Global Energy Budget and Radiation

The accounting of incoming solar radiation, reflected sunlight, and outgoing thermal radiation that determines how much energy the climate system retains.

Definition

The global energy budget is the quantitative balance of energy entering, leaving, and being redistributed within the climate system, dominated by the exchange of solar and terrestrial radiation at the top of the atmosphere and the surface.

Scope

This topic covers the flows of radiant and turbulent energy through the climate system: the solar radiation intercepted at the top of the atmosphere, the fraction reflected by clouds, aerosols, and the surface that sets the planetary albedo, the absorption of sunlight by the atmosphere and surface, and the outgoing longwave radiation. It includes the surface energy budget with its sensible and latent heat fluxes, and the observational closure of these flows using satellites and surface networks.

Core questions

• How much solar radiation is reflected versus absorbed by the planet?
• What sets the magnitude of outgoing longwave radiation to space?
• How are the surface energy fluxes partitioned among radiation, sensible heat, and evaporation?
• How well can the budget be closed from observations?

Key theories

Top-of-atmosphere balance

Averaged over the globe and over time, absorbed solar radiation closely balances outgoing longwave radiation, with any imbalance reflecting heat accumulating in or leaving the system.

Surface energy partitioning

At the surface, net radiation is balanced by sensible heat, latent heat of evaporation, and storage, and the partitioning controls local temperature and the hydrological cycle.

Mechanisms

About a third of incoming sunlight is reflected back to space by clouds, aerosols, and bright surfaces, while the rest is absorbed by the atmosphere and surface; the warmed surface returns energy upward as thermal radiation, evaporation, and sensible heat, and greenhouse gases trap part of the thermal radiation so that the system reaches balance only at an elevated temperature. Satellite radiometers measure the top-of-atmosphere fluxes and surface networks constrain the ground-level budget.

Clinical relevance

A small persistent imbalance at the top of the atmosphere, currently a net energy gain, is the fundamental measure of global warming, as the excess energy accumulates mainly in the ocean.

History

Early twentieth-century estimates of the energy budget relied on sparse surface measurements, but the launch of Earth-radiation-budget satellites from the 1980s onward, culminating in instruments such as CERES, allowed the global flows to be measured directly and the budget to be closed within observational uncertainty.

Debates

Closing the surface energy budget

The exact magnitude of surface fluxes, especially downward longwave radiation and evaporation, has been revised as new satellite and surface datasets refine the global numbers.

Key figures

• Kevin Trenberth
• Martin Wild
• Norman Loeb
• Veerabhadran Ramanathan

Seminal works

• trenberth2009
• hartmann2016

Frequently asked questions

What is planetary albedo?

It is the fraction of incoming sunlight that the Earth reflects back to space, currently about 0.3, set mainly by clouds, snow, ice, and aerosols.

Where does the extra trapped energy go?

More than ninety percent of the current energy imbalance is taken up by the ocean, with the rest warming the land, atmosphere, and melting ice.