Global Warming

Global warming is the term we use for the gradual increase in the average temperature at the Earth’s surface that has occurred over the past century or so.

Evidence for the increase in temperature

The following graph (Figure 1) shows there has been a gradual increase in the average annual temperature across Australia in the past century. The red graph shows the average maximum temperature for each year and the blue graph shows the average minimum temperature each year.

Notice the curve that traces through the middle of the red graph. This is called a trend line – a line that smoothes out the fluctuations to show the trend in values. The graph points for this trend line are calculated using statistics. Similarly a trend line is drawn for the blue graph. It is clear that while the temperatures fluctuate (go up and down), there is an increase in the mean annual temperature.

This increase in temperature everywhere across Australia also is illustrated in the following map (Figure 2). The map, prepared by the Australian Bureau of Meteorology, shows the average change in temperature per 10 years over the past century for each region. 

 

Like Figure 1 for Australia, the next graph (Figure 3) shows that there has been a gradual net rise in average temperatures across all countries over the past 130 years. 

Note: The vertical scale shows the ‘temperature anomaly’, not the actual average temperature. Temperature anomaly is a measure of how much the temperature is higher or lower than a long-term average. For example, a value of +1 means the average temperature was 1 ⁰C higher than the average temperature observed over a long time. A value of -0.6 means the average temperature was 0.6 ⁰C lower than the average temperature observed over a long time. This kind of graph is used by climate scientists to analyse trends in global temperatures.

The graph in Figure 3 was constructed from temperature data collected from a network of 6000 temperature stations across the globe. These are shown in the following map (Figure 4). The colours show how long the stations have been keeping records. Approximately 1650 of these stations have kept records for longer than 100 years.

Figure 4 Source: http://en.wikipedia.org/wiki/Instrumental_temperature_record Accessed: 22 June 2010

The next graph (Figure 5) shows the same trend over the months September-February, with satellite data shown in red. 

We can only conclude that there is overwhelming, reliable evidence that there has been a gradual increase in the temperature at the Earth’s surface over the past 130 years.

Acknowledgement: The above graphs were provided by Professor Neville Nicholls, of Monash University. Professor Nicholls is a Lead Author for the Intergovernmental Panel on Climate Change (IPCC), which was established in 1988.

What is the cause of this gradual temperature rise?

Scientific modelling by atmospheric scientists and meteorologists, using data they have collected from ice core studies and measurements of atmospheric temperatures and gas concentrations in the atmosphere, indicates that the increase in concentration of the greenhouse gases in the atmosphere above their natural levels is the prime cause of global warming. 

Greenhouse gases include carbon dioxide and water vapour. A number of other gases in the atmosphere also act as greenhouse gases, although the percentage of them in the lower atmosphere is much less than that of carbon dioxide and water. These gases include methane and nitrous oxide.

Greenhouse gases

Greenhouse gases all have one thing in common: their molecules contain 3 or more atoms. This is shown in the Table 1.

Table 1.

Gas	Approximate percentage in the air (if water vapour is removed)**	Chemical formula	Total number of atoms in each molecule	Is this a greenhouse gas?
Oxygen	20.9	O2	2	No
Nitrogen	78.1	N2	2	No
Argon	0.9	Ar	1*	No
Carbon dioxide	0.04	CO2	3	Yes
Methane	0.0002	CH4	5	Yes
Nitrous oxide	0.00003	N2O	3	Yes

* Argon is classified as a noble gas. Noble gases exist in Nature as individual atoms, so these are not called molecules.

** The percentage of water vapour in the air varies from place to place and at different times, but on average is about 1-4%. Its chemical formula is H₂O. It also is a greenhouse gas.

The larger number of atoms in the molecules of greenhouse gases enables them to absorb infrared radiation radiated by the Earth’s surface, and then emit some back to the surface. This warms the Earth even more. 

What is infrared radiation and why is it important?

The light energy that is radiated out in Space by the Sun is not just the light you can see. In fact, out in Space, unless you are looking directly at a star, you can only see what we call visible light when it reflects off objects, such as the Moon or the International Space Station, or when it is ‘scattered’ by particles, such as the particles in the Earth’s atmosphere, and enters your eyes.  

 

Figure 6 The International Space Station (ISS) hovering above Earth. CREDIT: Image supplied by NASA.  

Notice the huge solar panels that help supply electrical power required to run the space station, and the astronaut on a spacewalk.

 

The light energy that is radiated out in Space by the Sun is not just the light you can see. In fact, out in Space, unless you are looking directly at a star, you can only see what we call visible light when it reflects off objects, such as the Moon or the International Space Station, or when it is ‘scattered’ by particles, such as the particles in the Earth’s atmosphere, and enters your eyes.

