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News Article | November 14, 2016

Global temperature goes from heat record to heat record, yet the sun is at its dimmest for half a century. For a while, 2010 was the hottest year on record globally. But then it got overtopped by 2014. And 2014 was beaten again by 2015. And now 2016 is so warm that it is certain to be once again a record year. Three record years in a row – that is unprecedented even in all those decades of global warming. Strangely, one aspect of this gets barely mentioned: all those heat records occur despite a cold sun (Figs. 1 and 2). The last solar minimum (2008-2010) was the lowest since at least 1950, while the last solar maximum (2013-2015) can hardly be described as such. This is shown, among others, by the sunspot data (Fig. 1) as well as measurements of the solar luminosity from satellites (Fig. 2). Other indicators of solar activity indicate cooling as well (Lockwood and Fröhlich, Proc. Royal Society 2007). Fig. 1 Time evolution of global temperature, CO2 concentration and solar activity. Temperature and CO2 are scaled relative to each other according to the physically expected CO2 effect on climate (i.e. the best estimate of transient climate sensitivity). The amplitude of the solar curve is scaled to correspond to the observed correlation of solar and temperature data. (Details are explained here.) You can generate and adapt this graph to your taste here, where you can also copy a code with which the graph can be embedded as a widget on your own website (as on my home page). Thus it will be automatically updated each year with the latest data. Thanks to our reader Bernd Herd who programmed this. Fig. 2 Time series of the Sun’s luminosity from the World Radiation Center in Davos. As climate scientists we are by no means surprised at this development, as there has been clear evidence that the variations of the sun’s activity have played a completely subordinate role in climate change over the last 65 years. We’ve covered this issue many times, e.g. here, here and here. Global warming is driven by greenhouse gases, which is a long-standing consensus in science. The current IPCC report, for example, limits the natural contribution to global warming since 1950 to less than plus or minus 0.1 ° C (it might have been negative e.g. because of the fading sun). However, some unsupported claims by “climate skeptics” about the importance of solar variability are now clearly falsified. Climate skeptics have repeatedly predicted an imminent global cooling because of the weak sun. Attributing global warming to the sun has become untenable, because solar activity has not increased for the last 65 years. It has been essentially constant, except for the well-known 11-year Schwabe cycle (which also has little effect on global temperature) and a slight downward trend . The excuse of the skeptics here is usually that global warming is a time-delayed reaction to an increase in solar activity before 1950. The basic idea is not entirely wrong: the climate system has a certain inertia. If the solar luminosity were to be increased in a sudden step, the temperature would not rise immediately, as it would take a while to heat up the oceans. This inertia effect can be quantified with the help of model simulations. Caldeira and Myhrvold (ERL 2013) have shown that 60% of the temperature reaction occurs within the first 20 years. However, around 80% of global warming since the 19th century has only taken place after 1970. It is therefore unthinkable that the slight and gradual increase in solar activity before 1950 could have contributed significantly to the strong warming since the 1970s. Further evidence for this is the comparison of temperatures of land and sea. Everyone knows: when the sun rises in the morning, it takes only a few hours (certainly not decades) to heat the air strongly. Over 90% of the thermal inertia resides in the ocean, while the air over land quickly heats up. If the global warming since 1970 were a delayed response to a previous increase in solar luminosity, then we would now observe above all a catch-up warming of the oceans. The opposite is the case: the continents heat up more quickly and the ocean temperatures are lagging behind. Another point: even if solar variability, for some magical reason, had a noticeable warming effect over the last decades, this would have to come in addition to the CO2-effect and would not call it into question. The warming effect of CO2 on climate is physically well-understood, and the sensitivity of global temperature to CO2 is independently confirmed by paleoclimatic data, see e.g. Rohling et al. 2012 or the brand-new paper by Friedrich et al. 2016 (here is a nice write-up on this paper from Peter Hannam in the Sydney Morning Herald). Some “climate skeptics” have been courageous enough to make forecasts. A notable example is former German energy manager Fritz Vahrenholt (who once claimed in an interview that Greenland was nearly free of ice in the Middle Ages). In 2010 Vahrenholt (who was then in a leading position with the energy utility RWE, Europe’s largest CO2 emitter) published a newspaper article with the beautiful sentence: (Which it was not, but never mind.) He also knew the cause: In his 2012 book, Die kalte Sonne (co-written with Sebastian Lüning also from RWE; literally the title translates as The Cold Sun) he then presented his own forecast for the global temperature evolution until 2030. In Figure 3 we compare this to measured data. No comment required. Figure 3 Measurements of global temperature (NASA GISTEMP, moving average over 12 months) compared to the forecast for global temperature by 2030 by Vahrenholt and Lüning, after Figure 73 of their book. (Image by Stefan Rahmstorf, Creative Commons BY-SA 4.0.) Vahrenholt and Lüning’s book does have one clear merit, however, and that is its title. The Cold Sun nicely sums up the fact that the sun is currently weak – good to know at a time of unprecedented global warming! p.s. To compare to Vahrenholt’s forecast, here’s a comparison of earlier model projections of global temperature for the IPCC (prediction with the CMIP3 model ensemble used in the 4th IPCC assessment report, published in 2007) with the actual changes in temperature (the four colored curves). Graph by Gavin. (The agreement with the most recent set of models (CMIP5) was recently discussed here by Gavin.)

