March 21, 2006

Solar Warming?

Filed under: Climate Forcings, Solar

Just when you were starting to believe that variations in the amount of energy coming from the sun weren’t responsible for much of the observed surface warming during the past 20 years, comes along a paper in Geophysical Research Letters from two researchers at Duke University, Nicola Scafetta and Bruce West, that concludes otherwise:

We estimate that the sun contributed as much as 45–50% of the 1900–2000 global warming, and 25–35% of the 1980–2000 global warming. These results, while confirming that anthropogenic-added climate forcing might have progressively played a dominant role in climate change during the last century, also suggest that the solar impact on climate change during the same period is significantly stronger than what some theoretical models have predicted.

Scafetta and West arrive at their conclusions after applying a mathematical scheme that allows the cycles in solar variations to explain the cycles in temperature variations. They find this empirical method far superior to theoretical (i.e. climate models) methods because empirical methods take advantage of real behavior while theoretical methods are just that—theories—which very likely do not capture all of the real-world intricacies relating solar energy to climate processes.

The authors summarize:

The sun played a dominant role in climate change in the early past, as several empirical studies would suggest, and is still playing a significant, even if not a predominant role, during the last decades. The impact of solar variation on climate seems significantly stronger than predicted by some energy balance models…The significant discrepancy between empirical and theoretical model estimates might arise because the secular TSI [total solar irradiance] proxy reconstructions are disputed and/or because the empirical evidence deriving from the deconstruction of the surface temperature is deceptive for reasons unknown to us. Alternatively, the models might be inadequate because of the difficulty of modeling climate in general and a lack of knowledge of climate sensitivity to solar variations in particular. In fact, theoretical models usually acknowledge as solar forcing only the direct TSI forcing while empirical estimates would include all direct and indirect climate effects induced by solar variation. These solar effects might be embedded in several climate forcings because, for example, a TSI increase might indirectly induce a change in the chemistry of the atmosphere by increasing and modulating its greenhouse gas (H2O, CO2, CH4, etc.) concentration because of the warmer ocean, reduce the earth albedo by melting the glaciers and change the cloud cover patterns. In particular, the models might be inadequate: (a) in their parameterizations of climate feedbacks and atmosphere-ocean coupling; (b) in their neglect of indirect response by the stratosphere and of possible additional climate effects linked to solar magnetic field, UV radiation, solar flares and cosmic ray intensity modulations; (c) there might be other possible natural amplification mechanisms deriving from internal modes of climate variability which are not included in the models. All the above mechanisms would be automatically considered and indirectly included in the phenomenological approach presented herein.

The bigger the observed solar impact, the smaller the observed human impact. The smaller the human impact, the less sensitive the climate is to greenhouse gas emissions. The less sensitive the climate is to greenhouse gas emissions, the less the impact greenhouse changes (and greenhouse gas emissions restrictions) will have in the future.


Scafetta, N., and B. J. West, 2006. Phenomenological solar contribution to the 1900-2000 global surface warming. Geophysical Research Letters, doi: 1029/2005GL025539.

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