At last, the wreck of the rainforests is being tackled. One of the key parts of the Copenhagen climate agreement which the international community will try to construct in December is a comprehensive treaty aiming to reduce deforestation rates in the developing countries by at least 50 per cent by 2020.

Not before time. It has been 20 years since we woke up to the reality of large-scale rainforest loss: in the late 1980s, the terrible scale of destruction in regions such as the Brazilian Amazon, and later, in Indonesia and other areas, dawned on the world, but in the time since then, all we have been able to do, in effect, has been to wring our hands.

Deforestation was clearly terrible for wildlife, for the indigenous peoples of the forests, and for the "ecosystem services", as the modern jargon has it, which forests provide, such as climates which bring rain. But the whole process seemed so vast, the huge socio-economic forces behind deforestation in the developing countries so intractable, that it seemed impossible that anything could be done to stop it or even slow it.

Download the new Independent Premium app Sharing the full story, not just the headlines

Things have changed, and the change can be summarised in a single word: carbon. For, as the threat of climate change has become more and more clear, there has been an growing perception that the biggest benefit of all that rainforests provide is their function as a carbon store, and the biggest danger from their destruction is the release of carbon dioxide into the atmosphere when they are cut down and burnt.

This is happening on a tremendous scale. In a band around the tropics, shown in the red in our map, about 13 million hectares of natural forest are being chainsawed and burnt every year – an area about the size of England – and the CO2 emissions released total about 5.8bn tonnes annually.

This is almost as much as the emissions of the US or China, the two biggest carbon emitters; it is nearly 20 per cent of the global total, more than the whole of the transport sector across the world, which has always been considered one of the major difficulties in dealing with climate change.

The deforestation emissions of Indonesia and Brazil, for example, are now so great that they propel those countries to fourth and fifth place respectively in the world emissions table although, if their places are based just on burning fossil fuels, they are much lower.

It has become clear to policymakers that the now generally accepted goal of reducing global emissions by 80 per cent by 2050, in the hope of keeping the rise in temperatures to two degrees above the pre-industrial level – thought of as the danger threshold – will be impossible without tackling forest emissions.

So when the Coalition for Rainforest Nations, a grouping of 40 countries with substantial forest holdings from Costa Rica to Papua New Guinea, proposed in 2005 that there could be an agreement – we preserve our forests, you in the rich world pay us to do so – they were met with a sympathetic response.

The issue is known as Redd, reducing emissions from deforestation in developing countries,and you will be increasingly hearing the unfamiliar acronym as the Copenhagen meeting approaches. For Redd is now a key part of the treaty negotiating process under which, it is hoped, the developing countries will agree to tackle their own, mushrooming greenhouse gases, in return for billions of dollars of new aid.

The principal objective of the Redd agreement, put forward by the EU, with Britain leading in the negotiations, is that "all parties should collectively aim at ... reducing gross deforestation in developing countries by at least 50 per cent by 2020 compared to current levels."

That is widely supported by environmentalists. But another phrase in the objective, more supported by tropical nations with big logging industries, is also that "all parties should aim at halting forest cover loss in developing countries by 2030 at the latest". To decode the text, "reducing gross deforestation" means in essence slowing the rate at which you cut down your virgin, natural forests.

But "halting forest cover loss" means you can cut down the forests but replace them with other trees, so that "forest cover", the general area covered in trees, remains the same.

These other trees are likely to be commercial monocultures such as eucalyptus or oil palms, not remotely as valuable ecologically, or as a store of carbon, as virgin forest, and although it might be better to have those trees growing than bare ground, many environmentalists would stress that not cutting the virgin forest down in the first place is the best option of all.

The text is still up for negotiation, but the fact that two potentially conflicting stances can be part of the same first sentence of the proposed treaty shows what a difficult matter it is on which to reach agreement.

Even so, both parts of the objective constitute an ambitious aim, and the use of the phrase "all parties" indicates that we are all in this together; if they are going to stop deforestation in the developing world, we in the rich world have got to help them.

One of the key aspects of Redd is that is conceived of at a national level; before, attempts at preventing deforestation tended to be local projects. Now whole countries are being asked to lower their deforestation rates, if we finance it. How are we to do so?

There are three options. The first is to supply substantial new aid funding; the second is to let countries with high deforestation rates generate "carbon credits" from the forests they preserve, which could then be sold on the growing international carbon market; the third is a mixture of both.

Using the carbon market is the most controversial, because some policymakers feel this will provide a vast pool of emissions credits which western countries can buy and thus escape much of the obligation to cut back on emissions of their own (Brazil's rule of thumb is that a hectare of forest holds a tonne of carbon).

There are further objections, such as the "permanence" of the forest which has generated a carbon credit by being left uncut. What happens if it is subsequently cut down, or burnt, or even dies off because of climate change? But others feel the carbon market will be an essential tool, not least as the total funding needed is likely to be very substantial.

Last year, Johan Eliasch, a Swedish-born, London-based businessman with environmental interests, was asked by Gordon Brown to report on how the preservation of global forests could be financed. Mr Eliasch estimated that, according to one scenario, this could cost between $18n and $26bn annually, with perhaps $7bn of the funding coming from the carbon market. The rest would have to be found by the developed countries.

It'is an enormous sum, but Eliasch thought it was worth it. "Saving forests is critical for tckling climate change," he said. "Without action on deforestation, avoiding the worst aspects of climate change will be next to impossible."

Next week: finance