Is There a Human Effect on the Climate System?

As discussed in depth in the NRC (2005) report, the human influences on the climate system are diverse and include, in addition to the radiative effect of the well-mixed greenhouse gases such as carbon dioxide and methane, diverse influences from aerosols, land-use/land-cover change, the biogeochemical effects of enhanced CO2 and of nitrogen deposition. As concluded in the multi-authored paper Nonlinearities, Feedbacks, and Critical Thresholds within the Earth’s Climate System:

“The Earth’s climate system is highly nonlinear: inputs and outputs are not proportional, change is often episodic and abrupt, rather than slow and gradual, and multiple equilbria are the norm……..It is imperative that the Earth’s climate system research community embrace this nonlinear paradigm if we are to move forward in the assessment of the human influence on climate.”

The IPCC studies have convincingly shown that there are long-term effects on the climate system due to the human input of carbon dioxide into the atmosphere. Indeed, it should be no surprise that when we change the composition of the Earth’s atmosphere, we alter its energy budget, and thus other aspects of the climate system. The IPCC conclusions should be interpreted, however, as process studies as discussed in the July 15 blog on this website (What are Climate Models? What do They Do?). In that context they tell us that elevated atmospheric concentrations of well-mixed greenhouse gases due to human activity do affect our climate system.

However, carbon dioxide is just one human climate forcing. It is not the only first-order climate forcing, as clearly articulated in the NRC (2005) report. The term “global warming” has been used as a synonym for climate change, and is now the basis for extensive economic activity (for an example, see the July 7, 2005 article on carbon permits in the Economist; subscription required). Such a narrow focus, and use of the term “global warming” fails to recognize that other first order climate forcings exist, in addition to the radiative forcing of carbon dioxide. The issuance of carbon permits will not satisfactorily address the more complex (and realistic) questions as to how human activity is altering the climate system. “Global warming” is a grossly inadequate term to characterize the actual human effect on the climate system.

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