The 7 Billion Day is a sobering reminder of our planet's predicament. We are increasing by 10,000 an hour. The median UN forecast is 9.3 billion by 2050, but the range varies by 2.5 billion – the total world population in 1950 – depending on how we work it out.

Every additional person needs food, water and energy, and produces more waste and pollution, so ratchets up our total impact on the planet, and ratchets down everyone else's share – the rich far more than the poor. By definition, total impact and consumption are worked out by measuring the average per person multiplied by the number of people. Thus all environmental (and many economic and social) problems are easier to solve with fewer people, and ultimately impossible with ever more.

Since we passed one billion in 1800, our rising numbers and consumption have already caused climate change, rising sea levels, expanding deserts and the "sixth extinction" of wildlife. Our growth has been largely funded by rapidly depleting natural capital (fossil fuels, minerals, groundwater, soil fertility, forests, fisheries and biodiversity) rather than sustainable natural income. Our global food supply is heavily dependent on cheap oil and water. Yet peak oil means rising prices, while irrigation is quarrying out vital aquifers in many countries.

Thus our population rises at the same time as the number of people Earth can sustain shrinks, while spreading industrialisation and western consumption patterns only accelerate this process.The poor should get richer; but high birth rates, compounded by resource depletion and environmental degradation actively hinder development.

The crunch point is that indefinite population growth is physically impossible on a finite planet – it will certainly stop at some point. Either sooner through fewer births by contraception and (non-coercive) population policy, the "humane" way – or later through more deaths by famine, disease, war, the "natural" way. As Maurice Strong, secretary general of the 1992 Earth Summit put it: "Either we reduce our numbers voluntarily, or nature will do it for us brutally."

Some people, notably George Monbiot, argue that western over-consumption is the sole culprit, so criticising expanding population means "blaming the victims". Of course he is right that our self-indulgent lifestyles are grossly inequitable, and must become much more modest – each additional Briton has the carbon footprint of 22 more Malawians, so the 10 million more UK people the ONS projects for 2033 would equate to 220 million more Malawians. But all poor people aspire to become richer; if they succeed, their numbers will matter immensely.

That is why Population Matters campaigns to stabilise the UK's as well as the global population, effecting a culture shift in favour of smaller families here, while massively increasing the priority and resources for family planning and women's empowerment programmes in developing countries, enabling the 215 million women with an unmet need for contraception to control their own fertility.

Perhaps we can feed 9.3 billion people in 39 years' time – I don't know. We're barely feeding seven billion now. But Norman Borlaug, accepting his Nobel peace prize in 1970 for his "green revolution", said: "I have only bought you a 40-year breathing space to stabilise your populations."

On a finite planet, the optimum population providing the best quality of life for all, is clearly much smaller than the maximum, permitting bare survival. The more we are, the less for each; fewer people mean better lives.

Roger Martin is chair of the charity Population Matters