It is a sad and unacceptable fact that in this age of relative plenty nearly 1 billion people go hungry every day. Despite an abundance of food worldwide, one in seven people do not have enough to eat and the hidden crisis of malnutrition underlies one in three child deaths. At the G8 meeting this week in the US, world leaders have the opportunity to do something about this by renewing their 2009 pledge to invest more money in agriculture and nutrition, with a focus on the world's poorest people.

In South Sudan, from where I have just returned, people are living under the shadow of conflict, deprived of some of the most basic services that we all take for granted, and struggling to afford enough food to feed their families. In spite of their deprivations and traumatisation, they are resilient, hopeful and determined to survive and thrive. Their faith is infectious and should inspire us all.

The Bible rightly says that "man shall not live by bread alone" but without bread – or more accurately, without a balanced and nutritious diet – many lives are seriously compromised. Poor nutrition has knock-on effects for the rest of a child's life. An estimated 170 million children worldwide risk life-long impairment of their physical and cognitive development – a condition known as "stunting" – because of their diets.

This is not just morally repugnant at an individual level – it also has important economic implications. Stunted children earn an average of 20 per cent less when they become adults, and malnutrition can cause a 2-3 per cent reduction in a country's national income. This is why eight of the world's leading economists, including five Nobel Laureates, agreed in 2008 that combating malnutrition was the best investment we could make in development.

Sadly, in spite of progress over the past 20 years, many factors – climate change, volatile food prices, economic uncertainty and demographic shifts – are now jeopardising the fight against malnutrition. Up to 450 million lives will be blighted by stunting in the next 15 years, according to research by Save the Children.

It does not have to be like this. We know the solutions to poor nutrition and can do much more to promote sustainable agriculture. In fact, world governments promised three years ago to do just this. At a meeting in L'Aquila, Italy, in 2009, the G8 pledged an extra $22bn (£14bn) to agriculture, targeting smallholders and working with national governments.

Three years on, the headline pledges have not been achieved. A recent report from ActionAid revealed that although spending by donors increased by 60 per cent after they made their pledge, several countries, including Italy and France, actually cut their aid after the meeting. Furthermore, the promise to concentrate on countries with national plans to eradicate hunger has not been consistently followed, and there remains little correlation between countries with high rates of hunger and those that receive the most aid.

As another G8 meeting looms, aid agencies and experts agree that world leaders need to increase ambition, boost political will and refocus efforts. Politicians must revamp and extend the L'Aquila initiative, with a clear focus on smallholder farmers and women, and a commitment to address not just productivity but nutrition. They should agree an ambitious global target to improve nutrition.

With opportunities like this within reach, we all have a moral responsibility to take action and hold politicians to account. And it is in our own interests too. The world simply cannot afford the ongoing economic costs of malnutrition.

Our Prime Minister has shown great leadership by defending vital aid spending, but this commitment must continue. The UK has made a promise to the world's poor and I hope Mr Cameron will press fellow world leaders to use this G8 meeting to get back on track to addressing child malnutrition.

We must ensure that we have a fairer system globally, where those who are most in need have the opportunity to help themselves. With the UK holding the next presidency of the G8, now is the time to make that system a reality.

Dr John Sentamu is the Archbishopof York