4.4 Climate Change

Question 1

What are greenhouse gases?

Answer 1

Carbon dioxide and water vapour are the most significant greenhouse gases.

The Earth is kept much warmer than it otherwise would be by gases in the atmosphere that retain heat. The effect of these gases has been likened to that of the glass that retains heat in a greenhouse and they are therefore known as greenhouse gases, though the mechanism of heat reaction is not the same.

Question 2

Which greenhouse gases have the biggest warming effect?

Answer 2

Carbon dioxide and water vapour.

• Carbon dioxide is released into the atmosphere by cell respiration in living organisms and also by combustion of biomass and fossil fuels. It is removed from the atmosphere by photosynthesis and by dissolving in the oceans.
• Water vapour is formed by evaporation from the oceans and also transpiration in plants. It is removed from the atmosphere by rainfall and snow.

Question 3

How is water vapour removed from the atmosphere?

Answer 3

Snow and rainfall.

Question 4

Why is water vapour dangerous as a greenhouse gas?

Answer 4

Water continues to retain heat after it condenses to form droplets of liquid water in clouds. The water absorbs heat energy and radiates it back to the Earth’s surface and also reflects the heat energy back. This explains why the temperature drops so much more quickly at night in areas with clear skies than in areas with cloud cover.

Question 5

What are the other greenhouse gases?

Answer 5

• Methane - Methane is the third most significant greenhouse gas. It is emitted from marshes and other waterlogged habitats and from landfill sites where organic wastes have been dumped. It is released during the extraction of fossil fuels and from melting ice in polar regions.
• Nitrous oxide is another significant greenhouse gas. It is released naturally by bacteria in some habitats and also by agriculture and vehicle exhausts.

Question 6

How many of the gases in the atmosphere are greenhouse gases?

Answer 6

less than 1%, because oxygen nitrogen are NOT greenhouse gases. This is because they do not absorb longer-wave radiation.

Question 7

What factors do we have to investigate to see how harmful a greenhouse gas may be?

Answer 7

The impact of a gas depends on its ability to absorb long-wave radiation as well as on its concentration in the atmosphere.
Two factors determine the warming impact of a greenhouse gas:
- How readily the gas absorbs long wave radiation
- And the concentration of the gas in the atmosphere

For example, methane causes much more warming per molecule than carbon dioxide but as it is at a much lower concentration in the atmosphere its impact on global warming is less.

The concentration of a gas depends on the rate at which it is released into the atmosphere and how long on average it remains there. The rate in which water vapour enters the atmosphere is immensely rapid, but it remains there only nine days on average, whereas methane remains in the atmosphere for twelve years and carbon dioxide for even longer.

Question 8

Which is more dangerous in the atmosphere carbon dioxide or methane?

Answer 8

Methane causes much more warming per molecule than carbon dioxide but as it is at a much lower concentration in the atmosphere its impact on global warming is less.

Question 9

How does radiation come to the earth and leave?

Answer 9

The Earth absorbs short wavelength energy from the sun and then re-emits it, but at a much longer wavelength. Most of the re-emitted radiation is infrared, with a peak wavelength of 10,000nm, the sun’s radiation has a peak of 400nm.

Question 10

What do greenhouse gases actually do?

Answer 10

The longer wave radiation that is emitted from the earth is reabsorbed by greenhouse gases which means heat is retained in the atmosphere.

Question 11

Compare energy coming into the earth, and absorbed by the atmosphere and then energy emitted by the earth and absorbed by greenhouse gases?

Answer 11

25%-30% of the short-wavelength radiation from the sun that is passing through the atmosphere is absorbed before it reaches the Earth’s surface. Most of the solar radiation absorbed is ultraviolet light which is absorbed by ozone. 70-75% of solar radiation then reaches the Earth’s surface and much of this is converted to heat.

A far higher percentage of the longer wavelength radiation re-emitted by the surface of the Earth is re-absorbed before it has passed out to space. Between 70-85% is captured by greenhouse gases in the atmosphere. This energy is re-emitted, some towards the earth. The effect is global warming. Without it the mean temperature at the Earth’s surface would be able -18 degrees.

Greenhouse gases in the Earth’s atmosphere only absorb energy in specific wavebands. The wavelengths re-emitted by the Earth are between 5 and 70nm. Water vapour, carbon dioxide, methane, nitrous oxides all absorb these wavelengths so are greenhouse gases.

Question 12

What absorbs 25-30% of the radiation from the sun?

Answer 12

Ozone in the atmosphere before it reaches the Earth’s surface.

Question 13

How can we compare global temperatures and carbon dioxide concentrations?

