IAMAS: a century of international cooperation in atmospheric sciences

The International Association of Meteorology and Atmospheric Sciences (IAMAS) was founded in 1919 as the Section of Meteorology of the International Union of Geodesy and Geophysics (IUGG). Significant advances over human history, particularly during the 19th century, in the gathering, communication, assembly and analysis of observations of the changing weather and in theoretical understanding of the fundamental physical relationships and processes governing atmospheric circulation had been driven by the need for improved weather and climate forecasts to support the expansion of global trade, better public warnings of extreme weather, and safer and more effective military operations. Since its foundation, in parallel and cooperation with intergovernmental development under the auspices of what is now the World Meteorological Organization (WMO), IAMAS and its 10 international commissions have provided the international organizational framework for the convening of the general and scientific assemblies and other meetings that bring together expert scientists from around the world to further advance scientific understanding and prediction of the behaviour of the atmosphere and its connections to and effects on other components of the Earth’s intercoupled geophysical system.


Introduction
The successes and failures of societies around the world have been and continue to be dependent on weather and climate, especially because of their critical role in determining the availability of vital agricultural, ecological and hydrological 5 resources. Many of the deities of the earliest civilizations represented weather-and climate-related phenomena, becoming a mechanism for organizing, explaining and passing along relationships and linkages that had been gained from prolonged observation of the timing and variability of seasons, mon-10 soons and other phenomena to future generations. Significant departures from the expected patterns were often memorialized as conflicts between conflicting deities with such fidelity that many of the events have since been identified in paleoclimatic records.
In Greece in the fourth century BCE, Aristotle 1 and Theophrastus prepared treatises describing the collective wisdom of their time, hypothesizing that various interactions among the four bodies of fire, air, water and earth provided explanations for the observed weather and climate. About 20 50 BCE, Andronikos Cyrrhestes constructed a 12 m high, octagonal horologium (known now as the Tower of Wind), which is viewed as the first meteorological observatory, serving as a sundial, water clock, compass and weather vane, with stone carvings representing each of the eight directions 25 from which Athens' winds came over the course of the year. For many centuries, expectations regarding atmospheric phenomena were based almost solely on relationships involving Sun angle and the changing positions of constellations. Speculations were mainly qualitative with little capability of 30 linking together what was happening in different locations correlations of phenomena across regions and with geophysical events such as volcanic eruptions. Making progress, however, required both (1) instruments that could quantify observations for analysis and display as maps and (2) advances in understanding of atmospheric physics and chemistry that 10 would eventually provide a more rigorous basis for analysis and forecasting. Advances in both areas were spurred by increasing demands for better understanding of the range of meteorological conditions that could affect public safety, military planning and actions, and expansion of trade and the 15 global economy.
The second and third sections of this paper highlight a number of early advances with relevance to the atmospheric sciences and the gradual awareness about the need for increasing international cooperation to accelerate progress in 20 understanding of atmospheric behaviour and for forecasting. These advances provided essential groundwork that enabled the foundation of the International Association of Meteorology and Atmospheric Sciences (IAMAS) after the severe historical incision of World War I. The fourth and fifth sections 25 describe the formative years of IAMAS following World War I and then key developments of the association since that time. Tabulated biographical information about the 35 individuals who have served as the president and/or secretarygeneral is also included to provide a sense of the broad spec- 30 trum of interests of those in the international scientific community that have contributed to the development of IAMAS. The sixth section provides a summary of the establishment, primary focus and key scientists that played roles in the formation and development of the 10 international commissions 35 that form the backbone of IAMAS. The final section then contains a few concluding remarks.

The early development of instruments and assembly of observations
The quest for useful observations was significantly advanced 40 in the 1600s when Italian polymath Galileo Galilei invented the thermometer, Italian physicist Evangelista Torricelli invented the barometer and Dutch physicist Christiaan Huygens invented the pendulum clock. During the 18th century, French nobleman Antoine-Laurent de Lavoisier, in addition to organizing understanding of chemistry by establishing the periodic table, was instrumental in identifying that the atmosphere contained, among other components, both oxygen and water vapour, the latter becoming an especially important variable to observe.

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By the early 1800s, international trade led to sufficient observations being taken that initial climatological maps could start to be prepared, giving indications of typical conditions by latitude and longitude. It was not until the second half of the 19th century, however, that there were sufficient data and 55 insights to allow preparation of synoptic maps showing such features as cyclonic and anti-cyclonic circulations and indications of their movement in time and changes in intensity. As information was accumulated, climatologies were developed to provide information for improving the routing and 60 safety of the rapidly increasing number of sailing ships. The increasing utility of observations led directors of national meteorological services to agree in the late 19th century to standardize measurement techniques.
Assembly of information was further advanced in 1837 by 65 American Samuel Morse's invention of the telegraph, which soon led to communication lines for rapidly transmitting and receiving observations around the world. The first reliable cable across the Atlantic Ocean began service in 1866, with further installations helping to reduce the time for assembling 70 observations from days and weeks to hours and, starting with the first transatlantic radio communication in 1901, to minutes.
An important limitation to advancing understanding, however, was that virtually all of the regular observations were of 75 conditions at the surface, the few exceptions including cloud type, height and cover (changes of which became an important basis for early weather forecasting); atmospheric scattering (leading to the empirical forecasting adage: "Red sky at night, sailors' delight. Red sky at morning, sailors take 80 warning"; cf. Library of Congress, https://www.loc.gov/rr/ scitech/mysteries/weather-sailor.html, last accessed 25 January 2019); sunspot number (as a proxy for changes in solar radiation); and, beginning at a very limited number of locations in the late 19th century, observations of winds aloft 85 using pilot balloons.
Driven by their desire to better understand the weather and climate in the adjacent North Atlantic, Norwegian meteorologist Vilhelm Bjerknes and oceanographer Bjørn Helland-Hansen each established institutes at the University of 90 Bergen in 1917 that were particularly important in advancing understanding of atmospheric fronts, air-sea interactions, and other features that proved essential for forecasting system behaviour over ensuing days. During World War I, aircraft observations aloft started to become available, setting 95 the stage for significant improvement in weather forecasts.

