  Multiple hypotheses/models have been put forward regarding the cooling
history of the Earth. The search for life beyond Earth has brought these models
into a new light as they connect to one of the two energy sources life can tap.
The ability to discriminate between different Earth cooling models, and the
utility of adopting such models to aid in the assessment of planetary
habitability, has been hampered by a lack of uncertainty analysis. This
motivates a layered uncertainty analysis for a range of thermal history models
that have been applied to the Earth. The analysis evaluates coupled model
input, initial condition, and structural uncertainty. Layered model
uncertainty, together with data uncertainty and multiple working hypotheses
(another form of uncertainty), means that results must be evaluated in a
probabilistic sense even if the models are deterministic. For the Earth's
cooling history uncertainty leads to ambiguity - multiple models, based on
different hypotheses, can match data constraints. This has implications for
using such models to forecast conditions for exoplanets that share Earth
characteristics but are older than the Earth, i.e., it has implications for
modeling the long-term life potential of terrestrial planets. Even for the most
Earth-like planet we know of, the Earth itself, model uncertainty and ambiguity
leads to large forecast spreads. Given that this comes from the planet with the
most data constraints we should expect larger spreads for models of terrestrial
planets in general. The layered uncertainty approach can be expanded by
coupling planetary cooling models to climate models and propagating uncertainty
between them to assess habitability from a probabilistic versus a binary view.
