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. 2007 Mar 6;104(10):3713-8.
doi: 10.1073/pnas.0605064103. Epub 2007 Mar 5.

Solar Influence on Climate During the Past Millennium: Results From Transient Simulations With the NCAR Climate System Model

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Solar Influence on Climate During the Past Millennium: Results From Transient Simulations With the NCAR Climate System Model

Caspar M Ammann et al. Proc Natl Acad Sci U S A. .
Free PMC article

Abstract

The potential role of solar variations in modulating recent climate has been debated for many decades and recent papers suggest that solar forcing may be less than previously believed. Because solar variability before the satellite period must be scaled from proxy data, large uncertainty exists about phase and magnitude of the forcing. We used a coupled climate system model to determine whether proxy-based irradiance series are capable of inducing climatic variations that resemble variations found in climate reconstructions, and if part of the previously estimated large range of past solar irradiance changes could be excluded. Transient simulations, covering the published range of solar irradiance estimates, were integrated from 850 AD to the present. Solar forcing as well as volcanic and anthropogenic forcing are detectable in the model results despite internal variability. The resulting climates are generally consistent with temperature reconstructions. Smaller, rather than larger, long-term trends in solar irradiance appear more plausible and produced modeled climates in better agreement with the range of Northern Hemisphere temperature proxy records both with respect to phase and magnitude. Despite the direct response of the model to solar forcing, even large solar irradiance change combined with realistic volcanic forcing over past centuries could not explain the late 20th century warming without inclusion of greenhouse gas forcing. Although solar and volcanic effects appear to dominate most of the slow climate variations within the past thousand years, the impacts of greenhouse gases have dominated since the second half of the last century.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Prescribed forcings. (Top) Volcanic forcing is indicated as global visible optical depth. (Middle) Solar forcing is obtained by scaling 10Be data of ref. to a Maunder Minimum reduction in solar irradiance relative to today of 0.1% (green dash-dot, low solar simulation from 1550 to 2000 AD), 0.25% (red, solid, medium solar simulation from 850 to 2000 AD), and 0.65% (blue, dash, high solar simulation from 850 to 2000 AD). The 11-year solar cycle is not included in the forcing. Anthropogenic SO2 emissions (dash-dot) and forcing by greenhouse gases (Bottom, solid), expressed as CO2 equivalent, are from ref. . The atmospheric CO2 concentration is shown by the black, dashed line. Three long simulations were run with prescribed solar (high, medium, and low scaling) and identical volcanic, greenhouse gas, and tropospheric sulfate forcing. In addition, three “natural-only” simulations were branched off from each long integration at year 1870 AD, with greenhouse gas concentrations and anthropogenic sulfate kept constant at 1870 AD levels (green lines).
Fig. 2.
Fig. 2.
Global average surface temperature (50-year Gaussian weighted average) simulated with the NCAR CSM 1.4 with prescribed high (blue), medium (red) and low (green) solar forcing. Annual data are shown for the medium solar forcing run (red, thin line) and for the instrumental record of global average surface temperature (ref. , black). Volcanic, greenhouse gas, and tropospheric sulfate forcing are the same in all three simulations. Shaded ranges depict two positive and negative standard deviations around the 50-year averages computed from 1,000 years of an unforced control simulation (SD = 0.107°C).
Fig. 3.
Fig. 3.
Comparison of NCAR CSM simulations with proxy reconstructions and instrumental data. (a) Reconstructed NH average surface temperature anomalies over the past millennium (–9, 51, 52). All series are as published originally and no additional scaling has been performed, but annual records have been smoothed with a 50-year-long Gaussian filter. All series are relative to 1901–1960 averages computed from original data. The instrumental series is from ref. (dark gray). (b) Northern hemisphere surface temperature from the low- (green), medium- (red), and high-scaled (blue) solar forcing simulations compared with the range spanned by the annual proxy-based reconstructions (refs. , –, , and ; gray shaded area). This range does not include a systematic error analysis, it only illustrates the current debate regarding the amplitude of hemispheric multidecadal to century-scale temperature variations of the past. The smooth borehole-based estimate from ref. is shown by the solid black line. Instrumental series, reference period, and filter as in a. (c) Simulated versus the instrumental (gray) record of global average surface temperature (gray, thick solid line). The time series of the low (green), medium (red), and high (blue) solar forcing experiments were smoothed by using an 11-year Gaussian filter. Anthropogenic forcings were included in the primary experiments (solid lines) but held at 1870 AD conditions in 1870–2000 AD branch experiments (dashed lines).
Fig. 4.
Fig. 4.
The spatial pattern of solar-induced temperature changes in °C per W m−2 for the medium solar forcing simulation over the period from 850 to 1849 AD. Values shown are determined by using linear regression of local annual temperature onto solar irradiance anomalies and are restricted to areas that pass significance tests with 95% confidence taking into account serial correlation of the time series. Global mean response (from significant areas only) is equivalent to 0.062°C for each W m−2 irradiance change at the top of the atmosphere. High and low scaled solar forcing results are shown in SI Fig. 8.

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