To what extent does solar variability contribute to climate change? Dr. David H. Hathaway NASA/Marshall Space Flight Center National Space Science and Technology Center

Outline Solar VariabilitySolar Variability The Solar Sunspot CycleThe Solar Sunspot Cycle Solar Variability and NH TemperatureSolar Variability and NH Temperature Total Solar Irradiance VariationsTotal Solar Irradiance Variations Solar Spectral Irradiance VariationsSolar Spectral Irradiance Variations Galactic Cosmic Ray VariationsGalactic Cosmic Ray Variations

Solar Activity – Space Weather Solar activity includes flares, prominence eruptions, and coronal mass ejections (CMEs) along with associated electromagnetic emission at wavelengths from gamma-rays to radio-waves and energetic particles from electrons and protons to heavy nuclei.

Solar Evolution Over its 4.5 billion year history the Sun has slowly increased in brightness (Luminosity) as it has burned hydrogen to form helium in its core. It is now some 30% brighter than is was 4 billion years ago.

The Sunspot Cycle The sunspot cycle was discovered in 1844 by a Swiss Apothecary – Heinrich Schwabe. The number of sunspots rises and falls with a period of about 11- years. Individual cycles vary in amplitude, cycle length, and cycle shape. In 1854 Rudolf Wolf proposed the “Relative Sunspot Number” index and used historical records to extend the record back to 1749.

It’s All Magnetic! Sunspots are regions where intense magnetic fields loop up out of the Sun and back into its interior. These fields are shuffled around by flows at, and below, the surface. This leads to the generation of waves and magnetic reconnection above the surface – heating the corona and driving flares, prominence eruptions, and CMEs.

The Sun’s Magnetic Cycle Active regions erupt in two bands on either side of the Sun’s equator that start at about 30° north and south and slowly drift toward the equator over each sunspot cycle. A broad equatorial jet and a poleward meridional flow advect magnetic elements across the surface while cellular convective flows shred the field and scatter the elements in random directions.

The Sun’s Magnetic Cycle 30-years of magnetic measurements reveal the characteristics of the Sun’s magnetic cycle: different polarities at low- and high-latitudes that reverse from hemisphere-to-hemisphere and cycle-to-cycle; poleward drift of high- latitude polarity that reverses the polar fields at about the time of cycle maximum.

Solar Activity and Sunspots Sunspot number is well correlated with solar activity. The 400-year length of the sunspot number record helps to characterize the solar cycle. The connection with cosmic rays leaves even longer records of solar activity in tree rings ( 14 C) and ice cores ( 10 Be). Sunspot Area 10.7cm Radio Flux GOES X-Ray Flares Climax Cosmic-Ray Flux Geomagnetic aa index Total Irradiance

Solar Variability and Climate Yearly sunspot numbers back to 1610 (the invention of the telescope) show significant variations in the amplitudes of the sunspot cycles. This includes periods of inactivity like the Maunder Minimum ( ) and the Dalton Minimum ( ). Similar variations are seen in estimates of the Northern Hemisphere temperatures as given by Mann et al. (1998), Moberg et al. (2005), and others (variations of a few tenths of a degree Celsius).

The Total Solar Irradiance Formerly known as the “Solar Constant,” the Total Solar Irradiance (TSI) varies by about 0.1% over the last three solar cycles. (Larger dips in TSI are due to the passage of large sunspots across the visible disk of the Sun.) This 0.1% variation only produces a 0.1 °C change in Northern Hemisphere Temperature when introduced in most climate models.

TSI Successes Efforts to model the TSI appear to be quite successful. Models include dark sunspot umbrae and lighter (but still dark) sunspot penumbrae from “white light” images of the Sun. And bright faculae as determined by locations of small-scale, weaker, magnetic features.

TSI Modelling During Cycle 23 Magnetic features explain more than 90% of the variability. Both the day-to- day variations and the overall rise from minimum to maximum are modeled very well with just one parameter – a filling factor for faculae. Krivova et al A&A Lett

TSI Complications TSI measurements have been taken by satellite-based instruments for 30 years. The measurements are very precise (able to distinguish small variations) but the accuracy (actual value) varies from one instrument to another. The ACRIM reconstruction showed a significant difference in TSI for the 1986 and 1996 minima. The PMOD reconstruction showed no such variation but now shows a significant variation between the 1996 and 2007 minimum. This variation cannot be explained by magnetic features and raises the question: Could TSI have varied by more than 0.1% during the Maunder and Dalton Minima?

Spectral Irradiance Variations Although TSI only varies by about 0.1% over the last three cycles, the Sun’s UV irradiance (at wavelengths that influence ozone production in the stratosphere) varies by several percent. These variations may produce more significant tropospheric variations than TSI.

Judith Lean (2008 SORCE Science Meeting)

Cosmic Ray Modulation The flux of cosmic rays has been measured at mountain-top observatories for several decades. These measurements show significant (~20%) decreases at mid-latitudes that are associated with solar activity.

The Modulation Mechanism

Cosmic Rays and 14 C & 10 Be Carslaw et al. (2002) as lifted from Lockwood (2002) The solar cycle modulation of cosmic rays leaves a record of solar activity in 14 C in tree rings and 10 Be in ice cores.

Cosmic Rays and Clouds Carslaw et al. (2002) as lifted from Marsh & Svensmark (2000) There are indications that the modulation of cosmic rays may modulate cloud cover.

Cosmic Ray/Cloud Mechanism Carslaw et al. (2002)

Conclusions Solar variability appears to produce a significant signal in the tropospheric temperature recordSolar variability appears to produce a significant signal in the tropospheric temperature record Recent changes in the total solar irradiance should produce smaller temperature variationsRecent changes in the total solar irradiance should produce smaller temperature variations Long-term changes in total solar irradiance may be larger than suggested by recent behaviorLong-term changes in total solar irradiance may be larger than suggested by recent behavior Solar UV variability may produce larger temperature variationsSolar UV variability may produce larger temperature variations Solar cycle modulation of cosmic rays may influence cloud cover and produce larger temperature variationsSolar cycle modulation of cosmic rays may influence cloud cover and produce larger temperature variations More work is needed with Global Circulation Models that include: 1) stratospheric layers with solar UV forcing and 2) Cosmic ray induced cloud cover changesMore work is needed with Global Circulation Models that include: 1) stratospheric layers with solar UV forcing and 2) Cosmic ray induced cloud cover changes