Kretzschmar M.,CNRS Physics Laboratory | De Wit T.D.,CNRS Physics Laboratory | Schmutz W.,World Radiation Center | Mekaoui S.,Royal Meteorological Institute of Belgium | And 2 more authors.
Nature Physics | Year: 2010

Flares are powerful bursts of energy released by relatively poorly understood processes that take place in the atmospheres of stars1. However, although solar flares, from our own Sun, are the most energetic events in the solar system, in comparison to the total output of the Sun they are barely noticeable 2,3. Consequently, the total amount of radiant energy they generate is not precisely known, and their potential contribution to variations in the total solar irradiance 4 incident on the Earth has so far been overlooked. In this work, we identify a measurable signal from relatively moderate solar flares in total solar irradiance data. We find that the total energy radiated by flares exceeds by two orders of magnitude the flare energy radiated in the soft-X-ray domain only, indicating a major contribution in the visible domain. These results have implications for our understanding of solar-flare activity and the variability of our star. © 2010 Macmillan Publishers Limited. All rights reserved.

Krivova N.A.,Max Planck Institute for Solar System Research | Solanki S.K.,Max Planck Institute for Solar System Research | Solanki S.K.,Kyung Hee University | Schmutz W.,World Radiation Center
Astronomy and Astrophysics | Year: 2011

Context. The most recent minimum of solar activity was deeper and longer than the previous two minima as indicated by different proxies of solar activity. This is also true for the total solar irradiance (TSI) according to the PMOD composite. Aims. The apparently unusual behaviour of the TSI has been interpreted as evidence against solar surface magnetism as the main driver of the secular change in the TSI. We test claims that the evolution of the solar surface magnetic field does not reproduce the observed TSI in cycle 23. Methods. We use sensitive, 60-min averaged MDI magnetograms and quasi-simultaneous continuum images as an input to our SATIRE-S model and calculate the TSI variation over cycle 23, sampled roughly every two weeks. The computed TSI is then compared with the PMOD composite of TSI measurements and with the data from two individual instruments, SORCE/TIM and UARS/ACRIM II, that monitored the TSI during the declining phase of cycle 23 and over the previous minimum in 1996, respectively. Results. Excellent agreement is found between the trends shown by the model and almost all sets of measurements. The only exception is the early, i.e. 1996 to 1998, PMOD data. Whereas the agreement between the model and the PMOD composite over the period 1999-2009 is almost perfect, the modelled TSI shows a steeper increase between 1996 and 1999 than implied by the PMOD composite. On the other hand, the steeper trend in the model agrees remarkably well with the ACRIM II data. A closer look at the VIRGO data, which are the basis of the PMOD composite after 1996, reveals that only one of the two VIRGO instruments, the PMO6V, shows the shallower trend present in the composite, whereas the DIARAD measurements indicate a steeper trend. Conclusions. Based on these results, we conclude that (1) the sensitivity changes of the PMO6V radiometers within VIRGO during the first two years have very likely not been correctly evaluated; and that (2) the TSI variations over cycle 23 and the change in the TSI levels between the minima in 1996 and 2008 are consistent with the solar surface magnetism mechanism. © 2011 ESO.

