Climate change is one of the most complex and important issues facing us today. It involves multiple disciplines – natural sciences, economics, sociology, politics - and is a global thing with different regional or local impacts. It has already had observable effects on the environment: glaciers have shrunk worldwide, sea ice has retreated, biodiversity is being declined, plant and animal ranges have shifted towards higher latitudes and altitudes, extreme events such as heat waves or floods, have become more intense and frequent, sea level has risen, and many others leading to as much different impacts on population and society. Complexity of climate change is a result of the complexity of climate in itself. Climate is an example of complex and dynamical system indeed, described by a number of different components – the atmosphere, hydrosphere, cryosphere, biosphere, anthroposphere – all interacting with each other also through feedback mechanisms. Such a complexity calls for a number of different approaches, the best probably being the integration between measurements and numerical models, to analyze and understand the processes at work now as well as in the past, including paleoclimate, and to make projections of the future climate evolution. The Intergovernmental Panel on Climate Change (IPCC), the most eminent international body for assessing the science related to climate change, uses exactly this integrated approach to provide policymakers with periodical assessments of the scientific basis of climate change, its causes, impacts and future risks for the ecosystems and the society, including options for adaptation and mitigation. Responding to climate change, in fact, involves a twofold approach, that is, reducing emissions and stabilizing the concentrations of greenhouse gases in the atmosphere (mitigation) and adapting to the climate changes already occurring (adaptation). The capacity building of a next generation of decision-makers who will be asked to intervene on these two actions should be built not only on the knowledge of the policy processes to address climate change but also on the understanding of the complexity behind the functioning of the climate system (circulations in the atmosphere and oceans, processes involving vegetation, the role of clouds, etc.), the knows and unknowns in our understanding of its mechanisms, as well as certainties and uncertainties in our projections of the future climate evolution and in the numerical climate models we use to make them. Both mitigation and adaptation can imply long-term impacts and investments, which in turn can face or drive abrupt changes according to the complexity of the involved system and the decisions taken.