Thwarting death – and particularly child mortality – has been one of the greatest human achievements of the last 200 years. Average life expectancy across the world has risen from 47 to 68 in the last 60 years alone. Growing old remains a privilege, though. Citizens of richer countries still live around ten years longer than those in poorer ones.

Unfortunately, lowering birth rates is not so prestigious and attracts significantly less investment. Although family size has dropped from a global average of over five children in 1950 to just 2.6 today, we are still on course for a world population of 10.5 billion by 2050. According to many ecologists, it would be impossible for the earth to sustain so many. Already, at 6.8 million, we are over-consuming and over-polluting to a dangerous degree.

There are plenty of incentives for politicians to hasten an 'ecological demographic transition' – in which birth rates come down to equal death rates, and populations stabilise at a lower level. As solutions to climate change and resource shortages go, this is one of the easiest and cheapest available – and would bring many additional benefits to women, children and economies.

When rich countries discuss demographics, a commonly voiced fear is the cost of an ageing population. But, as Adair Turner, former Chair of the UK Pensions Commission, has calculated, raising children represents a far greater cost to families and to the state than caring for the frail and dying.

Globally, up to 40% of pregnancies are unplanned, revealing a huge unmet need for contraception and family planning services. Just by filling this gap, we would have a chance of bringing world population down to the lower UN projection of 8 billion by 2050.

In the poorest countries, where the average family size is over four children, women need the most help to plan their families and be confident that the children they do have will thrive. Improved access to contraceptives and advice in Iran, Thailand and Rwanda, for example, has been welcomed greatly by women – and has proved very successful at reducing growth rates. For determined governments, distance is no obstacle to making effective interventions. In remote upland regions of Mexico, contraceptives arrive in the villages in panniers carried by donkeys – with medical supplies in the same load.

Yet in the UK, where over a third of pregnancies are unplanned, Primary Care Trusts are failing to provide adequate services.

Currently, rich countries are alone in showing a rise in average family size. With a disproportionate impact on the environment, this puts them on fragile ecological, moral and diplomatic territory. A child born in Europe accounts for 11 times more greenhouse gas emissions than one born in Africa; for North America, the figure is 24 times. (These do not include 'offshore' emissions embedded in imports.)

Ultimately, it is clear that, with increasing pressure on key resources, such as land, food and water, everywhere, planning our family is a personal responsibility, as well as a global dilemma.

• Forum for the Future's new report, Growing Pains, warns that the UK will struggle to meet the needs of a population officially projected to reach 70 million by 2030, and calls on politicians to start planning now.