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Climate sensitivity

Filed under: — gavin @ 28 November 2004 - (Français)

Climate sensitivity is a measure of the equilibrium global surface air temperature change for a particular forcing. It is usually given as a °C change per W/m2 forcing. A standard experiment to determine this value in a climate model is to look at the doubled CO2 climate, and so equivalently, the climate sensitivity is sometimes given as the warming for doubled CO2 (i.e. from 280 ppm to 560 ppm). The forcing from doubled CO2 is around 4 W/m2 and so a sensitivity of 3°C for a doubling, is equivalent to a sensitivity of 0.75 °C/W/m2. The principal idea is that if you know the sum of the forcings, you can estimate what the eventual temperature change will be.

We should underscore that the concepts of radiative forcing and climate sensitivity are simply an empirical shorthand that climatologists find useful for estimating how different changes to the planet’s radiative balance will lead to eventual temperature changes. There are however some subtleties which rarely get mentioned. Firstly, there are a number of ways to define the forcings. The easiest is the ‘instantaneous forcing’ – the change is made and the difference in the net radiation at the tropopause is estimated. But it turns out that other definitions such as the ‘adjusted forcing’ actually give a better estimate of the eventual temperature change. These other forcings progressively allow more ‘fast’ feedbacks to operate (stratospheric temperatures are allowed to adjust for instance), but the calculations get progressively more involved.

Secondly, not all forcings are equal. Because of differences in vertical or horizontal distribution of forcings, some changes can have a more than proportional effect on temperatures. This can be described using a relative ‘efficacy’ factor that depends on the individual forcing. For instance, the effect of soot making snow and sea ice darker has a higher efficacy than an equivalent change in CO2 with the same forcing, mainly because there is a more important ice-albedo feedback in the soot case. The ideal metric of course would be a forcing that can be calculated easily and where every perturbation to the radiative balance had an relative efficacy of 1. Unfortunately, that metric has not yet been found!

CO2 Fertilization

Filed under: — mike @ 28 November 2004

It has sometimes been argued that the earth’s biosphere (in large part, the terrestrial biosphere) may have the capacity to sequestor much of the increased carbon dioxide (CO2) in the atmosphere associated with human fossil fuel burning. This effect is known as “CO2 fertilization” because, in the envisioned scenario, higher ambient CO2 concentrations in the atmosphere literally “fertilize” plant growth. Because plants in turn, in the process of photosynthesis, convert CO2 into oxygen, it is thus sometimes argued that such “co2 fertilization” could potentially provide a strong negative feedback on changing CO2 concentrations.

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El Niño/Southern Oscillation (“ENSO”)

Filed under: — group @ 28 November 2004 - (Français)

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.

Empirical Orthogonal Function (“EOF”)

Filed under: — group @ 28 November 2004

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

Energy Balance Model (“EBM”)

Filed under: — group @ 28 November 2004

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

Forcings

Filed under: — group @ 28 November 2004 - (Français)

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).

General Circulation Model (“GCM”)

Filed under: — group @ 28 November 2004 - (Français)

Typically refers to a three-dimensional model of the global atmosphere used in climate modeling (often erroneously called “Global Climate Model”). This term often requires additional qualification (e.g., as to whether or not the atmosphere is fully coupled to an ocean–see ‘Atmosphere-Ocean General Circulation Model’).

The length scales that are resolved in these models is typically on the order of 100s of kilometers (i.e. features that size or smaller are not directly resolved). The timestep for the models (how often the fields are updated) is usually 20 minutes to an hour. Thus in any day there would be 24 to 72 loops of the main calculations.

The basic variables are the temperature, humidity, liquid/ice water content and atmospheric mass. The physics usually consists of advection, radiation calculations, surface fluxes (latent, sensible heat etc.), convection, turbulence and clouds. More elaborate Earth System models often contain tracers related to atmospheric chemistry and aerosols (including dust and sea salt).

Greenhouse Gases (“GHGs”)

Filed under: — group @ 28 November 2004

Greenhouse Gases (GHGs) refer to any atmospheric gases that absorb long wave radiation (emitted from the surface), thereby causing the planet’s surface to be warmer than it would be otherwise. These gases include water vapour, CO2, CH4, N2O, many CFCs (chloro-fluro-carbons). Ozone (O3) as well as being a shortwave absorber (in the ultra-violet range) also has a small longwave greenhouse effect. Other components of the atmosphere also absorb longwave radition (specifically aerosols and clouds) and hence have a greenhouse effect while not being gases themselves.
Oxygen (O2) and nitrogen (N2) while being the dominant gases in the atmosphere do not have significant absorption lines in the relevant longwave range and so are not greenhouse gases.

Hockey Stick

Filed under: — group @ 28 November 2004

Instrumental data describing large-scale surface temperature changes are only available for roughly the past 150 years. Estimates of surface temperature changes further back in time must therefore make use of the few long available instrumental records and natural archives or ‘climate proxy’ indicators, such as tree rings, corals, ice cores and lake sediments, and historical documents, to reconstruct patterns of past surface temperature change. Due to the paucity of data in the Southern Hemisphere, recent studies have emphasized the reconstruction of Northern Hemisphere (NH) mean, rather than global mean temperatures over roughly the past 1000 years.

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Isotopes

Filed under: — gavin @ 28 November 2004

Isotopes can be thought of as different ‘flavours’ of a particular element (such as oxygen or carbon), that are distinguished by the number of neutrons in their nucleus (and hence their atomic mass). Carbon for instance most commonly has a mass of 12 (written as 12C), but there are also a small fraction of carbon atoms with mass 13 and 14 (13C and 14C), similarly oxygen is normally 16O, but with small amounts of 17O and 18O. All of the isotopes of an element behave in similar way chemically. However, because the mass of each isotope is slightly different there are certain physical processes that will discriminate (or ‘fractionate’) between them. For instance, during evaporation of water, it is slightly easier for the lighter isotopes to escape from the liquid, and so water vapour generally has less 18O than the liquid water from which it came. Because of these physical effects, looking at the ratio of one isotope to another can often be very useful in tracing where these atoms came from.