Image:Climate Change Attribution.png

From Global Warming Art

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Description

Instrumental temperature record of the last 150 years. This data is the same as the black curve in the figure.

Estimates by the Intergovernmental Panel on Climate Change of the relative importance of various forcing factors since 1750.

This figure, based on Meehl et al. (2004), shows the ability with which a global climate model (the DOE PCM [1]) is able to reconstruct the historical temperature record and the degree to which the associated temperature changes can be decomposed into various forcing factors. The top part of the figure compares a five year average of global temperature measurements (Jones and Moberg 2001) to the Meehl et al. results incorporating the effects of five predetermined forcing factors: greenhouse gases, man-made sulfate emissions, solar variability, ozone changes (both stratospheric and tropospheric), and volcanic emissions (including natural sulfates). The time history and radiative forcing effectiveness for each of these factors was specified in advance and was not adjusted to specifically match the temperature record.

Also shown are grey bands indicating the 68% and 95% range for natural variability in the five year average of temperature as determined from multiple simulations with different initial conditions. In other words, the bands indicate the estimated size of fluctuations that are expected to result from changes in weather rather than changes in climate. Ideally the model should be able to reconstruct temperature variations to within about the tolerance specified by these bands. Though the model captures the gross features of twentieth century climate change, it remains likely that some of the differences between model and observation reflect the limitations of the model and/or our understanding of the histories of the observed forcing factors.

In the lower portion of the figure are the results of additional simulations in which the model was operated with only one forcing factor at a time. A key conclusion of the Meehl et al. (2004) work is that the model response to all factors combined is approximately equal to the sum of the responses to each of the factors taken individually. They conclude therefore that it is reasonable to discuss how the evolving man-made and natural influences individually impact climate. Meehl et al. attribute most of the 0.52 °C global warming between 1900 and 1994 to a 0.69 °C temperature forcing from greenhouse gases partially offset by a 0.27 °C cooling due to man-made sulfate emissions and with other factors contributing the balance. This contrasts with the warming from 1900 to 1940 for which the model only attributes a net increases of 0.06 °C to the combined effects of greenhouse gases and sulfate emissions. The zeros on both plots are set equal to 1900 temperatures.

	Temperature change relative to 1900
	1940	1970	1994
Greenhouse gases	0.10	0.38	0.69
Sulfate emissions	-0.04	-0.19	-0.27
Solar forcing	0.18	0.10	0.21
Volcanic forcing	0.11	-0.04	-0.14
Ozone	-0.06	0.05	0.08
Net	0.19	0.17	0.53
Observed	0.26	0.21	0.52

Note that "Net" reflects the model runs with all factors included and is not identical to simply summing the individual factors.

References

Meehl, G.A., W.M. Washington, C.A. Ammann, J.M. Arblaster, T.M.L. Wigleym and C. Tebaldi (2004). "Combinations of Natural and Anthropogenic Forcings in Twentieth-Century Climate". Journal of Climate 17: 3721-3727.
Jones, P.D. and Moberg, A. (2003). "Hemispheric and large-scale surface air temperature variations: An extensive revision and an update to 2001". Journal of Climate 16: 206-223.

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This figure was created by Robert A. Rohde from published data.

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