Two tragedies are unfolding in the horn of Africa. The first is the very visible one, the tragedy of families who've walked for weeks, their children growing weak with hunger, desperate for our help.

Then there is the larger tragedy of a failing humanitarian system built around responding to emergencies, not preventing them.

Over the years of Somalia's long implosion aid workers have come to expect a steady trickle of desperate people fleeing their former lives as herders and pastoralists. But now the trickle has become a flood.

Over 2,300 people are leaving Somalia each day, and that number is increasing. Aid workers who've spent their careers in east Africa say it's the worst crisis they've seen in two decades.

Save the Children is on the ground delivering life-saving help to tens of thousands of children. The families we are helping now are incredibly tough, but they've been hit by a triple whammy. The rains have failed this year in Kenya and Ethiopia, and for the last two years in Somalia. Food prices have shot up across the region, with staples rising by up to 240%. Finally the ongoing conflict in Somalia has severely weakened people's resilience and ability to absorb shocks.

Families are struggling to get even one meal a day as livestock die and crops wilt. Small children are especially vulnerable to malnutrition, which leaves them stunted for life, physically and mentally.

We saw this disaster coming as early as last year. There's a warning system in place, fed by satellite imagery, rainfall measurements and crop growth figures. Food prices have been rising for years. Save the Children was already trucking in emergency water supplies in February.

But the fate of many of the children now malnourished was already sealed. We have a broken humanitarian system based on responding, not preventing. Paddy Ashdown raises these same issues in his recent review of the global humanitarian system for the UK government.

It's a collective failure. Aid agencies go to huge lengths to prepare the communities they work with to adapt to droughts and other shocks, but there's a limit to what they can do to raise money from governments and the UN in the early stages of a crisis. It's very hard to talk up a situation before it becomes a full-blown emergency.

If aid agencies successfully stop a problem developing into something worse then we're open to criticism of crying wolf. It's easier for an organisation to prove it has dealt with an emergency rather than averted one.

Politicians and policy makers in rich countries are often sceptical of taking preventative action because they think aid agencies are inflating the problem. Developing country governments are embarrassed about being seen as unable to feed their own people. This leads to huge pressures on aid agencies to keep the problem under wraps, and exacerbates their concerns over data, always difficult to collect in unstable countries.

The net effects of all this is that the crisis gets worse, until it becomes so serious journalists can't ignore it. But by the time the crisis is on the evening news it's too late.

What is doubly frustrating is that by acting earlier we could have had far more effect for much less money. The UN estimates that every £1 spent in prevention saves £7 in emergency spending.

While a critical to do, taking early action is a big, difficult choice for cash-strapped African governments. Do they use their limited hard currency to buy grain reserves, or do they spend it on something else and hope for the best?

Aid agencies like Save the Children go to huge lengths to run programmes which help people adapt to drought conditions in the long-term. By planting drought-resistant trees, storing rain when it does fall, and helping local economies by handing out cash grants and food vouchers, we can build the resilience of people who have been brought low by conflict and an historically unprecedented series of droughts.

Save the Children works on a large scale but it can't operate on a national scale. For that, we need proper support from donor countries, and for national governments to plan and pay for disaster prevention schemes.

The horn of Africa is one of those regions on the cutting edge of climate change. Droughts will get more common, but that's no reason to forget the people who live here and call this place home. We already have the techniques which will make this a viable place to live. Now early warning must become early action.

That's the system we want. But as this crisis unfolds we have to work with the system we have. Save the Children have launched a £40 million emergency appeal. The people struggling in to the camps across the horn of Africa have emergency survival techniques to get through the bad times, but not the terrible times we're seeing right now.

We need to provide help now. But we cannot forget that these children are wasting away in a disaster that we could - and should - have prevented.