Stations of IBGE’s Geodetic Permanent Tide Gauge Network (RMPG) recorded, in the period between December 2001 and December 2006, average annual sea level rise of 2.5mm in Imbituba and of 37mm in Macaé (RJ).

Stations of IBGE’s Geodetic Permanent Tide Gauge Network (RMPG) recorded, in the period between December 2001 and December 2006, average annual sea level rise of 2.5 mm in Imbituba and of 37 mm in Macaé (RJ).In the former, the increase is within the limits established internationally by the United Nations (UNO). In the latter, the probable explanation for the significant rise is the existence of local geological features. The measurements have been used as a reference for international studies related to climactic changes, mainly because all the RMPG stations are now considered international stations of the Global Sea Level Observing System of the Intergovernmental Oceanographic Commission of UNESCO (GLOSS/IOC). The information collected by IBGE’s RMPG in Rio de Janeiro, in Santa Catarina and, more recently, in Amapá and in Bahia, are available on the IBGE web site, in the “Geosciences/Geodesy” section.

In recent years, there has been a new vision of the role of the sea, which is one of the main providers of live resources and biodiversity on the planet, in terms of the global climate. The search for the understanding of its interaction with the atmosphere and the perception of its biophysical and chemical aspects are related to the impacts caused by the sea on ecosystems, on society and on the economy.

The great public interest for global climactic changes is triggered by the rise of the sea level and the subsequent increase of risks of floods in coastal areas. These risks, however, depend much more on the changes of tides and on wave patterns, which are very common in coasts, than on the rise of sea level itself. 

IBGE has decided to monitor the sea level, in order to fulfill the demands of national geodesy¹. Since 1968, after the end of the tide gauge activities of IAGS (Inter-American Geodetic Survey) in Brazil, there had been no institution to continue this activity.

The institute operated a tide gauge station, installed by Coppe/UFRJ (Alberto Luiz Coimbra Institute of Post-Graduation and Research in Engineering, of the Federal University of Rio de Janeiro) in Copacabana, Rio de Janeiro, between April 1993 and March 1994. Since then, the other RMPG stations along the Brazilian coast have been implanted gradually. There are stations in Santana – AP (2005), Salvador – BA (2001), Imbituba – SC (2001) e Macaé – RJ (since 1994, when a conventional tide gauge of Petrobrás was taken by IBGE and gradually improved). The installation of a fifth tide gauge station in Fortaleza (CE) is expected for this year.

The stations of Macaé and Imbituba have had five years of simultaneous tide gauge registrations. Because of this, they were chosen for a preliminary study of tide level. Both localities measure the “observed tide” level, that is, the sea level. This corresponds to “gross” information, with all the meteorological-oceanographic (changes in the Earth’s crust, accommodation of layers of land, meteorological and astronomical changes, etc.) The results for the rise of tides observed are simple averages (graphics below), which eliminate the seasonal effects.

In Imbituba (SC), there was a trend to annual rise by approximately 2.5 mm, considering that, for the period between 2002 and December 2006 as a whole, there was average rise by 1 cm. The area is very susceptible to cold waves, in which the tide gauge registrations are strongly affected by meteorological conditions.

This result is in accordance with the study conducted by the Federal University of Paraná (UFPR), for the same area, between 1997 and 2006, and which presented a trend to annual rise by 2.0 mm. During this period, UFPA conducted three campaigns – in 1997, 2000 and 2005, for monitoring of the vertical crustal change with GPS, and obtained a figure of about -0.8 mm/year. If we consider the influence of this variation for the registrations of the tide gauge, we will reach a trend of annual rise of 1.2 mm.

The studies which have been released, at global level, about the tendency to rise of sea level, point to annual figures of about 1 to 2 mm. Consequently, the data obtained in Imbituba were very close to what was predicted. The graph generated did not show any anomalous situation (big discrepancies), which does not mean, however, there was no need of further study of complementation of the control conditions of the station. In this respect, IBGE installed, in May this year, a GPS equipment – a system of navigation and positioning by satellite) of permanent operation, in order to continuously monitor the vertical movement around the tide gauge.

For Macaé (RJ), in the same period of observation, there was significant tendency to annual rise, by approximately 37 mm. The overall rise between December 2002 and 2006 was 15 cm. This is an adverse situation (discrepancy), which requires a lot of attention. The increase of the level of water has caused preoccupation to the municipal government of Macaé, mainly due to the possible occurrence of floods.

Some of the possible “causes” of this phenomenon are: a) effects of local geology, especially the existence of active faults around the city – which will be the focus of study of the DRM/RJ (Department of Mineral Resources of Rio de Janeiro) and of IEAPM (Institute of Sea Study Mar Almirante Paulo Moreira); b) systematic effects of a wind component, related to the geographic location of Macaé, which would cause the “piling” of coastal water. c) hydrodynamic effects caused by accelerated rise of the rivers which disembogue in Macaé and its surroundings, thus reducing the flow of waters which are responsible for the balance of the approach of the coast by the sea.

It is not possible to affirm, however, the precise reason for the results of Macaé. Efforts for the conduction of these analyses are being made by the departments of geodesy and natural resources of IBGE, which has maintained close contact with Coppe/UFRJ, DRM/RJ and IEAPM, aiming to provide detailed studies and make it possible to implement a system of proper monitoring and control according to international standards. It will contribute, therefore, to the global researches on the sea level. Among these initiatives, the highlight is the installation of a GPS unit until September this year.

In order to obtain the measurements of sea level, IBGE relies on the cooperation of partner institutions, such as the Navy Hydrograph Center (DHN/CHM), Petrobrás, the Imbituba Dock Company (CDI), the Santana Dock Company (CDSA) and the Federal University of Bahia (UFBA).

The measurements conducted in Brazil, however, are already being used as a reference for the studies related to climactic changes. International programs, such as the Global Sea Level Observing System of the Intergovernmental Oceanographic Commission of UNESCO (GLOSS/ IOC), have required access to tide gauge and meteorological data collected on the Brazilian coast.

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 ¹ Geodesy is the science related to the determination of the shape, size of the Earth and also of its gravitational field. In practical terms, IBGE, the institution in charge of these activities in Brazil, has as its function to implement and monitor the Brazilian Geodetic System (SGB). The System is formed by a group of stations whose apply as precise reference to several projects in the fields of engineering, mapping, geophysics, research, among others.