Climate change, the threat of extreme rainfall and the lack of city planning mean the city government needs to relocate more than 30,000 people living on the top of the Jarillon, a 23-kilometre dike that protects Cali from the river’s powerful waters. Decades of people living atop the structure have weakened it to the point that officials worry intense rains could cause the dike to break. Cali is just a few metres above the river level. If the dike fails one million people would be affected, homes and roads in eastern Cali would be flooded and 80 per cent of the city would be left without potable water.

Cali Mayor Maurice Armitage says the evictions are crucial in order to protect residents from the river. Alejandro Gomez

Authorities estimate such a flood would cost more than $2 billion (U.S.) and the city would take more than 20 years to recover.

“If the Jarillon of the Cauca River breaks today, I have no idea what will happen,” says Maurice Armitage, Cali’s mayor. “It would be a catastrophe of such magnitude that Cali surely isn’t ready for. It would be worse than (hurricane) Katrina in the United States.”

In 2011, Colombia suffered the worst rainy season in its history, affecting three million Colombians, uprooting thousands from their homes and destroying 3.5 million hectares of farmland. Cali was one of the worst hit regions. According to a United Nations report, climate change is making this kind of extreme rainfall more frequent, raising the risk of prolonged inundation in regions of the country that would not normally flood.

Cali has not yet seen another rainy season as intense as six years ago, but the threat has authorities concerned. This March, a landslide caused by rainfall killed 300 and destroyed dozens of neighbourhoods in the city of Mocoa, about 400 kilometres south of Cali.

Former IDEAM director Ricardo Lozano says the plan to reinforce of the dike does not take into account the Cauca River’s ecological structure.

These changes show how vulnerable Colombian cities are to climate change, which has caused temperatures to rise at a rate of around 0.1C per year since 1951, according to the Intergovernmental Panel on Climate Change. “When temperature changes, air currents change, and if air currents change, the whole rainfall regime changes,” says Ricardo Lozano, the director of IDEAM, Colombia’s leading environmental investigation institute. As climate change has intensified Colombia’s rain patterns, the effects of El Nino and La Nina, Pacific Ocean climate cycles that alter global weather, have worsened.

Last year, Colombia endured heavy droughts caused by the second strongest El Nino in 50 years. The fear now is whether the next La Nina will cause an intense rainy season, similar to 2011.

That year, the Colombian government created a fund for large-scale projects to protect several parts of the country against climate change-associated floods and landslides. The plan to fix the Jarillon is one these projects.