Some of the world's most protected forests are emitting more carbon than they absorb, driven by things like logging and wildfires, according to a new report.

At least 10 forests designated World Heritage sites - including Yosemite National Park in the United States – have been net carbon emitters over the last two decades, the report said.

"That even some of the most iconic and best protected forests such as those found in World Heritage sites can actually contribute to climate change is alarming and brings to light evidence of the severity of this climate emergency," Tales Carvalho Resende, report co-author and project officer for the United Nations Educational, Scientific and Cultural Organization (UNESCO), said in a statement.

Forests are considered vital for curbing climate change due to their ability to work as so-called carbon sinks.

Trees and other plants absorb carbon dioxide and emit oxygen, removing greenhouse gases from the atmosphere.

All 257 forests together do act as a net carbon sink, according to the research which analysed a period from 2001 to 2020.

Still, human activities like logging and intense climate-related events such as wildfires are hindering their ability to capture and store more carbon than they emit, which experts say is a cause for grave concern.

The Waterton-Glacier International Peace Park on the Canada-US border where 14,500 acres was burned in a wildfire in 2018

As well as in the United States, forests found to be net carbon emitters were also located in Indonesia, Australia and Russia, among other countries.

UNESCO investigators and researchers from advocacy groups the World Resources Institute (WRI) and the International Union for Conservation of Nature (IUCN) combined satellite data with on-site monitoring and found that together the heritage sites saw net absorption of 190 million tons of CO2 annually over the 20-year period.

Over the course of centuries the forests have stored some 13 billion tons of carbon, equivalent to Kuwait's proven oil reserves, the report said.

The findings drew on data published by the journal Nature Climate Change in January, which mapped greenhouse gas emissions and absorption by forests globally.

The researchers used this data and on-the-ground monitoring of the heritage sites to understand what is putting forests at risk, including logging, agricultural incursions, droughts and shifting temperatures.

"I would expect all of them to be removing carbon for the atmosphere, and not to be sources of carbon," Carlos Sanquetta, a forestry engineering professor at the Federal University of Parana in Brazil, said.

"Instead of playing a role in carbon sequestration they are playing a role in carbon emissions."

Though it produced important findings, the report could have presented its methodology in greater depth, he said.

While just 10 of the UNESCO-protected forests were found to have been carbon emitters, the report said other sites also showed clear upward trajectories in emissions.

"This is one more clear sign that even forests we traditionally assumed to be safe are now under increasing threat," David Kaimowitz, one of the forest directors at the UN's Food and Agriculture Organization, said.

The report did not place enough emphasis on supporting indigenous and local communities as well as activists who oppose forest destruction than is laid out in the report, he said.

He also questioned whether the report was representative of all forests.

"Readers should...not assume that the specific numbers presented here apply to forests everywhere," he said.