MEVE 011: Unit 02 - Physical Basis of Climate Change

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 UNIT 2: PHYSICAL BASIS OF CLIMATE CHANGE

2.1 Introduction              

This unit explores the scientific foundation of climate change by focusing on how energy from the sun interacts with the Earth's atmosphere and surface. It introduces the concept of radiation balance and how various natural and human-made factors disturb this balance, leading to climate change. It explains different types of climate forcing mechanisms—both internal and external—and the crucial role played by greenhouse gases. The unit also examines the concept of global warming potential and identifies key drivers responsible for ongoing climate change.

2.2 Objectives

The objectives of this unit are to:

  • Understand the Earth's radiation balance and radiative forcing
  • Identify different types of climate forcing mechanisms
  • Learn about greenhouse gases and their role in the greenhouse effect
  • Understand the concept of Global Warming Potential (GWP)
  • Identify natural and anthropogenic drivers of climate change

2.3 Radiation Balance and Radiative Forcing

The Earth's radiation balance refers to the equilibrium between the incoming solar radiation and the outgoing infrared radiation emitted by the Earth. When the amount of energy coming in is equal to the energy going out, Earth's climate remains stable.
Radiative forcing is a measure of how various factors (like greenhouse gases or aerosols) disturb this balance. Positive radiative forcing (e.g., from CO₂ or methane) leads to warming, while negative forcing (e.g., from aerosols) leads to cooling. This concept is essential to understanding how human activities alter the Earth’s climate system.

2.4 Climate Forcing Mechanism: External and Internal Forcing

Climate forcing refers to any factor that influences Earth’s climate by changing the energy balance. These can be external (originating from outside the climate system) or internal (within the system).

2.4.1 Orbital Variations

Also known as Milankovitch cycles, these are long-term changes in Earth’s orbit and tilt. They influence how much solar energy the Earth receives and are linked to natural glacial and interglacial cycles.

2.4.2 Solar Variations

The Sun's energy output is not constant. Small fluctuations in solar radiation can affect Earth’s climate. Periods of high or low solar activity can influence temperatures on Earth over decades.

2.4.3 Volcanic Eruptions

Large volcanic eruptions release dust, ash, and sulphate aerosols into the atmosphere. These can reflect sunlight and cool the Earth’s surface temporarily by creating a negative radiative forcing.

2.4.4 Atmospheric Composition

Changes in the levels of greenhouse gases, water vapor, aerosols, and ozone in the atmosphere alter the energy balance. The composition is affected by both natural processes and human activities like industrial emissions and deforestation.

2.5 Role of Greenhouse Gases and Greenhouse Effect

Greenhouse gases (GHGs) such as carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and water vapor trap heat in the Earth’s atmosphere. They allow incoming shortwave solar radiation to pass through but absorb and re-emit outgoing longwave radiation, thus warming the lower atmosphere.
This natural greenhouse effect keeps the Earth’s surface warm enough to support life. However, excessive emissions of GHGs due to human activities enhance this effect, leading to global warming.

2.6 Global Warming Potential (GWP)

GWP is a measure used to compare the heat-trapping ability of different greenhouse gases over a specific time frame (usually 100 years), relative to CO₂.

  • CO₂ has a GWP of 1
  • Methane (CH₄) has a GWP of about 28–36
  • Nitrous oxide (N₂O) has a GWP of around 265–298
    This helps policymakers understand which gases are more harmful in terms of long-term warming potential and prioritize emissions control accordingly.

2.7 Drivers of Climate Change

Drivers are the underlying causes that trigger climate change. These include:

  • Natural drivers like volcanic eruptions, ocean circulation patterns, and solar cycles
  • Anthropogenic (human-made) drivers such as fossil fuel combustion, deforestation, industrial processes, agriculture, and land-use changes
    Human activities are now recognized as the dominant drivers of the rapid and unprecedented climate change observed over the past century.

2.8 Let Us Sum Up

This unit explained how climate change is rooted in physical processes involving the Earth's radiation balance. Factors like solar radiation, greenhouse gases, volcanic eruptions, and human-induced emissions alter this balance through radiative forcing. The greenhouse effect, though natural and necessary, has been intensified due to increased greenhouse gas concentrations, leading to global warming. Concepts like Global Warming Potential (GWP) help quantify and compare the impact of different gases. The unit concludes by identifying both natural and human drivers that contribute to climate change.

 

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