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Climate Predictions and the Sun

By: Dr. Ricky Rood, 6:42 PM GMT on October 05, 2008

The Sun (5): This is the last of a series on the Sun in the Earth-Sun climate system. The first four entries are linked at the end. I want to first thank both Judith Lean at the Naval Research Laboratory and David Rind of NASA for their introduction to the current literature and insights into the problem. Lean and Rind have a new paper How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006. This appears in Geophysical Research Letters. To entice you to their paper I provide the following quote from their abstract: “We find a response to solar forcing quite different from that reported in several papers published recently in this journal, and zonally averaged responses to both natural and anthropogenic forcings that differ distinctly from those indicated by the Intergovernmental Panel on Climate Change, whose conclusions depended on model simulations.” Their calculation of the solar contribution to temperature trends is 0.007 degrees C per decade. The part of the trend from the enterprise of humans (OK “manmade”) is 0.05 degrees C per decade – 7 times larger. At the end, I list a set of references with much information on the solar influence on climate change. (Some of them are quite long, so don’t click casually!)

This series of blogs was motivated by several comments on previous blogs asking for general information about solar variability and the Earth’s climate, how solar variability is incorporated into climate predictions, and questions about the current lack of sunspots. This is, at last, the one about the inclusion of solar variability in models. The short answer is that many climate models include the variability associated with the sunspot cycle. Basically, they extend the sunspot cycle that has been observed in the past decades in to the future; they use a measure of average solar irradiance variability that is characteristic of the sunspot cycle. In general, a trend in solar irradiance is not included.

If one were to look into the literature of solar variability in climate models, one does find than many of the simulations of the past century include a representation of a trend in solar irradiance. Looking at those papers that have simulated the past 2000 years, there are changes associated with the Maunder Minimum and the Medieval Warm Period. There are also a set of papers that look at the impact of similar changes in the future climate. The bottom line from these experiments is that compared with temperature changes due to increasing carbon dioxide is that both observations and model estimates of solar-induced temperature changes is small. (A good summary of this is Chapter 2 of the IPCC report linked at the end.)

The earlier blogs, I hope, clarified the point about the relative size of the solar and greenhouse contributions to the warming of the Earth’s surface. Also, it documented that the current sunspot minimum is not, as of yet, outside of our realm of experience.

These blogs also stated that our ability to model the impacts of solar variability on the Earth’s climate has significant shortcomings. Broadly, the impact that we model is weaker than the impact that we observe. These leads to the idea that we need to understand what is missing in this link … current knowledge would suggest the need to find what “amplifies” the solar signal. This is a source of uncertainty in climate change projections, but it is quantifiable, and like the solar signal itself, the uncertainty is relatively small.

In the references listed below is a comprehensive review of solar variability and its inclusion in climate models by Leslie Gray and coworkers. At the end of that document is a list of challenges for climate change modelers. Their recommendations prescribe a method for including irradiance measures, considering the variation in several spectral bands, and a better representation of solar-induced changes in ozone. They also recommend several experiments to investigate the uncertainties associated with solar variability. This is a good example of how the climate community operates.


News from NASA: Blankest Year for Sunspots in the Space Age

Some references on solar variability:

How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006 by Judith Lean and David Rind. A newly published data and model paper that includes a excellent analysis of solar variability.

Living with a Variable Sun by Judith Lean. An excellent readable document for science-interested people.

The Sun and the Earth’s Climate by Joanna Haigh An online document that summarizes the state of knowledge of solar variability and the Earth’s climate.

The Influence of Solar Variability on the Earth’s Climate L. J. Gray, J. D. Haigh, and R. G. Harrison. (This is large file and document from the Hadley Center, published in 2005.)

Chapter 2 of the IPCC report A summary of the literature on solar variability in about 2005-2006, with documentation of some important changes for the previous five years. See page 188.

Blogs on the Sun.

The Sun (1)

The Sun (2)

The Sun (3)

The Sun (4)

This figure shows how solar (visible) and terrestrial (infrared) radiation flows through the atmosphere. This is an updated figure provided by Kevin Trenberth and will appear in the Bulletin of the American Meteorological Society in the article “Earth’s global energy budget,” by Kevin E. Trenberth, John T. Fasullo and Jeffrey Kiehl.

Blogs on radiative balance

Ice Water
Clouds Cool and Warm
Aerosols Cool and Warm

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.