SAILing to research Earth's climate

Susana Barbosa


António Maurício Camilo

  (2)Navy, CINAV Portuguese Navy Research Centre

Nuno Dias


Guilherme Amaral


António Ferreira


Carlos Almeida


In stormy weather situations, with thunder and lightning, we become aware (generally, and rightfully so, with a certain degree of fear) of the presence of electricity in the atmosphere. However, and although we don't always realise it, we are constantly surrounded by an intense electric field, even when the weather is good.

Simply put, we can perceive the Earth system as a pair of charged parallel plates. The upper plate is the ionosphere, the upper layer of the atmosphere (starting at 80km altitude), where solar radiation is powerful enough to ionise atoms and molecules (by removing some of their electrons). Hence, the ionosphere is a good electrical conductor, since it is made up of moving electrons and ions. The lower plate is the Earth itself, which is also conductive. The atmospheric electric field stems from the charge difference between the two plates - corresponding to approximately 250.000 volts.

What keeps these "plates" charged? Thunderstorms and electrical discharges, some of the most intense phenomena in the Earth's atmosphere. Inside a thunderstorm cloud, there are strong upward and downward air currents, and the up/down movement of water and ice particles causes the top of the cloud to hold a significant positive charge, while the lower part of the cloud gets a considerable negative charge. The transfer of charge from the cloud to the ionosphere above, and to the Earth's surface below, charges the plates and creates the Earth’s atmospheric electric field. Since the atmosphere is not a perfect insulator, this electric field generates an electric current that flows between the atmosphere and the Earth's surface in areas remote from thunderstorms, constituting Earth’s global electrical circuit. At any given moment, there are close to 1000 active thunderstorms on planet Earth that keep the planet's electric field, even in places far from the thunderstorm areas and in fair weather conditions.

The existence of a continuous electric field in the atmosphere, even in the absence of thunderstorms and electrical discharges, and its global nature, was demonstrated through campaigns measuring the atmospheric electric field aboard the Carnegie vessel. The data obtained on board Carnegie, between 1915 and 1929, showed that the electric field exhibited a diurnal variation, reaching highest values at 19:00 UTC (Coordinated Universal Time, corresponding to the Lisbon time zone). Regardless of the location – the Atlantic Ocean, the Pacific Ocean, or the Indian Ocean – those measurements showed that the maximum value of the electric field always occurred at 19:00, Lisbon time. This meant that the electric field depended on the absolute time on Earth, rather than the local time at the measuring location. Given that the measurements made on board the Carnegie vessel were the ones that allowed us to know that the atmospheric electric field varies systematically throughout the day, regardless of the location, this diurnal variation became known as the "Carnegie curve" (HARRISON, 2013; 2020). This curve is, to this day, used as a reference for the diurnal variation of the global atmospheric electric field. Why? Because there hasn't been any further systematic measurements of the electric field over the ocean at different points around the globe since the epic scientific expeditions of the Carnegie vessel, during the early 20th century. Why not measure the atmospheric electric field on land? The atmospheric electric field exists throughout the planet, and it can be measured anywhere. However, the electric field depends not only on the generators (thunderstorms) that maintain the potential difference between the ionosphere and the surface, but also on the atmospheric conductivity, which varies locally due to several factors - making it difficult to observe in land the planetary component of the electric field.

The atmosphere is able to conduct electric current due to the presence of ions between the surface and the ionosphere. Where do these ions come from? They are the result of the interaction of ionising radiation with the atoms and molecules present in the atmosphere. This radiation has two main sources: the Earth's surface, through natural radioactivity, and space, through cosmic radiation. The radioactive elements that exist on the earth's surface since the formation of the planet cause any soil or rock to emit ionising radiation, resulting from the radioactive decay of these elements; moreover, radon is released into the atmosphere and being a gas it can move in the lower part of the atmosphere (~2 km). This effect of environmental radioactivity occurs primarily over continents and not over the ocean. Ionising radiation reaching Earth from space, beyond the solar system, is the main source of ionisation over the oceans and at high altitudes. While the formation of ions is due to radiation coming from either the earth's surface or space,the removal of ions from the atmosphere is mainly due to aerosols (solid or liquid particles suspended in the air). The concentration of ions depends heavily on the amount and size of aerosols, as ions tend to bind to aerosols, reducing the number of ions or changing their size. Variations in ion concentration cause variations in the electrical resistance of the atmosphere and, consequently, in the atmospheric electric field at a given location.

