Deforestation cannot be blamed for widespread flooding such as the recent massive and deadly inundations in Central America, according to an international research report to be released Thursday.

The study, issued by a U.N. agency and the Indonesia-based Center for International Forestry Research, asserts that major floods tend to occur at regular intervals and are driven by major climatic patterns, rather than human activities such as logging. It notes that massive flooding occurred in northern Thailand in 1918 and 1953, when lush forests were abundant.

Catastrophic floods covering large areas of land almost always occur after prolonged rainfall saturates the soil and have little to do with the amount of forest cover, according to the report.

"The simple explanation for large-scale flooding is that it rains a huge amount and that flooding is part of natural processes,'' said David Kaimowitz, director general of the forestry research group, which co-wrote the study with the U.N. Food and Agricultural Organization. "There is a tendency to look for culprits when these natural events occur."

The report is based on a review of hundreds of published papers and is aimed at reversing policies that criminalize small farmers and loggers because of "the myth of the link," Kaimowitz said.

Officials have reported at least 652 deaths and 600 people missing in Guatemala after floods brought on by Hurricane Stan.

"The flooding in Central America is a natural process that would have happened, no matter what," Kaimowitz said. "Whether you had deforested, farmed or logged, the amounts of water involved and the severity of these floods is just overwhelming."

The conventional wisdom is that forests prevent floods by acting as giant sponges, soaking up water during heavy rainfall and releasing water during the dry season, the study says. In the wake of last week's floods in Central America, the environmental group Greenpeace International blamed rampant deforestation for worsening the disaster.

Greenpeace blasted the report. "I find this denial of a link between deforestation and floods quite dangerous -- as dangerous as people who would be saying the only reason for floods is deforestation," Christoph Thies, the group's forest policy coordinator, said from Hamburg.

"There is very, very broad acknowledgment in numerous scientific papers how important forests are in regulating water flow and protecting watersheds. So I think that to question that is not very credible," Thies said.

But Kaimowitz said deforestation is a major problem for other reasons. The burning of forests to clear land contributes one-fifth of the carbon emissions that cause climate change, he said. The loss of habitat leads to extinction of plant and animal species. And deforestation does contribute to small-scale flooding. Each year, the world loses forestland that would cover an area roughly the size of Greece, he said.