Figure 7  A schematic diagram of the electromagnetic spectrum. Source: http://upload.wikimedia.org/wikipedia/commons/8/8a/Electromagnetic_Spectrum.png  Accessed: 30 June 2010

Note in Figure 7: The band of wavelengths that match visible light has been expanded to show the colours that together make up the visible light. Note there is actually a continual range of colours, not just the seven distinct colours listed in the well-known “ROYGBIV” (Red, Orange, Yellow, Green, Blue, Indigo and Violet). Likewise, the band of wavelengths that are involved in radio and TV waves has been expanded to show some of the different broadcasting bands.

When the Sun’s radiation reaches Earth

Figure 8 shows what happens to the Sun’s radiation as it reaches the Earth.

Figure 9  Notice that when the system is in balance, the total energy going out from Earth into Space adds up to 100 %, which means it equals the total energy coming in. Source: http://eosweb.larc.nasa.gov/EDDOCS/images/Erb/components2.gif Accessed: 28 July 2010

When radiation from the Sun reaches the Earth:

• About 30% of the radiation is reflected back into Space by the particles in the atmosphere, clouds and the Earth’s surface. [In Figure 9 we see it is (6 + 20 + 4) %, which adds up to 30 %.]
• Some is absorbed by the water vapour in the atmosphere and by clouds. Some (about 3%) is absorbed by the ozone layer. (This layer is not shown in this diagram.) This adds up to about 19 %.
• The remaining 51 % of the Sun’s energy is absorbed by the Earth’s surface (land and oceans). If it kept being absorbed and none was ever given back out, the Earth would get hotter and hotter. The oceans would have boiled away long ago, and all our water would have evaporated! 

Fortunately for us, this does not happen, because the Earth radiates heat energy in the form of infrared radiation back into the atmosphere. And ultimately the atmosphere radiates heat energy back into Space. What happens to this infrared radiation in the atmosphere is of crucial importance to life on Earth.

The natural greenhouse effect

The greenhouse effect is the process whereby some of the infrared radiation emitted by the Earth’s surface is ‘trapped’ by greenhouse gases, which helps moderate the temperatures at the Earth’s surface. 

The greenhouse effect has occurred naturally on Earth for millions of years, as our atmosphere has contained greenhouse gases ever since it first formed. For this reason this process, now known as the natural greenhouse effect, has enabled life to evolve on this planet. Without it, the temperatures experienced on Earth would be like those on the Moon, which is the same distance from the Sun as we are. This means it would be far too hot by day and far too cold by night for life as we know it to survive. 

The Moon has no atmosphere and hence no greenhouse gases to help moderate the temperatures at its surface. This is why the average ground temperature on the Moon is -17 ºC, while on the Earth, which does have an atmosphere containing greenhouse gases, it is 16 ºC, which is 33 ºC higher. 

In fact, when we say ‘trapped’, we mean that the molecules of greenhouse gases absorb some of the infrared radiation. They also emit some infrared radiation back to the Earth. This makes the temperatures at the surface warmer than they otherwise would be. 

The natural greenhouse effect is illustrated in Figure 10. 

Figure 10 Source: maps.grida.no/go/graphic/greenhouse-effect   Accessed: 23 June 2010

Cycling greenhouse gases

Over the millions of years in which the greenhouse gases have been present in the Earth’s atmosphere, natural cycles have ensured that the proportion of the greenhouse gases in the atmosphere has remained steady. The ‘cycles’ consist of natural processes in which they are released into the air and other natural processes in which they are removed from the air. 

Between the two kinds of processes, molecules of a particular gas are slowly and steadily cycled around. For example, the carbon cycle refers the processes in which carbon dioxide is released into the air and the processes in which it is removed from the air. 

One of the natural processes in which carbon dioxide is released into the air is cellular respiration, a chemical reaction that can be summarised in the following word equation:

glucose  +  oxygen  →  carbon dioxide + water + energy

This reaction occurs in every cell within all those living organisms that undergo cellular respiration to obtain the energy they need to survive, including animals and plants.  The carbon dioxide is then released into the air, as it is a waste product that would be toxic to the cells if it were to accumulate within the cells.

One of the natural processes in which carbon dioxide is removed from the air is photosynthesis. This is a chemical reaction in which plants absorb carbon dioxide from the air and water from the ground to produce glucose.

carbon dioxide + water + energy →  glucose  +  oxygen  

The energy required for this process is obtained from light. The reaction also requires the presence of chlorophyll. 

Until the past century or so, all the natural processes in which greenhouse gases were released into the air and all the natural processes in which greenhouse gases were removed from the air balanced each other out. In other words, the gases were continually cycled around. As a result, the percentage of these gases in the atmosphere remained steady.  