The early development of scientific understanding
In parallel with the development of observations leading up to the 20th century, advances were made in understanding the physics and chemistry of the atmosphere. The perfect law 100 of gases (also known as Boyle's law after Anglo-Irishman Robert Boyle) and recognition of the three laws of motion put forth by England's Sir Isaac Newton in the late 17th and early 18th centuries provided the foundation for develop-ing an understanding of global atmospheric circulation patterns. In 1735 George Hadley, an amateur English meteorologist, published a paper expanding upon English astronomer Edmond Halley's earlier explanation for the trade winds. Hadley's explanation was based primarily on consideration 5 of the buoyancy of hot air and the Earth's rotation. The resulting circulation he derived featured a single cell extending from the Equator to high latitudes. American William Ferrel reworked the analysis in the 19th century, conserving angular rather than linear momentum, and proposed the three-cell 10 structure recognized today, which has the Ferrel cell located between the low-latitude Hadley cell and the high-latitude Arctic circulation.
In 1824, French scientist Joseph Fourier first published an explanation for the near stability of the Earth's temperature, 15 indicating that downward emission of infrared radiation from the atmosphere played a critical role (Fourier, 1824(Fourier, , 1827. The fundamental laws for thermodynamics were also formulated during the period, with many of the insights coming as a result of seeking to understand the behaviour of gases, partic-20 ularly those in the atmosphere. In particular, experiments by English physicist (and brewer) James Joule and theoretical work by German physicist Rudolf Clausius established the relationship between work and heat and, with later involvement of William Thomson (later Lord Kelvin), led to the de-25 velopment of the law of conservation of energy as the first law of thermodynamics. Clausius was also responsible for contributing to development of the second law of thermodynamics, which posited that entropy always increases (sometimes stated as heat flows from hot to cold) and deduced 30 what became the Clausius-Clapeyron relationship that, in the meteorological field, determines the saturation water vapour mixing ratio at various temperatures. Adding insights from experimental studies he began in the 1850s, Irish physicist John Tyndall presented proof that water vapour and carbon 35 dioxide (CO 2 ) were the most important atmospheric gases contributing to what is now called the "greenhouse effect" 2 , providing a strong intellectual framework for understanding the global energy balance and the thermodynamic effects of many atmospheric processes (Tyndall, 1861).