Answer 13

Columns of ice have been dug up from the Antarctic. This ice has built up over hundreds of years so ice from deeper down is older than ice near the surface. Bubbles of air trapped in the ice can be extracted and analysed to find the carbon dioxide concentration. Global temperatures can be deduced from ratios of hydrogen isotopes in water molecules.
It was found that during the Ice Age there was a repeating pattern of rapid periods of warming followed by much longer periods of gradual cooling. There is a very striking correlation between carbon dioxide concentration and global temperatures - the periods of higher carbon dioxide concentration repeatedly coincide with periods when the Earth was warmer.
This data is consistent with the hypothesis that carbon dioxide increases the greenhouse effect.

Question 14

How can you work out global temperature from thousands of years ago?

Answer 14

Take a sample of ice from the Antarctic from ages ago and look at the ratio of hydrogen isotopes in the water molecules.

Question 15

What effect will increasing greenhouse gases have?

Answer 15

• Increasing greenhouse gas concentrations will tend to cause higher global average temperatures and also more frequent and intense heat waves.
• If the temperature is higher then the evaporation of water will increase from the oceans. This means that there is likely to be more rain for longer periods and it be more frequent and the amount of rain delivered during thunderstorms and other intense bursts is likely to increase very significantly.
• In addition, higher ocean temperatures cause tropical storms and hurricanes to be more frequent and more powerful, with faster wind speeds.

The consequences of any rise in global average temperature are unlikely to be evenly spread. Not all areas would become warmer. The distribution of rain would also change with some areas more prone to drought and then flooding than others.

Predictions about changes in weather patterns are very uncertain but it is clear that just a few degrees of warming would cause very profound changes to the Earth’s climate patterns.

Question 16

What effect has industrialisation had on climate change?

Answer 16

There is a correlation between rising atmospheric concentrations of carbon dioxide since the start of the industrial revolution two hundred years ago and average global temperatures.
The combustion of coal, oil and natural gas has lead to increases in the atmospheric carbon dioxide concentration.

Question 17

What is the effect of burning fossil fuels?

Answer 17

Recent increases in atmospheric carbon dioxide are largely due to increases in the combustion of fossilised organic matter.
Burning fossil fuels releases carbon dioxide.

Question 18

What could be the reasons that people claim global warming is not happening?

Answer 18

• Scientists are trained to be cautious about their claims and to base their ideas on evidence. They are expected to admit when there are uncertainties and this can give the impression that evidence is weaker than it actually is.
• Global climate patterns are very complex and it is difficult to make predictions about the consequences of further increases in greenhouse gas concentrations. There can be tipping points in climate change patterns where sudden massive changes occur. This makes prediction even more difficult.
• The consequences of changes in global climate patterns could be very severe for humans and for other species so many feel that there is a need for immediate action even if uncertainties remain in climate change science. Companies make huge profits from coal, oil and natural gas and it is in their interests for fossil fuel combustion to continue to grow. It would not be surprising if they paid for reports to be written that minimised the risks of climate change.

Question 19

What is the effect of increasing carbon dioxide levels on coral reefs?

Answer 19

Carbon dioxide has already began to acidify the oceans, which are around pH 8, it used to be at 8.170 in the late 18th century and is now about 8.069. Carbon dioxide when dissolved in water forms carbonic acid which lowers the pH of the water.

• Marine animals that use calcium carbonate to make their skeletons need to absorb carbonate ions from the seawater. The concentration of carbonate ions in seawater is low because they are not very soluble. Dissolved carbon dioxide makes the carbonate ion concentration even lower as a result of it’s reaction with them. Carbon dioxide reacts with water to form carbonic acid (H2CO3), carbonic acid dissociates into hydrogen and hydrogen carbonate ions (HCO3 + H+). Hydrogen ions react with dissolved carbonate ions, reducing their concentration:

CO2 + H20 = H2CO3 (forms carbonic acid)
H2CO3 = H+ + HCO3- (carbonic acid dissociates)
H+ + CO23- = HCO3- (hydrogen ions then react with carbonate meaning that carbonate cannot react with the marine animals).

If carbonate ion concentrations drop it is more difficult for reef-building corals to absorb them and make their skeletons. Also if seawater ceases to be a saturated solution of carbonate ions, existing calcium carbonate tends to dissolve, so existing skeletons of reef-building corals are threatened.

Question 20

Any evidence for the danger of carbon dioxide to coral reefs?

Answer 20

Volcanic vents near the island of Ischia in the Gulf of Naples have been releasing carbon dioxide into the water for thousands of years, reducing the pH of the seawater. In the area of acidified water there are no corals, sea urchins or other animals that make their skeletons from calcium carbonate. In their place other organisms flourish such as sea grasses and invasive algae. This could be the future of coral reefs around the world if carbon dioxide continues to be emitted from burning fuels.