The Greenhouse Effect is the name given to the way in which the earth’s atmosphere acts to warm up the planet. The earth and the moon gain almost exactly the same incident radiation from the sun and yet average temperature on the earth is stable at around 14 degrees Celsius. On the moon the temperature fluctuates wildly from 1oo degrees Celsius during the day to minus 153 degrees Celsius at night. Life would be impossible in such extreme and variable conditions and so the greenhouse effect is definitely a “good thing” for life on our planet.
How does the greenhouse effect work?
Well the main idea here is that there are certain gases in the atmosphere that can trap the infrared radiation that the earth emits and prevent it escaping the atmosphere. These greenhouse gases are warmed as they absorb the infrared and so the atmosphere heats up.
Remember that because the sun is so hot, it emits radiation at a much higher frequency. This is mostly in the “visible light” part of the spectrum together with some ultraviolet. The gases in the atmosphere cannot trap visible light (air is transparent as you have probably noticed) and so most of the solar radiation passes through the atmosphere and hits the earth.
Which gases can act as greenhouse gases?
The two most prevalent gases in the atmosphere are nitrogen (N2) and oxygen (O2) and neither is able to trap infrared so cannot act as a greenhouse gas. The principle greenhouse gases in the atmosphere are
- carbon dioxide
- water vapour
- nitrous oxide
So what’s the problem?
Well of course the problem is that human activities over the past century or so have altered the composition of the atmosphere so that the concentration of greenhouse gases has risen. This has meant more heat is trapped and climate systems are altered in consequence. This enhanced-greenhouse effect is the problem and I will look at this in the next post…..
Before you can look at the science of climate change and how human activities are causing it, you first need to accept that we are currently undergoing a period in which our climate is changing at an unprecedented and rapid rate.
What would you expect to see in a warming world?
Well the first and most obvious point is that you would expect to see measurable changes in land and sea temperatures.
All three of these graphs represent in different ways the changes in temperature over the past century or so. It is true of course that there have been times in the past when global temperatures were much warmer than they are now but the rate of change seen since the Industrial revolution is totally unique in the 4.6 billion year history of our planet.
The top ten hottest years on record have all happened since 1998.
There is excellent evidence from all over the planet of glaciers shrinking; polar ice caps are shrinking and it may be that the North Pole is free of summer sea ice for the first time in 100,000 years some time in the coming decades. (This is controversial as it is a prediction based on computer models of future climate: different models make different predictions and with a system as chaotic and interwoven as global climate, it can be very difficult to predict)
As the oceans have warmed, this has impacted on extreme weather systems. This graph shows the incidence of North Atlantic Tropical Storms over the past century or so. It has continued to rise since 2007…..
When you look at all this evidence, it is hard to believe that our climate has been stable over the past century. In fact very few people try to dispute the fact of climate change. The dispute is whether human activities are causing this climate change and indeed whether it matters….. I hope in later posts I can convince you the answers to these two questions are yes and yes.
Climate change remains one of the more controversial topics in the IGCSE Biology specification. Just in the past few weeks, the USA (one of the finest nations on the planet) has elected as their President someone who has stated on record that he believes in some giant conspiracy theory about climate science centred around the Chinese….
The overwhelming majority of climate scientists do not support this interpretation of the facts. They are able to provide evidence of rapid climate change over the past few decades and link this to human-induced changes in the composition of the atmosphere due to pollution. When these facts are linked by a sensible scientific theory that proposes how and why certain gases might lead to an increased warming effect in the atmosphere (the so-called greenhouse effect) the evidence in support of human-induced climate change becomes compelling. Keeping US manufacturing competitive is important of course, but not at the expense of the enormous environmental and financial costs of allowing our pollution of the atmosphere to continue unchecked. I am not sure I will be able to convince President-Elect Trump (he probably doesn’t read my blog in any case) but perhaps I can show you the kind of understanding needed to generate A* answers in GCSE questions on this topic…?
I am going to organise this work into several sections and will post on each topic in the coming week….
- What is the evidence for climate change?
- What is the greenhouse effect?
- How are human activities altering the make up of the atmosphere?
- What are the predicted consequences of climate change in the coming years?
The extended topic on “Human Influences on the environment” is one that is well worth revising thoroughly. I haven’t done any analysis of past papers (life is too short) but my hunch is that questions on these topics have been over-represented in the past few years. This first blog post on air pollution is just going to summarise the consequences of pollution of the atmosphere by one gas – sulphur dioxide.
99% of the sulphur dioxide in the air comes from human sources. It is an acidic gas that acts as a pollutant in two ways. Firstly direct on human lungs and airways where it is an irritant and can cause wheezing, tightness in the chest and lead to lung disease. This can be a particular problem for people already suffering with asthma and other diseases of the lung. Sulphur dioxide (along with various oxides of nitrogen) is also a major contributor to the environmental problem of acid rain.
Sulphur dioxide is produced when any fossil fuel containing sulphur is burned. The biggest contributor of this pollutant gas (~70% of the total emissions) is the industrial combustion of coal and natural gas for electricity production. ~20% of total emission comes from other industrial processes and the remainder from burning petrol and other domestic fossil fuels.
The major environmental problem with sulphur dioxide is acid rain.
Sulphur dioxide and various nitrogen oxides are released from coal-fired power stations when coal is burned. These gases can react in the atmosphere under the influence of radiation from the sun and then dissolve in water to form sulphuric and nitric acids. These then cause the rainfall to be excessively acidic and this often occurs long distances from where the pollutants were released.
NB All rainfall is slightly acidic due to carbon dioxide in the atmosphere forming carbonic acid. Acid rain is therefore defined as rainfall (or other precipitation such as snow or hail) with a pH of less than pH5.6.
What are the consequences of acid rain?
As shown in the picture above, there are several biological impacts of acid rain.
Acid rain can cause coniferous trees (e.g. pine trees) to lose their needles (leaves). This will kill the tree as it cannot photosynthesise and so has led to deforestation in certain parts of the world.
Acid rain leaches minerals (e.g calcium and potassium )from the soil more effectively than normal rain. This leaves the soil very low in certain essential minerals and so makes it harder for plants to grow.
The minerals leached can then themselves become pollutants in fresh water. Aluminium ions in fresh water can cause fish to overproduce mucus in their gills. This will kill adult fish as they cannot get enough oxygen into their blood. Fish eggs will not survive in acidic water and many small invertebrates are also killed directly by the acidity. So freshwater ecosystems can collapse.