environmentweb climate change series
What is climate change and why does it happen?
The introduction to this series briefly touched on the topic of what is climate change, and the two possible answers to this question; firstly the more scientific answer that it refers to any alteration the long term weather patterns of the Earth and the second answer, which is in some ways more relevant, that it refers to the current warming trend that has been observed across much of the globe and linked to increases in greenhouse gas concentrations. Obviously the reasons for climate change are going to depend on which definition of climate change we’re using. So let’s begin with the scientific definition and look at some of the examples of climate change that have occurred naturally throughout the Earth’s history and the reasons for them.
Natural climate change
So in this context climate change refers to changes in the long term weather pattern of the Earth. By long term people usually mean about 30 years, which is the period used by the World Meteorological Organisation (WMO) for calculating climate. However as we go further back in time it obviously becomes harder to calculate climate this precisely and it becomes a more general term for average weather conditions in a period of history.
Studies have been carried out looking at climate variability going back hundreds of thousands of years. The diagram below shows some results obtained from the Vostok ice core showing records of temperature, carbon dioxide and dust going back over 400,000 years. While some of the variations might be down to chance, or errors in the methods used to calculate the figures, it’s pretty obvious that some big changes have taken place over time – both in temperature and in carbon dioxide, which is well known for its role as a greenhouse gas. It also seems quite clear that there is a relationship between the two, with periods with high levels of carbon dioxide having high temperatures and vice versa. This explains to some extent the worries about current increases in greenhouse gas levels, as they seem to be so strongly related to temperatures, but also gives rise to some more questions. In particular why have these changes happened? Obviously people weren’t burning fossil fuels and driving round in cars 400,000 years ago, but with all the attention on these sources of emissions it’s easy to forget that there are also many natural sources of carbon emissions. In fact natural sources account for more than 20 times more carbon dioxide than man-made, or anthropogenic, sources. The major sources of carbon dioxide include animals and plants, which create carbon dioxide through respiration, and the ocean which both releases and stores carbon dioxide. Volcanoes also release carbon dioxide when they erupt and although this contributes a relatively small percentage of the total amount of carbon dioxide emitted the suddenness with which it occurs can have a significant impact on climate.
One of the most important causes of long term climate change is the sun. The orbit of the earth around the sun isn’t constant, but follows cycles that take thousands, or even hundreds of thousands of years to complete. It is these cycles, and in particular the eccentricity cycle which lasts about 100,000 years, that are thought to be responsible for many of the long term climate changes that have been observed in ice cores and other climate records. The eccentricity cycle is important as it corresponds well with the series of ice ages that have occurred over the last 400,000 years and can be seen in the Vostok ice core.
Although the sun is the major factor in long term climate change there are also other factors that can influence the climate. One of these is dust in the atmosphere, which is also shown in the Vostok ice core results. High levels of dust in the atmosphere block out sunlight and so result in cooler temperatures than would otherwise be expected, and this relationship can again be seen in the diagram. Sources of dust include volcanic activity and impact from meteors, both of which throw huge quantities of material in to the atmosphere. Because of the nature of these events they are much harder to predict than regular events such as solar cycles, making it difficult to know exactly what impact they could have on climate change in the future.
Further factors include reflectivity of ice sheets, as this reflects solar radiation back in to space resulting in a cooling effect. Clearly cooling is likely to result in more ice sheets creating a feedback effect and this may play an important role in the rapid onset of ice ages. Levels of greenhouse gases in the atmosphere, as discussed earlier, and the quantity and type of clouds in the air also have an impact on climate.
Click image to see enlarged
So from this it’s clear that it’s natural for the climate to be constantly changing. The processes discussed above are only those responsible for major, long term changes in climate. There are many smaller scale processes that are responsible for climate variations over centuries and decades. This leads us in to the second type of climate change...
Anthropogenic (man-made) climate change
Anthropogenic climate change refers to climate change as a result of human activities, in particular global warming as a result of increasing greenhouse gas levels. These include a range of gases but by far the best known and most talked about is carbon dioxide. While carbon dioxide is neither the most damaging or most persistent of the greenhouse gases it is often focused upon because it is produced in such vast quantities by burning fossil fuels and has also been shown to have strong links to past changes in climate, as discussed above. Because of this much of the work on climate change has focused around carbon emissions, either trying to predict future trends and the impacts they will have or trying to find the best ways of reducing or offsetting them. But other gases also have a major impact on climate, in particular methane. Methane is usually produced by decaying matter, so landfill sites are a major source. Another major producer of methane is cows, who produce methane when they digest their food and release it as flatulence. While this may sound slightly ridiculous the quantities of methane released by cows reared for food and dairy are quite significant.
The real worry is that the increase in greenhouse gases caused by humans has resulted in a rapid increase in atmospheric greenhouse gas levels over the last 50 years, as shown in the diagram below. It is thought that while natural mechanisms have the ability to balance out naturally released greenhouse gases with naturally available sinks and storage the additional greenhouse gases produced by humans could throw this balance out and lead to catastrophic changes in the way the climate behaves. Sinks and stores include, for example, the world’s oceans. These absorb vast quantities of carbon dioxide from the air, and so reduce the levels in the atmosphere that cause the greenhouse effect. The resulting increase in greenhouse gases could lead to not only rising temperatures but also changes to weather patterns all over the world. These changes include more pronounced seasons, so winters will be colder and summers hotter, and along with this a greater chance of extreme weather events like floods and droughts.
The final worry is that global warming will actually cause the climate to flip completely and result in the onset of another ice age. This may sound a bit Day After Tomorrow, but it is possible that the melting of polar ice could drastically alter ocean currents and change the climate so severely that the result is a huge drop in temperature until things settle out again (though it probably won’t all happen in a fortnight!).
The last word
The problem with all this is that the mechanisms controlling climate change or so incredibly complex that it’s hard to predict them with much certainty. But the best efforts to date have indicated that we do need to be careful, the climate is more fragile than we might like.