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During the same period, Scottish geologist Charles Lyell, building on the reasoning of fellow countryman James Hutton, argued that the Earth's features were a result of slow processes acting over long periods of time (uniformitarianism), rather than being mainly responses to short-term, catas-45 trophic changes, thus rejecting notions derived from biblical genealogies that the Earth's age was only several thousand years. In books first published in the early 1830s, Lyell (1830Lyell ( , 1832Lyell ( , 1833) went on to define and distinguish geological strata (and the climatic conditions that must have been 50 prevailing), clarifying the differences among earlier geolog-2 The term "greenhouse" has become widely used even though the actual physics keeping greenhouses warm is a result of limiting the escape of water vapour from the glass-enclosed structure.
Lyell's contributions over the next few decades also con-55 tributed critical information underpinning the scientific studies of England's Charles Darwin, setting atmospheric (and geophysical) scientists on a quest to document and explain changes in climate in terms of their causes. The proposal by Swedish scientist Svante Arrhenius in 1896 that changes 60 in the atmosphere's CO 2 concentration could help explain Earth's climatic history led directly to his prediction that the future climate would become warmer as a result of ongoing fossil-fuel emissions (Arrhenius, 1896). Through laborious calculations, he became the first scientist to calculate the 65 sensitivity (i.e. responsiveness) of the climate to a doubling of the CO 2 concentration, arriving at a value only slightly higher than today's central estimate. That it took over half a century for his hypothesis to be accepted was the result of two key criticisms: (1) the large carbon-holding capac-70 ity of the oceans would limit the potential rise in the atmospheric CO 2 concentration (a criticism not convincingly refuted until the ocean tracer studies by American oceanographer Roger Revelle and Austro-American chemist Hans Suess in the 1950s; Revelle and Seuss, 1957) and (2) CO 2 ab-75 sorption in the atmosphere was already saturated (a criticism not convincingly refuted until the one-dimensional radiativeconvective model simulations of Japanese-American meteorologist Syukuro Manabe and American meteorologist Richard Wetherald in the 1960s showed the importance of 80 the change in altitude at which saturation occurred; Manabe and Wetherald, 1967).
With quantitative understanding developing, British mathematician (and later meteorologist) Lewis Fry Richardson made the first attempt to actually calculate the evolution of 85 the weather over one 6 h time step by quantitatively representing each of the many processes thought to alter atmospheric temperature, water vapour, pressure and winds (Richardson, 1922). Gathering all the available data for 20 May 1910 for a region in central Europe, he spent idle 90 time while serving as an ambulance driver during World War I undertaking the extensive calculations by hand. While the forecast was unsuccessful for various reasons, the stage was set for developing numerical forecasting capabilities over ensuing decades. Organized international cooperation in meteorology goes back to the mid-19th century, with the first international conference convening in Brussels in 1853 (Ismail-Zadeh, 2016). 100 Naval representatives from 10 nations (eight western European nations plus the USA and Russia) focused their attention on the need to improve and expand observations for An early example of such international cooperation is documented in the group photograph taken during the sum-10 mer of 1891 at the International Conference of Directors of Meteorological Services held in Munich, Germany (Fig. 1). Among the 34 attendees from more than a dozen nations on three continents were Georg Neumayer (pioneer of Southern Hemisphere meteorology) and Wilhelm von Bezold (ther-15 modynamicist who defined potential temperature) from Ger-many, Cleveland Abbe (early proponent of dynamical meteorology) from the USA, Julius Hann (global-scale collector of vast climatological datasets) from Austria, and Léon Teiserrenc de Bort (specialist in vertical soundings 20 and co-discoverer of the stratosphere a decade later) from France. Although most participants represented fledging meteorological services, they met as experts in a personal, nongovernmental capacity and set out to distribute tasks among technical commissions (Davies, 1990, p. 4), as is still done by 25 both IAMAS and WMO. The International Commission on Radiation and Insolation formed in 1896 (Bolle, 2008) and the International Commission for the Scientific Investigation of the Upper Air were two of the very early commissions.
As World War I was coming to a close in late 1918, lead-30 ing scientists in the allied nations began advancing plans for peaceful cooperation and scientific advancement under in Bergen, Norway, 2 years after the formation of the Section of Meteorology. Numbering is in five columns, from left to right, with the following information for each person: number, first name, surname (age at conference, country of work -not necessarily nationality -and lifespan). The high-resolution scan was provided by the University Library in Bergen, Norway.
the League of Nations. That international cooperation in the development and application of meteorological observations and forecasting had been going on for a half-century provided a strong basis for the effort to expand such cooperation across a broader spectrum of the geophysical sciences. Led 5 by representatives of the national academies of science of the various nations 3 , discussions and planning meetings led, within a year, to the formation of the international scientific structure that has persisted for virtually all of the period since (Shaw, 1923). The International Research Council (IRC) was of the International Council of Scientific Unions -ICSU, recently merged with the International Social Science Council to form the International Science Council). The International 15 Union for Geodesy and Geophysics (IUGG) was established within the IRC. It was composed initially of six sections, one being the Meteorology Section (Bauer, 1919a).
At the Brussels plenary, Sir Napier Shaw of UK's Royal Society was appointed President of the Bureau for 20 the Section de Météorologie, hereafter referred to as the Meteorology Section. Shaw had emphasized greater reliance on science (as opposed to empirical analysis) in his leadership of the British Meteorological Office during World War I (including by employing Lewis Richardson). 25 Charles Alfred Angot, earlier Directeur du Bureau Central Météorologique de France, was selected as vice president, and Charles F. Marvin, then chief of the US Weather Bureau, was selected as secretary (Bauer, 1919b). While each had been or was a leader of their government's meteorolog-30 6 M. C. MacCracken and H. Volkert: IAMAS ical service, the Meteorology Section was a creation of the national academies of science of the participating countries rather than of government meteorological organizations. The focus of the non-governmental effort was intended to be on the scientific questions arising in the study of meteorology and geophysics in contrast to the more empirical and observational emphasis of the governmental efforts that were focused mainly on operational forecasting and ensuring public safety. That these efforts had been ongoing together for so long provided a strong basis for their ongoing cooperation 10 and connection.
Each nation was called upon to have a national committee for IUGG, providing a member to serve as a representative to each of its sections; this requirement proved instrumental in spurring organization and cooperation among each of 15 the disciplines in the member countries. Members of the atmospheric science community were active participants in the organizational efforts (Shaw, 1923). For example, climatologist Hubert Lamb was included along with those affiliated with the UK Meteorological Service as UK's representatives; 20 France formed an IUGG national committee, of which 49 of its 124 members were affiliated with the Section of Meteorology; and in the USA, formation of the national committee spurred organization in 1919 of both the American Meteorological Society and what later became the American Geo-25 physical Union (Wood, 1920).
The photograph of participants at the eighth meeting of the International Commission for the Scientific Investigation of the Upper Air ( Fig. 2) held in Bergen, Norway, in 1921 shows many of those who were engaged with the early devel-30 opment of the Section of Meteorology, including Sir Napier Shaw (the section's founding president), Vilhelm Bjerknes (who served as president from 1935-1936) and his son Jacob Bjerknes (who served as secretary-general from 1936 to 1948 and president from 1951-1954). Right from the begin-35 ning, the enthusiasm for international cooperation on a voluntary basis appears to have been a natural and steady ingredient of most gatherings of IUGG's sections and their successors (cf. Gold, 1921;Volkert, 2017). A brief but vivid eyewitness account was provided some 55 years after the meeting 40 in Bergen by Schereschewsky (1977), including succinct appraisals of later achievements by some of the participants.
By providing their sections with an annual allocation from the collective dues paid by the national academy of each of the 20 initial member nations, IUGG provided resources for 45 their Sections to actually organize research activities. This was in contrast to the IMO, which did not at that time provide funds to promote cooperative activities. That this effort to promote international research was seen as a particularly important endeavour by the participating nations is suggested 50 by the fact that the US Congress explicitly endorsed the effort and specifically allocated funding for IUGG membership in its annual budget actions over IUGG's first several years.
The first major action of the Meteorology Section's Bureau was to call for proposals from the national committees for re-55 search topics and questions to be placed on the agenda for the first IUGG general assembly to be held in Rome in 1922. In his report to the Royal Meteorological Society, Shaw (1923) explained 11 topical areas (or "gaps of knowledge") that had been submitted by the national committees of Great Britain, 60 France and Italy (collected in Table 1). Many of the suggested topics continued to be of such interest that they later became the basis for formation of many of the commissions that now exist.
Nine nations are reported to have sent delegates to the first 65 meeting of the Meteorology Section at the Rome general assembly: Australia, Belgium, France, Great Britain, Italy (in large numbers), Portugal, Spain, Sweden and the USA. Resolutions approved were related to the following subjects (Kimball, 1922), which were closely linked to the above-70 mentioned topical areas: 1. allocating much of the Section's funding to initiate an enhanced network of balloon sondes to explore the upper atmosphere; 2. allocating much of the remaining resources for study of 75 the stratosphere through expansion of the pilot-balloon network, especially in the desert regions and around the Equator; 3. developing a cooperative field and theoretical programme on convection; 10. meeting with the International Meteorological Committee to discuss potential overlaps and cooperation.

