(Gerard Wynn is a Reuters market analyst. The views expressed are his own.)
By Gerard Wynn
LONDON Oct 14 Voluntary curbs on population through improved education could ease poverty and environmental pressures including carbon emissions but is ignored politically as the world passes 7 billion people because of long-standing taboos.
Discussion of fertility rates is unpalatable to religious institutions, for example opposed to contraception, and is often viewed as a private matter by conservative governments.
A link between choice of family size and environmental limits, meanwhile, has lost favour since the 1970s after peak oil and food concerns faded.
But as carbon emissions soar it may be time to review a connection with an increasingly urgent climate problem caused by burning fossil fuels and converting forests for food and energy.
The world population this month hits 7 billion, according to the United Nations Population Fund, up from 2.5 billion in 1950 and is forecast to reach about 9 billion by 2050.
Columbia University economist Jeffrey Sachs has suggested a cooperative world effort to limit growth to 8 billion through access to education, family planning and healthcare, and avoid a possible "calamity" of poverty, hunger and conflict for example in Africa where population alone is expected to rise by more than 1 billion extra people by 2050.
One narrow way to quantify the cost and benefits of a achieving a smaller population is through its impact on carbon emissions.
Global carbon dioxide emissions from burning fossil fuels will this year hit about 34 billion tonnes.
A recent study calculated that annual emissions should be no more than about 10 billion tonnes CO2 in 2050 to limit global average warming to below 2 degrees Celsius above pre-industrial levels.
Fossil fuel emissions of about 40 billion tonnes CO2 on the other hand would imply about 2.5 degrees warming, it estimated.
See study: link.reuters.com/cas44s
Such numbers are uncertain, depending on cumulative emissions in previous years and given an inexact link between emissions levels and resulting temperature rises.
Historical records suggest sea levels may have been more than 20 metres higher in a 1 degree hotter world 5 million years ago, according to climate scientist NASA's James Hansen. Such changes if they happened now would occur over centuries and millennia, as ice sheets melted.
See study: link.reuters.com/fas44s
Applying the above emissions limits in 2050, a population of eight rather than nine billion would emit 1.1-4.4 billion fewer tonnes of CO2, all else being equal.
That figure could be revised down to account for the fact that population is growing fastest among the world's poorest who consume least fossil fuels. On the other hand, feeding them will inevitably involve biting into marginal land including forests. Accordingly, we might conservatively halve the benefit of avoided emissions to 0.6-2.2 billion tonnes.
Regarding a financial benefit of avoided CO2 emissions, present carbon prices in the European Union's emissions trading scheme of 10.3 euros imply avoided costs of $9-32 billion.
It's impossible to guess what the real cost of cutting carbon emissions might be in 2050, although it may be expected to be higher than today if economic growth continued, driving up industrial output and pollution.
Columbia University's Sachs calculated the cost of a "bold population policy" at less than 0.1 percent of the annual income of rich countries, implying a figure under $100 billion.
Insofar as they are addressing climate change, however, governments have favoured a technology or market approach whether U.S. support for advanced solar power or the EU's carbon market.
No direct comparisons have been made with population growth.
Seven years ago Princeton University's Stephen Pacala and Robert Socolow coined the idea of "wedges" of effort to curb carbon emissions, divided between a range of low-carbon technologies and lifestyle changes, such as replacing coal with cleaner natural gas, and driving less.
Each wedge would cut emissions by 3.7 billion tonnes CO2 annually by 2054, they said, a figure in the same ball park as the rough estimate above of the impact of limiting population.
In a subsequent review of the concept Pacala and Socolow mentioned that limiting world population growth to 8 billion could be a possible "wedge".
The wedges were uncosted, but included (with added, basic estimates for present capital cost): 800 gigawatts of carbon capture with coal ($800 billion); an extra 700 GW of nuclear power ($3,500 billion); 2,000 GW of solar power ($6,000 billion).
See paper: link.reuters.com/mas44s
These costs are one or two orders of magnitude greater than Sach's estimate of a population policy.
The notion of battling a rising population has little or no political resonance with the exception of China, which has said that its one-child policy avoided 300 million births, and added that implied about 1.3 billion fewer tonnes of CO2 emissions annually in 2005.
China's direct, top-down approach would be impossible in liberal democracies, where voluntary efforts are also few, however. The main multilateral initiative is the U.N. Population Fund which has far lesser profile than U.N. agencies in food and climate.
Few would doubt that advances in technology are central to limit environmental and social pressures from population growth.
However, back of the envelope maths suggests that limiting world population could have an important climate bonus at low cost, not including the benefits of empowering women to complete their education and have better off children.
It makes sense at least therefore to review the issue formally in multilateral for including the G20 or U.N. climate talks, where it's largely ignored. ($1 = 0.730 Euros) (Reporting by Gerard Wynn)