(CNN)Human activity and climate change-fueled disasters have turned 10 of the planet's internationally recognized forests, also known as World Heritage sites, from carbon absorbers into carbon emitters, researchers have found.

The report from UNESCO found these sites can absorb approximately 190 million tons of carbon dioxide from the atmosphere each year -- roughly half the amount of the United Kingdom's annual fossil fuel emissions.

But in the past 20 years, many of these sites showed an increase in emissions, some even exceeded how much carbon they were removing from the atmosphere.

UNESCO researchers said two main factors are causing forests to flip from sinks to sources: climate change-fueled extreme weather events including wildfires, storm and drought; and human land-use pressures such as illegal logging, wood harvesting and agricultural practices such as livestock grazing.

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