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Following the general assembly in Rome in 1922, the Meteorology Section met every 3 years (except for the years during Table 1. Numbered topics ("gaps in our knowledge") outside of the routine investigations of meteorological services, suggested by three National Committees of IAM for discussion at the general assembly of 1922 in Rome (Shaw, 1923).

Country no. Topic
Great Britain 01 Lack of observations of temperature and wind velocity in the upper air over the sea and over deserts or inhospitable regions 02 Which observers worldwide pay attention to the actual detail of convection as prerequisite for rainfall? 03 What are immediate and longer-term effects of active convection on the surrounding atmosphere? 04 Role of radiation in the sequence of weather and for the large-scale vertical circulation 05 Impairment of visibility, measurements of hygroscopic aerosols to address atmospheric pollution 06 Composition of the atmosphere above 100 km. Is hydrogen main constituent at such heights?
France 07 Different sorts of thunderstorms and, more generally, electrical phenomena 08 Transparency of the atmosphere and its optical phenomena 09 Different sorts of clouds 10 Forecasting weather, in particular the method of tendencies (isallobars) traditional scientific disciplines in atmospheric studies, the name was officially changed to the International Association of Meteorology and Atmospheric Sciences (IAMAS) at the 21st general assembly held in Boulder in 1995. Hereafter in this chapter, the association's current name will be used.

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Since its beginning, IAMAS has been led by a president, with administrative and organizational activities directed by a secretary-general nominated by the national committees that are members of IUGG. New officers are elected at each of the IUGG general assemblies to serve from the conclusion of 20 that gathering through the next general assembly. Succinct biographical information and portraits are provided for all 23 presidents in Table 2 and Fig. 3 and for all 12 secretariesgeneral in Table 3 and Fig. 4. All of the officers have been trained as scientists, but they come from a broad range of 25 disciplines, including geography, meteorology, physics, astronomy and mathematics. Up to the 1970s, many of the officers were employees of the national meteorological services, while since then their primary employment has been by universities and research laboratories. 30 From the time of its formation, IAMAS has been cooperating with the international meteorological organizations organized by the world's national governments. With the formation of the United Nations, IMO's role was taken over by the World Meteorological Organization (WMO) in 1950 35 (Davies, 1990). A formal agreement between IUGG and WMO was worked out in 1953 that has governed their respective roles since that time, with WMO taking the leading role in organizing meetings of the international meteorological service organizations and IUGG (through IAMAS) orga-40 nizing meetings aimed at advancing the science of meteorology. To ensure ongoing cooperation, IUGG has, since signing the agreement, appointed a formal liaison with WMO, often a scientist drawn from IAMAS.
A major undertaking of IUGG and its associations in co-45 operation with other international unions and entities was sponsorship and organization of the International Geophysical Year (IGY) in 1957-1958. Along with later satellitebased data, data from the expanded observation network established during IGY has made clear that Planet Earth must 50 be considered as an integrated whole. To ensure ongoing cooperation in observations and research, ICSU created the Scientific Committee on Oceanic Research (SCOR) in 1957 and the Scientific Committee on Antarctic Research (SCAR) in 1958 to promote interdisciplinary science on the oceans and 55 Antarctica, respectively. Ever since then, IAMAS has had representation in these organizations, participating in their governance on behalf of IUGG.
Beginning in the 1970s, IAMAS and the other IUGG Associations began holding their own (sometimes joint) Scien-60 tific Assemblies in the years between the quadrennial IUGG general assemblies (Table 4). The first such IAMAS Scientific Assembly was convened jointly with the International Association of the Physical Sciences of the Ocean (IAPSO) in Melbourne in 1974. The assembly drew participation from 65 nations on both sides of the "Cold War," an openness to all that IUGG and its associations now enshrine. Given the geophysical continua of the solid, liquid, and gaseous Earth, atmospheric studies under the umbrella of IAMAS benefitted significantly from the advances in observations and under-70 standing led by a number of its sister associations, primarily involving IAGA (Mandea and Petrosky, 2019), IAPSO    non-governmental scientific community. The JSC and a sister body under the International Geosphere Biosphere Programme (IGBP) organized a number of major projects, often in cooperation with IUGG and its various components (including IAMAS and its commissions), covering the Earth system from the surface to the stratosphere and the tropics to the poles as well as physical, chemical and biological processes from the hydrologic cycle to cycling of atmospheric pollutants. Over ensuing decades, IAMAS has appointed representatives to participate in both planning meet-10 ings and project activities, especially when the area of study has included the atmosphere.

