English (UK)
The light captured by our telescopes is not exactly the same as the light that originated from the stars. On its journey to reach us, light can pass through regions of gas and dust that modify its spectrum. This effect is especially noticeable when light crosses our atmosphere because, far from being completely transparent, the atmosphere introduces lines (like its own “bar code”) into the observed stellar spectra.
These lines, known as telluric lines because they originate from our planet, are considered a nuisance by astronomers, as they are forced to “decontaminate” the spectra measured by their instruments in order to reconstruct their original form.
To complicate matters further, the exact shape of telluric lines varies when the temperature, pressure, humidity, or chemical composition of the atmosphere changes. In some way, we can imagine that telluric lines imprint information about the current state of the atmosphere on the spectrum of the stars. In particular, the main greenhouse gases (hereinafter GHGs) such as carbon dioxide (CO2), methane (CH4), and water vapor (H2O).
Thanks to this, what is “waste” for astronomers is very useful for studying the concentration and evolution of GHGs on our planet, which are responsible for global warming (that is, an increase in the average temperature of planet Earth). But to do this, we must first learn how to best use telluric lines, and this is precisely what has been done at Calar Alto for ten days and ten nights in July 2025.
The atmospheric measurement campaign “Combining CO2 and CH4 column retrievals from Astroclimes with collocated COCCON measurements” was carried out in Almería from July 7 to 16, 2025, in collaboration with the Calar Alto Astronomical Observatory, the University of Almería (UAL), the University of Warwick (United Kingdom), and the Spanish State Meteorological Agency (AEMET).
The main objective is to demonstrate the unique potential of combining solar measurements (during the day) and measurements of other stars (at night) using remote sensing techniques to study the carbon cycle, the role of GHGs in the current context of global warming, and the reinforcement of observation systems for these gases.
Specifically, this campaign took advantage of the unique opportunity to measure the complete daily cycle (night and day) of GHGs at the Calar Alto Observatory, through observations of stars with the CARMENES spectrograph on the 3.5 m telescope and solar observations with a low-resolution Fourier Transform Infrared (FTIR) spectrometer (portable model EM27/SUN) belonging to the COCCON (COllaborative Carbon Column Observing Network)-Spain network.
The nighttime atmospheric columns of CO2 and CH4 are derived from observations made with the CARMENES spectrograph, retrieved using the innovative Astroclimes algorithm developed by Marcelo Aron Fetzner Keniger, from the Astronomy and Astrophysics group at the University of Warwick. With its infrared channel, CARMENES is the ideal high-resolution spectrograph for accurately detecting greenhouse gas lines and feeding the Astroclimes algorithm developed at the University of Warwick. According to Fetzner Keniger, “If we can calibrate the algorithm with the help of COCCON measurements, Astroclimes could provide a new network for measuring GHG abundance, complementing current networks with nighttime measurements.”
The diurnal cycle of GHGs was observed by an FTIR instrument from the COCCON-Spain network, also located at Calar Alto. This instrument is capable of deriving atmospheric column concentrations of different GHGs. In turn, these diurnal observations will be used to calibrate the Astroclimes algorithm. The daytime “altitude” observations (about 2,100 m) were complemented by a second instrument from the COCCON-Spain network located at the University of Almería, in order to complete the atmospheric column from sea level.
For the University of Almería (UAL), this project represents a very important commitment to characterizing the local atmosphere, where the study of these GHGs is essential. For the UAL research group, which has been studying atmospheric physics for decades, having a project of this magnitude reaffirms its commitment to energy resources and their derivatives, which are essential to the environment around us. “We are grateful to AEMET for counting on us for such an important project, which we believe will be a benchmark in the energy transition,” says Joaquín Alonso Montesinos, University Professor, head of the research group ‘TEP165: solar energy resources, climatology, and atmospheric physics’ and representative of the UAL in the COCCON-Spain project.
“The COCCON-Spain national network aims to address the latent lack of atmospheric GHG observations in Spain through the implementation of a network of stations for measurement on a national scale. One of the main objectives of the COCCON-Spain network is to improve current knowledge of GHG sources and sinks, thus contributing to the development of mitigation and adaptation strategies for climate change.” emphasizes Omaira García-Rodríguez (AEMET-CIAI), coordinator of the network.
Jesús Aceituno, director of the observatory, concludes, “Calar Alto, with its photovoltaic plant and biomass boiler, aims at reaching energy sustainability. These greenhouse gas detections made with CARMENES demonstrate that an astronomical observatory can also serve to monitor our planet's climate.”
Spectrum of a star captured by the CARMENES infrared channel
COCCON-Spain is coordinated by AEMET and is part of the project to modernize observation networks and digitize production processes for the development of smart meteorological services in the context of climate change (C05.I03. P51), which is part of the Spanish Government's Recovery, Transformation and Resilience Plan (PRTR), to be financed with funds approved by the European Council on July 21, 2020, to address the consequences of the international pandemic caused by COVID-19.
The Calar Alto Observatory is one of the infrastructures that belong to the national map of Unique Scientific and Technical Infrastructures (Spanish acronym: ICTS), approved on March 11th, 2022, by the Science, Technology and Innovation Policy Council (CPCTI).
CONTACTS:
Calar Alto (CAHA) Observatory - Gilles Bergond
COMMUNICATION - CALAR ALTO OBSERVATORY