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​The Greenhouse Effect

Greenhouse gases (e.g. carbon dioxide, methane, nitrous oxide and water vapour) are present in our atmosphere and are a critical component of our climate, as they allow energy from the sun to reach the Earth’s surface, yet stop some of the energy released from Earth escaping into space.  As such, greenhouse gases effectively trap heat within the Earth-atmosphere system, a process called the ‘greenhouse effect’.  The greenhouse effect occurs naturally and is necessary for life as we know it. The mean air temperature across the Earth’s surface is 14°C.  In the absence of a greenhouse effect, it would be a frigid -18°C.

 

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Greenhouse gases such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N20) allow incoming shortwave radiation from the sun to pass through the atmosphere, but trap outgoing longwave radiation.  This results in the capture of energy (heat) which would otherwise be emitted to space - a process called the greenhouse effect  

​Clearly then, the greenhouse effect is a good thing.  The issue lies in human activity - predominantly the burning of fossil fuels - which is pumping huge amounts of greenhouse gases into our atmosphere, giving rise to an enhanced greenhouse effect.  As greenhouse gas concentrations increase, an increasing amount of heat is trapped by the atmosphere, ultimately resulting in our warming climate.

The IPCC AR5 states that atmospheric concentrations of carbon dioxide, methane and nitrous oxide had all increased since 1750 due to human activity.  In 2011, the concentrations of these greenhouse gases exceed pre-industrial levels by about 40%, 150% and 20% respectively.  NIWA has the longest-running continuous measurements of carbon dioxide in the southern hemisphere.  Since their measurements began, the combustion of fossil fuels has roughly doubled.  Not only has the concentration of carbon dioxide risen in that time, the growth rate has accelerated. The annual mean growth rate in atmospheric carbon dioxide measured by NIWA has reached over two parts per million seven times since records began; five of these peaks occurred in the past decade. 

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 NIWA’s carbon dioxide measurements obtained at Baring Head, Wellington.
 

Methane is about 23 times more powerful than carbon dioxide as a greenhouse gas.  From around the years 2000 to 2006, atmospheric methane concentrations flattened out.  The causes remain unclear but could include global deforestation, fewer gas pipeline leaks in former Soviet countries, and drying wetlands (due to high temperatures and draining).  Nevertheless, overall methane concentrations have increased steadily since 2007.2007 MethanatBaringHead.png

NIWA’s methane measurements obtained at Baring Head, Wellington.
 
 

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