Land grabs have grabbed global attention. It's on the agenda at the World Economic Forum this week, and as the trend for large land acquisitions accelerates, it has moved from being primarily a story about Middle Eastern petrodollars pouring into Africa, to a much more widely spread phenomenon affecting many parts of south-east Asia, such as the Phillipines, as well as Latin America.

In Cambodia, 15% of land has been signed over to private companies since 2005, a third of which are foreign. A new set of research studies from the International Land Coalition find the competition for land increasingly global and unequal.

Many of the deals are shrouded in secrecy, so the scale of what is happening is not clear, nor is it clear who is benefiting from these deals; a number of new reports try to tease these issues out, such as the International Institute for Environment and Development's analysis of legal contract, which is published on Monday.

It's not hard to see why the subject generates so much attention. It's partly the secrecy element, partly the fear: who is buying up the future? Large-scale land acquisition prompts all too vividly visions of a dystopian future in which millions of the hungry are excluded from the land of their forefathers by barbed wire fences and security guards as food is exported to feed the rich world.

This is no longer just a fear for the future. The US environmentalist Lester Brown points out in his new book, World on the Edge, that in 2009 Saudi Arabia received its first shipment of rice produced on land it had acquired in Ethiopia while at the same time the World Food Programme was feeding 5 million Ethiopians. Similarly in the Democratic Republic of the Congo, China has acquired 7 million hectares for palm oil production and yet millions of people in the DRC are dependent on international aid for food.

Brown warns that "land grabbing is an integral part of the global power struggle for food security". He argues that geopolitics for several centuries have been dominated by the issue of access to markets, but increasingly in the future this will be replaced by the overriding importance of access to supplies. Food importing countries are anxiously securing their food supplies, all too aware that exporting countries can impose export bans to meet their needs. In 2007 both Russia and Argentina, major grain exporters, put in place export bans and it sent waves of panic around the world, which have probably played a big part in fuelling land acquisition deals.

Much of the attention so far has focused on Africa. Most of the biggest deals have been in countries such as Ethiopia, Mali and Sudan. The imminently independent south Sudan has seen investors queuing up to exploit one of the areas of greatest potential for as yet under developed agricultural land. In comparison with many other areas of the world, land in Africa is very cheap; in Ethiopia, land can be leased for as little as $1 an acre.

China is acquiring land at the fastest rate, but South Korea is not far behind. It has now set up an agency specifically dedicated to making direct agreements with farmers and landowners to secure supplies.

Many African governments are defensive about the deals. Ethiopian President Meles Zenawi is expected to talk on the subject in Davos this week; in the past he has argued that investment in African agriculture is crucial to improve the continent's low agricultural productivity. He has argued that foreign investors bring in mechanisation and expertise which is vital for development. Many campaigners would agree that investment is badly needed, but insist that the future for African agriculture is not mechanised monocultures for export but supporting sustainable smallholder agriculture. They argue that the latter is far more likely to ensure food security for the poorest Africans.

Some land deals claim to try to meet the needs of smallholders and bring investment at the same time. When I visited Mali recently, a number of local campaigners argued that the Millennium Challenge Account project had invested in the irrigation needed and was training local farmers.

But this small example was outweighed by the enormous anxiety in Mali about the foreign investors who were leasing hundreds of thousands of hectares in a country where the population is rapidly expanding and the land suitable for agriculture is shrinking as the desert expands. Lester Brown rightly points out that the real issue here is not so much land deals as water deals. What is driving the land grabs is the scarcity of water. Saudi Arabia used to produce a lot of wheat, but it is the decline of its aquifers that is forcing it to look abroad to secure its future food.

Leasing and buying land are always ultimately about access to water, and in many parts of Africa this could be a major source of future conflict. Sudan and Ethiopia both feed water into the Nile; intensifying production in these areas could divert water. The Libyan lease of 100,000 hectares in Mali has involved the construction of a massive dam, diverting water from the Niger, a river on which several countries, including Niger and Nigeria, depend.

So what can be done? The World Bank has proposed guidelines for these kinds of deals, but has no way of enforcing them. Many campaigners, such as the international NGO GRAIN work with groups in affected countries who demand accountability and transparency from their governments. In Mali I heard how the CNOP, Coordination Nationale des Organisation Paysannes de Mali was bravely challenging the government, but it was far from clear what success it had had in checking the pace of land acquisitions.

This phenomenon reflects all too starkly the powerlessness of smallholder farmers across the world. They lack the formal land rights or the access to political power in their countries which would enable them to ensure these deals worked in their interests. Instead, the future of their children is being sold over their heads.