Human Influences on Greenhouse Effect: A* understanding for IGCSE Biology 4.12 4.13 4.14

I established in a previous post that the Greenhouse Effect is a good thing for life on the planet.  So what is the problem?  Well the simple idea is that human activities and the massive growth in human populations seen over the past two hundred years have changed the composition of the atmosphere.  The concentration of greenhouse gases has risen and this enhanced greenhouse effect is causing climate change.

The principal gases in the atmosphere responsible for the greenhouse effect are carbon dioxide, methane and water vapour.  Have a look at these two tables taken from the following website from the Center for Climate and Energy solutions:

http://www.c2es.org/facts-figures/main-ghgs

The first image shows the main gases in the atmosphere that contribute to the Greenhouse Effect.

number-1

Anthropogenic means “caused by mankind” and so you can see what humans are doing to generate an enhanced greenhouse effect.  The GWP figure stands for Global Warming Potential and gives a relative value for how each gas might contribute to climate change.  One molecule of CFC-12 is as powerful as a greenhouse agent as 10,900 molecules of carbon dioxide.

number-2

This second table shown above demonstrates how the composition of the atmosphere has changed from pre-indutrial to modern times.   I am going to focus on the two greenhouse gases at the top of the list:  carbon dioxide and methane.

Carbon Dioxide concentrations in the atmosphere

Scientists at the Maunua Loa Observatory in Hawaii have been measuring atmospheric carbon dioxide concentrations since the 1950s.  Here is a graph of their results.

keelingcurve-large

What do you notice about this graph?

  1. There is a gradual upward trend such that the average concentration has risen steadily over the 50 year period.
  2. Within each year, there is an annual peak and an annual trough in the carbon dioxide concentration.  The “peak” corresponds to northern hemisphere winters when there is less photosynthesis by plants and more fossil fuels are burned.  The “trough” is northern hemispere summer when photosynthesis rates are high and so carbon dioxide is removed from the atmosphere

If you want data going back further into the past, you need to look at ice core data.  Tiny volumes of the atmospheric gases are trapped within ice as it forms in Antartica and by drilling out a core and analysing the gases it contains, one can determine the concentration of the atmosphere when the ice was formed.  The deeper parts of the core formed longer ago so a journey through an ice core is like travelling back in time…..

co2_10000_years

What human activities might be responsible for these changes in carbon dioxide?

  1. Deforestation (see my post on this topic)
  2. Combustion of Fossil Fuels (coal, oil, gas)
  3. Cement production

 Methane concentrations in the atmosphere

world_cattle_inventory_vs_human_population_country_1_635794680294204000

Methane is also a potent greenhouse gas. It is produced as waste product of the bacterial reactions that happen inside the rumen and intestines of cattle.  (The rumen is the large first chamber of their stomach in which bacteria digest cellulose in the cow’s food)  I was once told that each cow produces 65 litres of methane a day but I have never measured it myself…….  Seeing as the world population of cattle is estimated at 1.4 billion, that is a lot of methane each day being released into the atmosphere.

Methane is also produced by bacteria that break down our domestic waste in land fill sites and by anaerobic bacteria that live in paddy fields in which rice is grown.    More humans means more cattle, more rice and more landfill and all of these are responsible for the rise in methane concentrations seen in the atmosphere in recent times.

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