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Forcing due to human, rather than natural, factors. Such factors include increased greenhouse gas concentrations associated with fossil fuel burning, sulphate aerosols produced as an industrial by-product, human-induced changes in land surface properties among other things.

Measure of the pressure gradient between the polar and subpolar regions of the Northern Hemisphere. The term was introduced by Thompson and Wallace (2000). More information on the AO can be found here. See also North Atlantic Oscillation”(NAO”).

A multidecadal (50-80 year timescale) pattern of North Atlantic ocean-atmosphere variability whose existence has been argued for based on statistical analyses of observational and proxy climate data, and coupled Atmosphere-Ocean General Circulation Model (“AOGCM”) simulations. This pattern is believed to describe some of the observed early 20th century (1920s-1930s) high-latitude Northern Hemisphere warming and some, but not all, of the high-latitude warming observed in the late 20th century. The term was introduced in a summary by Kerr (2000) of a study by Delworth and Mann (2000).

Fully coupled atmosphere-ocean model of the three-dimensional global climate. See also ‘General Circulation Model (GCM)’.

Approach to reconstructing a target large-scale climate field from predictors employing multivariate regression methods. CFR methods have been applied both to filling spatial gaps in early instrumental climate data sets, and to the problem of reconstructing past climate patterns from ‘climate proxy’ data.

Climate ‘proxies’ are sources of climate information from natural archives such as tree rings, ice cores, corals, lake and ocean sediments, tree pollen, or human archives such as historical records or diaries, which can be used to estimate climate conditions prior to the modern period (e.g. mid 19th century to date) during which widespread instrumental measurements are available. Proxy indicators typically must be calibrated against modern instrumental information to yield a quantitative reconstruction of past climate.

A natural coupled mode of climate variability associated with both surface temperature variations tied to El Niño and atmospheric circulation changes across the equatorial Pacific (see also ‘Southern Oscillation Index’). Term was first coined by Rasmusson and Carpenter (1982). More information on ENSO can be found here.

Spatial pattern tied to a particular mode of time/space variance in a spatiotemporal data set (see also “Principal Components Analysis or “PCA”).

Simple climate model consisting of a uniform ocean and atmosphere that respond thermodynamically, but not dynamically, to changes in radiative forcing.

Forcings in the climate sense are external boundary conditions or inputs to a climate model. Obviously changes to the sun’s radiation are external, and so that is always a forcing. The same is true for changes to the Earth’s orbit (“Milankovitch cycles”). Things get a little more ambigous as you get closer to the surface. In models that do not contain a carbon cycle (and that is most of them), the level of CO₂ is set externally, and so that can be considered a forcing too. However, in models that contain a carbon cycle, changes in CO₂ concentrations will occur as a function of the climate itself and in changes in emissions from industrial activity. In that case, CO₂ levels will be a feedback, and not a forcing. Almost all of the elements that make up the atmosphere can be considered feedbacks on some timescale, and so defining the forcing is really a function of what feedbacks you allow in the model and for what purpose you are using it. A good discussion of recent forcings can be found in Hansen et al (2002) and in Schmidt et al (2004).