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.
