The spectre of famine has returned to the Horn of Africa nearly a quarter of a century after the world's pop stars gathered to banish it at Live Aid, raising £150m for relief efforts in 1985. Millions of impoverished Ethiopians face the threat of malnutrition and possibly starvation this winter in what is shaping up to be the country's worst food crisis for decades.

Estimates of the number of people who need emergency food aid have risen steadily this year from 4.9 million in January to 5.3 million in May and 6.2 million in June. Another 7.5 million are getting aid in return for work on community projects, as part of the National Productive Safety Net Program for people whose food supplies are chronically insecure, bringing the total being fed to 13.7 million.

Donor countries provided sustenance to 12 million Ethiopians last year, more than half of it through the UN's World Food Programme (WFP). Having passed that total only eight months into this year, and with the main harvest already in doubt, aid agencies fear the worst is still to come. "We're extremely worried," said Howard Taylor, who heads the Department for International Development's office in Ethiopia. DfID has given £54m in aid to the country this year, and Britain has also contributed through the EU. "This is exactly the time when we shouldn't turn away from the people in need," he said.

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"Critical water shortages" were reported in some areas by the UN's Office for the Co-ordination of Humanitarian Affairs last week with water-borne diseases such as acute diarrhoea spreading as communities resort to drinking from insanitary wells and ponds. Unicef said that the outbreaks are putting extra pressure on its Out-Patient Therapeutic Programme, which provides healthcare in some of the most needy areas.

In Somali, the hardest hit region with a third of the humanitarian caseload and complications caused by a low-intensity insurgency, the mortality rate for infants has risen above two per 10,000 per day according to a regional nutrition survey, which gives newborns roughly a one-third chance of dying before their fifth birthdays. While there is no clear definition, one widely used threshold for famine is four infant deaths per 10,000 per day.

Declaring a famine is a political decision. While it can galvanise public opinion and bring millions into aid programmes, it is widely seen as a political failure. President George Bush challenged his officials to avoid the word, a policy known as "No famine on my watch". Ethiopia's Disaster Prevention and Preparedness Commission is charged with preventing famines of the 1984-85 type, the sort that bring down governments, argued Tufts University academics Sue Lautze and Angela Raven-Roberts in a 2004 paper.

Dismissing the warning signals, Ethiopia's Prime Minister, Meles Zenawi, said earlier this month that there was no danger of famine this year. And Berhanu Kebede, Ethiopia's ambassador to Britain, said at the weekend: "We are addressing the problem. Food is in the pipeline."

The main practical difference between a food crisis and a famine is whether enough aid arrives to keep the starving alive. So while the scope of the problem can be measured in the number of hungry people, the severity depends on the generosity of those in the rich world. And this year they have been miserly. Despite the promise of G8 leaders at their summit in L'Aquila, Italy, last month to provide $20bn (£12bn) to improve food security in poor countries, contributions have slumped dramatically this year as donor states have shifted priorities to supporting banks and stimulating their own economies. "The international community is not living up to its promise to the World Food Programme," Mr Kebede said.

The WFP had little trouble raising its $6bn budget last year, but in 2009 it has collected less than half of that. Its Ethiopian operation, which had $500m in 2008, is short $127m this year, equivalent to 167,000 tonnes of food. The Famine Early Warning Network forecast this month that the shortfall would reach 300,000 tonnes by December. Rations for the 6.2 million people receiving emergency food aid have, as a result, been slashed by a third from a meagre 15kg of cereals, beans and oil a month to just 10kg. Even if the shortfall were made up today, it would take three months for supplies to be loaded on to ships bound for Djibouti, then transferred to trucks for the arduous overland journey to land-locked Ethiopia.

Aid agencies are worried about the main harvest this autumn, arguing that the time for action is now, not when the food runs out in November – usually the driest month – let alone when starving children with distended bellies capture the attention of the West's television viewing public. Despite its good intentions, Bob Geldof's Live Aid came towards the end of the 1984-85 famine, which killed more than a million people. Since then, Ethiopia's population has doubled to 80 million.

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Mr Zenawi's government has set up a strategic food reserve which has at times reached 500,000 tonnes – though it is currently thought to be just 200,000 tonnes – which it uses to speed up delivery. As soon as they get funds, aid agencies can borrow food from this reserve, replacing it with supplies from abroad when they arrive. Although the government could release this food without promises of replenishment, it would soon run out; after covering the WFP's 167,000 tonne shortfall, the stockpile would be barely enough to feed a million people for three months.

The underlying problem for Ethiopia is the erratic behaviour of the country's climate, or rather its regional micro-climates. Moisture-bearing clouds scudding in from the Indian Ocean can pass over the parched eastern lowlands to dump generous amounts of rain on the fertile western highlands. The famine of 1984-85, revealed by BBC reporter Michael Buerk, was actually two separate famines, one in Tigray, in the north, the other in Somali, in the south-east.

Two main rains sustain the people of Ethiopia, the belg in spring and the kiremt, which usually start in July. Both are influenced by variations in sea-surface temperature. The El Niño phenomena in the eastern Pacific usually bring droughts to Ethiopia, and America's National Oceanic and Atmospheric Administration predicts that the current El Niño will strengthen over the next six months. The belg has failed for two years running now, while the kiremt started three weeks late this summer and the amount of rainfall when they did come was below normal. Aid agencies fear that the season could end early, or, equally bad, produce delayed downpours just when farmers need dry weather for the harvest. Even if the kiremt ends on time in October, some crops may not reach maturity because of the late planting.

Ethiopia is overwhelmingly dependent on agriculture, and some 90 per cent of its crops are watered by nature rather than by man-made irrigation systems. During droughts, farmers and nomadic herders tend to sell off their assets to buy food, leaving them with nothing when the next growing season begins. It can take three to five years for pastoral tribes to rebuild their herds.

Although Ethiopia is particularly hard hit, drought has also affected neighbouring countries. Resources in Somali are under additional strain because nomadic tribesmen from Somalia and Kenya have driven unusually large numbers of cattle across the border in search of water and pasture. Estimates of the number of cattle coming into the country range from 95,000 to 200,000.

The spike in global food prices in 2008 exacerbated a worsening situation, hitting the urban poor particularly hard. While they have fallen back this year, the price for grains in the markets of Adis Ababa are still some 50 per cent higher than their average in the four years to 2007.

The Ethiopian government is acutely aware of the danger of famine, not least to itself. Emperor Haile Selassie was deposed a year after the 1973 famine and the Derg military junta led by Lt Col Mengistu Haile Mariam was overthrown in 1991 after a civil war driven in part by the 1984-85 famine. While most other countries with food shortages allow charities to distribute food, Ethiopia's government insists that the bulk of food aid must pass through its hands.

The irony is that the Zenawi regime has done a reasonable job of boosting food production, achieving self-sufficiency in the late 1990s. One agency described it as the "bread basket" of Africa, harvesting more grain in a good year than South Africa. The government promotes best practices and distributes fertiliser to farmers. It also has an ambitious scheme to relocate 2.2 million people to more fertile areas. But even it can't control the rains.

Many Africans blame climate change for the erratic weather patterns and resulting food shortages. Jean Ping, the chairman of the African Union, said last week in Adis Ababa: "Although Africa is least responsible for global warming, it suffers most from a problem it didn't create."

The BBC R4 Reunion series, at 11.15am today, examines the 1984 Ethiopian famine. Guests include Michael Buerk