When I was 11 years old, I was forced to become a refugee in my own country, Rwanda. I could see how innocent children and mothers suffered from a conflict they have never started. People died including my own brother. Innocent children were massacred. From then on, I developed a spirit of giving justice to those who are helpless, giving a voice to the voiceless, giving protection to the most vulnerable.
This is not just a humanitarian imperative; it is in all our interests to act. In the globalised 21st Century conflicts are not easily contained by borders. As the Stern Review made clear, tackling climate change will ultimately be cheaper than allowing it to proceed unchecked. But it is the human cost of these crises, the children of Gaza, the homeless Philippines and the South Sudanese families who do not know where their next meal is coming from that really demand our action. The UK public have shown they are up to the task; it is time for world leaders to do likewise.
I know what it's like to lose your childhood to war. When I was five and conflict raged in Sudan, my family and I were amongst the lucky ones to leave for Egypt. Four years later we were granted asylum in the United Kingdom. Inspired by legendary South Sudanese basketball player Manute Bol, my siblings and I took up basketball which helped us fit in. Like Manute, I was lucky enough to turn the sport I loved into a career as a professional NBA player in the United States.
When I was born, 18 years ago, my country did not exist. Sudan and South Sudan were all one big nation, the largest in Africa, but already there was always fighting between the south and the rest. My family left our home in Akobo state, one of the most violent areas and moved to Khartoum, the capital of Sudan, which was much safer.
South Sudan's children are suffering - and the crisis is set to get worse - much worse - in the coming months if more action is not taken urgently. The world's newest nation is on the brink of devastation with a brutal conflict destroying hundreds of thousands of lives and a growing humanitarian emergency putting the entire future of the country and its children in jeopardy. The United Nations Secretary General has predicted that by the end of this year, an incredible half of South Sudan's 12million people will be either in flight, facing starvation, or dead. At least half of these will be children.
Who said children don't vote? Well, they don't cast their votes by standing in long lines and on ballet papers, but they cast their votes from under-funded relief camps and feeding centres and ill equipped schools and hospitals. Every hungry child is a vote of non-confidence on humanity. I hope the donors don't ignore them.
I smelled the decaying bodies in Haiti after the earthquake and I got ill because of the terrible fumes of the dead buried under buildings in Lebanon after the bombings. That was nothing compared to the massacres and violence happening in South Sudan now. I won't go into the details; they're too gory, too much everything.
Life has disappeared from Malakal, a key town in oil-rich Upper Nile state, South Sudan. The clashes between government and opposition forces have turned Malakal, a square grid bordering the river Nile, into a ghost town. But some people didn't manage to escape - they were forced to witness the horror.
I have been an aid worker for over 25 years. In that time I've witnessed and experienced events and horrors that are beyond most people's imaginations. But I don't think I have ever felt the mixture of emotions that hit me recently as I boarded a tiny plane to take me out of the city of Malakal in South Sudan, back to the relative safety of the capital Juba.
Ordinary South Sudan citizens have been extraordinarily affected by the violent events of the past weeks. The destruction of hospitals and markets, as well as the increased pressure on host communities due to mass displacement, brings me to this conclusion: South Sudan will face a humanitarian emergency for the months to come, and its people will need all the help they can get.
It is a tactic beloved of despots: while the world's attention is on one bloody conflict, you can slaughter with impunity elsewhere... Since mid-December the media has watched as the world's newest nation, South Sudan, has torn itself apart. Meanwhile, its old oppressor next door in Sudan is enthusiastically grasping the chance to "end" its own troublesome "rebellion".