The backbone of IAMAS: its 10 international commissions
The wide range of topics considered at the Rome general 15 assembly made clear that the scope of important topics encompassed in atmospheric sciences is large. With observation and research efforts intensifying through the 20th century, the number of specialists in each of the particular areas grew, leading to IAMAS adopting an internal structure 20 that would promote focused development and cooperation in each area while still encouraging integration across the study of meteorology and atmospheric sciences. To accomplish this, IAMAS has, over its history, established 10 scientific commissions (see Table 5). Each com-25 mission has a particular scientific focus and its own set of officers and members. In addition to describing the scientific scope of each commission, the following paragraphs list a few of the many scientists from around the world who have been responsible for advancing understanding in each area, 30 generally focusing on those involved in the early to mid-20th century and even before. A good number of these commissions have become so large that they convene their own scientific meetings, typically bringing together hundreds of scientists, most often in years other than those when IUGG gen-35 eral assemblies and IAMAS scientific assemblies are held. In addition to meetings held under IUGG and IAMAS auspices, many of the commissions also arrange to meet jointly with other IUGG associations or are involved with them in related or co-sponsored international scientific programmes. 40 In addition, commission members often serve as representatives to or are on the leadership teams for scientific projects sponsored by other international bodies. A comparison of the leading scientists mentioned in the commission highlights below with the names of officers listed in Tables 2 and 3 45 reflects the organizing notion of IUGG that its associations have officers drawn from those who can be prominent and knowledgeable spokespersons for the scientific areas encompassed by its commissions.  In its earliest days, the focus was on the development and accuracy of instruments and measurement techniques in order to generate better understanding of the transmission of solar radiation through both clear and cloudy conditions, particularly seeking to determine if and how the  Through the 20th century, there has been increasing attention devoted to the determination and calculation of infrared radiation and how the many gases in the atmosphere, especially gases being augmented by human-caused emissions, are influencing and strengthening the atmosphere's absorp-35 tion and emission of infrared radiation, both upward toward space and downward toward the surface, thus strengthening the natural greenhouse effect. The understanding gained regarding the Earth's radiation and energy balance has proved essential in improving understanding of both natural vari-40 ability and long-term climate change, including how past changes in solar insolation, volcanic aerosol loading, land cover change and human activities, particularly fossil-fuel combustion, have together affected the Earth's energy balance and the resulting weather and climate. With solar radi-45 ation essential for food production, ecosystem growth, and direct and indirect forms of renewable energy, among many other societal and ecosystem interactions, the scope of IRC's scientific coverage remains broad and critical.

International Ozone Commission
Noticing the sharp cut-off in the radiation spectrum at UV wavelengths, Marie Alfred Cornu (France) was the first to suggest in 1879 that this was due to atmospheric absorption (Cornu, 1879). A year later, Sir Walter Noel Hartley (UK) 5 suggested that absorption by ozone was the cause; the absorption band is now named after him (Hartley, 1880). Together they recognized that the ozone must be present in the upper atmosphere, being formed there by the missing UV radiation. While early observations showed variations through 10 the year, it took until the invention of the spectrometer in 1924 by British physicist and meteorologist Gordon Dobson for there to be an instrument that could be used to make reliable and comparable measurements around the world. The first international Conference on Ozone was held in 15 Paris in 1929 and the second in Oxford in 1936 (Meetham, 1936). An expression of interest in becoming affiliated with the Section of Meteorology led initially to formation of a Committee on Ozone by the Commission on Solar Radiation; it was this committee that was recognized as the Inter-20 national Ozone Commission (IOC) in 1948 (Bojkov, 2012 The availability of a strong understanding of stratospheric ozone chemistry became critical in the 1970s as threats to its vital role emerged as a result of proposals for establishing a fleet of supersonic passenger aircraft and then as a re-30 sult of the rising emissions of chlorofluorocarbons (CFCs) and other halocarbons. Observations and analyses of stratospheric ozone depletion and then the Southern Hemisphere's springtime "ozone hole" prompted international adoption of the Montreal Protocol and subsequent amendments. Since 35 then, there has been the start of a recovery (not to mention that cutting CFC emissions has also been an important con-tribution to cutting greenhouse gas emissions). Scientific advances in this field were internationally recognized in 1995 with the award of the Nobel Prize in chemistry given to Paul Crutzen (the Netherlands and Germany), Mario Molina (Mexico and USA) and F. Sherwood Rowland (USA).

6.3 International Commission on Clouds and Precipitation
The International Commission on Clouds and Precipitation (ICCP) was the third IAMAS Commission to be established, being approved in 1953 and holding its first official meeting  With fresh water being an absolutely vital resource for the world's growing population, developing the capabilities for forecasting precipitation and changes in its distribution and intensity over time will be essential for sustaining the world's peoples and communities.

International Commission on Atmospheric Chemistry and Global Pollution
The fourth commission created by IAMAS was the International Commission on Atmospheric Chemistry and Radioactivity, which was approved in 1957 and was most focused on 40 urban pollution and fallout from nuclear testing. It was renamed the International Commission on Atmospheric Chemistry and Global Pollution (ICACGP, later iCACGP) in 1971, recognizing that the chemistry and composition of the atmosphere was really an issue that reached from urban to global 45 scales and that, with the phasing out of nuclear testing in the mid-1960s, surface deposition of air pollutants, especially the sulphate compounds that caused acid precipitation, had replaced radioactivity as a more immediate issue of societal and ecological concern. 50 iCACGP's members played a leading role in establishing the International Geosphere Biosphere Programme (IGBP), which coordinated studies in the area from 1987 to 2015, and initiating and sponsoring a number of important international research projects, including IGAC (International 55 Global Atmospheric Chemistry), SOLAS (Surface Ocean-Lower Atmosphere Study) and iLEAPS (the Integrated Land Ecosystem-Atmosphere Processes Study). IGAC, SOLAS and iLEAPS are all now core projects within ISC's Future Earth 4 , which has succeeded IGBP. Some of the leading sci-60 entists in this research area in the mid-20th century and latter half of the 20th century included Christian Junge (Germany and USA); Bert Bolin, Henning Rodhe and Erik Eriksson (all Sweden); Paul Crutzen (the Netherlands and Germany); C. David Keeling, Lester Machta, F. Sherwood Rowland, 65 Robert Duce and Robert Charlson (all USA); Mario Molina (Mexico and USA); Hans-Walter Georgii and Dieter Ehhalt (both Germany); Sean Twomey and Ian Galbally (both Australia); and Davendra Lal (India).
iCACGP's efforts are now especially focused on how at-70 mospheric composition and deposition of various pollutants interface with and affect such basic societal issues as water supply and quality, food production, and human and ecosystem health in a changing climate. With the world population growing rapidly, urbanization increasing, forest fires becom-75 ing more intense and dependence continuing on sources of energy that pollute the atmosphere, understanding how to ensure a healthy atmosphere has become a very wide ranging research area and a challenge of existential importance for science and society as the Anthropocene evolves. 80 A celebration of the 60th anniversary of the commission's creation was held at the recent joint 14th iCACGP quadrennial symposium and 15th IGAC science conference (http: //icacgp-igac2018.org, last access: 25 January 2019), which brought together ∼ 700 participants in Takamatsu, Japan. 85 This gathering exemplified the scientific respect and organizational strength of the IAMAS commissions. The Japanese hosts underscored the importance of existing international organizations in promoting the development of a nation's scientific community, as has happened, for example, for atmo-90 spheric chemistry in Japan over the past several decades.