Therefore, there are many natural and anthropogenic processes that influence the atmospheric electric field, e.g., the amount of radon gas emitted from the earth's surface, sandstorms and snowstorms, volcanic eruptions, or urban pollution. In this sense, ground-based observations of the electric field reflect a wide variety of local contributions. In contrast, observations of the electric field made over the ocean, covering several locations and periods, allow to evaluate the Earth’s global atmospheric electrical circuit, which is why the Carnegie curve is still used as a reference of the global electrical circuit - although it is known that the current conditions are different from those existing at the beginning of the 20th century, for a particularly important reason: the climate! Global climate change is expected to influence significantly the global electrical circuit. The increase in the global Earth temperature increases the energy in the atmosphere and the intensity of storms, amplifying the global electrical circuit. Moreover, changes in the number of atmospheric aerosols, due to the intensification of sandstorms and atmospheric pollution, also influence the global electric field. It is particularly difficult to distinguish causes and effects in a complex and interconnected system such as the Earth system; in the case of the global electrical circuit, not only changes in climate alter the global electrical circuit, but changes in the atmospheric electric field have the potential to influence climate - for instance, by influencing aerosol production and cloud formation. Hence, it is of utmost scientific importance for climate and climate change studies to measure the atmospheric electric field in the ocean over time, and in various locations. Obtaining data on the current state of the Earth's atmospheric electric field is crucial since existing measurements date back to the early 20th and the Carnegie expeditions.

The opportunity to address this gap appeared with the Portuguese programme commemorating the 500th anniversary of the first circumnavigation of the Earth, carried out between 1519 and 1522 by the Portuguese explorer Fernão de Magalhães,. The celebrations included, among other events, a circumnavigation trip of the Portuguese navy vessel NRP Sagres - which, besides the usual naval, diplomatic, and training missions, would also serve, for the first time, as a platform for scientific projects on board. This raised the opportunity to measure the atmospheric electric field at sea for the first time in the 21st century - 100 years after the Carnegie expedition -leading to the establishment of the SAIL - Space-Atmosphere-Ocean Interactions in the marine boundary Layer project, in partnership with the Portuguese navy.

A curious fact is the reason for the 100-year interval between the first measurements of the global electric circuit aboard the Carnegie, and the modern measurements aboard the Sagres. Given the scientific relevance of the global electrical circuit, in particular its importance in terms of climate change, it is somewhat surprising that the scientific community still uses data from the beginning of the last century, and that a programme of systematic monitoring of the atmospheric electric field has not been implemented. This issue haunted the early stages of development of the SAIL project. Was it because of the lack of a ship? One of the main objectives of the Carnegie campaigns was the measurement of the Earth's magnetic field; hence, the Carnegie was a wooden vessel specifically designed with non-magnetic materials to minimise potential interferences in the measurements of the geomagnetic field. Sagres, on the other hand, is an iron-hulled vessel, equipped with a diesel engine and sails. Perhaps even more concerning: Sagres features the usual instrumentation found on a modern ship in terms of radars, navigation systems, and communications... would it be possible to measure the electric field under these conditions, which are so different - in terms of radiation spectrum - compared to those of the Carnegie? When posing the question to several international experts in the atmospheric electric field, the answer was surprisingly unanimous and encouraging... despite the differences pointed out between Sagres and the Carnegie, it would be worth measuring the electric field on board! Hence, the planning activities of the SAIL project continued with increased spirit and much enthusiasm and support from the international scientific community. However, the doubt persisted… why only 100 years later? Especially since there aren't significant "technical" barriers?

The reasons are, after all, much more prosaic than the ship's structure or electronic interference. One of the current difficulties is the time available to execute a scientific project, which is usually three to four years at most. During this period, one is supposed to plan the experiments, perform them, and publish the results. And part of this period is used to raise funding to support the continuity of the work after the end of the project - which is often not achieved, making it impossible to perform long-term measurements. It seems difficult to justify allocating taxpayers' money to something that has already been measured, and it is much more appealing to ask for funding for something new, rather than a work of continuity. The painstaking work of acquiring data over time is less likely to be funded and to attract recognition. In comparison, the Carnegie was built in 1909 and carried out six scientific expeditions between 1909 and 1921. After the ship's equipment was renovated, it went on a seventh and final expedition between 1928 and 1929, meticulously acquiring data over a period of more than 20 years. Data from the last expedition were only published in 1946 (TORRESON, 1946) and are still in use today.

