A recent paper in Geophysical Research Letters by K. Zickfeld and colleagues (“Is the Indian summer monsoon stable against global change?” provides an example of investigating multiple climate forcings. According to their study, sulfur emissions and/or land-use changes as they affect planetary albedo, or natural variations in insolation and CO2 concentrations, could trigger abrupt transitions between different monsoon regimes. While the paper uses a simple box model of the tropical atmosphere, it is a start at investigating a set of multiple climate forcings as causing rapid transitions of climate in India. Such rapid transitions are already part of the natural system; see Rial, J., R.A. Pielke Sr., M. Beniston, M. Claussen, J. Canadell, P. Cox, H. Held, N. de Noblet-Ducoudre, R. Prinn, J. Reynolds, and J.D. Salas, 2004: Nonlinearities, feedbacks and critical thresholds within the Earth’s climate system. Climatic Change, 65, 11-38.)
In contrast to the nearly linear, and monotonic predicted trends from the anthropogenic increases of CO2 such as reported by the IPCC, the occurrence of such sudden changes are more the norm and would have major societal impacts. Such nonlinear climate system responses, however, are likely to be impossible to skillfully predict. This provides further impetus to adopt the vulnerability perspective such as promoted by Jon Foley
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The August 11 2005 paper published in Science Express Reports (Sherwood S., J. Lanzante ,and Cathryn Meyer: Radiosonde daytime biases and late-20th century warming; subscription required), for example, perpetuate the emphasis on large-scale linear trend analysis, in their study of tropospheric temperature trends. This is because the General Circulation Models focus on global- and zonally-averaged temperature trends, and linear trends from the observations are being compared with linear trends from the GCMs. As quoted by one of the authors (Sherwood) in a New York times interview (free subscription required).
“Things being debated now are details about the models,” said Steven Sherwood, the lead author of the paper on the balloon data and an atmospheric physicist at Yale. “Nobody is debating any more that significant climate changes are coming.”
This statement is based on a linear analysis.
However, while their study of the accuracy of linear trends determined from radiosondes is scientifically interesting, if the Geophysical Research Letters study by Zickfeld et al. has merit, it is to show us that assessing large-scale linear trends is of little practical use in estimating our real threat from future climate change.