​His research has led to major advances in understanding the relationships between terrestrial greenhouse gas fluxes and climate. Philippe Ciais was notably a pioneer in integrating managed ecosystems into global ecosystem models to better understand how carbon sinks can be managed to limit global warming.

Throughout his career, he has coordinated numerous national and European projects, including his contribution to the large-scale European program CARBOEUROPE in the 2000s that was the first attempt to quantify the greenhouse gas balance of the European continent.

Philippe Ciais has also coordinated the establishment of the European research infrastructure ICOS (Integrated Carbon Observation System), which is now a flagship infrastructure for climate change research. At the same time, he co-chaired the Group on Earth Observations (GEO) Carbon Task and co-chaired the Global Carbon Project. He notably served as lead coordinating author of the chapter on the carbon cycle and other biogeochemical cycles in Working Group 1 of the IPCC Fifth Assessment Report published in 2013.

Philippe Ciais is the author of over 1300 publications in A ranked journal over the past 17 years. He is the most prolific researcher in the field of climate change and among the most cited scientists in geosciences and environmental research, with 270,000 citations. Over the course of his career, he has supervised more than one hundred PhD students and postdoctoral researchers.