More on the Relative Importance of the Radiative Forcing of CO2

In the weblog of April 27 entitled “What Fraction of Global Warming is Due to the Radiative Forcing of Increased Atmospheric Concentrations of CO2?” it was shown that, with respect to the human climate forcings, the globally averaged positive (warming) radiative forcing contribution due to CO2 is about 28%. This value is a conservative estimate, and could be lower.

However, this is a global average. As discussed on the Climate Science weblog (e.g. see), it is the regional tropospheric temperature variability and trends which have a much greater influence on weather patterns that matter to society and the environment (e.g. droughts, hurricanes).

The concept of a regional radiative forcing metric to expand our assessment of human- and natural climate forcings has been introduced on this weblog (see), based on a newly accepted peer reviewed paper (see).

The abstract of the paper reads,

“This paper diagnoses the spatial mean and the spatial gradient of the aerosol radiative forcing in comparison with those of well-mixed green-house gases (GHG). Unlike GHG, aerosols have much greater spatial heterogeneity in their radiative forcing. The heterogeneous diabatic heating can modulate the gradient in horizontal pressure field and atmospheric circulations, thus altering the regional climate. For this, we diagnose the Normalized Gradient of Radiative Forcing (NGoRF), as a fraction of the present global heterogeneous insolation attributed to human activity. Although the GHG has a larger forcing (+1.7 Wm-2) as measured than those of aerosol direct (-1.59 Wm-2) and possible indirect effect (-1.38 Wm-2) in terms of a spatially averaged top of atmosphere (TOA) value, the aerosol direct and indirect effects have far greater NGoRF values (~0.18) than that of GHG (~0.003).”

This new study documents for aerosol radiative forcing, using the concept of a normalized gradient of radiatve heating, that the direct and indirect aerosol effect is 60 time greater than that of the well mixed greenhouse gases!

Other spatially heterogeneous climate forcings will have similarly large weather impacts, as shown by Feddema et al. 2005: The importance of land-cover change in simulating future climates., 310, 1674-1678.

The focus on a globally averaged radiative forcing is missing the much more important influences of spatial variations in climate forcings. If the IPCC were less of an advocacy document, and more of a balanced scientific assessment, this issue would be highlighted in their Report.

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