5.3 Global warming
Media attention has been such that it would be hard to have missed the fact that global warming is considered to be a ‘bad thing’. Why should this be so? What is so wrong with being a bit warmer? Anyway, is global warming really occurring and, if it is, what are the causal factors responsible for it?
Let us deal with this last question first. As we sit on a beach in summer, or in a sunny window seat in winter, we are aware of the Earth being warmed by the Sun. In fact the Earth is warmer than would be expected from the amount of solar radiation that it receives, because heat is trapped by various gases in the atmosphere. Provided that the solar radiation remains constant and the composition of the atmosphere is unaltered, these so-called ‘greenhouse gases’ maintain a surface temperature hospitable to life on Earth. But it is being suggested that the composition of the atmosphere is altering, and that this is leading to global warming. How can we find out if this is the case?
In the 1950s the physicist Charles Keeling developed the instrumentation to measure atmospheric carbon dioxide, one of the most important of the greenhouse gases. Since then continuous records have been kept by the Mauna Loa Climate Observatory in Hawaii. A portion of that record is reproduced below as Figure 7, which shows a regular pattern of fluctuations in carbon dioxide concentration.
How often do peaks occur? (Hint: You will find it easier to work this out if you take a small chunk of the graph, say from 1962–1966, and count the number of peaks over that time period.)
Between 1962 and 1966 there are four peaks, evenly spaced. This means that peaks occur annually.
The peaks occur in the Northern Hemisphere spring. In other words, during spring the amount of carbon dioxide in the atmosphere starts to fall.
Figure 8 shows the ways in which carbon dioxide enters and leaves the atmosphere. Use this to list the ways in which carbon dioxide leaves the atmosphere, and then decide what might be responsible for the drop in atmospheric carbon dioxide over the spring and summer period.
Carbon dioxide passes from the atmosphere into the oceans.
Carbon dioxide is taken up by plants for photosynthesis.
In spring and summer, plants grow more vigorously than they do in autumn and winter. Thus their rate of photosynthesis increases and so more carbon dioxide is taken from the atmosphere. Consequently, this biological activity is likely to be responsible for the drop in atmospheric carbon dioxide over the spring and summer period.
You might have wondered whether oceans hold more carbon dioxide as their temperature rises. If you have noticed that ‘fizzy’ drinks are described as being ‘carbonated’ then you will know the answer! They don't! Warm, fizzy drinks gush out of their containers once opened; the ‘fizz’ is bubbles of carbon dioxide. In spring and summer, the oceans warm slightly and thus hold marginally less carbon dioxide than they do in winter. This would tend to counteract the effect of increased photosynthesis, making the annual fluctuations less dramatic.
Look again at Figure 7, at the 30-year period from 1958–1988. Does this show an increase, decrease or no change in the amount of carbon dioxide in the atmosphere?
Over the 30-year period there has been an increase in the amount of carbon dioxide in the atmosphere.
In fact, the average annual concentration was 315 parts per million (p.p.m.) in 1958 and had risen to 350 p.p.m. by 1988. (Expressing these concentrations in p.p.m. is just a bit neater than saying that the composition of the atmosphere has changed from 0.0315% to 0.0350% carbon dioxide.)
What is the percentage increase in atmospheric carbon dioxide concentration between 1958 and 1988?
The increase was by 35 p.p.m., which is 1 35/315×100=11%.
Having seen that there is an annual fluctuation in the concentration of carbon dioxide in the atmosphere, it is reasonable to ask whether the recent rise in atmospheric carbon dioxide levels is part of a longer-term pattern or cycle of fluctuations. In order to answer this question, we would need to find ways of estimating past levels of atmospheric carbon dioxide. Somewhat surprisingly, it is possible to do this. A historical record of atmospheric composition can be found in polar ice. As snow is added to ice sheets, air is trapped in pores and isolated from the atmosphere. In this way samples of the Earth's atmosphere have been preserved in frozen layers.
Figure 9 shows atmospheric carbon dioxide concentration for the past 160,000 years calculated from samples of a core of ice over two kilometres (km) deep, taken from Vostock, Antarctica. (Imagine this calculated from samples of a core of ice over two kilometres (km) deep, taken from Vostock, Antarctica. (Imagine the equipment you need to extract ice from that depth without damage!) The information is not very precise, because ice pores undoubtedly remained in contact with the atmosphere for variable amounts of time, probably between 10 and 11,000 years, after snowfall. In addition, the age of the piece of core being analysed has to be determined by estimating ice flow mechanisms and the rate of ice accumulation. Nevertheless, we can see that atmospheric carbon dioxide concentration has twice risen from about 190 p.p.m. to around 280 p.p.m.
How much of an increase does this represent?
The increase was by 90 p.p.m.
Over what time period has this change taken place (approximately)?
So, as can be seen from Figure 9, there have been marked fluctuations in levels of atmospheric carbon dioxide in the past.
Do you think the current increase in atmospheric carbon dioxide levels is part of this natural variation? (Hint: Compare the results from your last two calculations.)
If you think not, you are in agreement with most climatologists. The changes in the chemical composition of the atmosphere seem to be taking place far more rapidly today. (An increase of 11% in carbon dioxide concentration over the past 30 years, compared to a 47% increase over about 10,000 years.)
