CHAPTER 2.2 SOLAR RADIATIVE FORCING Natalie A. Krivova1 and Ilaria Ermolli2 1 Radiative forcing Radiative forcing (RF) quantifies the impact of various factors on Earth's surface temperature and climate. In equilibrium, the global average temperature of the Earth is determined by a balance between the incoming energy absorbed in the atmosphere and by the surface and the energy of the thermal infrared radiation emitted back into space, so that the net radiative flux (globally and annually averaged) at the top of the atmosphere3 is zero. When the amount of the incoming or outcoming radiation changes, then before a new equilibrium is established the net flux at the top of the atmosphere is not zero. Thus, radiative forcing is the instantaneous change in the value of the net downward radiative flux. Radiative forcing is defined positive when the amount of energy retained by the Earth- atmosphere system increases. This could be due to an increase in the incoming or a decrease in the outcoming energy and leads to an increase in the Earth's temperature. 2 Solar irradiance The dominant energy source to Earth's climate system is the Sun. Solar energy is produced in the Sun's core, where hydrogen atoms fuse to form, via a chain reaction, helium. Photons released in this process travel inside the Sun until, tens or even hundreds of thousands years later, they reach up to the so-called solar visible surface (or the photosphere), above which the solar atmosphere is transparent for the photons. They escape as visible light and part of them reaches the Earth just minutes later. 1 Max-Planck-Institut f¨urSonnensystemforschung, 37077 G¨ottingen,Germany 2 INAF, Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy 3 Radiative forcing can also be defined at the tropopause. c EDP Sciences 2015 DOI: 10.1051/978-2-7598-1733-7.c108 68 Earth's climate response to a changing Sun The total solar electromagnetic energy flux received at the top of the Earth's atmosphere and reduced to the mean distance from the Sun to the Earth (one astronomical unit, AU) is called the total solar irradiance (TSI). This definition takes care to differentiate between changes in the solar radiative output itself and changes in the Earth-Sun distance (see Infobox 2.1) or the interaction between the solar radiation and the Earth's atmosphere. Solar electromagnetic energy is emitted over essentially entire spectrum. Al- most half of this energy originates in the range visible to the human eye (roughly 400 to 800 nm), more than 40% come from the infrared, IR, range (wavelengths above 800 nm), and only less than 10% are contributed by the ultraviolet, UV, radiation (wavelengths below 400 nm). Solar irradiance measured at a certain wavelength or within a wavelength interval (in units of W m−2 per unit wave- length interval, e.g. W m−2nm−1) is called spectral solar irradiance (SSI). Total solar irradiance is one of the more recently discovered variables of the Sun, generally being referred to as the solar constant in the older literature. 3 Measurements of solar irradiance 3.1 TSI Measurements of TSI are done with absolute radiometers. They include a cav- ity with a blackened inner side, operating as a black body to efficiently absorb the incident radiation, which heats the cavity. A built-in electrical calibration heater maintains the heat flux constant by adjusting the supplied heating power. The electrical power required to maintain the equilibrium is proportional to the incoming radiative power absorbed by the cavity. Due to atmospheric extinction, measurements of total solar irradiance sufficiently precise to reveal its variability are only possible from space and have been regularly carried out since 1978 by over a dozen of different radiometers (Figure1). On an absolute scale, results from individual experiments differ significantly. Recent tests and experiments have helped to identify main sources of the offsets. The TSI value of 1360:8 0:5 Wm−2 recorded by SORCE/TIM in 2008 is currently believed to best represent± the solar minimum conditions. The relative TSI changes measured by space-borne radiometers agree quite good with each other on time scales of days to years. Thus, TSI varies on all observable time scales, from minutes to decades. Most prominent are the short- term fluctuations on time scales of days to roughly a week and the modulation in phase with the 11-year solar activity cycle (see Figure1). But there are also differences between the data coming from individual ex- periments, most importantly in the longer-term trends. This is because changes in the sensitivity (i.e., the calibration) of the instruments due to their aging and degradation once they are launched and exposed to the solar light, are quite in- dividual and often tricky to assess. Owing to the offsets in the absolute values, combining the data from individual instruments into a single composite record is non-trivial. Natalie A. Krivova and Ilaria Ermolli: Solar radiative forcing 69 Fig. 1. Space-borne measurements