Many developing countries will try to curb carbon emissions by setting aside forested areas as reserves.
But experts are worried that creating national parks often involves removing the people who live in these areas.
The study indicated designating forest reserves in Liberia and the Democratic Republic of Congo could displace as many as 1.3 million people.
With funding from Norway, Liberia has proposed 30% of their forests become protected areas by 2020.
DR Congo, funded by Germany and the Global Environmental Facility, aims to set aside 12-15% of their forested lands.
Consultants TMP Systems concluded:
"Governments have targets to expand their protected areas, and now with new climate funding being available the risk is they will use this to expand in a way that doesn't respect local rights," said Andy White, from the Rights and Resources Initiative, the campaign group that sponsored the research.
"It could result in the displacement of millions of people."
Analysts say that this type of displacement has already happened in sub-Saharan Africa, South East Asia and Latin America, and sometimes caused violent conflict.
"I don't think the international community wants to displace rural dwellers in Liberia - but I think if we go about it in the way we are talking about it right now, that is going to be the result," said Constance Teague, from Liberia's Sustainable Development Institute.
"We need to recognise that indigenous communities respect the forest and they have worked on [it] for hundreds of years.
"It may not look like what the international community may expect, but this effort to conserve the land does exist."
Liberia had the largest forest space left in West Africa, largely because of the indigenous communities, she added.
The report also looks into the costs of compensating people for the loss of their lands in both Liberia and DR Congo, which range from $200m (Â£137m) to more than Â£1bn.
The main argument for setting up reserves is to:
And Mr White said: "We need to make evidence available that makes it clear that the woods are full of people, and it makes more sense to help them rather than kick them out.
"Where indigenous peoples rights are protected, and they are able to use their forests for their own livelihoods, they have more carbon per hectare than protected areas.
"They are active protectors, you don't have to pay a park guard, because they protect their forests, and that is what the world needs."
Some 1.5 billion indigenous people inhabit or claim most of the land in the world - but, according to a study released last year, they have legal rights to just 10%.
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The Paris climate agreement could make millions of forest dwellers homeless, according to a new analysis.