Causes of Climate Change (HL IB Biology)

The greenhouse effect

• When radiation from the sun hits the earth, it is radiated back from the earth's surface 
• A greenhouse gas is a gas that absorbs this re-radiated radiation, trapping it in the earth's atmosphere so that it is not lost to space
  • Greenhouse gases in the atmosphere have a similar effect to the glass in a greenhouse, hence the term greenhouse gas, and their effect being known as the greenhouse effect
• The greenhouse effect is important to ensure that Earth is warm enough for life; if it were not for the insulating effect of greenhouse gases, Earth would see similar dramatic temperature fluctuations to its neighbouring planets
  • Temperatures on Mars range between 20°C and −153°C
• There are many greenhouse gases including
  • Carbon dioxide
  • Methane
• It is thought that increasing levels of carbon dioxide and methane are entering the atmosphere as a result of human activities, leading to increased rates of atmospheric warming
  • The atmospheric warming, and therefore the changing climate, for which humans are thought to be responsible is known as anthropogenic climate change

Greenhouse effect diagram

Greenhouse gases absorb radiation re-emitted from the earth's surface, trapping it in the atmosphere

Human activities & carbon dioxide 

• Atmospheric carbon dioxide levels have fluctuated throughout Earth's history due to events such as volcanic eruptions and the weathering of limestone rocks
• Since the industrial revolution, however, atmospheric carbon dioxide levels have risen to their highest in Earth's history
• The industrial revolution began in the late 1700s when the combustion of fossil fuels to power factories, transport, and homes became commonplace
  • Fossil fuel combustion releases carbon dioxide
• A clear correlation can be seen between increasing levels of carbon dioxide since the industrial revolution and increasing global temperatures, providing evidence for the role of human activities in causing global warming
  • Note that a correlation alone is not enough to prove causation, but this evidence can be taken alongside what we know about greenhouse gases and other evidence to provide a growing body of proof
• In addition to the burning of fossil fuels, carbon dioxide is also released into the atmosphere when natural stores of carbon are damaged or destroyed by human activities
  • These carbon stores are known as carbon sinks
  • Carbon sinks include trees, soils, peat bogs, and the oceans
    • Deforestation, soil degradation, peat harvesting, and ocean warming all contribute to the addition of carbon dioxide to the atmosphere

Human activities & methane

• Methane (CH₄) is a simple hydrocarbon
• It is present as a gas in the atmosphere, and underground, and is the main component of natural gas fossil fuel
• Methane can be produced by naturally occurring processes in some types of bacteria, but levels have risen significantly in the last 150 years due to human activities
• Methane can be produced by several human activities
  
  • Methane is released from the guts of ruminant mammals, such as cattle, that are farmed by humans
    • The intensive farming of such animals has greatly increased their contribution to atmospheric methane
  • Landfill sites release methane when organic matter such as food waste decomposes
  • Extraction of fossil fuels from underground releases methane
  • Anaerobic bacteria in waterlogged rice paddy fields release methane
• In addition to the list above, the warming of the poles that results from global warming also leads to the release of methane from natural stores such as permafrost
  • Permafrost is ground that remains frozen all year round

Atmospheric carbon dioxide and methane concentration graph

Atmospheric concentrations of both carbon dioxide and methane have increased since the industrial revolution due to human activities.  ppm = parts per million and ppb = parts per billion.

NOS: Students should be able to distinguish between positive and negative correlation and should also distinguish between correlation and causation

• Correlation analysis involves measuring two variables and assessing the relationship between the variables to look for an association
• A relationship that shows correlation can show
  • Positive correlation - as one variable increases, the other variable also increases
  • Negative correlation - as one variable increases the other variable decreases
• In climate change research, evidence from Antarctic ice cores shows a positive correlation between global temperatures and atmospheric carbon dioxide over hundreds of thousands of years

Global temperatures and atmospheric carbon dioxide correlation graph

Evidence from Antarctic ice cores shows a positive correlation between global temperatures and atmospheric carbon dioxide over hundreds of thousands of years

• While a correlation shows evidence of association, in itself it does not provide evidence of causation, i.e. that a change in one variable has caused a change in the other
  • It is not possible to say which variable has influenced the other
  • It is possible that an unknown third variable might be involved
• Note that in the case of carbon dioxide concentrations and global temperatures, there is additional evidence to support a causal link

• Positive feedback is any mechanism in a system that leads to additional and increased change away from the equilibrium
• Positive feedback loops occur when the output of a process feeds back into the system in a way that moves the system increasingly away from the average state
• In this way, positive feedback is destabilising; it amplifies deviation from the equilibrium and drives systems towards a tipping point where the state of the system suddenly shifts to a new equilibrium

Positive feedback and global warming

• Global warming has a positive feedback effect on the earth and its atmosphere
• This means that global warming leads to more global warming, which further increases global warming, etc.
• There are several factors that contribute to the positive feedback cycle of global warming

Loss of reflective snow and ice

• The extent to which a surface reflects light is known as its albedo; the higher the albedo, the more light is reflected
  • Light coloured surfaces such as snow and ice have a high albedo, while dark surfaces such as rock and soil have a low albedo
• As the polar ice caps melt due to global warming, the earth's overall albedo decreases, and more of the sun's energy is absorbed by exposed rock, soil, and the dark surface of the oceans; this increases global warming
  • This cycle continues, further increasing global warming

Impact of melting ice flow diagram

The loss of polar ice contributes to the positive feedback cycle of global warming 

Accelerating decomposition

• Decomposition is carried out by living organisms such as bacteria and fungi
• These organisms break down dead matter and waste in a series of enzyme-controlled reactions
• The enzyme-controlled reactions of decomposition occur faster at higher temperatures, meaning that as global warming increases, rates of decomposition increase 
• The respiration of decay microorganisms releases carbon dioxide into the atmosphere
  • Increased decomposition in peat bogs releases huge volumes of carbon dioxide
    • Peat bogs function as carbon sinks when they are stable
  • Increased decomposition in permafrost releases carbon dioxide
    • In parts of the world where temperatures remain low all through the year, the ground remains frozen for most of the year; this frozen ground is permafrost
    • Permafrost is a huge carbon sink because it contains organic material that cannot decompose at low temperatures
      • Decay organisms are inactive at low temperatures
• Increased atmospheric carbon dioxide further contributes to the greenhouse effect, increasing global warming

Release of methane

• Melting permafrost can also lead to the release of methane (CH₄), a potent greenhouse gas
• This is due to the activity of methanogenic microorganisms in the frozen soil, which increases as permafrost melts
  • Methanogenic microorganisms are species of archaea that produce methane as part of their metabolism

Impact of melting permafrost flow diagram

Melting permafrost leads to the release of carbon dioxide and methane, further increasing global warming

Increasing drought and forest fires

• As global warming increases the frequency of extreme weather events, droughts occur more often
• The dry vegetation that results from drought can catch fire easily, and wild fires become more likely
• Combustion of plant material releases carbon dioxide into the atmosphere, where it increases global warming
• The resulting reduction in the number of photosynthesising plants means that less carbon dioxide is removed from the atmosphere