International Commission on Polar Meteorology
In the late 19th and early 20th centuries, explorers were finally able to reach both poles. The International Polar Years of 1882-1883 and 1932-1933 led to significant advances in 95 understanding, but it was not until the International Geophysical Year (1957Year ( -1958 that permanent research and observation stations were established and thereafter permanently manned in the polar regions. Since then, appreciation has grown not only of the importance of what is occurring in po-100 lar regions but also of the associated influences that changes in the Arctic and Antarctic have on mid-latitude weather, the global climate and sea level.
Reconstructions of past changes in the polar climate make clear that its climate has changed more than the conditions of any other region, ranging from periods when it was warm enough for palm trees to grow on land areas bordering the Arctic Ocean to cold enough that glacial ice that was a few kilometres thick was piled over continental areas so large that the global sea level was pulled down by over 100 m.

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As the 20th century proceeded, airline routes were established across the Arctic, the population in high latitudes grew and the potential for extracting resources was recognized. The International Commission on Polar Meteorology (ICPM) was approved in 1963 to focus research attention 15 on high-latitude meteorology. Leading scientists in this field during the early to mid-20th century included, among others, Albert Crary, Joseph Fletcher and Morton Rubin (USA); Svenn Orvig (Norway and Canada); and Norbert Untersteiner (Austria and USA). 20 With ongoing human-induced warming causing amplified temperature increases in high latitudes, the Arctic's climate is rapidly changing, with sea ice and snow cover retreating, permafrost thawing, and accelerating mass loss from mountain glaciers and the Antarctic and Greenland ice sheets. In 25 addition to affecting mid-latitude weather, polar changes are influencing the Northern Annular Mode and Southern Annular Mode, affecting seasonal to interannual variability. With so much occurring, the importance of improving understanding of polar meteorology and the consequences for the sea 30 level is essential to provide better estimates of emerging impacts having effects around the world.

International Commission on Dynamical Meteorology
The the inherent chaotic results that emerged as a result of seeking to solve the coupled set of nonlinear equations governing atmospheric behaviour (Lorenz, 1963). Leading scientists in early research in this area included Jacob Bjerknes (Norway and USA; no. 17 in Fig. 2 Since ICDM's creation, and with substantial efforts by the national meteorological services, the time horizon for skilful 55 model-based weather forecasts has been extended from a few days to 1-2 weeks. Of particular importance in advancing forecast quality has been ensuring a better conditioning of initial conditions (the primary problem in the early modelling effort by Lewis Richardson;no. 11 in Fig. 2), treating the en-60 tire global atmosphere, using a finer scale grid (which has also required shortening the time step of the model), implementing less diffusive numerical schemes, better representing subgrid-scale influences and turbulence (treating nonadiabatic processes that were initially not being included), 65 and dealing with the chaotic behaviour caused by the nonlinear equations using an ensemble simulation approach.
With both more extensive observations and better models, there have also been significant advances in the understanding and simulations of not only the global atmosphere 70 but also of the treatment of storms. For example, Schultz et al. (2019) provide a description of how international cooperative activities and exchanges have advanced treatment of extratropical cyclones since the early 20th century. While much attention is given to how the underlying climate is changing, 75 the roles and effects of internal oscillations, ranging from the Madden-Julian Oscillation to Atlantic and Pacific multidecadal oscillations, are currently drawing intense research attention. This is the case both because of how such oscillations affect interannual and interdecadal variations and 80 because of interest in understanding how these oscillations might be influenced by climate change. Improvement of forecast skill at finer spatial scales, nowcasting, better simulation of severe storms and extreme conditions, and greater understanding of apparent cycles and long-term linkages all have 85 the potential to reduce losses of life and improve societal preparation and resilience, so providing exciting challenges for research in dynamical meteorology.