Another reason is the increasing scientific specialisation and compartmentalisation of knowledge into different domains. While it is true that multi/inter/transdisciplinarity are highly valued today, they are also increasingly difficult because of the ultra-specialisation of scientific practice in progressively narrow and specific areas. Since the atmospheric electric field is such a comprehensive phenomenon in terms of the physical processes involved, from space beyond the solar system to the surface of the earth, and directly related to scientific areas as diverse as the ionosphere, composition of the atmosphere, aerosols, environmental radioactivity, climate, etc., it becomes difficult to gather all the scientific competencies necessary to address the topic in an integrated way. In Carnegie 's time, the distinction between different scientific themes was much more subtle and often one person was proficient in several areas, allowing one to approach a subject like the atmospheric electric field in a more holistic way. The specialisation is not only translated into the scientific knowledge itself, but also into the infrastructure used. Since the electric field is a phenomenon traditionally more connected to the atmosphere than to the sea, resorting to vessels for atmospheric studies is not... obvious. Particularly given how difficult - and expensive - it is to use ships for oceanographic studies - let alone atmospheric and less conventional studies. Despite not being an insurmountable obstacle, it does not facilitate the implementation of a monitoring programme for the marine boundary layer (the part of the atmosphere influenced by the ocean), despite its clear importance, as the majority of the Earth's atmosphere is, in fact, maritime.

Finally, the main reason why there were no new measurements of the atmospheric electric field at sea until 2020, in addition to the reasons already pointed out, is that such endeavour requires an extraordinary collaborative effort. The idea for the SAIL project came up in 2019, and the circumnavigation trip would start in January 2020, so the entire project had to be planned and implemented in less than a year. The existence of a tight and unavoidable deadline led to exceptional human and institutional efforts to carry out what seemed like an impossible mission. In record time, an integrated monitoring system was installed on board Sagres, together with the development of a specific software to collect all the data. An extraordinarily committed team from INESC TEC Centre for Robotics and Autonomous Systems (CRAS) worked day and night to ensure that on January 5, 2020, Sagres set off from Lisbon to measure the atmospheric electric field (and atmospheric visibility, and environmental radioactivity, and solar radiation, and ion concentration). This was only possible thanks to the unusually close and unrivalled collaboration of the Portuguese Navy, which put Sagres at the service of science, by fully supporting the project. In addition to hosting the scientific instrumentation, Sagres also welcomed INESC TEC researchers, who participated in several stages of the circumnavigation trip - just as Carnegie had a scientific team on board. The entire Sagres crew supported the project, integrating the researchers into the ship's affairs, sharing navy traditions (for example, when crossing the equator), and doing everything in their power to ensure the success of the scientific mission.

Sagres' trip, initially planned to last 371 days, came to an unexpected end, as did Carnegie's last expedition. In November 1929 an explosion during Carnegie's fuel supply in Apia (Samoa) sank the ship and killed its iconic captain, James Ault (PAUL, 1932). Sagres' fate was less tragic, but also abrupt and unexpected. Due to the COVID-19 epidemic, the ship was forced to stop the circumnavigation trip, and in March 2020, the vessel sailed from South Africa to Lisbon, instead of sailing to the Indian Ocean - having arrived in Lisbon in early May 2020. Despite the setback caused by the pandemic, the on-board monitoring system has remained operational to this day, collecting about 10 GB of data per day, for over more than three years - an amount and quality of data unthinkable in Carnegie's times! And the expectation is that - as in the case of Carnegie - Sagres' scientific legacy, which is now beginning to take its first steps (BARBOSA et al, 2022; 2023), will endure for a long time.


BARBOSA, Susana, DIAS, Nuno, ALMEIDA, Carlos, SILVA, Guilherme, FERREIRA, António, CAMILO, António, SILVA, Eduardo (2023). Precipitation-driven gamma radiation enhancement over the Atlantic Ocean. Journal of Geophysical Research: Atmospheres, 128, e2022JD037570.

BARBOSA, Susana, DIAS, Nuno, ALMEIDA, Carlos, AMARAL, Guilherme, FERREIRA, António, LIMA, Luís, SILVA, Igor, MARTINS, Alfredo, ALMEIDA, José, CAMILO, António, SILVA, Eduardo (2022). An holistic monitoring system for measurement of the atmospheric electric field over the ocean–the SAIL campaign. In OCEANS 2022-CHENNAI ( pp. 1-5 IEEE.

HARRISON, R. Giles (2013). The Carnegie curve. Surveys in Geophysics, 34 (2), 209-232.

HARRISON, R. Giles (2020). Behind the curve: a comparison of historical sources for the Carnegie curve of the global atmospheric electric circuit. History of Geo-and Space Sciences, 11 (2), 207-213.

PAUL, J. Harland (1932). The last cruise of the Carnegie. The Williams & Wilkins Company, Baltimore

TORRESON, O. William (1946). Ocean atmospheric-electric results. Oceanography III: Scientific results of Cruise VII during 1928-1929 under Command of Captain JP Ault.