Today's announcement of the lowest level of Brazilian Amazon deforestation since the start of satellite monitoring in 1988 deserves to be celebrated, and it has been by out-going President Lula and his ministers - with perfect timing for them as they start negotiations at the Cancun climate talks where reduced deforestation is a major issue.

So the rest of this blog is going to seem very churlish as I add some context which takes a little bit of the shine off that main headline.

The first point to make is that the figure - 6,450 square kilometres of forest clear-cut in the year up to August 1st 2010 - is considerably higher than many of the predictions, which had forecast a fall to around 5,000 square kilometres based on the preliminary month-by-month monitoring. It is a cut of around 14% on the figure for the previous year (7,500 sq km) and still means an area larger than the state of Connecticut was cleared in 2009-10.

Even so, when you consider that an area of the Amazon more than four times that size went under the chainsaw and the torch in 2003-4, the scale of the progress must be acknowledged.

One reason for caution was highlighted by the director of the Brazilian National Space Research Agency (INPE), Gilberto Camara, as he delivered the presentation at the press conference to launch the monitoring results in Brasilia, with President Lula himself in attendance.

He helpfully emphasized the point on Twitter (@gcamara), advising his followers to check out the slide reproduced below.

To translate the text for non-Portuguese speakers, the heading reads "Deforestation by size" and the bottom notes, "Cutting less than 50 hectares: 35% of the total in 2002, 80% of the total in 2010.

In other words, there has been a very significant change in the pattern of deforestation since that high point in 2003-4: the very large mass-clearances, for example by soya farmers and big-time cattle ranchers, have to a large extent been brought under control, so what remains is a  large number of smaller acts of deforestation.

You could say that is good news, reflecting the pressure imposed on the big-time deforesters by Brazilian government action, international consumer pressure and indeed the policies of Brazilian stores including Walmart refusing to source beef from ranches associated with deforestation.

Even so, as Camara notes, it makes the continued decline of deforestation that much more challenging to achieve: how to monitor and control the remaining destruction when it is so dispersed?

"“After a fall of 45 per cent in deforestation in 2009, we had another 14 per cent in 2010. Each year, reducing deforestation will become more difficult,” Camara said on Twitter.

Another, linked trend is confirmed in these figures - a shift of focus away from the so-called "arc of deforestation", that swathe of land to the South and East of the Amazon biome which has seen the main advance of the agricultural frontier in recent decades.

As the two charts above show, the states in which that frontier is concentrated, Mato Grosso, Pará and Rondônia, have all seen declining trends in deforestation which have continued this year. States such as Amazonas and Acre, on the other hand, where a vast majority of the forest area is so far still intact, saw increases in deforestation this year, albeit still at a relatively low level. It is a warning sign that new frontiers may be opening up, and these trends could be very challenging to control.

I could go on to catalogue some of the remaining concerns: the possible impact of loosening Brazil's Forest Code, which could be voted imminently in the Congress (see last but one post); the impact of current projects to improve highways and build dams; the renewed pressure from a reviving world economy and additional demand for the agricultural commodities driving deforestation; and the likelihood of displacing deforestation to other regions, in particular the ultra-diverse Cerrado savanna which is disappearing at more than twice the current rate of the Amazon.

But who am I to be the Cassandra on the day Greenpeace Brasil issued a release headlined "Another nail in the coffin of deforestation"? It referred not to the INPE figures, but to an announcement by Banco do Brasil that it would deny credit to soya formers who planted in recently-deforested areas.

So for now, Brazil is well on track to meet its pledge to reduce Amazon deforestation by 80% before 2020. The world will be watching and hoping this trend continues.