A sudden and abnormal warming of Pacific waters off Peru has unleashed the deadliest downpours in decades, with landslides and raging rivers sweeping people away, clogging highways and destroying crops. At least 75 people have died, with another 20 reported missing, and more than 70,000 made homeless as the rainy season has dumped 10 times as much rain on Peru than normal.

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A woman is helped to cross a flooded street after the Huaycoloro river burst its banks, sending torrents of mud and water rushing through the streets of Huachipa Guadalupe Pardo/Reuters

A state of emergency has been declared in about half of Peru after flooding and landslides, mostly in the north where rainfall has broken records in several districts. The intense rains, raging rivers, mudslides and flooding being experienced in the country are the worst seen in two decades, Peruvian authorities said.

Even Peru's capital city of Lima, where a desert climate seldom leads to rain, police had to help hundreds of residents cross flooded roads by sending them one-by-one along a rope through choppy waters after a major river overflowed. Some residents left their homes with just a single plastic bag carrying their belongings.

The rains have overwhelmed the drainage system in the cities along Peru's Pacific coast and the health ministry has started fumigating around the pools of water that have formed in the streets to kill mosquitoes that carry diseases like dengue. In the Lambayeque region, 22 inmates at a juvenile detention centre took advantage of the rains to escape. And in the city of Trujillo, rains flooded a cemetery with the waters carrying bones through the streets.

The Andean nation is bracing itself for another month of flooding. A local El Nino phenomenon, the warming of surface sea temperatures in the Pacific, will probably continue along the northern coast at least through April, said Dimitri Gutierrez, a scientist with Peru's El Nino committee.

Coastal El Niños in Peru tend to be preceded by the El Niño phenomenon in the Equatorial Central Pacific, which can trigger flooding and droughts around the world, said Gutierrez. But this year's event in Peru has developed from local conditions. The US weather agency has put the chances of an El Nino developing in the second half of 2017 at 50-55%. Some scientists have said climate change will make El Niños more frequent and intense.

While precipitation in Peru has not exceeded the powerful El Niño of 1998, more rain is falling in shorter periods of time – rapidly filling streets and rivers, said Jorge Chavez, a general tasked with coordinating the government's response. "We've never seen anything like this before," said Chavez. "From one moment to the next, sea temperatures rose and winds that keep precipitation from reaching land subsided."