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Projections of future climate change cannot rely on a single model. It has become common to rely on multiple simulations generated by Multi-Model Ensembles (MMEs), especially to quantify the uncertainty about what would constitute an adequate model structure. But, as Parker points out (2018), one of the remaining philosophically interesting questions is: "How can ensemble studies be designed so that they probe uncertainty in desired ways?" This paper offers two interpretations of what General Circulation Models (GCMs) are and how MMEs made of GCMs should be designed. In the first interpretation, models are combinations of modules and parameterisations; an MME is obtained by "plugging and playing" with interchangeable modules and parameterisations. In the second interpretation, models are aggregations of expert judgements that result from a history of epistemic decisions made by scientists about the choice of representations; an MME is a sampling of expert judgements from modelling teams. We argue that, while the two interpretations involve distinct domains from philosophy of science and social epistemology, they both could be used in a complementary manner in order to explore ways of designing better MMEs.

Recent appraisals of the Gaia theory tend to focus on the claim that planetary life is a cybernetic regulator that would self-regulate Earth’s chemistry composition and climate dynamics, following either a weak (biotic and physical processes create feedback loops), or a strong (biological activity control and regulates the physical processes) interpretation of the Gaia hypothesis. Here, we contrast with the regulator interpretation and return to the initial motivation of the Gaia hypothesis: extending Schrödinger’s question about the nature of life at the planetary scale. To this end, we propose a relational and systemic biological approach using autopoiesis as the realization of the living and the (M,R)-system as the formal theory of biological systems. By applying a minimum of key categories to a set of interacting causal processes operating on a wide range of spatial time scales through the atmosphere, lithosphere, hydrosphere, and biosphere of the Earth system, we suggest a one-to-one realization map between the Gaia phenomenon and (M,R)-Autopoiesis. We show that metabolic molecular self-production by closure to efficient causation on a planetary scale is plausible. This suggests that the Gaia phenomenon may be the embodiment of Life itself in the planetary domain, a sui-generis biological unity and thus more fundamental than self-regulation by feedback mechanisms. Formulating the Gaia theory in biological terms provides a formal basis for the claim that planetary biology elsewhere in the universe must involve and have a formal equivalence to a self-referential physical process which cannot be implemented by a Turing machine and, therefore, has a non-computable character.

Non-epistemic values pervade climate modelling, as is now well documented and widely discussed in the philosophy of climate science. Recently, Parker and Winsberg have drawn attention to what can be termed "epistemic inequality": this is the risk that climate models might more accurately represent the future climates of the geographical regions prioritised by the values of the modellers. In this paper, we promote value management as a way of overcoming epistemic inequality. We argue that value management can be seriously considered as soon as the value-free ideal and inductive risk arguments commonly used to frame the discussions of value influence in climate science are replaced by alternative social accounts of objectivity. We consider objectivity in Longino's sense as well as strong objectivity in Harding's sense to be relevant options here, because they offer concrete proposals that can guide scientific practice in evaluating and designing so-called multi-model ensembles and, in fine, improve their capacity to quantify and express uncertainty in climate projections. Keywords. climate models; multi-model ensemble; uncertainty quantification; model pluralism; scientific objectivity; values in science; value-free ideal; strong objectivity. * The authors thank the working group on philosophy of science in Bern for fruitful discussions and helpful comments on a previous version of this paper, especially Claus Beisbart, Matthias Egg, Vincent Lam, and Tim Räz. They also thank the two anonymous reviewers for their helpful comments. JJ is funded by the Swiss National Science Foundation project PP00P1 170460 "The Epistemology of Climate Change". MC is a Research Director funded by the Belgian National Fund for Scientific Research.

The first records of Holocene CO 2 and CH 4 concentrations in the atmosphere were published two decades ago (Barnola et al., 1987;Chappellaz et al., 1990), and the observation that both gas concentrations had increased during the last several thousand years has drawn considerable attention from scientists. Competing hypotheses attribute these increases either to natural forcing or anthropogenic intervention in the climate system. This ongoing debate is the focus of this volume.Papers in Part 1 of this issue explore the history of Holocene population growth and land clearance. Part 2 examines possible sources of the CH 4 increase during the last 5000 years, and Part 3 examines possible sources of the CO 2 increase over the last 7000 years. Part 4 investigates whether or not the drops in CO 2 and CH 4 concentrations proposed in the early anthropogenic hypothesis would have been sufficient to cause glacial inception in the Northern Hemisphere. Part 5 explores possible causes of the decreases in CO 2 concentrations during the last 1000 years. A final paper provides one assessment of whether the natural or anthropogenic hypotheses for the late-Holocene greenhouse-gas increases can be rejected based on currently available evidence.When this issue was first planned, the editors sent out invitations to all scientists who had first-authored papers relevant to the topic or had otherwise played a prominent role in the discussion. This list was closely balanced among those favoring natural explanations, those favoring the anthropogenic explanation, and those who had not taken a strong position on either side of the debate. Roughly half of the scientists invited chose to participate, and papers that support the natural and anthropogenic explanations can be found in each of the five major parts of the issue. Some of the most prominent proponents of natural forcing did not choose to participate, but their views are frequently discussed in the 13 papers that form this issue. References