But now, the level of human activity we have today has altered this balance. Our large-scale burning of coal, natural gas and oil, our mass production of materials such as steel, cement and aluminium, and our huge piles of rotting garbage, not to mention burning trees to clear land or cutting them down to make goods (including paper), are releasing more greenhouse gas molecules into the air than can be removed in natural processes. 

Even growing more and more rice and increasing the number of ruminant animals (animals that eat grass), such as sheep and cattle, to feed our increasing populations contributes to the problem. Rotting garbage, rice paddies and animals that eat grass and other plant material all produce huge amounts of methane gas, which is a far more potent greenhouse gas than carbon dioxide. (That is, a molecule of methane will emit more infrared radiation than a molecule of carbon dioxide.) 

Insects that eat plant material or plant products such as wood or paper, add to this problem. Although each individual insect may only emit small amounts, because there are billions of them, this adds up! Termites alone contribute hugely to the problem.

In addition, new very potent greenhouse gases such as nitrogen trifluoride, NF3, are being introduced into the atmosphere as new technologies are developed.

The enhanced greenhouse effect and global warming

The enhanced greenhouse effect is the trapping of additional infrared radiation by the excessive amounts of greenhouse gases in the atmosphere that have been produced as a result of human activity. It is this process that many scientists are concerned about. 

Likely consequences of global warming

Scientists use sophisticated computer models of the Earth that predict likely changes due to global warming. Some examples of the consequences of global warming include: 

• Climate change. There are likely to be significant changes in climate around the world, including greater extremes of drought and heavy rainfall, and an increase in the severity of cyclones, typhoons and other extreme weather events.
• Melting of polar icecaps and glaciers. One consequence of this would be causing many species that are dependent on polar ice, such as the polar bear, to become endangered.
• Change in weather patterns. For example, some places may get more rain and storms while others may get less. 
• Increase in the temperature of the upper levels of the oceans. One consequence of this would be damage to marine ecosystems due to the loss of species that cannot survive or cannot reproduce in the warmer water. 
• Rising sea levels. This is mostly due to the expansion of the upper layers of the sea water due to the increase in temperature. The melting of the polar ice caps also contributes to this problem. This would result in the flooding of low-lying coastal areas. A large number of people would lose their homes and livelihoods. 
• The spread of tropical diseases. The higher mean temperatures may lead to a wider spread of tropical diseases such as malaria, which is caused by a certain species of mosquito.
• The spread of invasive species. The change in climate is likely to lead to the movement of species that can cause damage to crops or stock. This could significantly reduce food supplies, which could lead to serious social problems. When people are hungry and desperate and either cannot access or cannot afford to purchase food from other regions, they could resort to uncontrolled use of toxic chemical sprays on crops and pasture, or to destroying more forests to obtain more land.

 

Some of these consequences are already being observed!

Sea-level rising

Scientific data shows how much sea levels are already rising. The graph in Figure 12 shows the increase in Global Mean Sea Level from 1993 to 2010. This shows the average across all the oceans. 

The world map in Figure 13 shows which areas of the ocean are rising more rapidly, and which are rising less rapidly. Notice that the northern parts of Australia are at the greatest risk of coastal flooding.

Figure 12  This graph shows how much the sea level is above or below a long-term average. The second further reference at the end of this article shows a graph that displays the Global Mean Sea Level from 1870-2008, with back-up satellite data for the most recent years. Source: http://en.wikipedia.org/wiki/File:Global_mean_sea_level.png  Accessed:  24 June, 2010

Figure 13  Source: http://upload.wikimedia.org/wikipedia/commons/6/69/NOAA_sea_level_trend_1993_2010.png  Accessed: 24 June 2010

Melting of polar ice caps

One source of evidence for this is the large amount of satellite and other data collected by NASA scientists, seen in Figure 14. 

Figure 14 NASA scientists studying ice in the Arctic. CREDIT: NASA

 

Further references

• http://www.cmar.csiro.au/e-print/open/holper_2001b.html
• http://cmar.csiro.au/sealevel/
• http://www.nasa.gov/topics/earth/features/arctic_thinice.html
• http://www.realclimate.org
• http://www.climatedata.info/index.html
• http://scienceblogs.com/deltoid/
• http://tamino.wordpress.com/
• http://www.youtube.com/user/greenman3610#p/u
• http://www.skepticalscience.com/
• http://www.ipcc.ch

For information about the ozone layer and the holes in the ozone layer, refer to the Student Booklet. Note that the holes in the ozone layer do not contribute to global warming. However, the gases that helped cause the holes, including the propellants used in spray cans, and the gases that are now used in their place, as well as the ozone that is present in the lower atmosphere, are all greenhouse gases and do contribute to global warming. (Ozone gas is produced, for example, in photocopiers, and by the action of sunlight on the exhaust gases from vehicles. These are two of the sources of ozone in the lower atmosphere.)