This year, there will be 7 billion people on Earth. But how will the planet will cope with the expanding population – and is there anything we can, or should, do to stop it?

Later this year – on 31 October, to be precise – a boy will be born in a rural village in the Indian state of Uttar Pradesh. His parents will not know it, but his birth will prove to be a considerable landmark for our species as his arrival will mark the moment when the human population reaches 7 billion.

There is no way of knowing for sure, of course, the identity of this baby boy. But demographers say that this date, place and gender are the most likely. India has the largest number of births each year – 27 million, roughly one in five of all global births – and Uttar Pradesh, India's most populous state with nearly 200 million citizens, would be the sixth most-populated country in the world if it were a nation. The majority of the state's births occur in the rural areas and the natural sex ratio at birth favours boys by a narrow margin.

We do not need a guiding star to direct us to the symbolism of this boy's birth: the world has known about this approaching milestone for many years. After all, it is only 12 years since the six billion mark was reached. And just 100 years ago, the human population stood at 1.6 billion. The urgent search for solutions to population growth has been a hot topic ever since the Rev Thomas Malthus published An Essay on the Principle of Population in 1798, stating that the "power of population is indefinitely greater than the power in the earth to produce subsistence for man". Every generation since has seen a prophet predicting doom for our species if we don't curtail our numbers. And yet the rise in headcount has continued inexorably and exponentially.

But with rising greenhouse gas emissions and resource depletion ever-growing concerns, the approach of this year's population landmark has become an awkward, even unwelcome presence in the environmental debate. No one likes to talk about it, for there are no easy answers. Even a mention of it can see the questioner accused of racism, colonialism or misanthropy. Increasingly, environmental thinkers such as Jared Diamond, George Monbiot and Fred Pearce have made the case that population growth is not, in fact, the real problem (the UN predicts that growth will plateau at nine billion around mid-century before slowly starting to fall), rather that a rapid rise in consumption is our most pressing environmental issue. There are more than enough resources to feed the world, they say, even in 2050 when numbers peak – a point made this week by a report jointly published by France's national agricultural and development research agencies. The problem is that we see huge inequities in consumption whereby, for example, the average American has the same carbon footprint as 250 Ethiopians. The French report concluded bluntly that "the rich must stop consuming so much".

Stood shoulder to shoulder, the entire human population could fit within the city limits of Los Angeles. We've got more than enough land upon which to collectively sustain ourselves, we just need to use it more wisely and fairly. But, given the stubborn realities of global inequalities, the question remains: are there too many of us to achieve a sustainable future?

Another report published this week – by the Institution of Mechanical Engineers – provocatively posed just this question in its title: "One planet, too many people?" It concluded the answer was "no", but only if food output was vastly improved through biotechnology, mechanisation, food processing and irrigation. In essence, it said we need to innovate and think our way out of our "population explosion" using technology.

Reducing consumption

Paul Ehrlich, the Bing professor of population studies at Stanford University in California, has been a figurehead of this debate ever since his still highly controversial book The Population Bomb was published in 1968, when the human population stood at 3.5 billion. The book attracted international attention with its stark, Old Testament predictions about how devastating famines would ravage the human populace in the 1970s and 80s and how "all important animal life" in our seas would be made extinct by over-fishing and pollution. Growth must be stopped, he urged, "by compulsion if voluntary methods fail". In 1971, he famously said he would take "even money" on the UK not existing as a state in the year 2000, adding that if it did survive it would be an impoverished island containing 70 million people. Asked about his prediction in 2000, he admitted he would have lost the bet, but added: "If you look closely at England, what can I tell you? They're having all kinds of problems, just like everybody else."

Ehrlich still stands by many of his predictions, but says that the timings were postponed by innovations that he never anticipated. For example, the so-called "green revolution" in agriculture enabled a much more productive global grain harvest than he ever imagined. Could technological innovations facilitate our continued expansion?

"We're already way past the carrying capacity of this planet by a very simple standard," he says. "We are not living on the interest from our natural capital – we are living on the capital itself. The working parts of our life support system are going down the drain at thousands of times the rate that has been the norm over the past millions and millions of years."

You cannot view consumption and population growth as separate issues, says Ehrlich: "In one sense, it is the consumption that damages our life support system as opposed to the actual number of people expanding. But both multiply together."

Reducing consumption is a much easier task, though, than tackling population growth, he says: "What many of my colleagues share with me is the view that we would like to see a gradual decline in population, but a rapid decline in consumption habits. We utterly transformed our consumption habits and patterns of economy in the US between 1941 and 1945, and then back again. If you've got the right incentives, you can change patterns of consumption very rapidly."

So, if you accept the planet is over populated – a big "if" for many observers – what are the solutions? "We have two huge advantages when trying to tackle population growth compared with consumption levels. First, we know what to do about it. If you educate women about their means to control reproduction, the odds are you will see a decline in fertility rates. Second, everyone understands the problem: you can't keep growing the number of people on a finite planet.

