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:
The first image shows the main gases in the atmosphere that contribute to the Greenhouse Effect.
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.
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.
What do you notice about this graph?
- There is a gradual upward trend such that the average concentration has risen steadily over the 50 year period.
- 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…..
What human activities might be responsible for these changes in carbon dioxide?
- Deforestation (see my post on this topic)
- Combustion of Fossil Fuels (coal, oil, gas)
- Cement production
Methane concentrations in the atmosphere
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.
Variation within a population is a critical and required component for natural selection. If you have understood all the work on DNA, chromosomes and Mendelian genetics, you should now have a good understanding of where the genetic causes of this variation comes from. But remember that variation can also be caused by the environment. Indeed all variations in reality come from an interplay between genetic and environmental factors. Genes by themselves cannot cause variation as without the environment of a cell to produce the protein, genes alone cannot affect the phenotype.
Genetic causes of variation
Sexual reproduction is the key to genetic variation in a population. Meiosis produces gametes that are haploid and genetically different from each other. The gametes may have a different combination of randomly “shuffled” chromosomes. Crossing over in meiosis also allows alleles that would not otherwise be combined in a gamete. So there are massive genetic differences between one gamete and the next. And then there is random fertilisation so that any one male gamete is equally likely to fuse with any female gamete. Random fertilisation (of gametes that meiosis has made genetically different to each other) is the key to genetic variation in a population.
A final cause of genetic variation has nothing to do with sexual reproduction and is mutation. DNA replication is not 100% accurate – the enzymes in eukaryotes make one error every billion base-pairs. These mutations are random and can lead to new alleles appearing in a population. Chromosomal mutations can also occur where chromosomes do not separate properly in meiosis or parts of a chromosome break off and re-join somewhere else….
Environmental causes of variation
Some of the differences seen in populations are due not to differences in genes but due to the differing environments in which an organism lives. Peas which have inherited two copies of the T allele (for tallness) will never grow tall unless they are planted in well-watered soil and given access to sunlight. You would never be a school teacher as a career without understanding that the environment a brain develops in can affect a person’s outcomes. Environment is as important as genes in many variations in the human population, in particular to do with health and disease. This is why so much emphasis for health for example is placed on promoting balanced diets, altering smoking habits, and moderating alcohol consumption.
Genes and Environment always interact to determine Phenotype
Don’t allow yourself to fall into the lazy thinking of the “nature-nurture” debate. It is lazy thinking because the debate is a nonsense. Nothing is either determined by your genes or your environment – it is always both. So when you read of a ‘gene for obesity’ or a ‘gene for domestic violence’ treat with extreme caution (and switch newspapers……) If you read that playing violent computer games causes violent behaviour in humans, treat with caution. None of these variations in a population will be just due to genes, none will be just do to the environment. It will always be some complex interaction between the two.
This is a topic it is easy to overlook in your revision: water pollution by sewage is hardly glamorous and when you combine it with the limited excitement of learning about fertilisers, you don’t have to be a genius to see why many students leave it out. But examiners seem to like eutrophication so it is worth making sure you are going to score full marks on any question they set.
Starting point in your understanding for this topic should be that what limits plant growth in many circumstances, and often in aquatic ecosystems, is the availability to the plant of nitrate ions. A nitrate ion (NO3-) is essential for the plant to make amino acids, and hence proteins and also DNA. Cells need more DNA and proteins to divide and grow so farmers spray extra nitrate ions onto their fields in fertilisers.
Nitrate ions are very soluble in water so if it rains, they can easily dissolve as the rain water passes through the soil a process known as “leaching”. These nitrate ions can thus end up in freshwater, for example streams and ponds where they have exactly the same effect as in the soil – they cause excessive plant growth. The plants in freshwater are often types of algae and if you have far too much nitrate in the water, this can cause an algal bloom. This excessive growth of plants is called eutrophication (or more properly hypertrophication)
Now the eutrophication story then proceeds like this….
If there is an algal bloom, this can eventually cover the surface of a pond so that light doesn’t penetrate to the multicellular plants that live on the bottom, the beautifully named “bottom-dwellers”. No light for these plants means they can’t photosynthesise and so they die. In the water there will be aerobic bacteria that act as decomposers and so if there is loads of dead material in the water, the populations of these bacterial decomposers will rapidly rise to break down the detritus. These bacteria remove dissolved oxygen efficiently from the water as their numbers go up (remember they are aerobic bacteria), so water quality rapidly falls. The lowering oxygen concentration in the water will itself cause animals from small invertebrates to larger fish to die and so a vicious circle is set up: less oxygen = more dead organisms = more decomposers = less oxygen.
The consequences for a pond in these conditions are very damaging. The complex ecosystem involving producers, consumers, food chains and so on is replaced by one with algae, decomposing bacteria and a few organisms that can survive in anaerobic conditions.
Anything that increases the numbers of decomposers in a freshwater ecosystem can cause eutrophication and the consequences described. The commonest cause is agricultural run off from fertilisers (nitrates/phosphates etc.) but untreated sewage can cause similar consequences. The decomposers need to break down the sewage, releasing nitrate ions into the water that cause eutrophication. The murkiness of the water can also kill bottom-dwellers directly and set the whole cycle off.
Eutrophication is a sequential process and often examiners use this question to test your ability to organise your answer properly. I would always answer a four or five marker on this topic with bullet points rather than a paragraph of text. Why do you think this is?
Now make a summary diagram to show eutrophication and answer a few IGCSE questions from the booklet.