The concept of “irreversibility” and its counterpart “reversibility” have become prominent in environmental and ecological research on human-induced changes, thresholds, climate tipping points, ecosystem degradation, and losses in the cryosphere and biosphere. Through a systematic literature review, we show that in these research fields, these notions are not only descriptive terms, but can have different semantic functions and normative aspects. The results suggest that, in the context of environmental and ecological research the concepts of irreversibility and reversibility have taken on additional usages in comparison to their contexts in theoretical thermodynamics and mechanics. Irreversible as a classification of anthropogenic environmental change can be used categorically, in the sense of a finite end, or relatively, i.e. on time or spatial scales of interest. Surprisingly, most of the analyzed scientific articles that use the terminology of (ir)reversibility substantively do not provide an explicit conceptualization or definition (74.7%). The research on potential (ir)reversibility of environmental change may affect the social and political willingness to bear the costs of interventions to mitigate or prevent undesirable environmental change. In particular, classifying a change as reversible or irreversible and determining the timescale(s) and spatial scale(s) involved has implications for policy and ecosystem management decisions, as suggested by its use in several high-level scientific and policy reports on ecosystem and climate change. Therefore, it is important to explicitly present a clear definition of irreversibility or reversibility for the readers from other fields, even if it could be the case that within a specific community an implicit definition was considered to be sufficient. We propose further recommendations for inter- and transdisciplinary reflection and conceptual use in the context of environmental, ecological, and sustainability research.

This chapter discusses the societal and ethical challenges of climate engineering or large-scale intentional intervention in the climate system. Climate engineering is highly controversial, and raises many questions about the values of human societies and the desirability of technological visions of the future. Yet existing ethical theories and concepts may not be equipped to deal with the resulting ethical issues. To understand the potential social and political disruptiveness of climate engineering, we argue it must be placed in the context of global environmental changes caused by human activity. However, climate engineering is also accompanied by a high degree of uncertainty and risk in terms of potential and actual unintended impacts on natural processes and society. An important challenge stems from epistemic and normative uncertainties about the reversibility and variability in spatial and temporal scales of deployment. Epistemic uncertainties arise in the methodological framework of climate science, while normative uncertainties arise from the challenge of reconciling a plurality of values. A key question is how forms of climate engineering enforce or hinder disruption in social practices and institutional settings in the direction of a sustainable future. Climate engineering technologies can affect and potentially disrupt existing conceptions of climate and environmental justice, due to the scale and scope of impacts upon people currently living on the planet, future generations, and non-human species and ecosystems. The availability of climate engineering may also require rethinking the responsibility for climate mitigation, as well as applications of the precautionary principle. Climate engineering also raises the question of how the perspectives of affected communities can be adequately represented. While it remains unclear whether climate engineering techniques can genuinely assist in lessening the impacts of climate change, the question is whether and to what extent it should be used as a complementary approach to systemic changes in social, economic, and political practices.