Thursday, 17 January 2013

The good, the bad and the ugly: how do different greenhouse gases compare?

Which greenhouse gas contributes the most to global warming? The bad news, for those who want a simple answer, is that it really depends on how you define the question. 

One oft-quoted statistic is that three quarters of the world's greenhouse gas emissions are made up of carbon dioxide. Another is that in New Zealand almost half our greenhouse gas emissions come from agriculture. And these emissions aren't carbon dioxide - rather, roughly 2/3 are methane and 1/3 are nitrous oxide. 

But what do these figures mean? Does agriculture produce roughly one out of every two kilograms of gas we emit? Or is agriculture ultimately responsible for roughly half our contribution to global warming? 

Somewhat mysteriously, the answer is – neither. There are three important variables to keep in mind when comparing carbon dioxide, methane and nitrous oxide. The first is the quantity in kilograms of gas emitted - more gas causes more warming. The second is how long gas molecules linger in the atmosphere - and this varies significantly across types of gas. The third is how much energy a kilogram of each type of gas traps in the atmosphere per unit time.

The statistics referred to above don't tell us about the number of kilograms of each gas emitted, nor about the total amount of energy that each type of gas will trap in the atmosphere in the long run. Rather, they are calculated by taking the number of kilograms of each gas that we emit, and multiplying it by a "global warming potential" (GWP). A GWP is the amount of energy that one kilogram of that gas traps in the atmosphere over a 100 year period relative to carbon dioxide. For instance methane's GWP is more than 20, which means that in the 100 years after a kilogram of methane is emitted, it will trap over 20 times as much energy in the earth's atmosphere as one kilogram of carbon dioxide will.

The thing about GWPs is that they only look 100 years into the future. If we care only about the short term, then GWP - or an even shorter term measure - might be the best to use. But if we want to consider the impact of gases more than a hundred years forward, then we might think that GWP over- or understates the effect a gas has on warming. How far ahead should we look? The question remains open for debate.
In this video, Dave Frame from the Victoria University of Wellington Climate Change Institute discusses these differences between the three types of gas. The results might be surprising! Note that Dave’s comparisons are per-kilogram, and don’t necessarily reflect the aggregate contribution to global warming of each gas.