In fact, using other ice core data shown in Figure 10, we believe that this accelerating increase in levels of atmospheric carbon dioxide started around the time of the first industrial activities some 200 years ago.
Use Figure 8 to suggest the human activities that have led to an increased release of carbon dioxide into the atmosphere.
Burning fossil fuels, burning forests and ploughing.
When the annual global output of carbon dioxide from the burning of fossil fuels is calculated, it is discovered that only about half of it contributes to the increase in the atmosphere. Where is the ‘missing half’? A small amount may be used by plants growing faster, but the majority seems to dissolve in the oceans. The ability of the oceans to be flexible in the amount they absorb depends, in the first instance, upon a relationship between the ocean's plant life (the phytoplankton) and movement of water between the warmer surface layers, where the plants must live to receive enough light for photosynthesis, and the colder layers that are rich in the nutrients that the plants need and which must, therefore, be circulated to the surface. A disturbing suggestion made in 1994, was that warming of the oceans could lead to reduced mixing of these layers and hence reduced growth of phytoplankton.
What effect would this have on the numbers of animal plankton (zooplankton)?
The numbers would decrease because the phytoplankton are their food source.
In 1995, a report from Dean Roemmich and John McGowan of the Scripps Institute of Oceanography noted an 80% decrease in zooplankton since 1951, together with a 1.51°C increase in ocean surface temperatures in the Pacific Ocean off the coast of California. It seems that the ocean's ability to absorb carbon dioxide may already be compromised.
Suggest another reason to be concerned about the decrease in zooplankton numbers.
Through the workings of food webs there will be a knock-on effect on the productivity of the oceans.
Table 1 showed that the NPP of oceans is quite low. However, the oceans cover 70% of the Earth's surface and fishing is vital to the economy of some people.
In this section we have looked at the stability of the carbon dioxide content of the atmosphere and seen that fluctuations occur naturally. The subtleties of the control of the levels of atmospheric carbon dioxide are clearly very complex and a long way from being fully understood. We need to know how much more carbon dioxide can enter the atmosphere before triggering catastrophic climate change. The only hope of reaching an understanding is if scientists from different disciplines continue to work together on programmes such as the International Geosphere– Biosphere Programme (IGBP) established in 1986 and described by the Intergovernmental Panel on Climate Change (IPCC) as
… an inter-disciplinary research initiative … to understand the interactive physical, chemical and biological processes that regulate the total Earth system, … including those which control the concentration of carbon dioxide and other chemicals in the atmosphere.
By the mid-1990s it became clear that, whilst we cannot make accurate predictions of how far or how fast global warming is proceeding, climatic change is definitely occurring and is brought about by human activities. Furthermore, the predicted effects on human health are universally negative. They range from fairly direct effects such as the increase in the number of deaths from cardiovascular problems which occurs during heat waves, particularly when the temperature soars above 30°C, to the loss of life, homes and agricultural land that would result from flooding when the ocean levels rise.
To some extent, loss of life in very hot spells would be offset by fewer deaths in very cold spells. There is the possibility of adapting behaviour to cope with climatic change, particularly for those people who live in the EDCs. Routines could be changed to reduce the likelihood of overheating, and homes and offices could be air-conditioned. This assumes that the major climatic problem associated with global warming will be increased temperatures. There are models that predict that global warming will bring more episodes of violent weather changes such as hurricanes and prolonged monsoon seasons. This would wreak havoc over short timespans and it might not be possible to evacuate people, particularly from densely inhabited areas in ELDCs. As a result, there could be widespread loss of life followed by epidemics of the kinds of diseases that flourish when urban infrastructures collapse such as cholera. A similar scenario could be brought about by a different mechanism, namely as a result of flooding following a rise in the ocean level caused by a combination of thermal expansion of water and melting of polar ice. Some of the world's most densely populated areas and most productive farmlands lie along the coasts and rivers that would be the first to be affected by any such rises. In this latter scenario, there are even longer-term effects as productive land remained submerged, perhaps lost for ever. Food shortages would be added to the difficulties faced by the displaced populations. Although a climatic shift would make huge tracts of land in Canada and Siberia suitable for grain production, there are concerns that the soils would not be sufficiently fertile to sustain the high levels of production currently achieved in North America and in the wheatlands of Europe.
There are a number of diseases that would move into areas that are currently affluent and densely populated. In this context, we mean those requiring another species as host for part of their life cycle. The best-known is probably malaria where the infective agent, Plasmodium (a single-celled organism), requires the female Anopheles mosquito to complete its life cycle. But there are a number of other mosquito-borne diseases that could spread into temperate latitudes should these areas become warmer. These include yellow fever, Rift Valley fever, dengue fever and arbovirus encephalitides. They could cause considerable damage in populations unprepared for their arrival before any (costly) preventative health measures were taken.
Many scientists regard pollution as the greatest hazard threatening the continuation of any kind of life, let alone healthy life on this planet, and global warming as a kind of unstoppable time-bomb. Of all the greenhouse gases, carbon dioxide is the most troublesome because of the sheer scale of its production and the amount of time that it remains in the atmosphere. It is estimated that just to stabilize current levels would require an immediate drop of 60% in emissions. This is unlikely to happen whilst ELDCs are increasing industrialization, and in the EDCs neither the USA nor the UK have been prepared to offer more than a resolve to hold emissions at current levels.