Range of Uncertainty in IPCC Global Warming Challenged. (July 6, 2007)

    A challenge to one part of the latest climate assessment by the Intergovernmental Panel on Climate Change (IPCC) "is not a question of whether the Earth is warming or whether it will continue to warm" under human influence, according to University of Washington, Seattle atmospheric scientist Robert Charlson one of three authors (Stephen E. Schwartz, Robert J. Charlson & Henning Rodhe) of a commentary published online last week in Nature Reports: Climate Change.

In their summary they say: "The latest report from the Intergovernmental Panel on Climate Change assesses the skill of climate models by their ability to reproduce warming over the twentieth century, but in doing so may give a false sense of their predictive capability."

As Science puts it the authors: "argue that the simulation by 14 different climate models of the warming in the 20th century is not the reassuring success IPCC claims it to be. Future warming could be much worse than that modeling suggests, they say, or even more moderate. IPCC authors concede the group has a point, but they say their report--if you look in the right places--reflects the uncertainty the critics are pointing out." According to the authors greenhouse-gas changes are well known, but that's not the case for the counteracting cooling of pollutant hazes, i.e. aerosols. Aerosols cool the planet by reflecting away sunlight and increasing the reflectivity of clouds.

However, the authors also say: "The century-long lifetime of atmospheric CO₂ and the anticipated future decline in atmospheric aerosols mean that greenhouse gases will inevitably emerge as the dominant forcing of climate change, and in the absence of a draconian reduction in emissions, this forcing will be large. Such dominance can be seen, for example, in estimates from the third IPCC report of projected total forcing in 2100 for various emissions scenarios... Depending on which future emissions scenario prevails, the projected forcing is 4 to 9 W m^-2. This is comparable to forcings estimated for major climatic shifts, such as that for the end of the last ice age3. Developing effective strategies, both to limit emissions of CO₂ and to adapt to the inevitable changes in global climate will depend on climate sensitivity. The magnitude of forcing anticipated in 2100 thus highlights the urgency of reducing uncertainty in Earth's climate sensitivity.