Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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6.1.1. Anthropogenic Climate Change, Variability, and Detection
Other reports in this collection
What is climate change? The common definition of climate refers to the average of weather, yet the definition of the climate system must reach out to the broader
geophysical system that interacts with the atmosphere and our weather. The concept of climate change has acquired a number of different meanings in the scientific
literature and in the media. Often, "climate change" denotes variations resulting from human interference, and "climate variability" refers to natural variations.
Sometimes "climate change" designates variations longer than a certain period. Finally, "climate change" is often taken to mean climate fluctuations of a global nature,
including effects from human activities such as the enhanced greenhouse effect and from natural causes such as volcanic aerosols.
For the purposes of the UNFCCC (and this report), the definition of climate change is: "A change of climate which is attributed directly or indirectly to human activity
that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods." This alteration of the
global atmosphere includes changes in land use as well as anthropogenic emissions of greenhouse gases and particles. This FCCC definition thus introduces the
concept of the difference between the effect of human activities (climate change) and climatic effects that would occur without such human interference (climate
variability).
What drives changes in climate? The Earth absorbs radiation from the sun, mainly at the
surface. This energy is then redistributed by atmospheric and oceanic circulations and radiated
to space at longer ("terrestrial" or "infrared") wavelengths. On average, for the Earth as a whole,
incoming solar energy is balanced by outgoing terrestrial radiation. Any factor that alters
radiation received from the sun or lost to space or the redistribution of energy within the
atmosphere and between atmosphere, land, and ocean can affect climate. A change in radiative
energy available to the global Earth/atmosphere system is termed here, as in previous IPCC
reports, radiative forcing (see Section 6.2 for more details). Radiative forcing (RF) is the global,
annual average of radiative imbalance (W m-2) in net heating of the Earth's lower atmosphere
as a result of human activities since the beginning of the industrial era almost 2 centuries ago.
Increases in the concentrations of greenhouse gases reduce the efficiency with which the
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