by Judith Curry
Two new workshop reports provide insights into what we know and don’t know about the effects of solar variability on climate.
NRC Workshop Report
The National Academies Press has released a draft publication entitled The effects of solar variability on Earth’s climate: A workshop report. The Workshop was under the auspices of the Space Studies Board Committee on the Effects of Solar Variability on Earth’s Climate, chaired by Gerald North.
From the Preface:
The workshop featured presentations on a variety of topics related to solar variability and climate change, organized as follows:
- The Sun and Solar Variability: Past and Present —Overview of solar and heliospheric variability —Observations of the Sun’s variable outputs —Techniques for revealing past solar changes
- Sun-Climate Connections on Different Timescales —Evidence of solar influences in the troposphere and stratosphere —How the climate system works and how it might respond to solar influences —Indications of influence based on paleoclimate records
- Mechanisms for Sun-Climate Connections —Mechanisms connecting variations in total solar irradiance directly to the troposphere —Mechanisms that influence upper parts of the atmosphere, such as variations in solar ultraviolet radiation and possibly solar energetic particles —Mechanisms that link variations in galactic cosmic rays to climate change.
This workshop report contains no recommendations, findings, or statements of consensus. Instead, this workshop report summarizes the views expressed by individual workshop participants (invited speakers and guests). Also included is background information intended to provide context to the reader on both the solar and climate science topics presented at the workshop; however, this is not intended to be an exhaustive review of the current state of the science.
An overview is provided of the sun-climate interactions:
The changes in TSI over the solar cycle provide a good starting point for discussing these challenges. Periodic, or quasi-periodic, forcing17 provides invaluable information on climate dynamics. Other than the seasonal variability on a yearly scale and the precession of the equinoxes (the change of the season in which the minimum Sun-Earth distance occurs) with scales of 20,000 years, the only quasi- periodic forcing term is the 11-year solar cycle. Based on the climate community’s best estimates of global climate sensitivity, the solar stimuli are much smaller than would be required to dominate the temperature record on decadal timescales.18 The search for the solar cycle signal in the temperature record, albeit small, continues to motivate much of the climate research in this area, and so far two basic mechanisms have been modeled. In the first, the 11-year cycle may affect the climate system via the bottom-up total solar irradiance path through which solar cycle effects can manifest themselves at the surface and its nearby environment. In general, this bottom-up driver is strongest in the tropics, where there are feedbacks (from clouds, ocean currents, sea surface temperature, and so on) present in the climate system that strengthen the effect and even show up at higher latitudes.
A second avenue of inquiry is the top-down mechanism that makes use of the modulated absorption of ultraviolet radiation in the stratosphere. Top-down mechanisms operate through changes in the more energetic, shorter-wavelength components of the solar spectrum that influence stratospheric temperatures and winds directly and through absorption by stratospheric ozone. Early work by Karen Labitzke and Harry Van Loon on interactions of the solar cycle and the Quasi-Biennial Oscillation (QBO) of the equatorial stratosphere helped direct attention to the top-down pathway.19,20 The modulation of stratospheric temperatures is clear from observations. Climate models also take this modulation as input and have demonstrated significant perturbations on tropospheric circulations. If borne out by future studies and shown to be of sufficient magnitude, this mechanism could be an important pathway in the Sun-climate connection, particularly in terms of regional impacts. However, it is important to realize that, unlike the bottom-up mechanism, it can in itself contribute very little to global temperature variations.
The effects on climate of centennial timescale variations in TSI have been an even more difficult and contentious issue. Since the work of Jack Eddy in 1976,21 the claim that the lower temperatures of the Little Ice Age from roughly 1600 to 1850 are connected to the secular changes in the Sun, as reflected in paleoclimate data derived from cosmogenic isotopes in sediments and the observed record of sunspots, remains an unresolved research topic (Figure 1.2). Recent findings that removal of small-scale photospheric fields could dim the Sun more than previously expected increase the likelihood of such variations in secular irradiance.22 It remains to be shown whether or not the field decreased significantly below levels observed during normal 11-year activity minima. Ongoing discussion of the role of solar variations in the early 20th century has given rise to the unfounded conjecture that the observed increase in temperature in the last half century could also be due to changes in TSI rather than to anthropogenic influences. The IPCC Fourth Assessment and the recent National Research Council report on climate choices agree that there is no substantive scientific evidence that solar variability is the cause of climate change in the last 50 years. However, the mechanisms by which solar variations can affect climate over longer timescales remain an open area of research.
JC comment: This type of statement, which is endemic to the IPCC brings to mind a statement by Kerry Emanuel: Absence of evidence is not evidence of absence. This is another example of failure to account for ignorance in assessing our knowledge about these topics.
