Climate change has been at the forefront of environmental concerns for over a quarter of a century. The sequence of reports published by the Intergovernmental Panel on Climate Change (IPCC, 1990, 1996, 2001, 2007, 2013) has been instrumental in bringing climate issues to the attention of the public and policy-makers alike. In 1992, the United Nations Conference on Environment and Development (UNCED-1992, Rio de Janeiro, Brazil) identified climate change as one of the major environmental challenges facing humankind and the Framework Convention on Climate Change (FCCC) was ratified on this occasion, thereby acknowledging the concerns of governments related to the impacts and risks to environmental resources, local to national economies, and society resulting from a changing climate. The two other major UN conventions drafted at UNCED-1992, namely the biodiversity and the desertification conventions, have never received the attention that the FCCC has, particularly since the Kyoto Protocol, aimed at reducing anthropogenic carbon emissions into the atmosphere, was signed in 1997. Despite much criticism as to the slow pace of negotiations and the limited success in the implementation of the Protocol, it remains to this day a unique attempt to address the greenhouse-gas problem at the global scale. The issues of climate change have led to heated debates, both within academia as well as between scientists and a large community of skeptics. While the sources of skepticism range from simple denial to organized “anti-global warming” lobbies, the opposition to what is considered to be a consensus view on anthropogenic climate change has had a beneficial side-effect in terms of encouraging improved research and communication. Large efforts have been deployed to improve the knowledge base on the functioning of the climate system, observations of the Earth system, numerical modeling techniques, and data handling and analysis. Despite large scientific progress in the last two decades, many domains of uncertainty still remain as to the functioning of the climate system. The reduction of these uncertainties would be unlikely to radically change the fairly robust conclusion that contemporary climate change is to a significant degree driven by anthropogenic emissions of greenhouse gases. However, furthering our understanding of climate-relevant processes would improve not only the predictive capability of climate models at space and time scales useful to decision-making, but also help reduce criticisms when communicating on climate issues. The outstanding issues that need continued scientific focus are today collectively termed as “grand challenges,” since many of these issues have been addressed over many years by many research teams, but still have large uncertainties associated with them. The World Climate Research Program (WCRP), one of the flagship domains of the World Meteorological Organization (WMO) has identified a number of these challenges that are currently receiving enhanced scientific attention that will be outlined in the next section. Indeed, the CLIVAR (Climate predictability and variability) program of the WCRP has proposed the definition that “A Grand Challenge is both highly specific and highly focused identifying a specific barrier preventing progress in a critical area of climate science.” In addition to the fundamental science issues identified by the WCRP, two further issues are identified as grand challenges in this paper, namely those associated improved communication of science, and those linked to access to environmental and socio-economic data for research purposes. The current state of knowledge of the climate system and its functioning contains a range of uncertainties inherent to most complex non-linear systems. These need to be further addressed by the research community in order to enhance the confidence in the understanding and prediction of the system (Asrar and Hurrell, 2013). The outstanding areas identified by the WCRP as “grand challenges” requiring particular attention, are briefly outlined below. The potential impacts of sea-level rise on coastal cities through permanent flooding or greater vulnerability to storm surges, as well as on the increasing risks of salt intrusion into ground water and the collateral effects on the fertility of agricultural soils, for example, have been widely reported in the IPCC (2007) reports. While earlier assessments of sea-level rise as a function of a warming climate seemed to be fairly robust, there is today much greater uncertainty; current estimates of sea-level rise range from 20 cm by 2100 (Meehl et al., 2007) to 2 m or more (Vermeer and Rahmstorf, 2009). One of the principal reasons for this increased uncertainty in sea-level projections is related to the behavior of the Greenland ice sheet that has been showing signs of accelerated melting and calving at a scale that was unanticipated a decade ago. Ocean processes linked to the thermohaline circulation and the contribution of modes of climate variability such as the North Atlantic Oscillation require further understanding in order to enable more refined estimates of changes in sea-level at the regional scale and thereby to prepare for appropriate adaptation strategies. The major changes currently observed notably on the Arctic sea-ice and on the Greenland ice sheet are part of a two-way interaction between climate and the high-latitude cryosphere that can lead to amplification of warming and higher rates of sea-level rise than hitherto anticipated. Permafrost beneath large ice-sheets could increasingly melt as surface waters infiltrate to the base of the ice sheet. As the permafrost recedes, melt-waters could enter into porous bedrock regions that would reduce the amount of water flowing into the oceans, and thereby confound gravity-based estimates of the contribution of ice melting to sea-level rise. Thawing permafrost in...