People in sub-Saharan African can expect to live for 11 years longer than the generation that went before them, new statistics show.

Factors including recovery from HIV epidemics, reductions in child mortality, improved nutrition and access to drinking water have driven life expectancy from 49.7 years in 1990 to 60.7 years in 2017. The number of years children typically spend in school has also increased across the region, growing 14% over the past decade.

The UN’s annual human development index, which ranks 189 countries on areas such as life expectancy, education and standard of living, found substantial global progress since 1990. But researchers warned that the progress made is vulnerable to threats including climate change, conflict and epidemics, as well as worsening living conditions among the middle classes.

While inequality debates have been dominated by commentary about the wealth owned by the richest 1%, said the report’s authors, this focus “risks obscuring another growing concern in inequality – the decline and stagnation in the livelihood conditions of middle-income households.”

The middle classes – defined as the middle 40% – in countries such as Brazil, India and Bangladesh are “the engine of economic growth”, according to Selim Jahan, the human development report’s director.

“[It’s] a big group and that is also a group that tries to balance between bottom and top. It’s a highly educated group, a highly trained group, where most of the entrepreneurship comes,” he said.

Gender inequality was described in the report as “one of the greatest barriers to human development”. The study also warned of major inequality both within and between countries, which the authors said risked fuelling extremism.

Over the past five years, Ireland, Botswana, the Dominican Republic and Turkey have experienced the most rapid growth across various development measures, the researchers found.

Ireland is now ranked the fourth most developed country in the world, behind Norway, Switzerland and Australia. Its position was boosted by higher average incomes: the gross national income per capita in Ireland is $53,754 (£41,199) compared with $39,116 in the UK, which is ranked 14th.

Violent conflict in Syria, Libya and Yemen caused rapid decline in the development of those countries, as did the economic crisis in Venezuela. Niger was at the bottom of the development index, followed by Central African Republic, South Sudan, Chad and Burundi.

Overall progress has slowed over the past decade, the report said, hindered by the global food, financial and economic crises of 2008–09. Limits to further advancement in highly developed countries – where measures such as life expectancy, which is now 79.5 years, cannot grow indefinitely – have also impacted rates of change.

The degradation of the environment and atmosphere, coupled with significant declines in biodiversity, pose a profoundly serious risk to future progress, added the report’s authors, who said the consequences of climate change range from “declining food and water supplies to losses of livelihood and to losses of life from extreme weather events”.

Regionally, south Asia has developed at the fastest rate since the first index was published in 1990, followed by east Asia and the Pacific. Sub-Saharan Africa was also making progress, Jahan added. “That’s ultimately the kind of story that we have not seen so far in some of the earlier reports,” he said.

Successful projects in the region included school feeding programmes in Tanzania and Ghana that have helped to keep children in education, improve child nutrition and boost local food production.

“We know that built into development literature and also in the media we often talk about sub-Saharan Africa as a continent which is not doing so well,” said Jahan. “There are problems, but when you look at the data there are some signs which indicate that there has been good progress on many fronts.”