The report presents no conclusions, but some statements from the panel discussion are interesting, notably Gerald North’s summary statement:
At the end of the panel discussion Gerald North summarized the issues that were developed during the workshop. He made three principal points:
1. NASA has led the way in providing a model for ready access to data from many sources—the challenge is to provide better access to paleoclimate data while recognizing the effort it takes to acquire and archive those data in a form accessible to the community.
2. Coupled models, with their inherent complexity, are the future and need to be used more widely for well-designed studies. It is fortunate that climate modeling has advanced to the point that such projects can be undertaken with some confidence.
3. The directly measured record is limited and not without its issues, and so the challenge is to make sure that a means is developed to infer the time history of TSI variability and the limitations on the ability to specify that past behavior.
North also summarized other issues that he felt had been addressed during the workshop and that were particularly noteworthy. Those issues included, among others, the need to be careful in making inferences from the isotope record, which may reflect influences of atmospheric circulation; the need to understand the role galactic cosmic rays may play in cloud nucleation; and the influence of variations in geomagnetic field on the paleo-climate record. North noted that Peter Foukal’s discussion of the Sun was particularly interesting because of the unresolved issues with understanding variability and the sources of variability in TSI arising from the details of the quiet network. Also, a better understanding is needed of how solar brightness, TSI, and the spectral and spatial distribution of energy are affected by the faculae and the dynamics of the Sun.
SORCE SSI Workshop Summary
The latest issue of NASA’s Earth Observer Magazine has an article on SORCE SSI Workshop Summary (link to entire issue; see pages 17-20.) Excerpts:
The Agenda included:
- Reviewing various SSI (solar spectral irradiance) instrument observations, capabilities, and their estimated irradiance uncertainties;
- discussing how each instrument team analyzed the spectral data, to separate instrument effects (e.g. degradation) from intrinsic solar variations;
- discussing the reported SSI differences and refinement of the uncertainties, to gain a better under- standing of them; and
- planning future methods, to identify the significant differences (e.g., new studies, new calibrations, etc.) and refine uncertainties.
A critical requirement for this workshop was that participants examine the methods used to perform long-term instrument degradation corrections. The techniques used for correcting on-orbit irradiance vary from instrument to instrument, so analysis of how these corrections are performed and uncertainty estimates for those corrections are necessary.
Each instrument had its own unique challenges regarding calibration and degradation; key in-strument degradation trend challenges are listed here:
- Degradation trends are complicated because there are many variables that must be accounted for (e.g., multiple drivers, multiple parameters, differ-ent time scales).
- Most instrument calibration channels have differ-ent trend relationships with exposure time that are different from their daily channels.
- Laboratory measurements indicate that photodiodes can have significant recovery—up to 50%— after being exposed to intense levels of UV radiation if they are kept unexposed for a period of time after the UV exposure.
- Carbon deposition degradation rate has many de-pendences (e.g., pressure, temperature, contamina-tion materials), so the same optical elements can have different trends.
- Solar spectral changes at different points in a solar cycle can enhance degradation .
The sun as a variable star
A relevant and very educational paper was published in the latest issue of the Bulletin of the American Meteorological Society: Understanding space weather: the sun as a variable star, by Keith Strong, Julia Saba, Therese Kucera. Excerpts related to TSI:
In the late 1970s and early 1980s, instruments on two National Aeronautics and Space Administration (NASA) missions (Nimbus-7 and Solar Maximum Mission) began to monitor the solar output with vastly improved precision; their measurements led to the discovery that TSI was positively correlated with the sunspot cycle. Thus, the faculae predominate in determining the variation of TSI throughout the cycle. The long-term variation of the TSI is punctuated by short-lived deep dips (~0.3%) when particularly large sunspot groups transit the solar disk. The problem with looking for longer trends in TSI is that the observations have been taken by several different spacecraft and the task of making a composite to establish a longer baseline is difficult; there have been a number of attempts to do so, but there are wide discrepancies between them, and thus we have no consensus.
We now have just over 30 yr of TSI observations, but this represents only three activity cycles, and it is dangerous to put too much weight on any broad conclusions drawn from them. There are a variety of longer-term proxy data for solar activity, but many of the proxy estimates are based largely on less reliable SSN measurements and so may be adding as much noise as signal to our understanding of solar activity patterns.
The change in TSI over a solar cycle is <0.1%. However, the emission in various wavebands can vary much more substantially. For example, UV irradiance can vary by 10%–40% over a cycle. However, the change in the overall energy input to the Earth from UV variability is very small.
Substantial uncertainties remain in our understanding of the effects of solar variability on climate, and there is uncertainty in how to interpret the satellite observations. Some previous solar threads at Climate Etc.
So, what would it take to convince you that the sun has NOT played an important role in 20th century climate change? I am not convinced by the type of analysis done by the IPCC and also Muller’s observation-based attribution, as I have argued elsewhere. I think there is much that we don’t know about the sun’s variations and their impact on climate, and I don’t see the IPCC taking a serious look at this issue, other than to say ‘absence of evidence’.