From atmosphere,to climate,to Earth system science

In the current context of climatic changes, climate science is becoming increasingly a focus of attention for large portions of society. At the same time, our basic understanding of climate processes is evolving very rapidly, following tremendous advances in Earth observation as well as in computing power. Still, the very notion of ‘climate’ appears not all that well defined, the physical contours of a ‘climatic system’ are often rather dependent on the given scientific question, and there is little obviously in common between – for example – researchers studying the climatic conditions experienced by the dinosaurs, those attempting to use satellite information to better quantify the present-day climate, and those who use the largest computers available to predict possible dire consequences of current global warming. But appearances are sometimes misleading. Indeed, climatology is in its essence highly interdisciplinary, and it has numerous firm connections with many other scientific disciplines in the physical, natural and human sciences. But first of all, it is necessary to discuss what is hidden behind the word ‘climate’. In particular, in order to move from a phenomenological towards a more physically-based view, a key question is to draw the contours of a climate system. In this respect, it is interesting to note that climate modellers are now increasingly defining their object in terms of an ‘Earth system’ instead of a ‘climate system’. I go on to present some recent progress in climate sciences over the last few decades. In particular, based on recent paleoclimatological discoveries, the climate appears now as a very dynamic system, with its own internal variations. The finding that past climate has changed ‘spontaneously’ and very abruptly, on a decadal timescale, has profoundly changed our vision of climate dynamics. Finally, I outline some possible future directions for climate sciences: ‘Earth system science’ is an emerging discipline that has the ambition to describe, in the same mathematical framework, the past and the future evolution of our planet.