1 Introduction
The early man was fascinated by the Sun, its daily sunrise and sunset, its role as a purveyor of energy and light. Its cult in Egypt was even a first step towards monotheism. A scientific view of the part played by the Sun in human health appears in the Greek medicine as it was practiced in the Asclepion of Hippocrates on the island of Kos: exposure to sunshine was seen as a part of a curative envi-ronment as well as a healthy regime.
The scientific renaissance began with the Copernicus heliocentric system fol-lowed by the systematic observation of sunspots by Galileo beginning in 1610.
The relation betweensolar radiationand climate got wide publicity at the end of the 18th century when Benjamin Franklin (1784) made the link between the June 1783 eruption of the Laki volcano in Iceland, the dry haze which followed and the abnormally cold winter of 1783-1784 (Franklin, 1784). These conditions lead to a shortage in cereals and price increases which can be related to the revolutions of the end of the decade. Presently, a much more detailed meteorological analysis is possible and some authors (D’Arrigo et al., 2011) indicate that the volcanic eruption might have only amplified a global perturbation caused by an El-Ni˜no event.
The next step was taken byHerschel(1801) when he compared thesunspot numberto the first economical statistical report: the “Essay on the wealth of the nations” by Adam Smith. Herschel hypothesised a relation between crop yield and the price statistics, he noticed then that the price values were anti-correlated with thesunspot number. He did not discover the 11-year cycle in this analysis as his series still included theMaunder minimumof the 17th century, which dominated the 11-year cycle.
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EDP Sciences 2015 DOI: 10.1051/978-2-7598-1733-7.c105
Schwabe(1844) was the first to put it in evidence after three textbook solar cycles. The Hershel anti-correlation was met with criticism since its first publica-tion and revisited in an appendix of the analysis ofsunspotsmade byCarrington (1863). Besides being a superior solar observer, Richard Carrington was also a brewer and had his income tied to the cereals price. He concluded that in the 19th century, wheat prices, and especially the import duties, were dependent on tax laws passed by the British parliament. These taxes were amounting to import subsidies in order to have British farmworkers leave their job to participate in the new industrial expansion in the cities and be sufficiently fed with minimal wages.
However the work of Herschel after more than 200 years is still open for discussion in both the study of the influence of geophysical parameters on economy and in the use of statistics to determine causality (see for example, (Love,2013)).
More recently, scholars studying the Maya classical periods found that the successive collapses of the Maya civilisations were periodical with a period close to 200 years and corresponding to a possible solar forcing (Hodell et al., 2001).
Here again, after 30 years of abundant publications, the issue is still open (Turner and Sabloff,2012) because it involves the governance and agricultural practices of a closed society which could have built its own instability and repeated the same errors at each recovery. The Maya cycles constitute in any way an important case study for the validation of regional climate models including the interaction with human activities (Cook et al.,2012).
The best known historical example is of course theMaunder minimumwhere a coincidence between the absence ofsunspotsand the “little ice age” in Northern Europe was observed with a peak in the 17th century. The “little ice age” corre-sponds to real historical observations, such as the growth of glaciers, population extinctions in Greenland and retreat of crops towards the South (Le Roy-Ladurie, 1967). Possible mechanisms are developed in the review paper by Gray et al.
(2010), associating solar UV forcing with theNorth Atlantic Oscillation, this last article concludes that further research is required both on the reconstruction of past spectral solar irradiances and on the possible triggers of this cooling.
2 UV climate and its biological and societal effects
Until the middle of the 20th century, medicine followed the Hippocrates hypothe-sis that exposition to solar radiationwas beneficial. However, these effects could only be quantified when physicists developed both lamps and UV sensors, and used them in a hospital environment. Subsequent studies showed damages compa-rable to the effects of ionising radiation on plants and test animals and ended up establishing a relation between UV and the genesis of human skin cancer (Latarjet, 1959). The progress of UV sensors led not only to this new devel-opment on the biological effects ofsolar radiationbut also to studies of what was called at the time “the UV limit of the solar spectrum”, these were precursors of the determination of the UV effects on atmospheric chemistry and its climate effects.
C. Muller: The role of the Sun in climate change: a societal viewpoint 37
From a biological point of view, UV radiation is divided between UV-A (400–
315 nm), UV-B (315–280 nm) and UV-C (280–100 nm). It is commonly admitted that UV-A is necessary in moderate doses to initiate metabolic processes, such as for example the production of vitamin D in the human skin.
Stratosphericozonefilters entirely UV-C and the largest part of UV-B;ozone is not the only effective UV filter, as carbon dioxide filters all UV below 220 nm, but a reduction inozonewould expose the biosphere to UV-B with consequences on both plants and animals.
The decrease of theozone layerobserved since the 1970s and especially since the Antarctic spring ozonehole has led to the 1987 Montr´eal protocol on ozone depleting species. Solar variations play a role in the equilibrium of the ozone layer but this effect is buffered by the fact that atomic oxygen produced by the dissociation of molecular oxygen more than compensates the destruction ofozone by UV radiation of longer wavelengths. Thus, the solar climate has very little effect on the UV radiation received at the Earth’s surface, but there again scientific research should never overlook the effects of perturbations and never consider a part of the Earth’s system as definitively auto-stabilising.
3 Present: assessment of anthropogenic influence on climate
The Intergovernmental Panel on Climate Change (IPCC) was established by the United Nations Environment Programme (UNEP) and the World Meteorological Organisation (WMO) in 1988 to provide the world with a clear scientific view on the current state of knowledge in climate changeand its potential environmental and socio-economic impacts. In the same year, the UN General Assembly endorsed the action by WMO and UNEP in jointly establishing the IPCC.
