New Zealand accounts for a fraction of the world's greenhouse gas emissions, about 0.17 per cent in 2014.

But, on a per capita basis we have an out-sized carbon footprint, emitting 18 tonnes of greenhouse gasses per person, every year.

That makes New Zealand the 21st biggest per capita contributor to climate change in the world. Amongst the OECD (other rich nations which we benchmark ourselves against) we have the fifth highest per capita emissions.

READ MORE:

* What's the beef with methane?

* Climate change blame game only stalls progress

* Climate change report a wake-up call for NZ farming

That's because New Zealand has a small population (4.8 million) and a lot of cows (10m) and sheep (30m).

Cows and sheep emit methane into the atmosphere and methane is considered 28 times worse for the planet than CO₂. Agriculture is responsible for about half of New Zealand's gross emissions.

Not all greenhouse gasses have the same impact on the atmosphere, so in order to calculate a country's total carbon footprint, the impact of all the different gasses are expressed in terms of their CO₂ equivalent.

One tonne of methane is 28 tonnes of CO₂-equivalent when it comes to calculating New Zealand's greenhouse gas emissions.

Specifically, that means one tonne of methane has as much "global warming potential" over 100 years as 28 tonnes of CO₂.

In the long-run (beyond 100 years) CO₂ persists in the atmosphere many centuries longer than methane and continues contributing to warming. While methane is very damaging in the short-term, it disappears relatively quickly.

GRANT MATTHEW/STUFF Cows are a big contributor to New Zealand's carbon footprint.

But with the deadline for reducing global emissions to avoid "catastrophic climate change" only decades away, it makes sense to heavily weight the short-term impact of gasses like methane.