This chapter provides an overview of Earth system models, the various model ‘flavours’, their state of development including model evaluation, benchmarking and optimization against observational data and their application to climate change issues.
The Earth system can be conceptualized as a suite of interacting physical, chemical, biological and anthropogenic processes that regulate the planet’s low of matter and energy. Earth system models (ESMs; Box 5.1 ) are built to mirror these processes. In fact, ESMs are the only tool available to the scientific community to investigate the system properties of the Earth, as we do not have an alternative planet to manipulate that could serve as a scientist’s laboratory.
The term ‘Earth system model’ is commonly used to describe coupled land–ocean–atmosphere models that include interactive biogeochemical components. Such models have developed progressively from the physical climate models first created in the 1960s and 1970s. Conventional climate models apply physical laws to simulate the general circulation of atmosphere and ocean. As our understanding of the natural and anthropogenic controls on climate has grown, and given the steady advances in computing power, global climate models have been extended to include more comprehensive representations of biological and geochemical processes, involving the addition of the various interacting components of the Earth system with their own feedback mechanisms. Figure 5.1 shows the conceptual differences between a conventional global coupled atmosphere–ocean general circulation model (AOGCM) and an ESM. In terms of the coupling between components, ESMs are more complex, and they have correspondingly higher computational demands.
In articles published in physics journals, the multiverse hypothesis is strictly regarded from a non-theistic perspective, as a possible explanatory hypothesis for the life-permitting values of the constants of physics. Further, there have been several attempts to make specific predictions with regard to the values of these constants from a multiverse hypothesis, such as the value of the cosmological constant [1—3]. Such approaches reflect the legitimate methodological naturalism of physics. However, in wider-ranging philosophical discussions of the multiverse hypothesis — as found in various books on the topic — the issue arises as to what is the relation between the multiverse hypothesis and much larger philosophical issues, particularly whether reality is ultimately impersonal or personal in nature. In such contexts, the multiverse hypothesis is often presented as the atheistic alternative to a theistic explanation — such as that offered by John Polkinghorne  — of the purported fine-tuning of the cosmos for intelligent life. In this contribution, I will attempt to explain why, contrary to the impression one often gets, contemporary physics and cosmology are not only compatible with theism, but could arguably be thought to suggest a theistic explanation of the Universe or multiverse. I do not expect necessarily to convince anyone of the theistic point of view, realizing that many factors — both theoretical and personal — underlie our views of the ultimate nature of reality.
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