The IPCC has up to now generated five global reports each divided in three parts: 1) physical scientific basis, 2) vulnerability of socio-economic and natural systems, and 3) mitigating climate change through limiting or preventing green-house gas emissions and enhancing activities that remove them from the atmo-sphere.
The successive IPCC reports evolved in both complexity and assertiveness to-gether with the collection of new evidence of the surface temperature increase, the first four reports were more and more precise in comparing the observed warming with anthropogenic forcings until reaching an encyclopaedic size in 2007 with the Assessment Report 4 (AR4). AR4 was criticised in detail points, as ignoring the growth of some high altitudes Himalayas glaciers and largely dismissing fluctu-ations in natural forcings. It is in this respect that the 5th Assessment Report (AR5) increased its coverage to include and discuss the solar forcings.
4 Natural and extra-terrestrial forcings in AR5 of the IPCC
The Sun is the main driving force of our climate, but in general its changes appear in cycles or on a relatively long scale. Short term fluctuations as in Natural forcings
and especially extra-terrestrial forcings are detailed in chapter 8 of the first report.
The IPCC documents being consensus reports, the solar variations are considered from the point of view of the total solar irradiance in the modelling chapters, as non-linearity is not yet present in the models. The conclusions of IPCC WG-1 concerning the balance of forcings are shown in Figure 1: since 1750, solar ir-radiance has been a negligible cause of warming compared to the anthropogenic warming by the greenhouse gases.
Chapter 7 (Clouds and Aerosols) of AR5 dedicates a whole subsection on the impact ofcosmic rayson aerosolsand clouds, coming to the following synthesis:
cosmic rays enhance new particle formation in the free troposphere, but the ef-fect on the concentration of cloud condensation nuclei is too weak to have any detectable climatic influence during asolar cycleor over the last century.
Fig. 1.Radiative forcingestimates in 2011 relative to 1750 and aggregated uncertainties for the main drivers ofclimate change. Volcanic forcing is not included as its episodic nature makes is difficult to compare to other forcing mechanisms. Total anthropogenic radiative forcing is provided for three different years relative to 1750. The change in solar irradiance forcing correspond to the “grand maximum” ofsolar activityof the 20th century. (Figure from (IPCC,2014)).
C. Muller: The role of the Sun in climate change: a societal viewpoint 39
4.1 Anthropogenic forcing and acceptance of AR5
From the summary for policy makers of (IPCC, 2014), we learn that the WG 1 contribution to AR5 considers new evidence ofclimate changebased on many inde-pendent scientific analyses from observations of theclimate system, paleoclimate archives, theoretical studies of climate processes and simulations using climate models. Warming of theclimate systemis unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmo-sphereand ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.
Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system.
AR5 has been until now much less criticised by climate skeptics than the pre-vious reports as it contains almost all the scientific arguments prepre-viously put in evidence to dismiss the values of the conclusions. Governments have now come to accept that the 2015 ministerial conference planned in Paris will lead to a binding agreement on the emission of anthropogenic greenhouse gases and begin the prepa-ration of corresponding legislation. Businesses representing more than 92 trillions of dollars have federated since 2000 in the carbon disclosure process2in the United States, China, Japan, and the entire American continent to consider the risks and opportunity aspects of both global warming and regulation. Similar studies are more discrete in Europe where the adaptation information comes mostly from doc-uments of the European Commission. 3 However, a business-based process similar to the carbon disclosure process does not seem to exist. It is clear that European companies are now considering global warming and urban heat islands in their extension plans and planning of their air conditioning needs.
5 Evolution of solar-societal aspects non-related to climate:
UV and visible solar radiation
The IPCC and climate research in general do not consider the human need for natural visible light and moderate UV-A, most of the legislations do not go further than the European Council directive : “Workplaces must as far as possible receive sufficient natural light” (Council Directive of 30 November 1989 concerning the minimum safety and health requirements for the workplace). National regulations even recommend the obstruction of natural light sources to prevent the glare on computer screens. However, more and more architects incorporate in buildings light wells and atriums to achieve a feeling of comfort and spaciousness but it is far from being a general practice in housing projects and workplaces. For the UV part, the dangers of exposure to UV-B are better known than they were before
2https://www.cdp.net/en-US/Pages/HomePage.aspx
3See for examplehttp://ec.europa.eu/clima/policies/adaptation/what/docs/com_2013_
216_en.pdf
theozonelayer was endangered by the release of atomic chlorine and nitric oxide in thestratosphere, excessive exposure is however not yet recognised as a health hazard related to profession. It can be said that both the information campaigns and fashion evolution have mitigated the increase in human skin cancer. On the other side of the spectrum, morbidity associated to deprivation of UV-A is rarely studied and not considered at all in work legislation.
6 Future research related to climate
The IPCC did not yet set a schedule for a sixth report but many improvements are possible in the domain of solar forcing, the first being to use a forecast of thesolar cycles in the models instead of repeating a typical solar cycle (in AR5:
cycle 23). Future IPCC reports will certainly consider regional variations and will thus increase their societal impacts as they would influence insurance rates. These future reports will also be supported by improved data: solar total and spectral irradiance observations continue with better instruments on both ends of the fre-quency spectrum: from the short term variations associated withspace weatherto several 11 years cycles, these provide already better inputs to weather and climate models. Climate and General Circulation models are also rapidly progressing in spatial and time scales. Moreover, more complex earth system models including the ocean, land and cryosphere are appearing. These new developments lead to hope for a better description of the influence of all manifestations ofsolar activity on climate.
Further reading
The societal effects of climate changegenerated a large number of articles. The most complete and certainly the most verified is the IPCC synthesis report (IPCC, 2014), which summarises conclusions from all three working groups and which can be directly downloaded from the website of the IPCChttp://www.ipcc.ch/pdf/
assessment-report/ar5/syr/SYR_AR5_SPMcorr1.pdf