If the demand for meat and dairy keeps pace with projections, it would lead to massive growth in livestock production that in turn could generate as much as 70 per cent of the total greenhouse gas emissions by 2050, a new study has claimed.

This will leave little room for all of the other sources of greenhouse gases like transportation and electricity, which now account for more than 80 per cent of emissions, claimed researchers at Dalhousie University in Halifax, Nova Scotia.

Livestock could generate an even greater proportion of the sustainable threshold for other environmental indicators, they said.

"It's sobering," said lead author Nathan Pelletier who did the research with Peter Tyedmers at Dalhousie University.

He emphasised that they were "not suggesting that everyone in the world become vegan or vegetarian". But, "we really stress the importance of policies aimed at production and consumption over time by changing not just how much we eat, but what we eat and how frequently we eat it," he was quoted as saying by the Discovery News.

For their study, the scientists duo considered three aspects of global livestock production: greenhouse gas emissions, biomass consumption and nitrogen emissions.

They looked at estimates of current and future levels for each of these and compared with projections for the earth's limits, beyond which these systems may become dangerously or irreversibly out of whack.

A 2006 UN report has estimated that livestock production in 2000 produced about 14 per cent of global greenhouse gas emissions. Meanwhile, many scientists agree that a global average temperature increase of two degrees Celsius (3.6 degrees Fahrenheit) is the threshold past which serious and irreversible negative impacts of climate change are likely to occur.

Based on that threshold and UN projections for future global livestock demand, the team calculated that livestock would account for around 70 per cent of the greenhouse gas emissions in 2050 that would keep the planet below the two degree threshold.

Similarly, Pelletier and Tyedmers determined that future livestock demand would consume 88 per cent of all of the biomass that humans can safely harvest from the earth's lands.

Humans already are exceeding the third safety threshold, reactive nitrogen production, according to other researchers.

The new study, published in the journal Proceedings of the National Academy of Sciences, projected that this threshold would be surpassed by almost threefold.

"If our models are even close and these sustainability thresholds are right, these technical measures are not going to be enough to allow us to remain within these sustainability thresholds," Pelletier said.