Today, a government deciding where to install infrastructure to avoid urban flooding would logically favour a wealthy district.
Natural disasters drive at least 26 million people into poverty each year and cause more than $500 billion in lost consumption, far exceeding the value of damage to property alone, according to a World Bank report released Monday.
Those numbers will be driven up in the coming decades as climate change amplifies the destructive power of cyclones, flooding and drought, said the report, released on the margins of high-level UN climate talks in Marrakesh.
Up to now, global calculations of the damage wrought by Nature on communities have not adequately taken into account disparities in wealth, according to the 190-page report, entitled "Unbreakable: Building the resilience of the poor in the face of natural disasters".
The new approach has huge implications for how and where to best spend money to make cities and rural areas more resilient to such shocks.
"One dollar in losses does not mean the same thing to a rich person as a poor person," said lead author Stephane Hallegatte.
"The same loss affects poor and marginalised people far more because their livelihoods depend on few assets, and their consumption is closer to subsistence level."
Today, a government deciding where to install infrastructure to avoid urban flooding would logically favour a wealthy district that suffered $20 million of property damage over a poor one where asset losses totalled $10 million.
'Gains in well-being'
But the calculation changes as soon as the often long-lasting human misery left in the wake of flooding in a slum area is factored in.
Building dikes and drainage systems in the poorer area "would generate lower gains in avoided assets loss, but larger gains in well-being," the report said.
The true cost of natural disasters have been vastly underestimated, it concluded.
A recent UN study of 117 countries, both rich and developing, estimated total global asset losses from natural disasters at $327 billion (304 billion euros) a year.
But if lost consumption -- when medicine or schooling for example that was barely within reach before becomes unaffordable -- is included, the bill totals about $520 billion annually, the World Bank found.
Based on a global survey of 1.2 million people in 89 nations, the report also showed that 26 million people fall below the income threshold of $1.9 (1.75 euros) a day, a widely accepted measure of poverty.
"This is surely a conservative figure," Hallegatte told AFP.
Myanmar's Cyclone Nargis in 2008, which claimed some 140,000 lives, caused some four billion dollars in damage, according to the UN.
But it also forced up to half of the country's poor farmers to sell off land and other assets to relieve debt following the cyclone, pushing them deeper and more irretrievably into hardship -- making the true cost much higher.
The most deadly disasters since the start of the century -- Nargis, the Indian ocean tsunami of 2004, earthquakes in China and Haiti -- have not been caused by extreme weather events.
As climate change kicks in, however, the destructive power of nature will increase, scientists say.
"Severe climate shocks threaten to roll back decades of progress against poverty," World Bank Group President Jim Yong Kim said in a statement.
"Building resilience to disasters not only makes economic sense, it is a moral imperative."