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Climate forcing and feedback

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Idea

Climate scientists use the terms ‘forcing’ and ‘feedback’, which are related to cause and effect (see also climate model)

Forcing denotes an external influence on a characteristic of the climate system. Example: Increased emission from the sun leads to an increase of the temperature.

Feedback denotes the reaction of the (climate) system to the forcing which, in return, leads to a change in the forcings. Example: a change in the Earth’s temperature may cause effects that lead to more radiation being absorbed or emitted. This then creates further changes in the Earth’s temperature. This ‘loop’ where a change in temperature creates a further change is called a climate feedback, or simply feedback.

A feedback is said to be positive if warming leads to further warming, and cooling to further cooling. Otherwise it is said to be negative. If the total feedback, counting all mechanisms, were positive, then the Earth’s climate would be unstable.

On this page we’ll discuss the main influences on the earth’s climate, whether forcing or feedback.

List of feedbacks

The five most important feedbacks are:

  • The Planck feedback: The higher the temperature of a radiating body, the more energy it radiates. This is a negative feedback, and it is very strong, so the total feedback is negative for our Earth. In climate science, the Planck feedback is often not mentioned and explained explicitly.

  • Water vapor feedback is the second largest feedback. When it gets warmer, the evaporation-condensation balance shifts in favor of relatively more evaporation, and the water vapor content of the atmosphere increases. But water vapor, like CO2, is a gas, which causes additional warming.

  • The lapse rate is the rate at which air temperature decreases with height. In the tropics, the lapse rate is expected to decrease in response to the enhanced greenhouse effect, amplifying the warming in the upper troposphere and suppressing it at the surface. This suppression causes a negative feedback on surface temperature. Toward the poles, the reverse happens (a positive feedback), but the tropics tend to dominate, producing an overall negative feedback.

  • Clouds create both positive and negative feedbacks. For example, low-level clouds, such as stratocumulus, reflect significant amounts of solar radiation. If our warming climate creates more low clouds, this would give a negative feedback. But if makes there be fewer low clouds, there would be a positive feedback. To complicate the situation further, clouds can also keep infrared radiation from leaving the Earth’s atmosphere.

  • The surface albedo effect refers to mechanisms like this: when it gets warmer, ice near the poles tends to melt. But ice is white, so it reflects sunlight. When ice melts, the landscape gets darker, and absorbs more sunlight, so it gets warmer. Overall this is a positive feedback. For more details, see ice albedo effect.

Details

Feedbacks are measured in units of watts per square meter per degree, denoted W/m2/K. The Planck feedback is called λ 0\lambda_0, and the sum of the rest is called λ\lambda. The definition of feedback is

T=F/(λ 0+λ) T = - F / (\lambda_0 + \lambda)

where TT is the change in temperature caused by a small radiative forcing FF.

Climatologists tend to take the Planck feedback for granted, and focus attention on the non-Planck feedbacks, λ\lambda. One must be a bit careful, because different people use different conventions for feedback. The equation above corresponds to a convention where positive feedback (as explained above) gives a positive contribution to λ\lambda.

Estimated values of climate feedbacks can be found in this paper cited in the AR4 WG1 report?:

In brief:

  • The Planck feedback λ 0\lambda_0 is -3.2 W/m2/K.

  • Climate models predict a range of water vapor feedbacks of 1.48 to 2.14 W/m2/K.

  • Climate models predict a lapse rate feedback of -0.41 to -1.27 W/m2/K.

  • However, water vapor and lapse rate feedbacks are often combined into a single feedback, because stronger water vapor feedbacks also tend to produce stronger lapse rate feedbacks. The combined water vapor+lapse rate feedback ranges between 0.81 to 1.20 W/m2/K.

  • Clouds are the next largest feedback, 0.18 to 1.18 W/m2/K. Different models can predict very different cloud feedbacks. It is the largest feedback uncertainty, and this has been the source of much contention.

  • Estimates of the surface albedo feedback range from 0.07 to 0.34 W/m2/K.

Vegetation feedback

There are many complicating factors, e.g. the growth of new leaves as CO2 levels increase:

The following paper concludes that long term negative feedback from CO2‐induced increases in vegetation density could reduce temperature following a stabilization of CO2 concentration.

  • L. Bounoua, F. G. Hall, P. J. Sellers, A. Kumar, G. J. Collatz, C. J. Tucker, and M. L. Imhoff, Quantifying the negative feedback of vegetation to greenhouse warming: A modeling approach, Geophysical Research Letters 37 (2010)

The vegetation feedback is related to Carbon dioxide fertilization.

Aerosol forcing

From the latter:

According to the latest (2001) IPCC report, direct radiative forcing by anthropogenic aerosols cools the planet, but the forcing magnitude is highly uncertain, with a global, annual average between -0.35 and -1.35 W/m2 at the top of the atmosphere (TOA). The uncertainty of the total indirect effect is even larger. Aerosols eventually fall out of the atmosphere or are washed out by rainfall. The smaller particles having the largest radiative effect typically reside in the atmosphere for only a few days to a few weeks. This time is too short for them to be mixed uniformly throughout the globe (unlike CO2), so there are large regional variations in aerosol radiative forcing, with the largest effects predictably downwind of industrial centers like the east coast of North America, Europe, and East Asia. Consequently, aerosol effects upon climate are larger in particular regions, where they are key to understanding twentieth century climate change.

References

Also see:

Watch out: Roe’s λ\lambda is the negative of the reciprocal of the Soden & Held λ\lambda used above: i.e., it’s a direct proportionality between forcing and temperature.

Note that climate feedback as we have been describing it is a concept from linear perturbation theory: we are assuming that a small change in the forcing will cause a small change in the Earth’s average temperature. For nonlinear aspects, see:

Also see Climate sensitivity, Climate model, and Uncertainty in climate science.

category: climate