{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,3,10]],"date-time":"2026-03-10T12:39:11Z","timestamp":1773146351960,"version":"3.50.1"},"reference-count":57,"publisher":"MDPI AG","issue":"11","license":[{"start":{"date-parts":[[2025,11,20]],"date-time":"2025-11-20T00:00:00Z","timestamp":1763596800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Atmosphere"],"abstract":"<jats:p>Atmospheric carbon dioxide (CO2) levels are rising globally, yet their multiscale variability in remote oceanic regions remains poorly characterized. This study examines a 45-year monthly CO2 record (1980\u20132024) from the Azores, a subtropical North Atlantic site, using a spectral and statistical framework. The series was decomposed into high- and low-frequency components via Butterworth filtering and analyzed with the Correlogram-Based Periodogram (CBP) and Monte Carlo significance testing. The residual component robustly recovered the expected seasonal cycle (~12 months), validating the methodology. The trend component revealed an apparent enhancement in low-frequency spectral power, largely explained by the accelerating long-term increase. Control tests with a synthetic quadratic trend and polynomial detrending indicate a weak ~11-year enhancement in low-frequency power that is not robust under a red-noise (AR(1)) null. Segmented regressions showed a sustained and accelerating increase in CO2 accumulation over the past four decades, consistent with Mauna Loa. These results demonstrate the importance of long-term monitoring in remote regions while highlighting both the potential and limitations of spectral methods for detecting weak low-frequency signals in greenhouse gas records.<\/jats:p>","DOI":"10.3390\/atmos16111308","type":"journal-article","created":{"date-parts":[[2025,11,20]],"date-time":"2025-11-20T09:44:53Z","timestamp":1763631893000},"page":"1308","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":1,"title":["Multiscale Variability of Atmospheric CO2 at the Azores: Detecting Seasonal and Decadal Oscillations"],"prefix":"10.3390","volume":"16","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-9356-3045","authenticated-orcid":false,"given":"Maria Gabriela","family":"Meirelles","sequence":"first","affiliation":[{"name":"Faculty of Science and Technology, University of the Azores, 9500-321 Ponta Delgada, Portugal"},{"name":"Research Institute of Marine Sciences, University of the Azores (OKEANOS), 9901-862 Horta, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5935-870X","authenticated-orcid":false,"given":"Helena Cristina","family":"Vasconcelos","sequence":"additional","affiliation":[{"name":"Faculty of Science and Technology, University of the Azores, 9500-321 Ponta Delgada, Portugal"},{"name":"Laboratory of Instrumentation, Biomedical Engineering and Radiation Physics (LIBPhys, UNL), Department of Physics, NOVA School of Science and Technology, 2829-516 Caparica, Portugal"}]}],"member":"1968","published-online":{"date-parts":[[2025,11,20]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1783","DOI":"10.5194\/essd-11-1783-2019","article-title":"Global Carbon Budget 2019","volume":"11","author":"Friedlingstein","year":"2019","journal-title":"Earth Syst. 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