"But many economists still want people to consume more to get our economy back, but this will just see more resources destroyed. We also don't have what I'd call 'consumption condoms'. One of my colleagues once joked that the government ought to send round a truck to your home the day after you've been on a spending spree and offer to take everything you've bought back to the store. It would be the equivalent of a consumption morning-after pill."

The seven billion figure is eye-catching, but behind it lies a complicated demographic reality. For example, population growth in developed nations has largely stagnated. Even in places traditionally associated with rapid population growth, such as Bangladesh, birth rates have fallen considerably over the last generation, yet remain well above the natural replenishment rate of just above two children per woman. The only place where birth rates still remain at pre-industrial-age rates – six or more children per woman – is sub-Saharan Africa.

Every region requires its own solution, says Ehrlich. "In the US, where the population has risen by 10% in a decade, largely due to immigration, it is super critical that we tackle the population rise because we are super consumers. But, in general, in the rich countries where population growth has stopped or fallen, we should now be concentrating on reducing per capita consumption levels."

Projected population growth to 2050

Ehrlich says that he is far more pessimistic now than he was when he wrote The Population Bomb. Increased immigration is an inevitability caused by increasing population and it will, he says, "become an ever-increasing political nightmare".

He also laments the lost opportunities: "The only thing we have done which was beneficial – but possibly fatal in the long run – was the 'green revolution'. But technological rabbits pulled out of the hat often have very nasty droppings. Frankly, I don't think most people are even remotely aware of what needs to be done to make our world a pleasant place to live in by, say, 2050."

James Lovelock, the independent scientist who first proposed the Gaia theory, is another prominent environmental thinker who has prescribed a bleak future for the human species if it continues to grow without restriction. He now advises people to "enjoy it while you can" because the outlook for future generations is, he believes, so stark.

"We do keep expecting a crash, as Malthus said, but then technology steps in, or something else, and alters the whole game," he says. "But it has its limit: it doesn't go to infinity. I expect we'll muddle through for the next 50 years, but sooner or later it will catch up with us."

Lovelock believes nations such as the UK now resemble a lifeboat: "In the UK, our population is growing slightly. It's containable. We do grow quite a bit of our food, so we should be OK, provided our climate doesn't change drastically. We could drop our calorie intake without noticing it in health terms. In fact, we'd improve our health like we did in the second world war.

"But I think we should call a halt to all immigration, or encourage people to go abroad. The average American has about 10 times as much land as we do. We're one of the most densely populated places in the world. In some respects, England is one large city. If you want to keep stuffing people in, you'll have to pay the price. I see us as a lifeboat with the person in charge saying: 'We can't take any more, or else we'll all sink.' America, meanwhile, could handle lots more immigration. Not politically, perhaps, but in terms of shared resources and land."

Optimism in Istanbul

Rotating spotlights illuminate the heavy rain clouds above a former coal-fired power station in the heart of Istanbul. Inside, a DJ ups the pace of the music as new arrivals browse the finger buffet. Business cards are being swapped as a speaker calls for hush. A promotional video begins to play.

The Silahtaraga Power Station – now an energy museum on the campus at Istanbul Bilgi University – could be the venue for a product launch anywhere in the world. But the crowd of business leaders, media and government officials has gathered for the launch of a report called Future Agenda: The World in 2020. Commissioned by Vodafone, Future Agenda claims to be the world's largest "open foresight project": an exercise in future-gazing involving 2,000 global participants, including the British Council, Google, Shell, PepsiCo and academics, with the aim of "analysing the crucial themes of the next 10 years".

But, in contrast to the pessimism offered by Lovelock and Ehrlich, there is a sense of pragmatism, even optimism, about both the opportunities and challenges a rising population will bring. And it's no accident that Istanbul has been chosen as the launch city. As continental Europe's only "megacity" – a population greater than 10 million – Istanbul believes itself to be an international beacon of how a city can grow both successfully and rapidly. The Economist has reported that Istanbul, with income growth of 5.5% and employment growth of 7.3% over the last year, is currently the world's "best-performing" city (although credit rating agencies question its status as a borrower).

Dr Tim Jones, the British author of Future Agenda, believes that by 2050, 75% of us will be living in cities. "The major trend we need to grasp is rural-urban migration," he says. "To put it simply, people are largely in the wrong place at the moment. They want to move where they perceive there to be opportunities. This means large cities. Immigration is a difficult political subject at the moment all over the world, but I believe migration will ultimately come to be seen in a positive light as the realisation is finally made that immigrants are a necessity to maintain ageing populations."

Our greatest challenge, says Jones, is to build cities that address the realities of rapid growth: "Sprawl is already being rejected as a deeply inefficient model for growing cities. Hong Kong and Paris are good examples where densities are key to success. They are seen as successful cities. For example, just 5% of Hong Kong's personal income is spent on transportation whereas in Houston it is 20% because everyone drives such huge distances commuting. Paris, with its six- and seven-storey housing, open spaces and street-based cafe culture is a model to aspire to. The Japanese are also role models when it comes to living densities. We must aspire to be like them. For example, we can't let China shoot past Japan and attempt to live like the Americans."

