Climate Change and Forcing
The natural and human drivers that perturb the planet's energy balance, the warming they produce, and how observed change is attributed to its causes.
Definition
Climate forcing is an externally imposed perturbation to the planetary energy balance, measured as radiative forcing, and climate change is the resulting long-term shift in the statistics of the climate system, dominated since the industrial era by human activities.
Scope
This area covers the forcings that drive climate change and the resulting observed and projected changes. It treats radiative forcing from greenhouse gases, aerosols, solar variations, and volcanic eruptions; the carbon cycle that governs how emissions translate into atmospheric concentrations; the formal detection and attribution of observed change to human and natural causes; and the possibility of crossing thresholds into abrupt or irreversible change.
Sub-topics
Core questions
- What forcings drive climate change and how large is each?
- How are emissions translated into atmospheric concentrations by the carbon cycle?
- How is observed change attributed to human and natural causes?
- What thresholds could trigger abrupt or irreversible change?
Key theories
- Radiative forcing framework
- Diverse drivers of climate change can be compared on a common scale of radiative forcing, the imposed change in the energy balance, which to first order predicts the resulting global temperature response.
- Detection and attribution
- By comparing observed patterns of change with the distinct fingerprints expected from each forcing, the human contribution to warming can be quantitatively separated from natural causes.
Mechanisms
Human emissions of greenhouse gases and aerosols, along with natural solar and volcanic variations, impose radiative forcings that push the energy balance away from equilibrium; the carbon cycle determines how much of the emitted carbon remains airborne, and feedbacks amplify the response. The resulting warming, sea-level rise, and circulation changes carry distinct spatial and temporal fingerprints that detection-and-attribution methods use to identify their causes.
Clinical relevance
Quantifying forcings and attributing observed change is the scientific basis for holding human activity responsible for current warming and for projecting future change under different emission scenarios, directly informing climate policy.
Evidence & guidelines
The IPCC Sixth Assessment Report concludes that it is unequivocal that human influence has warmed the atmosphere, ocean, and land, and quantifies the radiative forcing of each driver and the attributable human contribution to observed warming.
History
From Arrhenius's nineteenth-century estimate of carbon dioxide warming, through Callendar's and Keeling's twentieth-century measurements and the rise of climate modeling, the science of climate forcing matured into the formal detection-and-attribution framework that underpins successive IPCC assessments.
Debates
- Magnitude of aerosol forcing
- The cooling forcing from aerosols is the most uncertain term in the radiative budget, which propagates into uncertainty about the total human forcing and inferred climate sensitivity.
Key figures
- Syukuro Manabe
- James Hansen
- Veerabhadran Ramanathan
- Susan Solomon
Related topics
Seminal works
- ipccar6wg1
- weart2008
Frequently asked questions
- What is radiative forcing?
- It is the change in the planet's energy balance, in watts per square meter, caused by a driver such as added greenhouse gases, used to compare the strength of different climate influences.
- How do scientists know recent warming is human-caused?
- Detection-and-attribution studies match the observed patterns of change to the distinct fingerprints of each forcing, and only the inclusion of human emissions reproduces the observed warming.