International Commission on Climate
Climate variations and changes have long been of scientific 90 interest because of how they have affected human activities and played a role in determining the prevailing landscape. In proposing that a change in the atmospheric CO 2 concentration could have been a cause of changes in Earth's climatic history in the 1890s, Arrhenius also hypothesized that 95 human-caused emissions of CO 2 could affect the future climate (Arrhenius, 1896). Fundamental theoretical criticisms of his hypothesis were not resolved by the atmospheric and carbon cycle research communities until the mid-20th century. With the 1960s then becoming relatively cool, recog-100 nition that emissions of sulphur dioxide and its conversion to sulphate could also affect the climate broadened inquiry into how the full range of human activities was affecting and could affect past, present and future climates.
The 1970s brought the first analysis of climate change 105 as recorded in ocean-sediment cores (Hays et al., 1976); further studies soon provided records of changes in climate and, through isotopic analyses, the sea level stretching back millions of years. The sediment-core records provided initial confirmation of Serbian geophysicist Milutin Milankovitch's hypothesis from the 1930s that glacial-5 interglacial cycling was being driven by the combined influences of cyclic changes in the ellipticity of the Earth's orbit, tilt of its axis and precession of the orbit through the seasons (Milankovitch, 1941); studies since then have made clear that many additional processes and feedbacks 10 also play a role (Berger, 2001). Leading climate scientists in the early and mid-20th century included, among others, Hubert H. Lamb (UK); Hermann Flohn (Germany); Mikhail Budyko (USSR); Jacques van Mieghem and André Berger (Belgium); José Peixoto (Portugal); Ye Duzheng 15 (China); and Jerome Namias, Victor Starr, Barry Saltzmann and W. Lawrence Gates (all USA).
To provide a focus for the growing international community of climate researchers, IAMAS established the International Commission on Climate (ICCL) in 1977, with 20 its scope set broadly to include the characteristics, fluctuations and changes in climate on all timescales, covering the past, present and future. Two years later, the first World Climate Conference, organized primarily by WMO (1979), led to establishment of WCRP, which has since then provided 25 a framework for bringing together the research capabilities and interests of individual scientists, organized government agency and laboratory research activities.
In response to concerns regarding ongoing increases in fossil-fuel emissions (e.g. Study of Man's Impact on 30 Climate, 1971;WMO et al., 1985), the Intergovernmental Panel on Climate Change (IPCC) was established in 1988 by WMO and the United Nations Environment Programme (UNEP; later joined by UNESCO's Intergovernmental Oceanographic Commission) with the goal of period-35 ically summarizing and assessing research findings regarding human-driven effects on climate, consequent impacts on the environment and society, and potential technological and policy options for moderating the projected changes in climate. While science typically advances as it seeks to rec-40 oncile and resolve unexpected observations and different interpretations among scientists, the IPCC has succeeded in bringing the international scientific community together to assess and summarize both understanding of and uncertainties regarding climate change, its impacts on society and 45 the environment, and approaches and measures for avoiding "dangerous anthropogenic interference with the climate system" (United Nations Framework Convention on Climate Change, 1992). For its accomplishments and its now five assessments 5 , carried through with participation of many cli-50 5 For access to the IPCC assessments and special reports, see https://www.ipcc.ch/reports/ (last access: 25 January 2019). mate scientists, the IPCC was the co-winner of the 2007 Nobel Peace Prize 6 .
With fossil fuels currently providing roughly 80 % of the world's energy and human-induced changes in the climate now pushing the global average temperature toward 55 levels higher than observed in many tens of millions of years, research relating to climate variability and change and integrated impact studies are now the subject of numerous meetings and assessments sponsored not only by IA-MAS and other associations within IUGG but also by the 60 WCRP, IPCC, and many other national and international programmes and organizations.

International Commission on Planetary Atmospheres and their Evolution
Scientific hypothesizing about the climates of Mars and 65 Venus goes back at least to the discovery of the telescope in the early 17th century, with science fiction writers speculating on the potential for alien life during the late 19th and early 20th centuries. The establishment of highly capable observatories and careful spectral analyses provided the 70 first scientific insights into the actual compositions of planetary atmospheres. Within a decade of the launch of the first Earth-orbiting satellite in 1957, fly-by missions took satellites past Earth's neighbouring planets with attempts at landing occurring soon thereafter. These missions provided the 75 basis for improving understanding the energy balances and climates over a much wider range of possible conditions, enriching the types of evaluations of model representations of atmospheric composition and processes and confirming the applicability of physical and chemical relationships for ex-80 plaining resulting features. The International Commission on Planetary Atmospheres and their Evolution (ICPAE) was established in 1977, emerging as a result of meetings during the early 1970s of the IRC, which was faced with interpreting satellite-based ra-85 diation measurements being taken in planetary fly-bys. The scope of ICPAE was set broadly to include study of planetary, cometary and satellite atmospheres and their evolution (including the Earth's atmosphere, when it is considered in the context of atmospheres on other planets). Sig-90 nificant discoveries have included Venus and Mars hosting liquid water on their surfaces in the past, the extreme greenhouse effect on Venus, the many zonal jet streams in alternating directions on Jupiter and Saturn, erupting volcanoes of many kinds on the planets and their moons, and 95 seasonal extremes on Titan with methane rain filling lakes on the surface. Leading scientists in the early studies of planetary atmospheres included Carl Sagan, Joseph Chamberlain, Thomas Donahue, Seymour Hess, James Kasting, Toby Owen and James Pollack (all USA); Donald Hunten 100 (Canada and USA); Richard Goody (UK and USA); and many others.
In addition to ongoing analytic and Earth-based observational studies, ICPAE members have played leading roles in planetary missions exploring atmospheres in all their diverse 5 guises, from Pioneer Venus to Venus Express, a multitude of Mars orbiters and landers, and the Voyager Grand Tour of the outer solar system to Galileo and Juno at Jupiter, Cassini at Saturn, and New Horizons' exploration of Pluto's tenuous atmosphere. With the newest satellite-based observatories starting to provide information about the atmospheres of planets orbiting other stars, understanding the compositions and climates of planetary atmospheres and their evolution continues to provide insights into factors that affect the suitability of a planet for supporting life and the types of al-15 terations that might affect this suitability.

