What worries Bill Gates most? The booming population of Africa looms over his foundation’s latest global survey. By the end of this century there will be 4 billion more people on Earth – and 3 billion of these extra souls will be born in Africa. The challenge, he says, is that “Africa must almost quadruple its agricultural productivity to feed itself. That’s very daunting.” The philanthropist is torn between sending out a message of hope and a message of fear when I meet him at his foundation’s spacious campus in the heart of his hometown, Seattle. He is reaching for what works best to revive the west’s faltering conscience in the face of “America first” nationalism and rising pull-up-the-drawbridge populism in Europe. The spirit of generosity is under assault as government aid budgets come under constant sniper fire from right-wing politicians and their media. Half of the Bill & Melinda Gates Foundation spending goes to Africa. The funds put into the foundation by themselves and fellow philanthropist Warren Buffett now amount to more than than $50bn (£38bn). Until last year Gates, the Microsoft founder, was the world’s richest man. He has now been overtaken by Amazon’s Jeff Bezos. Gates’ first instinct is optimism. Just consider the astonishing story of how far and how fast people have been brought out of abject poverty in a very short time. Since 2000, a billion people have been taken well over the line of $1.90-a-day wretchedness (£1.45), with the same uplift among those previously living on $3.20 a day.

The foundation’s report bursts with remarkable data – too few people know about the galloping progress of humankind. Take India, where only 18 years ago almost one in five children were not enrolled in primary school – now, 97% attend classes. Look at the indicators on the report’s global scorecard for the UN’s sustainable development goals for 2030, and most things are improving almost everywhere. But there is a marked variation in the future trajectory: progress depends on the level of future investment. Today, the west takes some persuading that things are getting better, especially in Europe where countries like Britain have suffered a decade of falling real living standards and eye-watering austerity. The upbeat message of Gates – as well as those in his late friend Hans Rosling’s book Factfulness, and Steven Pinker’s Enlightenment Now – telling us most of the world is on an unprecedented upswing, jars with our reality. Yet it is so: the world has never seen such a rapid rise in prosperity among most of the poor. Eager to encourage western countries to keep giving aid, Gates is well aware that Britain – as elsewhere – suffers frequent political attacks on its aid budget. The UK, Sweden and Norway are among the few reaching the UN aid spending target of 0.7% of gross national income, and all feel the cold blast of an anti-foreigner political grudge. “If you are saving lives for a very small amount of money, people should feel good about that,” Gates says emphatically, protesting at current cynicism about international aid organisations. Look what can be done, he keeps saying, pointing as an example to the brilliant education system in Vietnam, a poor country whose results outstrip far richer ones.

If Britain needs encouragement, he says, “the data about the impact has been amazing. The UK has been very generous to the vaccine fund, for two miraculous vaccines – one for pneumonia, one for diarrhoea.” These sicknesses are now preventable “at extremely low cost to all the children of the world. The UK led that effort and saved over 10 million lives.” Do people across the UK know that? He admits a failure to spread the good news. “We’re being challenged to explain.” Everyone should know this message of hope. “We’re talking about uplifting the human condition in a fairly dramatic way.” But if hope doesn’t beat the new nativism infecting the western world, then fear is Gates’ ammunition of last resort. Ignore Africa at your peril. “The stability of Africa makes a huge difference to the entire world.” Here come the threats: “A pandemic like Ebola can spread very fast,” he warns, and many others spread even faster if there are no local health services to contain them. Migration is the other threat to touch Europe’s politics. Syria was a small country, he reminds us, yet its civil war exodus has “challenged the asylum system”. But watch out because “Africa is another order of magnitude”. The huge African youth boom is a strong theme in the report, where in a world of ageing and shrinking populations, Africa’s demographic bulge could be an asset or a threat. So it would be wise “to make their lives attractive” not just out of “pure human caring”. Invest in Africa’s young, their human capital, health and education, support more productive agriculture, protect subsistence farmers against climate change and see how self-interest blends with good works. Africa, he always stresses, is not one country. Of its 54 nations, many are leaping ahead, Ghana, Botswana and Rwanda among them. Those causing concern are the Democratic Republic of the Congo and Nigeria. On his recent visit he warned Nigerians against the country’s growing inequality, where oil wealth for a few is leaving millions behind. The foundation has spent $1bn so far on Nigeria. He says: “Their health system is worse than poorer countries, their agricultural advice largely broken down.” Government resources are low because “their level of taxation is one of the lowest in the world”. He sounds puzzled: “You would think politicians would compete on the basis of, ‘Hey, I’m going to run a primary health system really well, I’m going to get vaccination coverage up, I’m going to save lives …’ But other issues about religion and ethnicity come to the fore.” Gates is something of a political naif. After all, I point out, look at us in the west. Look at the gross and rising inequality in the US and UK, look at the need for better public services, yet politics is all too often about culture wars and identity issues – Brexit for us, guns and abortion for America. Look at the problem of homelessness, with tent cities on the streets of his rich Seattle, a most liberal city, where only 8% voted Trump. Yes, he’s trying to help the homelessness crisis: he gives £350m a year to US causes.

So we reachthe great blond elephant in the room, President Trump, who has stopped funding family planning organisations that also offer abortions, stalling access to contraception where women need it most. Trump also attacked the foreign aid budget, but “fortunately the Congress, including a lot of Republicans, said no”. How much can Gates influence him? Here, he steps with the caution of a barefoot man crossing broken glass, anxious to say nothing that could further imperil America’s aid budget. “We have to work with Trump himself and the whole administration on ‘What is your vision for Africa?’ And as a human being who cares about human beings, as a country who doesn’t want to go fight foreign wars or deal with pandemics that are out of control …” Did that pitch work? What helped was the famous warning of Trump’s defence secretary, General James Mattis, that soft power – state department diplomacy and aid – prevents the need for hard power: “If you don’t fully fund the state department then I need to buy more ammunition.” Is Trump persuadable? Gates pauses to pace a cautious reply: “Yes, one of the things you can say, a plus or minus, is in very few areas does he have a fixed ideology. If there’s something where he feels he can look smart … particularly if it’s doing things in a different way than was done before, then yes, I think he’s open-minded.” But the truth is that it would be hard to find two mega-wealthy men further apart in their view of the world, their mission or their morality than Trump and Gates. While Trump slashes tax for the rich, Gates constantly calls for the wealthy to be taxed more. “The fact that people aren’t for an estate tax [inheritance tax] is kind of mind-boggling to me.” He says creation of hereditary aristocracies dampens dynamism. “It’s amazing we allow people to have gigantic amounts of money when the state should take more of that.” He wants higher tax on corporate profits too. Famously, he boasts he has paid $6bn in tax, more than anyone on Earth, “and gladly so”. He sounds perplexed by the forces of darkness that fail to see the need for equality at home and globally, that oppose redistribution through taxes and that sneer at the spirit of philanthropy. Surely, he says at the end of our interview, “the improvement in the world is something people should be excited about”.

• The Now generation is a series produced in collaboration with the Bill & Melinda Gates Foundation. You can read more about it here