, were taken from Nepal down through Indonesia and across the Indian and Pacific oceans; the ice cores came from the Peruvian Andes. The researchers used materials they collected themselves, as well as samples from the archives of other scientists.

The data show that the most sustained cooling followed two events: an 1809 eruption that probably took place in the tropics, but whose exact location remains unknown; and the 1815 Tambora eruption, one of the most powerful recorded in human history, according to a Columbia university release.

‘Particularly warm decades may have partially overridden the cooling effect of some volcanic events,’ said D’Arrigo. ‘This study provides some of the first comprehensive information about how the tropical climate system responded to volcanism prior to the instrumental period,’ she concluded.

The findings were published in the current issue of Nature Geoscience.