The gleaming towers of Shanghai belie the Chinese commercial hub's vulnerability to climate change, and the city is spending billions to try to protect itself, but experts say the country's authoritarian system is a hidden weakness, AFP says.

According to a report last year by Climate Central, a US-based research group, the low-lying megacity is, in population terms, the world's most at risk from rising sea levels.

A two degree Celsius increase in global temperatures would inundate land currently lived on by 11.6 million people, it said -- by far the world's highest. A 4 C rise would see that leap to 22.4 million.

The United Nations' Intergovernmental Panel on Climate Change lists Shanghai among the cities in Asia expected to be most vulnerable to coastal flooding by the 2070s.

It is already scrambling to fortify itself against increased rainfall city officials say is outstripping current defences.

"In the past two years we have often seen more than 100 millimetres of rainfall within a single hour, but our city only has the capacity to deal with 36 millimetres," Zhang Zhenyu, the deputy director of the Shanghai Flood Control Headquarters told AFP, as staff pored over weather data.

"Especially this year with global warming, Shanghai's weather has seen a dramatic change."

Work will begin this year on a 40 billion yuan ($6 billion) underground tunnel beneath Shanghai's Suzhou Creek to manage excess rainfall, and 135 kilometres of a more than 500 kilometre long sea wall are to be reinforced.