Nearly a third of the world's economic output, some $44 trillion (32 trillion euros), will by 2025 be in countries at the highest risk of climate change effects, said research published Wednesday.

This would represent a 50 percent increase over today in the share of global GDP (gross domestic product) in high or extreme risk countries, said the assessment by British risk consultancy Maplecroft.

Most of these countries are ill prepared to deal with more severe floods, storms, droughts and sea-level rise likely to result from a warming planet, and the report said much investment is required in flood and other defences to protect infrastructure and assets.

"Adaptive measures... will, however, require the sustained commitment of governments," said a statement from Maplecroft.

The 67 countries at highest risk include economic giants India in 20th place and China at number 61.

Topping the list was Bangladesh, followed by Guinea-Bissau, Sierra Leone, Haiti, South Sudan, Nigeria, the Democratic Republic of Congo, Cambodia, Philippines, Ethiopia, the Central African Republic, Eritrea and Chad.

The United States and much of Europe are in the "low" risk category -- partly because they had more money to spend on adaptation measures.

"Many of the global growth markets are located in countries that are acutely vulnerable to climate change," said the report that measured risk of exposure in 193 countries along with their capacity to adapt.

"The increased spending power of middle-class populations in growth economies is resulting in significant international investment in highly vulnerable regions."

The analysis found that five cities facing a "extreme" climate risk: Dhaka in Bangladesh, Mumbai and Kolkota in India, Manila in the Philippines and Bangkok in Thailand, expected to see their GDP triple from $275 billion to $804 billion by 2025.

"The growing economic importance of cities in developing countries may increase exposure of assets, investments and supply chains to the impacts of climate change," said the report.

"Cities with some of the biggest economic growth potential are among those with the greatest vulnerability to climate change."

London and Paris are the only cities classified as "low" risk in an analysis of 50 cities.

The UN has set a target of limiting global warming to 2.0 degrees Celsius (3.6 deg Fahrenheit) over pre-Industrial Revolution levels in order to avoid its worst effects.

This goal is mainly being targeted by projects to reduce emissions of Earth-warming carbon dioxide (CO2) created through fossil-fuel burning for energy production and transport.

The Maplecroft report said it appeared "increasingly unlikely", as greenhouse gas emissions continue to grow, that the two-degree target could be met -- with potentially devastating effects like species extinctions, water shortages, crop die-offs, loss of land to the rising seas and disease spread.

Currently, there are more than 4.5 billion people, about 64 percent of the global population, living in countries at high or extreme climate change risk, and the figure was expected to exceed five billion by 2025.

The analysis showed south and east Asia and sub-Saharan Africa to be at highest risk.

While Canada and the United States are low risk countries, a breakdown showed several areas in the US as highly vulnerable -- including the coastal regions of Florida, Louisiana, Georgia, South and North Carolina which are exposed to cyclones and storm surge.