Increased use of satellite data and new tactics to deter loggers have led to drop, says Brazilian environment agency

Large-scale deforestation in the Amazon rainforest fell dramatically last year, according to official figures released yesterday.

Data from satellite sensors making fortnightly detections of only larger areas of forest destruction (greater than 25 hectares) was 1,500km2 between August 2009 and May 2010, compared with 3,000km2 in the same period a year earlier.

The Brazilian environment agency, Ibama, which is responsible for protecting the forests against illegal logging, said the drop was due to the increased use of satellite data to spot the felling of trees and new tactics to deter loggers, including ending their ability to hide under cloud cover.

The full figures for the year and all deforestation will be published on 31 July. The areas of forest destruction are expected to be 5,000-6,000km2, down from 7,500km2 the previous year, and from 27,000km2 in 2004.

"We are winning another victory over deforestation in the world's largest and most important biome," Luciano Evaristo, director of environmental protection, told the Guardian, which had been flown to Brazil by the Brazilian government for the annoucement. "Before [satellite data] we were looking blindly. But in 2010, all 244 actions were based on smart geo-processed data."

But Evaristo agreed with critics of the government that Ibama remains understaffed, with 700-800 enforcement officers on the ground at any one time across the vast country, which is nearly four times the size of western Europe. "I wish we had 4,000," he said, adding that the satellite data was making the work of officers more effective.

The ecologist Philip Fearnside, at the National Research Institute for the Amazon in Manaus, said the decline is partly due to control measures, but also due to a drop in demand as soy and beef consumption fell and the appreciation of the Brazilian real against the US dollar made export more expensive to foreign markets. "Deforestation is not under control," he said. "Prices of commodities will go up after the global recession. When that happens you discover you do not have control."

Evaristo rejected that argument: "The figures for 2010 show high commodity prices do not lead to an increase in deforestation."

The environment minister Izabella Teixeira said: "I think several factors can explain [the drop]. We now share the responsibility with 17 ministries."

Ibama has adopted new tactics in the fight against deforestation. Only 0.32% of the 250,000 fines issued by Ibama over the last 20 years have been paid. "It is true, thanks to the Brazilian legal system," said Evarista, blaming three different appeal systems.

Ibama seizes the tools and equipment of suspected illegal loggers while the legal process plays out, and also blocks their access to government credit, which is proving highly effective.

The ranchers can no longer hide under clouds either. Until recently, only visible light satellite images were taken. "The ranchers knew Ibama was much less active on the ground during the cloudy season," said the satellite data manager George Ferreira. Now, radar surveillance means the felling of trees can be spotted from space, rain or shine, day or night.

Raquel Taitson, an armed enforcement officer who has been attacked with an axe and had to escape being run down by a car, said she is driven by the desire to protect the forest. "My father is usually more scared than me, so usually I don't tell him. But we need more officers. We also have other tasks, such as controlling animal trafficking and illegal fishing."

A study published last week by the influential Chatham House thinktank said illegal logging dropped by between 50 and 75% across Cameroon, Indonesia and the Brazilian Amazon over the last decade.