24.3.2 External Forcing

In climate science, a forcing is any factor that alters the Earth's energy balance. External forcing refers to influences that originate outside the climate system itself — they drive the climate to change rather than being generated by internal climate dynamics.

What is Radiative Forcing?

Radiative Forcing (RF) quantifies the change in energy flux in the atmosphere caused by a forcing agent. It is expressed in Watts per square metre (${\text{W m}}^{-2}$).

$RF=\Delta F({\text{W m}}^{-2})$

• A positive RF means more energy enters the system than leaves → warming.
• A negative RF means less energy enters (or more leaves) → cooling.

Categories of External Forcing

1. Natural External Forcing

Volcanic Eruptions as Negative Forcing

When a major volcano erupts (e.g., Mt. Pinatubo, 1991), it injects large quantities of sulfur dioxide (${\text{SO}}_2$) into the stratosphere. This gas reacts with water vapour to form sulfate aerosol particles that:

1. Scatter and reflect incoming shortwave solar radiation back to space.
2. Increase Earth's albedo temporarily.
3. Cause a measurable drop in global surface temperatures (≈ 0.5 °C for 1–2 years).

This is a negative external forcing because it reduces the energy entering the climate system.

Milankovitch Cycles

The Serbian astronomer Milutin Milanković identified three orbital parameters that vary over tens of thousands of years:

• Eccentricity — how elliptical Earth's orbit is (~100,000-year cycle).
• Axial tilt (obliquity) — the angle of Earth's axis relative to its orbital plane (~41,000-year cycle).
• Precession — the wobble of Earth's rotational axis (~26,000-year cycle).

These cycles alter the distribution and intensity of solar radiation reaching Earth's surface, acting as the pacemaker of ice ages.

2. Anthropogenic External Forcing

Human activities have become the dominant external forcing since the Industrial Revolution (~1750):

The IPCC estimates the total anthropogenic RF since 1750 is approximately $+2.7{\text{W m}}^{-2}$, far exceeding natural forcings over the same period.

Natural vs Anthropogenic Forcing — Key Distinction

Summary

External forcings are the primary drivers of long-term climate change. Natural forcings (solar, volcanic, orbital) have shaped Earth's climate over geological time. Since industrialisation, anthropogenic forcings — especially ${\text{CO}}_2$ emissions — have become the dominant driver of the observed rapid warming, overwhelming natural variability.