by Judith Curry
During the 20th century, solar activity increased in magnitude to a so-called grand maximum. It is probable that this high level of solar activity is at or near its end. It is of great interest whether any future reduction in solar activity could have a significant impact on climate that could partially offset the projected anthropogenic warming. (Jones et al. 2012).
Two recent papers suggest that there will be little impact in the 21st century from a decrease in solar insolation similar to what was seen in the Maunder minimum.
What influence will future solar activity changes over the 21st century have on projected global near-surface temperature changes?
Gareth S. Jones, Mike Lockwood, and Peter A. Stott
During the 20th century, solar activity increased in magnitude to a so-called grand maximum. It is probable that this high level of solar activity is at or near its end. It is of great interest whether any future reduction in solar activity could have a significant impact on climate that could partially offset the projected anthropogenic warming. Observations and reconstructions of solar activity over the last 9000 years are used as a constraint on possible future variations to produce probability distributions of total solar irradiance over the next 100 years. Using this information, with a simple climate model, we present results of the potential implications for future projections of climate on decadal to multidecadal timescales. Using one of the most recent reconstructions of historic total solar irradiance, the likely reduction in the warming by 2100 is found to be between 0.06 and 0.1 K, a very small fraction of the projected anthropogenic warming. However, if past total solar irradiance variations are larger and climate models substantially underestimate the response to solar variations, then there is a potential for a reduction in solar activity to mitigate a small proportion of the future warming, a scenario we cannot totally rule out. While the Sun is not expected to provide substantial delays in the time to reach critical temperature thresholds, any small delays it might provide are likely to be greater for lower anthropogenic emissions scenarios than for higher-emissions scenarios.
Citation: Jones, G. S., M. Lockwood, and P. A. Stott (2012), What influence will future solar activity changes over the 21st century have on projected global near-surface temperature changes?, J. Geophys. Res., 117, D05103, doi:10.1029/2011JD017013. [Link]
On the effect of a new grand minimum of solar activity on the future climate on Earth
Georg Feulner and Stefan Rahmstorf
The current exceptionally long minimum of solar activity has led to the suggestion that the Sun might experience a new grand minimum in the next decades, a prolonged period of low activity similar to the Maunder minimum in the late 17th century. The Maunder minimum is connected to the Little Ice Age, a time of markedly lower temperatures, in particular in the Northern hemisphere. Here we use a coupled climate model to explore the effect of a 21st‐century grand minimum on future global temperatures, finding a moderate temperature offset of no more than −0.3°C in the year 2100 relative to a scenario with solar activity similar to recent decades. This temperature decrease is much smaller than the warming expected from anthropogenic greenhouse gas emissions by the end of the century.
Citation: Feulner, G., and S. Rahmstorf (2010), On the effect of a new grand minimum of solar activity on the future climate on Earth, Geophys. Res. Lett., 37, L05707, doi:10.1029/ 2010GL042710. [link]
Both of these papers generally come to the same conclusion: a small impact, nominally 0.1C, from an insolation change similar to a Maunder Minimum.
Feulner and Rahmstorf consider decreases in the solar constant determined from historic reconstructions of 0.08% and 0.25%. Whereas Jones et al. consider the reconstructions of Lean (2000), Krivova et al (2007) and Lean (2009). Referring also to the slide on Pending Maunder Minimum? of Judith Lean’s presentation discussed recently on the Solar Discussion II thread. For 21st projections, these different reconstructions imply the following insolation reductions (note the first threeeare my eyeball interpolations from Lean’s slide):
- Lean 2000: 2.2 W m-2
- Wang et al. 2005: 0.4 W m-2
- Krivova et al. 2007: 0.8 W m-2
- 0.25% reduction: 3.4 W m-2
- 0.08 reduction: 1.1 W m-2
Feulner and Rahmstorf select reconstructions with larger reductions in insolation than Jones et al.; however neither includes the recent reconstruction of Shapiro et al. (2012) that gives a 6 W m-2 reduction.
Fuelner and Rahmstorf use a low order coupled climate model, whereas Jones et al. use a simple energy balance climate model that is tuned to the Hadley AOGCM in terms of sensitivity and ocean heat diffusivity.