The gap between the world's rich and poor will worsen, he says, but that doesn't mean they will be forced apart geographically. "Residents within the world's megacities are already realising that they are all interdependent. In Mumbai, the rich want the inner-city slums to remain because they want the cheap labour close by. Equally, when slum dwellers have been given land on the outskirts of the city to tempt them away from the inner-city slums, many people have sold the land and moved back to the slum areas because they are closer to the work."

When it comes to consumption levels, Jones says there are already clear innovations emerging that will help to ease this problem. "The need and desire to actually own something is likely to reduce with rental of goods becoming more and more popular. We're already seeing this in cities with car [sharing] schemes such as Zipcar."

The rise of the city will also have a huge impact on geopolitics, predicts Jones, with some megacities wielding far more power than many nation states. "Already we are seeing that the C40 [a group of large cities committed to tackling climate change] is having more impact than the G20. I see far more political action being enacted by city mayors in the future."

Çaglar Keyder, a professor of sociology at Istanbul's Bogaziçi University, agrees that the citizens of Istanbul are very proud of their city's rise, but growth has not come without some problems: "There has been rapid urban regeneration; knocking down shanties and putting people in high rises. Retired, older people are moving further out and the young are moving in, but birth rates are falling. Traffic and environmental pollution is where the growth is most felt."

Standard of living

Carl Haub has been counting the world's people for the last three decades. The Conrad Taeuber chair of population information at the Population Reference Bureau in Washington DC and author of the World Population Data Sheet, an internationally respected annual report that provides population, health and environmental indicators for more than 200 countries, he has near-total recall of the myriad figures that underpin 2011's seven-billion landmark.

"In terms of future growth, everything depends on the birth rates in developing nations," he says. "There is a presumption that the global average will come down to less than two children per woman after 2050, but there are big question marks about this. For example, everyone is pessimistic about sub-Saharan Africa where birth rates overall are not coming down at all. The political situation is key. Both Zimbabwe and Cote d'Ivoire were seen as bright spots by demographers, but now things are much bleaker. Sub-Saharan Africa will double in size by 2050. Nigeria is 158 million now, but will be 326 million by 2050 and will continue rising. Starvation is actually quite rare at the moment in sub-Saharan Africa, but standards of living will continue to fall. And without Aids, there would be 200-300 million more people in Africa by 2050. Many people in the west just don't understand what the standard of living is like in these countries. Some of the consequences are invisible to us in the west, but for how long?"

Migration and ageing are two key, interconnected issues, argues Haub. "China has now got a serious problem with ageing," he says. "I predict they will relax their one-child policy within five years. In Japan, where ageing is a huge problem, they are now having a major nursing crisis with very few young women wanting to be nurses. They're having to import them from Vietnam and the Philippines."

City living does help to suppress birth rates, says Haub. This can clearly be seen in Bangladesh, he says, where rural-to-city migration is driven by desperation. Once in the city, the need to have lots of children to work the land disappears, they become expensive to support and access to family planning is readily available.

But migration inevitably brings with it political tensions. "Terrorism is the curve ball in the immigration debate. Turks and Slavs were tolerated in Germany for a long period, but not now. The chancellor Angela Merkel recently said that assimilation is not working. The birth rate in Germany is very low – about 1.4 children per woman, which is close to demographic suicide – and immigration has maintained the population. In cities such as Frankfurt, where there is a very sizeable Turkish population, there is now a fear of radicalism amid isolated communities. And then there is religion, of course. But, in general, I see this as a decreasing influence when it comes to family planning. In Africa, for example, cultural norms have a much greater impact. Here we see issues such as polygamy and men boasting about how many children they have."

So are there any signs of optimism?

"We have to be realistic," he says. "We are just not going to see fewer people on the planet in the near future. But there are some developing countries where the birth rate is under control. Thailand is seen as the No 1 developing country when it comes to family planning. The birth rate there is 1.8. Indonesia also has a very efficient family-planning system.

"But how far do you go? South Korea has a birth rate of 1.2 and in Taiwan it is 1.0. It is the lowest in the world and means the country is literally dying. If you do want a reduced birth rate, then well organised family planning campaigns are much more important than economic growth. It might be unfashionable to say so, but international aid acts as a catalyst for this. Monetary assistance is key at the beginning to get these campaigns going."

Haub is already thinking ahead to the eighth billion person being born sometime around 2025. "The 20th century saw many things happen that greatly helped to reduce the death rate, such as public-health campaigns, immunisation and provision of clean water. The challenge for the 21st century is different: it's all about managing birth rates."

• Future Agenda paid Leo Hickman's travel expenses to attend its event in Istanbul.