Radiative forcing

Physical Drivers of climate change

NASA Earth Science Division Operating Missions

ModtranRadiativeForcingDoubleCO2

Sunspot Numbers

CERES Global albedo anomaly

Radiative forcing is a measure of the influence a factor has on the balance of incoming and outgoing energy in the Earth's atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. In simple terms, it is the difference between solar irradiance (sunlight) absorbed by the Earth and energy radiated back to space. Changes in radiative forcing are typically quantified at the tropopause or at the top of the atmosphere in units of watts per square meter (W/m^2).

Overview

The concept of radiative forcing is critical in understanding the climate system and assessing the impact of human activities on global warming. Factors that can cause changes in Earth's radiative forcing include changes in solar radiation, variations in the albedo (reflectivity) of the Earth's surface and atmosphere, and changes in the concentrations of greenhouse gases. Positive radiative forcing leads to warming of the climate system, while negative forcing leads to cooling.

Types of Radiative Forcing

Radiative forcing can be categorized into several types based on its sources:

Greenhouse Gases

The increase in concentrations of greenhouse gases such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases leads to positive radiative forcing. These gases trap heat in the atmosphere, preventing it from escaping into space, thus warming the Earth.

Aerosols

Aerosols can have both cooling and warming effects on the Earth's climate. Some aerosols reflect sunlight back into space, leading to cooling (negative radiative forcing), while others, such as black carbon, absorb sunlight, leading to warming (positive radiative forcing).

Land Use Changes

Changes in land use, such as deforestation and urbanization, can alter the albedo of the Earth's surface, leading to changes in radiative forcing. Deforestation typically leads to a decrease in albedo (less reflection, more absorption of sunlight), resulting in positive radiative forcing, while urbanization can lead to both positive and negative forcing depending on the materials used and the specific changes in land cover.

Solar Variability

Changes in the output of the sun can also affect the Earth's climate. Increased solar activity leads to slight increases in solar irradiance, contributing to positive radiative forcing. However, solar variability has a relatively small impact on climate change compared to the effects of greenhouse gases.

Measurement and Modelling

Radiative forcing is measured and modeled using satellite observations, ground-based observations, and theoretical models. Climate models use radiative forcing as a key input to simulate the response of the climate system to various forcing mechanisms.

Implications for Climate Change

Understanding radiative forcing is essential for predicting future climate change and for developing strategies to mitigate or adapt to its impacts. Policies aimed at reducing emissions of greenhouse gases focus on reducing positive radiative forcing and thus limiting global warming.

See Also

• Climate change
• Global warming
• Greenhouse effect
• IPCC Reports