Grobner J.,World Radiation Center
Metrologia | Year: 2012

A new reference radiometer for downwelling atmospheric longwave irradiance has been designed and built. The new infrared integrating sphere (IRIS) radiometer is designed to acquire measurements with a time constant of less than 1s. Based on a thorough characterization, the IRIS radiometer is able to measure longwave irradiance with an expanded uncertainty of 1.8Wm -2 and 2.4Wm -2 in the summer and winter seasons, respectively, which is equivalent to a temperature range between +15°C and -15°C for typical conditions at Davos, Switzerland. The long-term stability of the IRIS radiometer was determined over a one year period, yielding a calibration reproducibility in the laboratory of 0.5%. Outdoor measurements with four IRIS radiometers were performed during 13 clear nights in April 2011. The four radiometers measured atmospheric longwave irradiance with differences ranging from -1.1Wm -2 to +0.7Wm -2, which were well within their estimated uncertainties. © 2012 BIPM & IOP Publishing Ltd.

Frohlich C.,World Radiation Center
Contributions of the Astronomical Observatory Skalnate Pleso | Year: 2011

The last solar activity minimum during 2008/09 was unusually long and with extended periods without sunspots. During this period the total solar irradiance (TSI) was much lower than during the previous minima and no solar activity proxies show similarly low values. Proxy models for TSI use a measure for the darkening of sunspots, the so-called photometric sunspot index (PSI) and for the brightening of faculae and network a chromospheric index. Because none of these can explain the low TSI, a further component is needed, which describes the trend between minima due to a still controversially discussed mechanism. A new algorithm for the calculation of PSI is described which uses individual factors for the different observing stations and a better representation of the size-dependent contrast of spots. The proxy model based on the new PSI, the long- and short-term MgII index and a trend based on the minima values of the open field explains almost 85% of the variance of TSI over the last three solar cycles. Moreover, it confirms the factor of ≈ 4 between the observed trend of TSI and those of the chromospheric and other solar activity indices.

Haberreiter M.,World Radiation Center | Haberreiter M.,University of Colorado at Boulder
Solar Physics | Year: 2011

We present spectral synthesis calculations of the solar extreme UV (EUV) in spherical symmetry carried out with the 'Solar Modeling in 3D' code. The calculations are based on one-dimensional atmospheric structures that represent a temporal and spatial mean of the chromosphere, transition region, and corona. The synthetic irradiance spectra are compared with the recent calibration spectrum taken with the EUV Variability Experiment during the Whole Heliospheric Interval. The good agreement between the synthetic and observed quiet Sun spectrum shows that the employed atmospheric structures are suitable for irradiance calculations. The validation of the quiet Sun spectrum for the present solar minimum is the first step toward the modeling of the EUV variations. © 2011 Springer Science+Business Media B.V.

Frohlich C.,World Radiation Center
Surveys in Geophysics | Year: 2012

The record of total solar irradiance (TSI) during the past 35 years has overlapping observations from space which can be merged to a composite, and three are available, namely the PMOD, the ACRIM and the IRMB composites. There are important differences between them, which are discussed in detail in order to find the best representation of solar variability during the last three cycles, for the following discussions of solar irradiance variability. Moreover, the absolute value of TSI from TIM on SORCE is 1,361 Wm -2, substantially lower than the value 1,365 Wm -2, which was observed by the classical radiometers. New results from specific experiments are now available, which are discussed in order to define the value to be used in, e. g., climate models. The most important issue regarding the recent TSI records is the low value observed during the minimum in 2009, which is 25% of a typical cycle amplitude lower than the value in 1996. The validity of this low value has been confirmed by comparing all existing TSI observations during cycle 23. On the other hand, activity indices, such as the sunspot number, the 10. 7-cm radio flux (F10. 7), the CaII and MgII indices and also the Ly-α irradiance or the frequency changes in low-order p modes, show a much smaller decreases relative to their respective typical cycle amplitude. It is most likely that an increasing contrast of the facular and network elements with decreasing magnetic field is responsible for this discrepancy. The value of TSI at minima is correlated with the open magnetic field of the Sun, B R, at minima. Using B R at minima, interpolated linearly in between as a fourth component of a proxy model based on the photometric sunspot index and on the MgII index improves the explanation of the variance of TSI over the full period of the last three solar cycles to 84. 7%. Results from other models are also discussed. © 2011 Springer Science+Business Media B.V.

