Radical plan to cut CO2 argues that paying for family planning is developing world is the best bet

Consumers in the developed world are to be offered a radical method of offsetting their carbon emissions in an ambitious attempt to tackle climate change - by paying for contraception measures in poorer countries to curb the rapidly growing global population.

The scheme - set up by an organisation backed by Sir David Attenborough, the former diplomat Sir Crispin Tickell and green figureheads such as Jonathon Porritt and James Lovelock - argues that family planning is the most effective way to reduce the likelihood of catastrophic global warming.

Optimum Population Trust (Opt) stresses that birth control will be provided only to those who have no access to it, and only unwanted births would be avoided. Opt estimates that 80 million pregnancies each year are unwanted.

The cost-benefit analysis commissioned by the trust claims that family planning is the cheapest way to reduce carbon emissions. Every £4 spent on contraception, it says, saves one tonne of CO2 being added to global warming, but a similar reduction in emissions would require an £8 investment in tree planting, £15 in wind power, £31 in solar energy and £56 in hybrid vehicle technology.

Calculations based on the trust's figures show the 10 tonnes emitted by a return flight from London to Sydney would be offset by enabling the avoidance of one unwanted birth in a country such as Kenya. Such action not only cuts emissions but reduces the number of people who will fall victim to climate change, it says.

"The scheme, called PopOffsets, understands the connection [between population increase and climate change]," says the trust director Roger Martin. "It offers a practical and sensible response. For the first time ever individuals, companies and organisations will have the opportunity to offset their carbon voluntarily by supporting projects to provide family planning services where there is currently unmet demand."

In papers released with the launch of the offset scheme, the trust claims that reducing CO 2 by 34 gigatonnes would cost about $220bn with family planning, but more than $1tn with low carbon technologies. The 34 gigatonnes is roughly what the world emits in a year, and would be achieved by cutting the projected global population in 2050 by 500 million.

The world's population, presently 6.8 billion, is increasing by nearly 84 million a year. The growth is equivalent to a new country the size of Germany each year, or a city the size of Birmingham every week. It is expected by the UN to peak at about 9 billion people in 2050. By this time, UN scientists say global carbon emissions must have reduced by at least 80% to avoid dangerous rises in temperature, meaning the carbon footprint of each citizen in 2050 will have to be very low.

"The current level of human population growth is unsustainable and places acute pressure on global resources. Human activity is exacerbating global warming, and higher population levels inevitably mean higher emissions and more climate change victims," said Martin.

The giant carbon footprints of developed countries mean prevented births will save far more carbon than those in developing nations.

However, some development groups opposed the plan. "We are keen that any money raised [from offsets] help the poorest who are most vulnerable to climate change. [But] it would be misleading if it was spent in this way. It should go to [immediate] things like disaster risk reduction, food security and water," said Paul Cook, advocacy director of Tearfund, a faith-based development group.

Population control is highly contentious in rich and poor countries alike Some, such as Jonathon Porritt, the former Sustainable Development Commission chair, have said promotion of reproductive health is one of the most progressive forms of intervention. "Had there been no 'one child family' policy in China there would now have been 400 million additional Chinese citizens," he has said.

But other thinkers, such as the Guardian columnist George Monbiot, say global population increase pales into insignificance when compared with the effect of increased consumption and economic growth.