International Commission on the Middle Atmosphere
The International Commission on the Middle Atmosphere (ICMA), originally named the International Commission on 20 Meteorology of the Upper Atmosphere, was approved in 1979. It was formed to provide a focus for international cooperation in research relating to the middle atmosphere, which is considered to reach roughly from the tropopause (∼ 12-18 km, depending on latitude and season) up to the lower 25 thermosphere (∼ 90-95 km). The composition and circulation of the layer are strongly affected by the most energetic radiation emitted by the Sun. Fortunately, the absorption of this energetic radiation by the ozone layer in the middle atmosphere protects the lower atmosphere from much of the 30 biologically damaging UV solar radiation and the consequences of solar storms. The middle atmosphere's composition is coupled to that of the troposphere via exchanges of trace constituents (e.g. water vapour and methane) and also reflects the tropospheric 35 trends introduced by anthropogenic contributions, notably of carbon dioxide and halocarbons. Middle atmospheric composition is also affected by direct emissions from aircraft and rockets and from the rapid injection of material from the most powerful volcanic eruptions. Trends in trace con-40 stituents over the last several decades have been linked to significant trends in stratospheric ozone, including the development of the "ozone hole" over the high southern latitudes in spring. Couplings are also dynamic, with conditions in the middle atmosphere affecting circulation in the tropo-45 sphere on subseasonal to interannual (and possibly decadal and longer) timescales, especially in the mid-latitudes and high latitudes.
Leading scientists as this field developed during the 20th century included Gordon Dobson and Sydney Chap-50 man (both UK); Michael McIntyre (New Zealand and UK); Alan Brewer (Canada and UK); Taroh Matsuno (Japan); Paul Crutzen (the Netherlands and Germany); Guy Brasseur (Belgium, USA and Germany); Colin Hines (Canada and USA); and James Holton, F. Sherwood Row-55 land, Richard Lindzen and Susan Solomon (all USA). Major international programmes of relevance to ICMA in the recent past include the Middle Atmosphere Program (MAP;1982-1985 led by SCOSTEP (Scientific Committee on Solar-Terrestrial Physics) and the SPARC (Stratosphere-60 troposphere Processes and their Role in Climate) project (1992-present) led by the WCRP.

International Commission on Atmospheric Electricity
Atmospheric electricity is a field of atmospheric sciences 65 that has a long and rich history, dating back to the time of American diplomat Benjamin Franklin's kite experiment and later to experiments by Lord Kelvin (Scotland and Ireland), Charles T. R. Wilson and Sir George Simpson (both UK), Sir Basil Schonland and D. J. Malan (both South Africa), 70 and, more recently, Bernard Vonnegut and Martin Uman (both USA) among many others. The field deals with the electrical nature of thunderstorms, particularly lightning but also fair-weather electricity, ions and radioactivity in the atmosphere. Lord Kelvin's early interest in the global electrical 75 circuit paved the way for the work at the Carnegie Institution and Wilson's fundamental formative studies (electron runaway, the cloud chamber, global circuit hypothesis, role of electrified shower clouds and electrostatic infrasound) that have many connections with current work. 80 International meetings under IUGG and IAMAS auspices go back to the mid-20th century, building up a cadre of researchers and providing a forum for expert scientists from around the world. To better focus international collaboration and cooperation, IAMAS approved the International 85 Commission on Atmospheric Electricity (ICAE) in 1989 to supervise, coordinate, and expand upon the conferenceorganizing efforts of the prior committee that had existed. With the growing dependence on electric power grids, lightning physics and protection has become a key area of inter-90 est of ICAE, while in recent years the link between climate change and atmospheric electricity has also received much focus. Members of ICAE were also key in the discovery and understanding of upper-atmospheric transient luminous events (TLEs) such as sprites, elves and terrestrial gamma 95 ray bursts (TGFs).

Concluding Remarks
With respect to the organization of scientific research relating to the Earth system, the year 1919 was one of very significant achievement, setting the stage for very significant ad-100 vances over the succeeding 100 years and beyond. As World War I ended and a serious flu epidemic waned, the rebirth and expansion of international scientific cooperation to its modern scope began. Observational systems were growing in coverage, and scientific understanding of why changes were occurring expanded. Leading scientists of the time showed exceptional foresight in moving to establish the foundation of the international structure that has carried forward scientific advancement in the century since. While much has been learned, the questions for the decades ahead remain as important and daunting as they were 100 years ago, providing a strong and continuing rationale for IAMAS and its 10 commissions to continue to promote international cooperation and advancement.

10
The IAMAS focus on advancing scientific understanding of the atmosphere has benefited greatly from both active participation of the world community of atmospheric scientists and also from the advances by the broader geosciences community represented within IUGG. With collective grat-15 itude to all who have served and participated in IAMAS over the past 10 decades, IAMAS welcomes all who would join in ongoing and emerging atmospheric and meteorological research. Further information on IAMAS is available at http://www.iamas.org (last access: 25 January 2019). 20 The multitude of scientific associations, commissions, and similar groupings within and outside of the parenting International Union of Geodesy and Geophysics may appear confusing at times. It reflects, however, the human endeavour to form various cooperating groups and to categorize phenom-25 ena arising from the interrelated and transient continua of the solid, liquid and gaseous compartments of the Earth system. Good (2000) reflected in detail about the evolving geophysical scientific disciplines that together form the union called IUGG. Upon IUGG's centennial, may this commemorative 30 article contribute further to a common memory and tradition into the future about personalities and groupings that have dealt scientifically with atmospheric issues during the past century.
Data availability. The bulk of information used in this article is 35 taken from the cited references or the currently active websites. In addition, information was drawn from a number of additional sources, including an article by P. H. Shaw in the Encyclopedia Britannica (15th edition); early meeting reports appearing in Science and The New York Times; appropriation bills printed in the Congres-