On 28 November, 195 parties and 17,000 delegates will descend on Durban for the latest round of global climate talks.

Africa is the continent most vulnerable to climate change, because of its vastness, its poverty and its diversity. Its people also stand to lose the most because they have the least resources to adapt.

Before the talks, John Vidal embarked on a journey between Africa's two most industrialised countries – South Africa and Egypt. The route included one of Africa's poorest nations (Malawi), its newest (Southern Sudan), its hungriest (Ethiopia), visiting some of its most remote tribes (in Uganda and Kenya), highest mountains (Uganda) and coastal areas (South Africa). All countries are experiencing climate change in different ways, and preparing and adapting at a different pace. But for all, the stakes are high, and the agreement is unanimous: climate change is real and it is happening.

This is the second in a series of articles recording that journey. Read the first part, on Egypt, here.

A thousand miles south of Cairo, Sudan is having another rotten year. To the east, Somalia, much of Ethiopia and the Horn of Africa are experiencing their third or fourth drought in a decade. This one is the deepest in 60 years and has led to millions of people becoming destitute, vast migrations and loss of animals. Sudan has this year been experiencing similar, but less dramatic conditions, with unpredictable rains, late harvests and severe crop losses. Nearly 4m people will need food aid, the harvest next year is expected to be well below average, and farmers are fleeing intense fighting between north and South Sudan troops in the Blue Nile and South Kordofan provinces.

None of this surprises Sumaya Zakieldeen, a researcher at Khartoum University's Institute of Environmental Studies who is on Sudan's delegation to the UN climate talks. She and her team have studied five regions, comparing historical data going back 70 years and found drought and extreme flooding becoming more frequent, temperatures rising in winter, extreme – good and bad – years now more common and rainfall patterns changing. She says that if temperatures continue to rise, as is predicted over the next 50 years, the country can expect more desertification, and more tension between traditionally hostile groups.

Four years ago, UN Environment programme researchers based in Nairobi reported that the conflict in Darfur, which has caused more than 200,000 Sudanese deaths, was being driven by climate change and environmental degradation. With the Sahara desert advancing a mile or more a year in places, and rainfall down 30-40% over just a few decades, the UN study said shifts in climatic patterns threatened to trigger a succession of new wars across Africa.

Access to water is the difference between life and destitution, and climate change, it said, was creating "unavoidable pressure on people through migration, displacement, food insecurity and impoverishment, possibly ending in conflict".

The point was well made. The scale of climate change impacts recorded in northern Darfur is almost unprecedented: the reduction in rainfall has turned millions of hectares of already marginal semi-desert grazing land into desert. This has forced people to migrate and added significantly to the stress on the livelihoods of pastoral societies, forcing them to move south to find pasture.

But the UN interpretation of the undisputed data was challenged as far too simplistic by many academics and Sudan experts. They argued that an ecological interpretation of Sudan's civil war let the Sudanese government off the political hook. Instead, they traced the roots of the Darfur conflict to historical tensions between pastoralists and agriculturalists, the proliferation of arms and the absence of democracy.

The debate goes on, but the steady erosion of natural resources caused by a changing climate is now more or less established as one cause amongst many for the social strife and conflict.

Zakieldeen agrees that when climate change comes on top of other existing problems, and has the ability to trigger or at least exacerbate trends. "Sudan is typical of other least developed countries in Africa in being highly vulnerable to climate change and climate variability. The interaction of multiple stresses – endemic poverty, ecosystem degradation, complex disasters and conflicts, and limited access to capital, markets, infrastructure and technology – have all weakened people's ability to adapt to changes in climate."

Five hundred miles further south in Juba, the capital of South Sudan, the Nile flows slowly past the burgeoning city, which is now experiencing a building boom. Oil provides 90% of the country's income, but water is the key to its survival. Joseph Lual Acuil, the new minister for humanitarian affairs and disaster management recently warned that a food security crisis "caused by the global climatic changes" is looming.

"Climatic changes have affected South Sudan in both floods and droughts. Rainfall that was supposed to have started early this year was delayed. This change in rainfall patterns has led to [the need for] repeated planting," he said.

Acuil is backed by Tipo Nyabeny, a young Sudanese community worker developing a climate adaptation programme with 15,000 people in Ayod county: "The rainfall pattern has changed in South Sudan in the past few years and crops and livestock are threatened by this phenomenon. About 80% of the population here are cattle keepers who lead a pastoralist lifestyle, they travel long distances during dry season to the low lands in search of pasture and water. They say that poor harvests are the main challenge they face."

What happens to Sudan is expected to become more common across sub-Saharan Africa. According to the Africa Partnership Forum, nearly half of Africa will face water stress within 20-30 years. It says: "Three-quarters of African countries are in zones where small reductions in rainfall could cause large declines in river water. Climate models show that 600,000 sq km classified as moderately water-constrained will experience severe water limitations. By 2020, [we expect] between 75m and 250m people to be exposed to an increase of water stress due to climate change."

This week, research sponsored by government-funded international science research group CGIAR concluded that higher temperatures and shifting rainfall patterns will bring further uncertainty and change to river basins in Africa, including the Nile. These changes could significantly alter water flows – presenting a new barrier to nascent efforts to better manage water for food production and to resolve potential cross-border water conflicts.

"Climate change introduces a new element of uncertainty precisely when governments and donors are starting to have more open discussions about sharing water resources and to consider long-term investments in boosting food production," CGIAR said.

• John Vidal will be tweeting from the climate talks in Durban as @john_vidal. His journey was supported by the Guardian, Oxfam, and the African Investigative Journalism Conference at Wits University.