Bj?rn Lomborg would plough the money into tackling Aids and malnutrition despite cries of protest from the environmental lobby. Here, he explains why

In theory, dealing with all the world's problems should be within our grasp. We should strive to win the war against hunger, end conflicts, stop communicable diseases, provide clean drinking water, make education more widely available and halt climate change. But we don't. We live in a world with limited resources, so we have to ask: which problem should we deal with first? We have a moral obligation to do the most good that we possibly can with every pound we spend. We need to start talking about prioritisation.

Surprisingly, an explicit economic prioritisation of the world's problems hasn't been done before. Why? There is a good deal of antagonism towards the idea that we need to prioritise. This is because when we prioritise, we put certain areas on hold (which is seen as cynical).

This view puzzles me; not talking about prioritisation doesn't make the need for it go away. It only makes dealing with the world's problems less clear, less democratic and less efficient. Refusing to prioritise and focusing on those problems that get the most "buzz" in the media is wrong.

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Imagine doctors at an overrun hospital refusing to perform triage on casualties, attending to patients as they arrive and fast-tracking those whose families make the most fuss. Not prioritising is unjust. It wastes resources and costs lives.

At the 2004 Copenhagen Consensus project, 30 specialist economists joined forces with eight of the world's top economists - including three Nobel laureates - to make a global priority list. They were asked to answer the question: if the world had, say, $50 billion extra to spend, where could that money best be deployed?

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The top priority was HIV/Aids. A comprehensive preventive programme would cost $ billion, yet the social benefits would be immense. If implemented now, it could prevent more than 28 million new cases by 2010. This makes it the best investment the world could make, reaping benefits that outweigh the costs by 40 to 1.

Similarly, providing micro-nutrients missing from more than half the world's diet would reduce diseases caused by deficiencies of iron, zinc, iodine and vitamin A. This also has an exceptionally high ratio of benefits to cost. Establishing genuinely free trade could be achieved at very low cost, with benefits of up to $2,400 billion a year, if we could only find the political will to push it through. Dealing with malaria would create benefits at least five times greater than the costs. Mosquito nets and effective medication could halve the incidence of malaria and would cost $13 billion. The list also focuses on agricultural and water technologies that tackle hunger and food production and on the lack of clean drinking water and sanitation.

The Copenhagen Consensus has been well received by many, the book from the meeting is becoming a scholarly bestseller and its conclusions are beginning to translate into real politics. Inspired by the Copenhagen Consensus, the Danish Government, for example, has increased its spending on HIV/Aids and has asked an international panel to give it specific recommendations based on the priority list.

However, some people have been critical of the project. Perhaps the most common criticism has been that proposing only an extra $50 billion to be spent over the next four years is not ambitious enough. Jeffrey Sachs, director of the Earth Institute at Columbia University, has been the key proponent of this view. He is adamant that we should not prioritise the world's problems and estimates that we should be spending upwards of $200 billion more a year.

Fifty billion dollars extra is indeed too little, but it is a realistic - perhaps even somewhat optimistic - target, if history is anything to go by.

More than 30 years ago, the United Nations called on the developed nations to double the percentage of money they spent on overseas development aid, but the sad fact is that, since 1965, that percentage has been halved.

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Substantial increases will, at the very least, be an uphill battle for most governments, given that they face various strains on their national budgets from domestic and other concerns. There is a risk that, in asking for an unrealistically optimistic aid increase, we will get gilded promises but little real commitment. Even if Sachs or someone else were to come up with some $800 billion over the next four years, for which I would extend my congratulations, the money would still have to be prioritised, and that would still mean following the Copenhagen Consensus list. The amount of extra money available simply determines how far down the list we go. It does not change the list.

Sachs has also criticised the fact that the Copenhagen Consensus consisted only of economists. All the estimates are based on the best integrated economic and natural science models. Yet only economists - not climatologists or malaria experts - can economically prioritise between battling global warming or communicable disease.

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And this leads us to the biggest problem some people have with the Copenhagen Consensus. The experts rated climate change extremely low on their to-do list. In fact, the panel called programmes aimed at tackling climate change - including the Kyoto Protocol - "Bad Projects" simply because they cost more than the benefits they reap. Implementing Kyoto, for example, would have fairly little impact - it would postpone global warming in 2100 by six years - and yet it would cost a hefty $150 billion a year.

This does not mean that we should ignore climate change, but that we should look for smarter long-term investments in, for example, renewable energy technologies.

Naturally, the fact that the Copenhagen Consensus has stated what most economists have known for a long time - that Kyoto is a bad deal - has not been well received in environmental circles. But since we all share a wish to implement policies that make for a better world, the Copenhagen Consensus forces us to ask: do we want to do a lot of good now or a little good a long time from now? And can we do more for the developing world by investing differently?

Far from suggesting a laissez-faire policy, these questions address the pressing problem of prioritisation head-on. Why did thousands die in Haiti during the recent hurricanes and not in Florida? Because Haitians are poor.

They cannot take preventive measures. Breaking the cycle of poverty, by addressing disease, hunger and polluted water will make people less vulnerable to the effects of climate change.

The Copenhagen Consensus is challenging our mindset and highlighting that the urgent problems facing the poor majority of this world are not primarily linked to climate change. Their problems are basic: dying from easily preventable diseases; being malnourished owing to a lack of simple micro-nutrients; being prevented from exploiting opportunities in the global economy by restrictions on free trade; not having access to condoms and good health education to prevent HIV or to simple vitamin supplements to prevent malnutrition. We are not talking about investing in space-age technologies, but paying for basic necessities. The message from the Copenhagen Consensus is that it is possible to solve some of the world's most serious problems - and that not only are we morally obliged to do this, but it is also a sound investment.

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Bj?rn Lomborg is the organiser of the Copenhagen Consensus, associate professor in political science at University of Aarhus, editor of Global Crises, Global Solutions and author of The Skeptical Environmentalist . He is listed as one of the world's 100 most influential people by Time magazine.