Over the next four decades Africa's progress in human development could outpace any other region of the world, but environmental challenges could threaten the continent's advancement, possibly reversing it, according to a comprehensive report by the United Nations Development Programme (UNDP) released on Wednesday.

The 2011 Human Development Report, Sustainability and Equity: a Better Future for All, says bold steps must be taken to reduce environmental risks and inequalities in Africa and other parts of the world.

The report said sub-Saharan Africa's average Human Development Index (HDI) rating could rise by an estimated 44 percent by 2050 if recent positive regional human development trends are projected forward. The HDI rating is the report's composite measure of health, education and income.

Such advancement for Africans would put many of them into medium or high levels of human development achievement, the report said.

But "environmental challenges" such as climate change effects and more severe water and air pollution could reduce this regional HDI growth by 12 percent by mid-century, the report said. Human development progress could halt or decline by 2050 if there were a more severe "environmental disaster" scenario.

"There is no scientific consensus on the net effects of climate change on precipitation, given different patterns around the world," the report said. "However, some broad regional trends emerge from the climate models. Africa is expected to see higher than average warming - with less rain in North Africa and the southern and western parts of the continent but more rain in East Africa."

Overall, the report showed that sub-Saharan Africa has had the highest average HDI improvement over the past decade of any region in the world.

Countries in the lowest 25 percent of countries ranked - the majority of them African - improved their overall HDI achievement by 82 percent between 1970 and 2010. That was twice the global average.

Among countries with the most important gains were Kenya and Nigeria, where poverty had declined, partly due to improvements in water, sanitation, health and other living standards.

The highest ranked sub-Saharan African countries on the HDI were Mauritius, Gabon, Botswana, Namibia and South Africa. But the continent also has the 10 lowest of the 187 ranked nations and territories worldwide on the HDI: Guinea, Central African Republic, Sierra Leone, Burkina Faso, Liberia, Chad, Mozambique, Burundi, Niger and the Democratic Republic of Congo.