4.1 Essential ideas

4.1.4 Climate change

Gases in the atmosphere are warmed by the energy reflecting from the surfaces of land and oceans. This natural greenhouse effect is what keeps the temperatures on Earth suitable for life.

Gases that are able to absorb the reflected longwave radiation (i.e. heat energy) are called greenhouse gases.

Primary greenhouse gases
Absorb most heat
Secondary greenhouse gases
Absorb some heat
Other atmospheric gases (not greenhouse gases)
Do not absorb heat
Carbon dioxide (CO2) Methane (CH4) Oxygen/ozone (O2/03)
Water vapour (H2O) Nitrogen oxides (NOx) Nitrogen (N2)

The enhanced greenhouse effect

Greenhouse gases that retain the most heat (carbon dioxide and water) have the greatest impact on global temperatures and climate patterns. When the concentration of atmospheric carbon dioxide increases, more longwave radiation is absorbed and more heat is retained.

greenhouse effectFigure 4.1.4a – The greenhouse effect

Not all the infrared energy is absorbed by the Earth. Some is reflected into the atmosphere, where it is trapped as greenhouse gases. When concentrations of atmospheric carbon dioxide increase, more heat is reflected back from the atmosphere to the surface.

Atmospheric concentrations of carbon dioxide are rising primarily as a result of combustion of fossil fuels. Most scientists agree that increasing carbon dioxide levels are the main reason for the increase of average global temperatures over the last 200 years.

carbon dioxide and temperatureFigure 4.1.4b – Carbon dioxide and temperature
There is a positive correlation between increasing atmospheric carbon dioxide concentrations and average global temperatures.

Consequences of ocean acidification

Carbon dioxide released by burning fossil fuels diffuses into the oceans. Some of the carbon dioxide gas dissociates in water to form weak carbonic acid. The result is that, along with atmospheric temperatures, the acidity of the oceans is also increasing.

CO2 (aq) + H2O ← → H2CO3 ← →  H+ +  HCO3-

Carbon dioxide + water ← → Carbonic acid ← → Hydrogen ions + Hydrogen carbonate ions

Increasing ocean temperatures also contribute to ocean acidification. Recall from Figure 4.1.3b that deep oceans are a significant store of carbon dioxide. As temperatures rise, carbon dioxide becomes less soluble in water, so more gas is released from the cold, deep ocean into the warm, shallow ocean.

solubility of CO2 in waterFigure 4.1.4c – Solubility of CO2 in water at different temperatures

Increasing ocean acidification has disastrous consequences for shallow-water corals and other shelled animals because carbonic acid dissolves calcium carbonate.

dissolving shellfishFigure 4.1.4d – Dissolving shellfish1
Corals, crustaceans and molluscs are all at risk because of ocean acidification

Coral reef ecosystems are especially vulnerable to small changes in the environment because they have a very small zone of tolerance for temperature and pH. Ocean acidification has been implicated as a cause of coral reef bleaching, when normally very colourful coral reef formations appear white or very pale. The change in colour is also associated with changes in mass and productivity.

coral bleachingFigure 4.1.4e – Coral reef bleaching

Corals normally live in symbiotic relationships with colourful algae. When the temperature or acidity is disturbed, corals expel the algae and appear white. Corals can survive a bleaching event. Usually, they episodes are short-lived, but are very stressful on the organisms involved.

Study tip

There is a layer of stratospheric ozone that protects the Earth from about 97% to 99% of ultraviolet radiation from the Sun. The ozone layer is shrinking due to human activities. This phenomenon, though important for other biological reasons, is not related to climate change.

Language tool

Longwave radiation is also called infrared radiation.

Analysing data

  • Outline the trend shown in Figure 4.1.4a.
  • Discuss whether this correlation is evidence that increasing carbon dioxide concentration is a cause of rising global temperatures  

positive feedback loopFigure 4.1.4f – A positive feedback loop

Consider this

When surface temperatures increase, more water evaporates. Water vapour is also a greenhouse gas, so temperatures increase again. This is an example of positive feedback – one result causes the amplification of the same result. The relationship between ocean acidification and rising ocean temperatures is another example of positive feedback.

International mindedness

Greenhouse gases are released on a small scale but are a global problem. Consider how international agreements, like the Kyoto Protocol or the UNFCCC (United Nations Framework Convention on Climate Change), make an impact. 

crabFigure 4.1.4g – Crab
clamsFigure 4.1.4h – Clams
In addition to coral, other hard-shelled animals such as crabs and clams are at risk from increasing ocean acidification.

Did you know?

Scientists predict there will be more frequent coral bleaching events in the future.

Further reading

Evaluating claims: scientists have also implicated changes in sun exposure, certain chemicals and human disturbance as causes of coral bleaching. To learn more about bleaching, see:
Sapp, J. (2003) What is Natural? Coral Reef Crisis. New York: Oxford University Press.

Sources

  1. Figure 4.1.4d from "Ocean Acidification: Global Warming's Doppelgänger." Seattle Magazine