of the total solar irradiance (TSI) covering the pe- riod from 1978 to 2014. Individual records are shown in different colors, as labeled in the plot. The bottom part of the plot shows the monthly mean sunspot number. Courtesy of G. Kopp (http://spot.colorado.edu/koppg/TSI/). Abbreviations: Active Cavity Radiometer Irradiance Monitor (ACRIM) instruments on the Solar Maximum Mission (SMM), Upper Atmosphere Research Satellite (UARS), and ACRIMSat; Earth Radiation Budget (ERB) instrument on Nimbus-7; Earth Radiation Budget Satellite (ERBS), the Earth Radiation Budget Experiment (ERBE) on the Earth Radiation Budget Satellite (ERBS) National Oceanic and Atmospheric Administration (NOAA), Precision Monitor Sensor (PREMOS) on PICARD, Solar Variability Experiment (SOVA) on EUropean Re- trievable Carrier of ESA (EURECA), Solar Variability and Irradiance Monitor (SOVIM) on the International Space Station (ISS), Total Irradiance Monitor (TIM) on Solar Ra- diation and Climate Experiment (SORCE), Variability of Solar Irradiance and Gravity Oscillations (VIRGO) on SoHO. Currently, three different composites exist (see Figure2). The most critical discrepancy concerns their conflicting secular trends, best seen as the difference in the TSI levels during activity minima in 1986, 1996 and 2008. Models of so- lar irradiance (see Section5) return an either downward trend (Figure2), as in the PMOD (Physikalisch-Meteorologisches Observatorium Davos) composite, or in case of most proxy models, no trend. None of the models returns an upward or alterating trend. 70 Earth's climate response to a changing Sun Fig. 2. The ACRIM (red), IRMB (green) and PMOD (blue) TSI composite records smoothed over 181 days. Also plotted is the SATIRE-S model reconstruction of TSI. The vertical dashed lines mark solar minima. From Yeo et al.(2014). 3.2 SSI Measurement of the solar spectral irradiance is more sophisticated. Very low am- plitudes of the variability in the visible and IR ranges, a rapid increase in the variability in combination with significant decrease of the amount of the radia- tion with decreasing wavelength (Figure3) and the need for a meaningful spectral resolution are only some of the complicating factors. Thus in addition to the high radiometric accuracy and precision, a spectrometer has to maintain a suf- ficient wavelength accuracy and account for the wavelength-dependent long-term degradation effects. Regular monitoring of the solar spectrum in the UV (below 400 nm) also started in 1978. Most of our knowledge is based on the results of two experiments, SOLSTICE (Solar Stellar Irradiance Comparison Experiment) and SUSIM (Solar Ultraviolet Spectral Irradiance Monitor), onboard UARS (the Upper Atmosphere Research Satellite), in operation from 1991 to 2001 and 2005, respectively. SSI monitoring over a broad range from the UV to the IR (up to about 2 400 nm) with absolute radiometric calibration has been carried out by SORCE/SIM since 2003. Whereas most of the solar radiant energy is emitted at visible and infrared wavelengths, the variability is significantly stronger in the UV spectral range (Figure3). The amplitude of the variation grows from about 0.1% in the visi- ble and even less in the IR to almost 100% around the Ly-α at 121.6 nm, which is the strongest line in the solar spectrum. The contribution of the UV (below 400 nm) part of the spectrum to the total irradiance variability most probably exceeds 50%, although the exact number is still a matter of debate (Figure4). Thus, the more recent observations by SORCE/SIM suggest stronger variability in the UV than had been measured before SORCE and predicted by the models. This exces- sive variability would need to be partly compensated by a presumably anti-phase (with the solar cycle) variation in the visible (400{700 nm). This pattern of SSI variability is, however, in contradiction not only with other, partly contemporary, observations, but also with the up-to-date models, which are at the same time Natalie A. Krivova and Ilaria Ermolli: Solar radiative forcing 71 Fig. 3. Top: Reference solar spectrum recorded in April 2008 within the Whole Heliosphere Interval (WHI) project. Bottom: Relative SSI variability as observed by UARS/SUSIM (red curve) between the maximum of cycle 23 (March 2000) and the preceding minimum (May 1996), as well as by SORCE/SOLSTICE (light blue) and SORCE/SIM (dark blue) (Harder et al., 2009) between April 2004 and December 2008. Also shown is the variability between 2000 and 1996 predicted by the SATIRE-S model (green). For each period, averages over one month are used. Negative values are indi- cated by dotted segments. Following Solanki et al.(2013). Abbreviations: UARS - Upper Atmosphere Research Satellite; SUSIM - Solar Ultraviolet Spectral Irradiance Monitor; SORCE - Solar Radiation and Climate Experiment; SOLSTICE - Solar Stellar Irradiance Comparison Experiment; SIM - Spectral IrradianceMonitor; SATIRE-S - Spectral and Total Irradiance Reconstruction for the Satellite era.