Rising levels of water vapour high above the earth are likely to intensify the effects of global warming in coming decades, say scientists.

The increasing amounts of water in the upper troposphere are a direct result of human activity, research suggests. Computer simulations predict that as burning fossil fuels warms the climate, concentrations of water vapour will rise.

The moistening of the atmosphere in turn absorbs more heat and raises the earth’s temperature further.

To investigate these effects scientists measured levels of water vapour in the upper troposphere, a region five to eleven kilometres above the Earth’s surface.

Their findings were compared with climate model predictions of water circulation between the ocean and atmosphere.

The results showed that increasing levels of atmospheric water vapour could not be explained by natural forces such as volcanoes or changes in solar activity.

But they did appear to be linked to emissions of greenhouse gases such as carbon dioxide.

The findings are publishedin the journal Proceedings of the National Academy of Sciences.