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Hydrographic survey vessel Polarcus in Dunedin in 2015, alongside the Fryatt St wharf in the upper harbour.PHOTO: GERARD O'BRIEN

Environmentalists have caused rumblings in Dunedin recently in protest against plans by Australian giant OMV to explore for natural gas in the Great South Basin.

They say natural gas should be left in the ground, and burning it as a fuel is a retrograde step.

To test their concerns we first need to understand what natural gas actually is.

The process for its creation is similar to other fossil fuels.

Buried plant matter can become compressed over millions of years, and eventually some of it turns into gas and becomes trapped.

Emissions of carbon dioxide are the concern when we burn fuels for electricity as it stays in the atmosphere and traps heat, contributing to a warming climate.

Advocates for natural gas' use as a fuel cite its lower emissions of carbon dioxide relative to the dirtier coal.

Per kilowatt hour of electricity coal emits 1.1kg of carbon dioxide, whereas natural gas emits 0.35kg.

Natural gas is also nearly three times as efficient, as 0.13kg is needed to create 1kWh of electricity, compared with 0.36kg for coal.

University of Otago physics senior lecturer Michael Jack, who is director of its energy management programme, says to understand why we need to look at the chemical makeup of both substances.

Coal is predominantly carbon, so when it is combusted a lot of carbon dioxide is released.

Natural gas is primarily methane, which has one carbon atom and four hydrogen atoms.

Because the energy is contained mostly in the hydrogen bonds, most of what is released when burned is harmless water.

"Just the chemistry of it means it burns less carbon."

The plants for burning natural gas are also generally cleaner than those burning coal, he says.

However, natural gas has its issues.

Natural gas, and hence methane, the same greenhouse gas emitted when cows belch, can leak when being extracted from the ground and during transportation.

Methane is roughly 30 times more potent than carbon dioxide in terms of trapping heat and warming the planet.

To complicate things further there is a side argument in the scientific community as to whether methane should be considered as harmful as it has been, considering it dissipates in the atmosphere much more quickly than carbon dioxide, Dr Jack says.

Rather than comparing natural gas emissions with those of coal, there is worth in seeing how they stack up against renewable energy such as hydro-electric dams and wind farms.

These require no combustion, therefore create no direct emissions of carbon dioxide, making them far better in terms of reducing the impacts of climate change.

Then there comes the complicated issue of distinguishing between chemical emissions of just burning the fuel and life cycle emissions of everything else that goes into creating the electricity, Dr Jack says.

One example was rotting vegetation that collected in reservoirs of hydro-electric dams, which could let off greenhouse gas.

"You have to make a wind turbine, you have to make a hydro dam. Making those things requires fuel and depending on how you make them there could be emissions associated with those."

This is much harder to determine and compare, he says.

Still, there have been studies which attempt the challenge.

The Intergovernmental Panel on Climate Change Life Cycle Assessment states wind and nuclear generate the lowest life cycle emissions per kWh.

Hydro is double that but still low, natural gas is 41 times worse and coal 68 times.

"With all the scientific evidence so far taking into account the life cycle, you get much, much lower with these plants than natural gas. Also the hope is one day we will be able to make these plants with renewable energy."

There is little doubt in the literature natural gas is inferior to renewable energy when it comes to greenhouse emissions.

So then comes the argument, touted by former United States president Barak Obama, that natural gas is useful as a transition fuel while the world pivots to more sustainable methods of power creation.

This idea is not shared by Otago Energy Research Centre co-director Dr Ivan Diaz-Rainey.

The associate professor of finance says firstly this would depend on your starting point as a country.

"If you're China and let's say you have 70% coal generation, or you're Australia a couple of decades ago, it was 80 or 90%, then when switching to gas there are some carbon-reduction gains."

In New Zealand, however, where the majority of electricity generation is renewable, you could not make the same argument.

In either case he fears extracting more gas would lead to less work being done on creating renewable alternatives.

The differences in how natural gas impacts emissions in different countries can be seen in the "dash for gas" of the 1980s and 1990s, when electricity markets across the world were liberalised.

"Prior to that, apart from America, most electricity systems were state-run. Then they introduced markets.

"Markets are good at squeezing efficiencies out and getting the cheapest option. That was gas because it was the cheapest technology and gas prices were relatively cheap."

New Zealand started increasing its gas use from the 1970s, as did Australia.

In the latter it lowered overall emissions.

"However, the dash for gas was disastrous in New Zealand in terms of carbon emissions."

Worldwide, putting effort into natural gas energy seems like a step back, he says.

"Given cost reductions in renewables and the increasing number of storage technologies, the gas as a transition fuel argument looks increasingly flimsy in most countries, but certainly does not have a leg to stand on in NZ."