Frohlich C.,World Radiation Center
Space Science Reviews | Year: 2013

The record of total solar irradiance (TSI) during the past 35 years shows similarities of the three solar cycles, but also important differences. During the recent minimum with an unusually long periods with no sunspots, TSI was also extremely low, namely 25% of a typical cycle amplitude lower than in 1996. Together with the values during the previous minima this points to a long-term change related to the strength of solar activity. On the other hand, activity indices as the 10.7 cm radio flux (F10.7), the CaII and MgII indices and also the Ly-α irradiance, show a much smaller decrease. This means that proxy models for TSI based on the photometric sunspot index (PSI), and on e.g. MgII index to represent faculae and network have to be complemented by a further component for the long-term change. TSI values at minima are correlated with the simultaneous values of the open magnetic field of the Sun at 1 AU and thus, these values may be used as a surrogate for the long-term change component. Such a 4-component model explains almost 85% of the variance of TSI over the three solar cycles available. This result supports also the idea that the long-term change of TSI is not due to manifestations of surface magnetism as the solar cycle modulation, but due to a change of the global temperature of Sun modulated by the strength of activity - being lower during low activity. To explain the difference between the minima in 1996 and 2008 we need a change of only 0.25 K. © 2011 Springer Science+Business Media B.V.

Wehrli C.,World Radiation Center | Schmutz W.,World Radiation Center | Shapiro A.I.,World Radiation Center
Astronomy and Astrophysics | Year: 2013

Context. The variability of solar spectral irradiance (SSI) over the rotational period and its trend over the solar activity cycle are important for understanding the Sun-Earth connection as well as for observational constraints for solar models. Recently the Spectral Irradiance Monitor (SIM) experiment on the Solar Radiation and Climate Experiment (SORCE) has published an unexpected negative correlation with total solar irradiance (TSI) of the visible spectral range. It is compensated by a strong and positive variability of the near UV range. Aims. We aim to verify whether the anti-correlated SIM-trend in the visible can be confirmed by independent observations of the Variability of solar IRadiance and Gravity Oscillations (VIRGO) experiment on the SOlar and Heliospheric Observatory (SOHO) satellite. The challenge of all space experiments measuring solar irradiance are sensitivity changes of their sensors due to exposure to intense UV radiation, which are difficult to assess in orbit. Methods. We exclude the first six years prior to 2002 where one or more fast processes contributed to instrumental changes and analyse a ten-year timeseries of VIRGO sun photometer data between 2002 and 2012. The variability of SSI is correlated with the variability of the TSI, which is taken as a proxy for solar activity. Results. Observational evidence indicates that after six years only one single long-term process governs the degradation of the backup sun photometer in VIRGO which is operated once a month. This degradation can be well approximated by a linear function over ten years. The analysis of the residuals from the linear trend yield robust positive correlations of spectral irradiance at 862, 500, and 402 nm with total irradiance. In the analysis of annual averages of these data the positive correlations change into weak negative correlations, but with little statistical significance for the 862 nm and 402 nm data. At 500 nm the annual spectral data are still positively correlated with TSI. The persisting positive correlation at 500 nm is in contradiction to the SIM results. © 2013 ESO.

Grobner J.,World Radiation Center | Wacker S.,World Radiation Center
AIP Conference Proceedings | Year: 2013

Measurements between a newly developed windowless longwave irradiance radiometer IRIS at PMOD/WRC and commercially available pyrgeometers based on thermopile detectors show seasonal variations of up to 6 Wm-2 between different instrument groups which are likely due to the spectral mismatch of the dome transmissions of the respective pyrgeometers. The measurements at PMOD/WRC suggest that Eppley PIR and PRE2003CG4 (Kipp & Zonen CG4 Pyrgeometers manufactured before 2003) require a longwave irradiance correction of about -0.5 Wm-2mm-1 IWV when the atmospheric integrated precipitable water vapor (IWV) falls below 10 mm, while POST2003CG4 do not show these discrepancies and give consistent results with respect to the IRIS Radiometers. The IRIS radiometers measure on average 4.6+2.5 Wm -2 higher longwave irradiances than the WISG (for IWV>10 mm), which needs to be confirmed by independent measurements. © 2013 AIP Publishing LLC.

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