Solar Influences on Global Change (1994) / Chapter Skim
Currently Skimming:
2 SOLAR VARIATIONS AND CLIMATE CHANGE
2 SOLAR VARIATIONS AND CLIMATE CHANGE
Pages 23-48
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 23... ...
. Observations of total solar irradiance by spacecraft radiometers (WilIson and Hudson, 1991; Hoyt et al., 1992)
|
From page 24... ...
. Total solar irradiance is increased during times of maximum solar activity (e.g., 1980 and 1990)
|
From page 25... ...
, net anthropogenic forcing from greenhouse gases, aerosols, clouds and ozone changes (dotted line) , and solar irradiance variations associated with the 11-year solar activity cycle alone (small squares)
|
From page 26... ...
Understanding solar influences on climate requires the interaction of two primary research areas that are currently quite distinct: the monitoring and assessment of solar irradiance variations, which is reviewed first in this chapter, and the perspective of solar variability and climate from both the paleoclimate record and for immature global change, which is discussed subsequently. Origins of the solar radiative output variations are addressed in the broacler context of the variable Sun in Chapter 6.
|
From page 27... ...
This experiment was specifically designed for, and dedicated to, long term, high precision solar total irradiance monitoring; it ceased operation in October 1989 when the SMM spacecraft reentered the Earth's atmosphere. The Nimbus 7/ERB and ACRIM I results provided the first unequivocal proof of intrinsic total solar irradiance variability, and variations have since been detected on every observable time scale (Figure 2.1~.
|
From page 28... ...
I) both show a decline through the solar minimum period and an increase with the increasing solar activity of solar cycle 22.
|
From page 29... ...
Implications from Observations of Solar Surrogates The direct correlation of solar radiative output with solar activity over the il-year solar cycle is a major discovery from the ACRIM and ERB long term solar monitoring programs. Variations in total solar irradiance occur continuously, on time scales of days to months, in response to episodes of activity throughout the ~ I-year solar cycle and the modulation of active region emission by the Sun's 27-day rotation.
|
From page 30... ...
To understand the forcing of the climate system by solar irradiance changes, it is necessary to have empirical models capable of extrapolating the radiative output variations to epochs beyond present solar cycles. Knowing that total solar irradiance is enhanced at times of maximum activity, and that these variations appear to arise from the competing effects of two different types of active regions (dark sunspots and bright faculae)
|
From page 31... ...
However, as discussed below, limits of solar variability, such as inferred from observations of Sun-like stars, provide circumstantial evidence for a brightness component that has been slowly increasing the total solar irradiance since the Maunder Minimum, a time of reduced solar activity from about 1645 to 1715. With changes in this additional brightness component superimposed on the ~ I-year cycle variations, a reduction of 0.24 percent is estimated for the Maunder Minimum, relative to the mean of the contemporary ~ I-year irradiance cycle (Lean et al., 1992a)
|
From page 32... ...
This conjecture is roughly supported by the occurrence of minima that punctuate the 14C geophysical record of solar activity. In the four Sun-like stars observed to be inactive, Ca IT emissions were almost always lower than in the stars that exhibited activity cycles (Baliunas and Jastrow, 1990~.
|
From page 33... ...
SOLAR FORCING OF CLIMATE CHANGE Variations in solar irradiance may affect the Earth's climate through a direct influence on the global mean temperature or in more subtle ways. The magnitude of climate change that can be associated directly with the changes in total solar irradiance measured during the recent solar activity cycle (about 0.
|
From page 34... ...
(1 992a) , were obtained by using the Ca II K data as a proxy for long term brightness variations in the total solar irradiance, and are based on ACRIM radiometry (see Figure 2.1~.
|
From page 35... ...
Paleoclimate, like the recent climate, displays numerous examples of potential interactions between solar radiative forcing and climate. On the longer time scales associated with variations in the Earth's orbital parameters, the insolation gradient between low and high latitudes may be modified, or its seasonal variation changed, which could lead to the growth of ice sheets and altered ocean circulation patterns.
|
From page 36... ...
Thus, enhanced solar activity corresponds to i4C minima, and the mechanism proposed by Eddy for the apparent relationship between climate and the '4C wiggles involved changes in the total solar irradiance linked to the long term envelope of the ~ I-year sunspot cycle and reflected in the 14C record. The extent to which cosmogonic isotope variations really indicate terrestrially relevant variations in solar energy outputs, either radiative or particle, and the scaling of the relationship over long times, is poorly known; the paleoclimate record is similarly somewhat uncertain (Bradley and Jones, 1993~.
|
From page 37... ...
The solar cycle envelope and the solar cycle length are related to each other, and both are indicators of the long term variability of solar activity (with longer cycles having smaller amplitudes)
|
From page 38... ...
Res., 96, 2835, 1991, copyright by the American Geophysical Union. In a one-dimensional model of the thermal structure of the ocean, consisting of a lOOm mixed layer coupled to a deep ocean, and including a thermohaline circulation, a change of 0.6 percent in the total solar irradiance is needed to reproduce the observed variation of 0.4°C in the sea-surface temperature anomalies.
|
From page 39... ...
The problem of assessing direct solar radiative forcing of climate change is additionally complicated because the extent to which total solar irradiance variability arises from radiative changes at ultraviolet rather than at visible wavelengths (Lean, 1989) determines the altitude of its direct impact on the global system.
|
From page 40... ...
, ~ . , Solar Activity Cycles and the Weather There have been many studies of the possible relationships between weather phenomena and the 11-year solar sunspot cycle or the 22-year solar magnetic cycle.
|
From page 41... ...
accepting the findings as proof of a causal relationship, primarily because the mechanisms providing the linkage have not been apparent. The subject has received new impetus in the past decade, due both to the observation of total and ultraviolet irradiance variations associated with the 11-year solar activity cycle and to observations of a distinct 10-to12-year oscillation (TTO)
|
From page 42... ...
Other studies have indicated correlations between solar activity and weather phenomena even when no stratification by QBO phase is made. For example, the mean latitude of winter storm tracks in the north Atlantic appears to shift equatorward at times of maximum solar activity relative to times of activity minima (Tinsley and Deen, 1991~.
|
From page 43... ...
and sea (60°W) pressures are correlated with the 11-year solar activity cycle.
|
From page 44... ...
In addition, the GCM studies demonstrated that dynamical changes induced by solar cycle variations can affect the radiative properties of the troposphere by influencing cloud ant! snow cover.
|
From page 45... ...
Melting of the ice sheets occurred 15,000 to 10,000 years BP, coincident with high northern hemisphere summer insolation, in agreement with this hypothesis. However, the southern hemisphere climate also experienced rapid warming in this time interval, when southern hemisphere insolation was at a minimum.
|
From page 46... ...
Examples of this mismatch can easily be found: the peak of the last ice age, about 20,000 years BP, coincides with a very weak minimum in Northern Hemisphere summer solar insolation, and the deglaciation Northern Hemisphere summer maximum at about 12,000 years BP is no larger than a similar feature at about 30,000 years BP, which did not lead to complete deglaciation. These facts suggest that processes other than direct solar forcing may be responsible for the observed climate record.
|
From page 47... ...
Both of these conclusions have important implications for global change projections: the former implies that contemporary GCMs might not be sufficiently sensitive to solar radiative forcing (whether of orbital or solar activity origin) , while the latter emphasizes that it is the climate system feedbacks that are most important in producing climate change, invalidating the use of simple transfer functions between radiative forcing perturbations and climatic responses.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.