Comment on “Climate forcing by the volcanic eruption of Mount Pinatubo” by David H. Douglass and Robert S. Knox

Abstract

[1] Douglass and Knox [2005, hereinafter referred to as DK] present a confusing and erroneous description of climate feedbacks and the climate response to the 1991 Mt. Pinatubo eruption. Their conclusions of a negative climate feedback and small climate sensitivity to volcanic forcing are not supported by their arguments or the observational evidence. As pointed out by Wigley et al. [2005a], this is the consequence of assuming a one-box representation for the climate system, and ignoring energy exchange with the deep ocean. [2] In the description of their analysis, DK make a fundamental mistake in describing the problem. They claim to use ‘‘standard linear response theory,’’ but they confuse the response with the forcing. They say, ‘‘The LW [longwave] effect is by definition the forcing function DF for the climate, represented by the measured surface temperature anomaly DT.’’ LW radiation changes, however, are produced by both the presence of a forcing agent, in this case stratospheric aerosols, and the response of the climate system. It is the instantaneous net radiation change with no response that is the forcing. The temperature anomaly is the response to forcing, and not the forcing itself, and the LW changes reflect both the true forcing and the effects of the temperature response. [3] In spite of their statement to the contrary, DK apparently do use the correct forcing, as characterized by their equation (5) and illustrated in their Figure 2. Confusion arises because the forcing they use is a scaled version of the estimated aerosol optical depth changes, which represents the true forcing, yet they continually refer to the scaled optical depth as the LW changes. That the two items are different is clear from DK’s Figure 1. [4] The forcing of climate change can be defined as the change in the net radiation at the top of the atmosphere, the tropopause, or the surface, without any response of the climate system, or allowing for the stratosphere to equilibrate. Stenchikov and Robock [1995], Houghton et al. [1996], and Hansen et al. [2005] discuss the standard definitions of radiative forcing and considerations for forcing from aerosols, which are not uniformly mixed in the atmosphere. For our purposes, it is sufficient to consider the forcing at the top of the atmosphere, allowing for no equilibration [Stenchikov and Robock, 1995; Stenchikov et al., 1998]. This forcing can be defined as: