1. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
2. Ottawa-Carleton Geoscience Centre, University of Ottawa, Ottawa, Ontario KIN 6N5, Canada
3. Department of Earth Sciences, Memorial University of Newfoundland, St John's, Newfoundland A1B 3X5, Canada
4. Department of Earth Sciences, Brock University, St Catharines, Ontario L2S 3A1, Canada
5. Waquoit Bay National Estuarine Research Reserve, Waquoit, Massachusetts 02536, USA

Correspondence to: Rosemarie E. Came¹ Correspondence and requests for materials should be addressed to R.E.C. (Email: rcame@gps.caltech.edu).

Atmospheric carbon dioxide concentrations seem to have been several times modern levels during much of the Palaeozoic era (543–248 million years ago), but decreased during the Carboniferous period to concentrations similar to that of today^{1, 2, 3}. Given that carbon dioxide is a greenhouse gas, it has been proposed that surface temperatures were significantly higher during the earlier portions of the Palaeozoic era¹. A reconstruction of tropical sea surface temperatures based on the ¹⁸O of carbonate fossils indicates, however, that the magnitude of temperature variability throughout this period was small⁴, suggesting that global climate may be independent of variations in atmospheric carbon dioxide concentration. Here we present estimates of sea surface temperatures that were obtained from fossil brachiopod and mollusc shells using the 'carbonate clumped isotope' method⁵—an approach that, unlike the ¹⁸O method, does not require independent estimates of the isotopic composition of the Palaeozoic ocean. Our results indicate that tropical sea surface temperatures were significantly higher than today during the Early Silurian period (443–423 Myr ago), when carbon dioxide concentrations are thought to have been relatively high, and were broadly similar to today during the Late Carboniferous period (314–300 Myr ago), when carbon dioxide concentrations are thought to have been similar to the present-day value. Our results are consistent with the proposal that increased atmospheric carbon dioxide concentrations drive or amplify increased global temperatures^{1, 6}.

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