21st century solar cooling

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.