This is the VOA Special English ENVIRONMENT REPORT.
Scientists have tried for years to understand what is causing frogs to disappear around the world. Since Nineteen-Eighty, at least twenty kinds of frogs have disappeared. Many explanations have been proposed. They include pollution, infections and unidentified environmental threats.
Scientists have warned about the possible harm to the environment caused by warming of the Earth. Now researchers say they have evidence that climate change could be a cause of the frog deaths. The findings are the first to link climate change with the decreasing frog population in North America.
A team of researchers studied frog eggs in small bodies of water in the northwestern American state of Oregon. Joseph Kiesecker led the study in Oregon's northern Cascade Mountains.
Scientists have known for years that frog eggs in the western states are dying from a fungal infection in water. The latest research suggests that the infection is the result of a complex series of events caused by warming temperatures.
The scientists believe the frogs' eggs become infected because water levels where the eggs are laid are too low. They say climate change has reduced the amount of rainfall in areas where the frogs lay their eggs. Lower water levels permit more dangerous ultraviolet light from the sun to reach the eggs. Ultraviolet radiation can cause the eggs to lose their ability to resist deadly fungal infections.
The researchers had done other experiments in the mountains. In those studies, scientists thought that ultraviolet radiation from the decreasing amount of ozone in the atmosphere was causing the frog deaths. Ozone in the upper atmosphere blocks ultraviolet radiation from the sun.
Some scientists disagree with the new study. They say the effects of ultraviolet radiation in Oregon probably would not happen anywhere else. They also disagree about whether the fungal infection is the direct cause of the frog deaths. Some scientists say the fungus could have developed in the eggs after they had already died of a different cause.
Scientists note that not all of the world's frogs lay their eggs in water. They say there are probably many reasons for the decreasing populations of frogs around the world.
This VOA Special English ENVIRONMENT REPORT was written by Cynthia Kirk.