Lecture 45: Internal and External Climate Forcing

Lecture 45: Internal and External Climate Forcing

Introduction

  • Overview of topics covered:

    • Internal and external forcing factors

    • Radiatively active gases and the greenhouse gas effect

    • Planetary albedo and its implications

    • External radiative forcing processes and impacts on Earth's climate

  • Reference to previous lecture (Friday's lecture) and its key topics:

    • Importance of climate forcing and Earth's energy balance in climatic patterns

    • Differentiation between weather and climate

Key Concepts

Radiative Forcing

  • Definition:

    • Refers to factors that change the Earth's net irradiance or energy balance, involving incoming energy and outgoing energy from the Earth's surface and atmosphere.

  • Types of Forcing:

    • Positive Forcing:

    • Causes warming of the Earth's surface.

    • Negative Forcing:

    • Causes cooling of the Earth's surface.

  • Sources of Climate Forcing:

    • Both human and natural processes contribute to climate forcing.

Examples of Climate Forcing Agents

  • Natural Processes:

    • Volcanic eruptions contribute greenhouse gases and aerosols, affecting radiative forces directly and indirectly.

  • Human Activities:

    • Fossil fuel burning, industrial activities, and agriculture contribute significantly, creating various feedback loops that serve as direct, indirect, or non-radiative forcings.

Internal and External Forcing

  • Internal Forcing:

    • Factors like the greenhouse effect and planetary albedo that occur within Earth's system, directly influencing energy balance.

  • External Forcing:

    • Orbital and astronomical factors outside the Earth's system affecting climate behavior.

Radiatively Active Gases

  • Definition:

    • Gases that absorb energy in certain wavelengths, impacting radiative balance in the atmosphere.

  • Examples of Greenhouse Gases:

    • Naturally occurring: carbon dioxide (CO$2$), methane (CH$4$), water vapor (H$2$O), and ozone (O$3$).

    • Anthropogenic: Chlorofluorocarbons (CFCs), which are ozone-depleting substances.

  • Importance of Water Vapor:

    • A significant greenhouse gas that fluctuates but plays a key role in the energy balance.

Mechanism of Radiative Forcing

  • Greenhouse gases absorb infrared radiation emitted from Earth's surface, re-emit it in all directions, including back towards Earth, leading to net warming effects.

  • Location:

    • Greenhouse gases primarily exist in the troposphere, impacting the lower atmosphere, unlike stratospheric ozone.

Energy Balance and Atmospheric Dynamics

  • Atmospheric Window:

    • Region of the electromagnetic spectrum allowing solar radiation to pass through the atmosphere to Earth and enabling Earth's outgoing thermal radiation to escape into space.

  • Solar Incoming Radiation:

    • Peaks in shorter wavelengths such as ultraviolet (UV) and visible light, while outgoing radiation primarily exists in the infrared (IR) spectrum.

  • Energy balance indicates that incoming solar radiation is approximately equal to outgoing terrestrial radiation, which is influenced by atmospheric filters (ozone layer and water vapor).

Albedo Effects

  • Planetary Albedo:

    • Reflectivity of Earth's surface, defined as the fraction of incoming solar radiation reflected back into space, approximately 30% for Earth.

  • Influencing Factors:

    • Varies by surface types—forests and oceans have low albedo, while ice and snow have high albedo, impacting climate modeling and predictions.

External Radiative Forcing Processes

  • Solar Output Variability:

    • Solar irradiance varies over different time scales from seconds to centuries, driven by solar activity cycles (e.g., Schwabe cycle of 11 years).

  • Milankovitch Cycles:

    • Periodic variations affecting Earth's climate related to Earth's orbital geometry with three main variations:

    • Precession:

    • Wobbling of Earth's axis affects seasonal timing, occurs over a cycle of 26,000 years.

    • Eccentricity:

    • Variation in the shape of Earth's orbit affecting distance from the sun, with a cycle of 100,000 years.

    • Obliquity (Axial Tilt):

    • Variation in the angle of the Earth's axial tilt, affecting sunlight distribution across latitudes, occurring over a cycle of 41,000 years.

Feedback Loops

  • Recognition of positive and negative feedback loops improves understanding of climate complexity.

  • Examples of Feedback Loops:

    • Positive Feedback: Warming leads to increased evaporation, raising water vapor levels (a greenhouse gas) and further warming.

    • Negative Feedback: Increased warming leads to more evaporation and cloud cover, which insulates the Earth and can block incoming solar radiation.

Summary of Key Points

  • Trace gases in the atmosphere are radiatively active, altering Earth's energy balance.

  • Greenhouse effect as an internal forcing process is crucial for maintaining planetary warmth.

  • Different greenhouse gases have varying warming potentials; measured relative to CO$_2$.

  • Planetary albedo is significant in determining how much solar radiation is reflected into space.

  • External astronomical factors lead to cyclical variations in energy distribution and long-term climate shifts.

  • Positive and negative feedback mechanisms complicate climate understanding, impacting predictions and models.

Conclusion

  • Future discussions will cover climate research, sampling proxies, models, and deeper exploration of climate change implications.