Liv Ullmann's greatest role is saving 41/2 million children

WHAT does Liv Ullmann do when she has come to talk about desperate hunger in Africa at a lunch where prosciutto melon is followed by marinated pork roast, wild rice, and chocolate mousse?

Well, in New York recently, she took a bite or two of the roast, waved off a waiter pouring wine, and pushed aside the mousse uneaten. Then she rose to talk.

The Norwegian actress, whose international fame derives from her expressiveness, did not emote. She did not even project. My tape of the occasion shows her words frequently blotted out by the horns of Mercedes limos and cabs rushing down Fifth Avenue on their way to important events or gridlock.

What Miss Ullmann did was recall in a quiet, deeply genuine way the mothers she had seen in Mali digging for water with their bare hands or wringing out rags to dribble a thin, muddy stream into a cup. Schoolrooms left empty because children hadn't eaten for days and couldn't drag themselves to class. Villagers who had had no meat for weeks, asking for simple equipment to grind millet.

Miss Ullmann's pellucid blue eyes looked out to a score of New York media moguls gathered to hear her report. But she seemed to be seeing those mothers and children - each one momentarily more real and close to her than the pinstripes and bustle of Manhattan - in their huts 4,000 miles away. Her Norwegian-tinted English brought what was until recently the lake district of Mali - now a parched, cracked landscape - into that room overlooking the verdure of Central Park.

For four years Liv Ullmann has served as UNICEF's good-will ambassador to countries where children are in need - and from them to the successful, industrialized world whose pennies and dollars can literally save endangered species of the human race. (In that role she follows the humanitarian comics Danny Kaye and Peter Ustinov, who brought hope and mirth to distant clans of our Earth tribe for three decades.)

She attended this luncheon at the ornate Metropolitan Club to focus media attention on what UNICEF (the United Nations Children's Fund) is doing to feed and teach hungry children and their parents.

It's easy to draw the wrong lessons from this scene. Anyone who grew up during the Great Depression remembers American parents goading their offspring to clean their plates by telling them not to waste in a world where others went hungry. Several of the network producers and editors gathered for the lunch said afterward they felt guilty about letting mousse go either to waist or waste while hearing about villagers who had not eaten for a week. It gave them a Marie Antoinette complex. But two inescapable lessons emerge from Miss Ullmann's account. Both call for action, not fasting.

(1) The relatively rich can help the hungry without undercutting their own prosperity. This is not money wasted in a hopeless fight against the encroaching desert. Miss Ullmann mentioned several success stories she had witnessed in previous campaigns against hunger - in Sudan, Yemen, Cambodia. In each case the headline picture had been one of hopelessness, but international action had turned the situation around.

(2) Far from adding to the world population problem, the saving of millions of families from famine in parched Africa and elsewhere actually serves to restrict future population growth. It's paradoxical. But empirical evidence shows that peoples rescued from such calamity do not go on to proliferate and create a new crisis through population pressure. The main factor seems to be relief from the desperate feeling that more children are the only insurance against pestilence.

Aid channeled through UNICEF has been, and remains, a bargain. The annual contribution of the single biggest donor, the US, amounts to the cost of just one F-15 fighter plane. In terms of jets-into-plowshares, that appropriation buys as much, penny for penny, as almost anything the Congress votes on.

At the moment, executive director James Grant explains, UNICEF is concentrating on Africa, where 4 1/2 million children are threatened. Ironically , while the industrial West has focused on Latin America's very real debt problems, the African continent is actually in greater difficulty. A combination of widespread drought from the Sahara to Mozambique, high interest rates on loans, slumping commodity prices for much of what Africa produces, and inexperienced administrators has left Africa in a more precarious position than Latin America.

Mr. Grant is not without hope. ''If the world community can react in a supportive way,'' he says, ''looking 10 years ahead, the prospects for Africa are quite bright indeed.'' That judgment is predicated on a continuing world recovery leading to higher prices for cocoa, copper, coconut oil, etc.

One big philosophical question hangs over the noble rescue effort in the lands south of the Sahara: Is mankind right to resist these global climate shifts? Or should we be helping populations to move elsewhere?

No world organizations were around to pose such questions during the ice ages - or when other great climatic changes occurred. It's hard to claim exact knowledge. But a strong case can be made that these particular branches of our worldwide brotherhood have learned to live in what has long been a tough and beautiful terrain. The Sahara is an extension of the desert-at-sea known as the Horse Latitudes or the Doldrums, about which Coleridge wrote his famous phrase: ''Water, water, everywhere, Nor any drop to drink.'' The people of Mali and their neighbors have adapted to the sub-Sahara's sparseness over centuries. There is no reason to believe that the desert's current southward march, at five miles per year, marks a permanent change of climate pattern, even though Mali's rivers are at the lowest level ever recorded.

So when UNICEF not only brings food and water to villagers, but teaches the children to plant trees to retain soil and water, it is helping them to vote for hope rather than flight.

What Grant calls the ''child survival revolution'' is an even more emphatic vote for human endurance.

Liv Ullmann puts the issue simply: ''Sand is creeping over their water, their livestock, their families, . . . and we can do so much to help. I would hate to come back in a year's time and not find the people, and have to say that man was here and now he's gone because we could not afford to keep him alive. . . .

''We have an illness today that is called indifference. The media is pouring down on us with statistics, . . . and in the end, people can't take any more, and they just guard themselves. . . . These people (in Africa) are deprived. But they are people who still have what we have lost. That is hospitality, generosity. I've never been to a deprived place where I haven't been asked to come in. Maybe there's not even water to share, but I've always been welcomed with kindness.''

Then Miss Ullmann relents on her judgment of modern society. Generosity is not, she notes, really missing. When people hear about individuals in need, ''the reaction is enormous. People say, as one woman did to me, 'At least I can put a blanket over my children at night. I'm on social security. But here is two dollars. I am very grateful and I want those children to have something.''

A few years ago, Liv Ullmann sat between Henry Kissinger and Andrei Gromyko at a dinner in the Soviet Embassy in Washington. What, one wonders, would have been the reaction of those two superpower Metternichs to her plea in 1984?