"Often it's accepted that hydropower is a climate friendly technology but in fact probably all reservoirs around the world emit greenhouse gases and some of them, especially some of the ones in the tropics, emit very high quantities of greenhouse gases even comparable to, in some cases even much worse than, fossil fuels like coal and gas," said International Rivers Network executive director Patrick McCully.
He said when water flow was stopped, vegetation and soil in the flooded area and from upstream was left to rot, as well as fish and other animals, which died in the dam.
These decaying animal and vegetable matter then released carbon dioxide, methane and nitrous oxide into the air.
"Basically they're factories for converting carbon into methane and methane is a very powerful greenhouse gas - it's less known than carbon dioxide but it's actually about 25 times stronger than carbon dioxide in terms of trapping heat in the atmosphere," said McCully.
He said studies had also shown that dams were responsible for a third of all methane emissions worldwide.
But it was an area that was under-researched so a clearer picture of how dams were contributing to global warming was not known, he added.
He said scientists should now make efforts to incorporate technology that could produce energy from methane from dams.
The findings were presented at the 10th annual Riversymposium in Brisbane this week, reports the Sydney Morning Herald.