INVITED REVIEW Observing terrestrial ecosystems and the carbon cycle from space DAVID SCHIMEL 1 , RYAN PAVLICK 1 , JOSHUA B. FISHER 1 , GREGORY P. ASNER 2 , SASSAN SAATCHI 1 , PHILIP TOWNSEND 3 , CHARLES MILLER 1 , CHRISTIAN FRANKENBERG 1 , KATHY HIBBARD 4 andPETER COX 5 1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91101, USA, 2 Department of Global Ecology, Carnegie Institution for Science, 260 Panama St., Stanford, CA 94305, USA, 3 University of Wisconsin-Madison, Madison, WI 53706, USA, 4 Pacific Northwest National Laboratory, PO Box 999 MSIN: K9-34, Richland, WA 99352, USA, 5 College of Engineering, Mathematics and Physical Sciences, University of Exeter, North Park Road, Streatham Campus, Harrison Building, Exeter EX4 4QF, UK Abstract Terrestrial ecosystem and carbon cycle feedbacks will significantly impact future climate, but their responses are highly uncertain. Models and tipping point analyses suggest the tropics and arctic/boreal zone carbon–climate feedbacks could be disproportionately large. In situ observations in those regions are sparse, resulting in high uncertainties in car- bon fluxes and fluxes. Key parameters controlling ecosystem carbon responses, such as plant traits, are also sparsely observed in the tropics, with the most diverse biome on the planet treated as a single type in models. We analyzed the spatial distribution of in situ data for carbon fluxes, stocks and plant traits globally and also evaluated the potential of remote sensing to observe these quantities. New satellite data products go beyond indices of greenness and can address spatial sampling gaps for specific ecosystem properties and parameters. Because environmental conditions and access limit in situ observations in tropical and arctic/boreal environments, use of space-based techniques can reduce sampling bias and uncertainty about tipping point feedbacks to climate. To reliably detect change and develop the understanding of ecosystems needed for prediction, significantly, more data are required in critical regions. This need can best be met with a strategic combination of remote and in situ data, with satellite observations providing the dense sampling in space and time required to characterize the heterogeneity of ecosystem structure and function. Keywords: arctic, boreal, carbon, climate feedback, diversity, fluroescence, spectroscopy, tropics Received 22 April 2014; revised version received 5 November 2014 and accepted 8 November 2014 Introduction Feedbacks from the global carbon cycle contribute substantially to uncertainty about future climates. Twenty years ago, Schimel (1995) summed it up, ‘Lack of knowledge about positive and negative feedbacks from the biosphere is a major limiting factor to credi- ble simulations of future atmospheric CO 2 concentra- tions’. Despite decades of research since then, and very substantial increases in knowledge, the statement remains true today. Ecosystems take up a significant fraction of carbon released to the atmosphere from fossil fuel burning and deforestation, but if this sub- sidy declines, the rate of increase in atmospheric CO 2 accumulation will sharply increase for any given emission scenario (Ciais et al., 2013). As a result, the importance and complexity of the world’s terrestrial ecosystems have come into sharp focus over the past few decades. Despite the significance of terrestrial carbon storage in the climate system, global ecosystem models persis- tently diverge on even fundamental predictions of the sign and magnitude of feedbacks (Piao et al., 2013; Friend et al., 2014; Hoffman et al., 2014), contributing substantial uncertainty to the overall accuracy of Earth system prediction (Bodman et al., 2013). Gaps in theory contribute to the failure of models (Wieder et al., 2013), but a lack of critical observations slows to the pace of development of theory, and its implementation into models (Keller et al., 2008). In one recent analysis of observing needs for the carbon cycle, the current state was characterized as a ‘sparse, exploratory framework’ and the need as being a ‘dense, robust, and sustained system’ (Ciais et al., 2014). In this study, we analyze the current state of observations for several critical terres- trial ecosystem variables relative to their known Correspondence: David Schimel, tel. +818 354 6803, fax +818 354 3223, e-mail: dschimel@jpl.nasa.gov 1762 © 2014 John Wiley & Sons Ltd Global Change Biology (2015) 21, 1762–1776, doi: 10.1111/gcb.12822