The world’s human population is growing. More people require more food, space, water, energy and other resources. Rates and even the direction of human population change vary greatly around the world, with the highest rates of growth (4 per cent per year or higher) in some Third World countries, low rates of growth (1 per cent per year or less) in some First World countries such as Italy and Japan, and even population decrease in countries facing major public health crises, such as Russia and Aids-affected African countries.

Although everybody agrees that the world population is increasing, they also agree that its annual percentage rate of increase is not as high as it was a decade or two ago. However, there is still disagreement about whether the world’s population will eventually stabilise at some value above its present level (double the present population?), and, if so, how many years (30, 50?) it will take for population to reach that level, or whether population will continue to grow.

What really counts, though, is not the number of people alone, but their impact on the environment. If most of the world’s six billion people today were in cryogenic storage and not eating, breathing or metabolising, that large population would cause no environmental problems. Instead, our numbers create difficulties in so far as we consume resources and generate wastes. That per capita impact — the resources consumed, and the wastes put out, by each person — varies greatly around the world, being highest in the First World and lowest in the Third World. On average, each citizen of the UK, Western Europe, the US, and Japan consumes 32 times more resources such as fossil fuels, and puts out 32 times more waste, than do inhabitants of the Third World.

But low-impact people are becoming high-impact people for two reasons: rises in living standards in Third World countries whose inhabitants see and covet First World lifestyles; and immigration, both legal and illegal, of Third World inhabitants into the First World, driven by political, economic, and social problems at home. Immigration from low-impact countries is now the main contributor to the increasing populations of Europe and the US. By the same token, the overwhelmingly most important population problem for the world as a whole is not the high rate of population increase in Kenya, Rwanda, and some other poor Third World countries — although much discussed. The biggest problem is the increase in total human impact, as the result of rising Third World living standards, and of Third World individuals moving to the First World and adopting First World living standards.

There are many “optimists” who argue that the world could support double its human population, and who consider only the increase in human numbers and not the average increase in per capita impact. But I have not met anyone who seriously argues that the world could support 12 times its current impact, although an increase of that factor would result from all Third World inhabitants adopting First World living standards. (That factor of 12 is less than the factor of 32 that I just mentioned, because there are already First World inhabitants with high-impact lifestyles, although they are greatly outnumbered by Third World inhabitants). Even if the people of China alone achieved a First World living standard while everyone else’s living standard remained constant, that would double human impact on the world.

People in the Third World aspire to First World living standards. They develop that aspiration through watching television, seeing advertisements for First World consumer products sold in their countries, and observing First World visitors to their countries. Even in the most remote villages and refugee camps today, people know about the outside world. Third World citizens are encouraged in that aspiration by First World and United Nations development agencies, which hold out to them the prospect of achieving their dream if they will only adopt the right policies, like balancing their national budgets, investing in education and infrastructure, and so on.

But no one at the UN or in First World governments is willing to acknowledge the dream’s impossibility: the unsustainability of a world in which the Third World’s large population were to reach and maintain current First World living standards. Nor is it possible for the First World to resolve that dilemma by blocking the Third World’s efforts to catch up: South Korea, Malaysia, Singapore, Hong Kong, Taiwan and Mauritius have already succeeded or are close; China and India are progressing rapidly; and the 15 rich Western European countries making up the European Union have just extended membership to ten poorer countries of Eastern Europe, in effect thereby pledging to help those ten countries catch up.

Even if the human populations of the Third World did not exist, it would be impossible for the First World alone to maintain its present course, because it is not in a steady state but is depleting its own resources as well as those imported from the Third World. At present, it is untenable politically for First World leaders to propose to their own citizens that they lower their living standards, as measured by lower resource consumption and waste production rates. What will happen when it finally dawns on all those people in the Third World that current First World standards are unreachable for them, and that the First World refuses to abandon those standards for itself? Life is full of agonising choices based on trade-offs, but that’s the cruellest trade-off that we shall have to resolve: helping all people to achieve a more equitable standard of living, without undermining that standard through overstressing resources.

As we contemplate these choices, it’s easy to become depressed about our prospects. Nevertheless, I remain cautiously optimistic, for two reasons. One basis for hope is that, realistically, we are not beset by insoluble problems. While we do face big risks, the most serious risks are not ones beyond our control, like a possible collision with an asteroid of a size that hits the Earth every hundred million years or so. Instead, they are ones that we are generating ourselves. Because we are the cause of our environmental problems, we are the ones in control of them, and we can choose or not choose to stop causing them and start solving them. The future is up for grabs, lying in our own hands. We don’t need new technologies to solve our problems; while new technologies can make some contribution, for the most part we “just” need the political will to apply solutions already available. Of course, that’s a big “just”. But many societies did find the necessary political will in the past. Example? Our modern societies have already found the will to solve some of our problems, and to achieve partial solutions to others.

My other cause for hope is a consequence of the globalised modern world’s interconnectedness. Past societies lacked archaeologists and television. While the Easter Islanders were busy deforesting the highlands of their overpopulated island for agricultural plantations in the 1400s, they had no way of knowing that, thousands of miles to the east and west at the same time, Greenland Norse society and the Khmer empire were simultaneously in terminal decline, while the Anasazi had collapsed a few centuries earlier, Classic Maya society a few more centuries before that, and Mycenaean Greece 2,000 years before that. Today, though, we turn on our televisions or pick up our newspapers, and we learn about what happened in Somalia or Afghanistan a few hours earlier. Our television documentaries and books show us in graphic detail why the Easter Islanders, Classic Maya, and other societies collapsed. Thus, we have the opportunity to learn from the mistakes of distant, past peoples. That’s an opportunity that no previous society enjoyed to such a degree. I hope that enough people will choose to profit from that opportunity to make a difference.

Collapse: How Societies Choose to Fail or Survive by Jared Diamond will be published by Allen Lane (£20; offer £16, 0870 1608080).

Jared Diamond will speak at the McDonald Institute, Downing Street, Cambridge, at 7.30pm on Jan 19; the Royal Society, 6-9 Carlton House Terrace, London SW1, at 6.30pm on Jan 20; Foyle’s Bookshop, 113-119 Charing Cross Road, WC2 at 6.30pm on Jan 21