While I find the model and experimental design of Feulner and Rahmstorf to be preferable, I find the text of Jones et al. to be more interesting in terms of raising issues. Some points of interest from Jones et al.:
While the IPCC assessed research that investigated the impact of natural forcing factors on past climate, researchers have not methodically examined what impact future changes in natural external forcing factors may have.
JC comment: there has been an implicit assumption by the IPCC that natural forcings are of minor importance. IMO this has been to the great detriment of our understanding of the climate system. Little effort has been made to investigate the impacts of varying forcing reconstructions (e.g. Schmidt et al.) on attribution of past climate change and variability. It seems that this issue has been receiving attention only in the past few years.
Climate modeling and detection and attribution studies show that changes in TSI have a relatively small influence on global temperatures changes over the 20th century, with anthropogenic influences dominating the observed warming.
JC comment: I find this statement to be unsupported by what the IPCC reports, since none of these detection and attribution analyses have seriously addressed the warming in the early part of the 20th century and have not systematically addressed the impacts of different volcano and solar reconstructions and allowed for uncertainties in solar indirect effects.
In the last thousand years or so of the preindustrial era solar activity may have played a relatively important role influencing climate, competing with volcanic activity and human driven land use changes for a dominant influence. A number of studies suggest significant zonal/regional and seasonal impacts on surface climate and at all altitudes over the 11 year solar cycle.
JC comment: Bottom line is that we don’t really know, and IMO this substantially reduces the confidence with which we can say anything about attribution in the latter half of the 20th century.
In the calibration of the simple energy balance climate model to the AOGCM, they find a climate sensitivity parameter for CO2 to be 0.88 K W-1 m2, with values 0.49 for solar and 0.48 for volcanic forcing.
Low climate sensitivity parameters, or efficacies <1, have been noted before for solar and volcanic forcing factors in HadCM3 and in some other models. Differences in the spatial distribution of the radiative forcings can cause differences in efficacies.
JC comment: Lets speculate on the implications of this for solar forcing. The largest insolation occurs in the polar regions during summer ( see fig 12.3 in Lecture 12 at this link). The amount received at the top of the atmosphere during the polar summer is very much larger than the globally and seasonally averaged value that is obtained by dividing by 4. The significance of the much larger irradiance in polar summer (a substantial amount of which makes it to the surface) is associated with ice/snow melt, and its subsequent impact on atmospheric and ocean circulations. So inferring anything about solar sensitivity from a 1D energy balance climate model, such as that a perturbation to irradiance at the top of the atmosphere only implies a 10% change in surface forcing, is not very useful, IMO.
So, is a possible reason for the larger sensitivity to CO2 (relative to solar) an artifact of incorrect treatment of physical processes in the polar regions, e.g. related to cloud and water vapor feedback or sea ice processes? Climate models arguably underestimate the water vapor feedback in the polar regions owing to deficiencies in the treatment of the microphysics of mixed phase clouds and failure of most climate models to adequately treat the “dirty window” in the water vapor rotation band (too complicated here to go into the details of all these here). Not to even get started on the issue of clouds in the polar regions.
So are these model-based studies getting it right in terms of what we might see in the 21st century from an irradiance reduction comparable to the Maunder Minimum? I think we have barely scratched the surface of this problem, since these papers rely on alot of assumptions, many of which I haven’t even mentioned. And I suspect that, because of the seasonal and latitudinal asymmetries in insolation, that the direct solar response may be greatest in the polar regions.
And none of this discussion addresses the issues associated with magnetic fields, cosmic rays, and the latest results from SORCE on spectral variations of irradiance.
What do observational analyses tell us? Observational analyses can implicitly include the effects that are ignored by climate models. Here is one paper that suggests a significantly larger role for solar forcing:
Surface warming by the solar cycle as revealed by the composite mean difference projection
Charles D. Camp Ka Kit Tung
By projecting surface temperature data (1959–2004) onto the spatial structure obtained objectively from the composite mean difference between solar max and solar min years, we obtain a global warming signal of almost 0.2°K attributable to the 11-year solar cycle. The statistical significance of such a globally coherent solar response at the surface is established for the first time.
Citation: Camp, C. D., and K. K. Tung (2007), Surface warming by the solar cycle as revealed by the composite mean difference projection, Geophys. Res. Lett., 34, L14703, doi:10.1029/2007GL030207. [link]
JC summary question: So, how many people are convinced that a reduction of solar irradiance comparable to the Maunder Minimum would translate to a cooling of about 0.1C?