Image caption An increasing number of people may not necessarily lead to a similar rise in emissions, the study says

Changing population dynamics could "substantially influence" future greenhouse gas emissions, a study has suggested.

A team of US and Austrian researchers found that urbanisation could increase emissions by up to 25% in some developing nations.

However, industrialised countries could see emissions fall by about 20% as a result of ageing populations.

The findings appear in the Proceedings of the National Academy of Sciences.

In their paper, they also showed that slowing population growth could deliver up to 30% of the cuts deemed necessary by 2050 to prevent dangerous climate change.

"If global population growth slows down, it is not going to solve the climate problem," said lead author Brian O'Neill, a scientist at the US National Center for Atmospheric Research (Ncar).

"But it can make a contribution, especially in the long-term."

According to the UN Population Division, the average annual global growth rate peaked at just over 2.0% between 1965 and 1970.

Since then, it has been steadily falling and currently stands at about 1.1%. By 2050, the UN projects that it will have fallen to an estimated 0.3%.

In contrast, the number of people over the age of 60 is increasing, and the UN predicts that it will almost triple, from 737m in 2009 to in excess of two billion by 2050.

'Not surprising'

Assuming average global per capita consumption will continue to rise over time, [this] will require population to actually decline over time from current levels Simon Ross,, Optimum Population Trust

Until now, most scenarios contained population projections but none had considered the "demographic influences" on emissions, the scientists wrote in their paper.

Although the scientists highlight what they see as the importance of including demographics in emissions scenarios, Dr O'Neill said it was not surprising that it had not been a key concern.

"When you set out to develop emissions scenarios, what you want to focus on are the factors that you anticipate will make the biggest difference," he told BBC News.

"Therefore, most scenarios have focused on alternative economic growth rates and alternative futures in terms of technological development."

Using UN-derived data, they developed a computer model that took into account population, environmental and technological factors, such as:

impact of population growth rates on economic growth rates;

age structure changes on labour supply;

urbanisation on productivity;

and anticipated demographic changes on saving and consumption patterns.

"When a population ages faster (as a result of people living longer and reduced fertility rates), emissions turn out to be less than they otherwise would be," explained Dr O'Neill.

Image caption Some campagin groups fear the planet is struggling to cope with human demands being placed upon it

"Although it is true that older households - for example - don't travel as much, we find that the dominant effect is that older [people] are less likely to be working.

"This reduced contribution to the labour force means that the overall economy grows more slowly. As a result, the overall use of energy within the economy goes down, and emissions go down."

As for the impact of urbanisation, Dr O'Neill added that urban households were, generally, less energy intensive that rural ones.

"The fuel choice, or electricity availability, is pretty much the same in rural areas as it is in urban areas, but what is different is that people maybe live in smaller houses or an apartment in cities, and if they have a car then perhaps they do not drive it as much."

Glow of the city

However, he added, urbanisation was a major source of greenhouse gases when an indirect effect on productivity was considered.

"Overall, we find that when countries urbanise, the labour supply is more productive, meaning that it contributes more to the growth of GDP.

"People are working in sectors that contribute more to economic growth, which increases energy demand, which increases emissions."

The UK-based Optimum Population Trust, a charity that is concerned about the impact of population growth on the environment, believes the Earth is already being stretched beyond its carrying capacity.

It says that the optimum human population - one that can be sustained in the long-term - is closer to three billion people.

Responding to the paper, chief executive Simon Ross said: "We welcome this analysis of the links between global population dynamics and... climate change.

"We believe this paper supports our assertions that reproductive health is an environmental issue, as well as a humanitarian and developmental one," he told BBC News.

However, he said a lower population growth alone would not be enough to prevent dangerous climate change.

"We need a combination of even lower population growth, reduced per capita consumption and better use of technology," Mr Ross observed.

"Assuming average global per capita consumption will continue to rise over time, [this] will require population to actually decline over time from current levels."

Writing in the paper, the scientists - which included researchers from the US National Oceanographic and Atmospheric Administration and Austria-based International Institute for Applied Systems Analysis - said the findings highlighted the importance of understanding population dynamics.

"Greater attention should be given to the implications of urbanisation and ageing, particularly in key regions of the world, including China, India, the US and the EU," they concluded.

"Better modelling of these trends would improve out understanding of the potential range of future energy demand and emissions."