President Magufuli pulled off an intriguing feat last year when, in a single speech, he managed to affront just about every liberal cause on the planet. The Tanzanian leader told a public rally not to listen to advice from foreigners on contraception because it had “sinister motives”. For good measure, he accused women who use birth control of being “lazy” – it was their duty to have large numbers of children.

By any standards, these were outrageous remarks – and worrying ones, for they indicate there has been a deep and potentially catastrophic failure by the west in promoting a measure on which the future health of our planet depends: limiting numbers of our species. Until this basic task is achieved, virtually every measure we take to tackle global heating will be negated by the energy demands of the extra billions we have added to global populations, say campaigners.

Hence the demonstrations – to be staged by groups such as Britain’s Population Matters and others – in capitals around the world on Thursday when the planet marks World Population Day. This will be the 30th anniversary of the annual event set up by the United Nations in 1989 – when there were a mere 5 billion people on Earth – to focus attention on the urgency of our impending population crisis.

Today there are about 7.7 billion men, women on children on Earth, a staggering figure given that a century ago, there were only 1.9 billion. And although populations have stabilised in many regions, in particular Europe and North America, figures released by the UN this month show global numbers are now growing at the alarming rate of about 100 million every 14 months. By 2050, the Earth’s population will have hit 9.7 billion and it will continue to rise, reaching a figure of about 10.9 billion by 2100.

These are the kind of population numbers we associate with simple organisms swimming in a pond, not those of a big-brained omnivore that requires 3,000 calories a day to survive. If there are 10 billion of us, every forest, valley and piece of land will have to be turned to agriculture to feed us. “Our planet cannot withstand such numbers,” says the palaeontologist Peter Ward in his book The End of Evolution.

Our planet cannot withstand such numbers Peter Ward, palaeontologist

Crucially, the vast majority of the extra 3 billion human beings that could be added to Earth’s population will be born in Africa. Today there are about 1.2 billion Africans. By 2100, there will be more than 4 billion. Our growing population crisis therefore needs to be tackled there as a priority: by boosting women’s rights, by making contraception easily available and by improving education for all. The remarks of President Magufuli suggest that this is going to be a very hard task. Nor have the actions of Donald Trump’s White House helped. By slashing funds to international birth control programmes, the US is now undermining hopes of limiting Africa’s population growth.

We should note that many countries in Africa are likely to become grimly inhospitable when global heating takes its grip on Earth. Millions may be driven from their homes as heatwaves, famines and droughts sweep their lands.

Yet very little of the burning of fossil fuels that has triggered this climate crisis occurred in Africa. It took place in the west, whose industrial emissions are a key cause of global heating. It is a fact we should remember when millions of climate refugees from scorched and overpopulated nations seek salvation on our shores.