Which climate driver is the strongest?

Carbon dioxide is the single largest contributor to human-caused radiative forcing, followed by other greenhouse gases such as methane, while aerosols provide a partly offsetting cooling.

Do volcanoes and the Sun drive recent warming?

No; volcanic eruptions cause only brief cooling and measured solar changes are small, so neither can account for the sustained warming, which tracks rising greenhouse gases.

Radiative Forcing and Climate Drivers

The greenhouse gases, aerosols, land-surface changes, and natural solar and volcanic variations that perturb the planet's energy balance, compared on a common scale.

Definition

Radiative forcing is the change in the net energy flux at the top of the atmosphere caused by an imposed driver, and climate drivers are the natural and human agents, from greenhouse gases to volcanoes, that produce such forcings.

Scope

This topic covers the agents that drive climate change and the radiative forcing framework used to compare them. It treats the warming forcings of carbon dioxide, methane, nitrous oxide, and halocarbons, the largely cooling forcing of aerosols and their cloud interactions, surface-albedo changes from land use, and natural drivers including solar irradiance variations and explosive volcanic eruptions, together with the concept of effective radiative forcing that includes rapid adjustments.

Core questions

How is radiative forcing defined and measured?
Which drivers warm and which cool the climate, and by how much?
How do aerosols and clouds complicate the forcing estimate?
How do natural solar and volcanic drivers compare with human ones?

Key theories

Common-scale comparison of drivers: Expressing every climate driver as a radiative forcing in watts per square meter allows their relative strengths to be compared and summed to estimate the total perturbation to the energy balance.
Effective radiative forcing: Including rapid atmospheric and surface adjustments that occur before the surface temperature responds gives a more accurate predictor of the eventual warming than the instantaneous forcing alone.

Mechanisms

Each driver alters the energy balance differently: greenhouse gases reduce outgoing thermal radiation, aerosols scatter or absorb sunlight and modify clouds, brighter or darker land surfaces change reflected sunlight, volcanic sulfate veils reflect sunlight for a few years, and solar variations change incoming energy. Summing these forcings, with rapid adjustments included as effective radiative forcing, yields the net push on the climate that the system responds to through feedbacks.

Clinical relevance

Knowing the size of each forcing identifies which human activities most strongly drive warming and which, such as aerosol emissions, partly mask it, directly guiding mitigation priorities and air-quality policy.

Evidence & guidelines

The IPCC Sixth Assessment Report assesses that the total human-caused effective radiative forcing since the pre-industrial era is strongly positive and dominated by carbon dioxide, with aerosols providing the largest offsetting and most uncertain cooling.

History

The radiative forcing concept was developed to compare diverse climate influences on a single scale and has been refined across IPCC assessments, most recently by adopting effective radiative forcing to better account for rapid adjustments, especially from aerosols and clouds.

Debates

Magnitude of aerosol-cloud forcing: How strongly aerosols cool the climate by modifying clouds is the largest uncertainty in the total forcing, and resolving it would sharpen estimates of human influence and climate sensitivity.

Key figures

Gunnar Myhre
Veerabhadran Ramanathan
Susan Solomon
James Hansen

Seminal works

myhre2013
ipccar6wg1

Frequently asked questions

Which climate driver is the strongest?: Carbon dioxide is the single largest contributor to human-caused radiative forcing, followed by other greenhouse gases such as methane, while aerosols provide a partly offsetting cooling.
Do volcanoes and the Sun drive recent warming?: No; volcanic eruptions cause only brief cooling and measured solar changes are small, so neither can account for the sustained warming, which tracks rising greenhouse gases.