to “megatrends” that were driving education of planners in Australia. A similar term, “fatigue proofing,” was used in 2007 to describe steel cover plates in bridge construction that would not fail due to fatigue cracking. In 2012, a New Zealand-based organization outlined eight principles of future-proof buildings: smart energy use, increased health and safety, increased life cycle duration, increased quality of materials and installation, increased security, increased sound control for noise pollution, adaptable spatial design, and reduced carbon footprint. Another approach to future-proofing suggests that only in more extensive refurbishments to a building should future-proofing be considered. Even then, the proposed time horizon for future-proofing events is 15 to 25 years. The explanation for this particular time horizon for future-proof improvements is unclear. In the valuation of real estate, there are three traditional forms of obsolescence which affect property values: physical, functional, and aesthetic. Physical obsolescence occurs when the physical material of the property deteriorates to the point where it needs to be replaced or renovated. Functional obsolescence occurs when the property is no longer capable of serving the intended use or function. Aesthetic obsolescence occurs when fashions change, when something is no longer in style. A potential fourth form has emerged as well: sustainable obsolescence. Sustainable obsolescence proposes to be a combination of the above forms in many ways. Sustainable obsolescence occurs when a property no longer meets one or more sustainable design goals. One reasonable approach to future-proof sustainable cities is an integrated multi-disciplinary combination of mitigation and adaptation to raise the level of resilience of the city. In the context of urban environments, resilience is less dependent on an exact understanding of the future than on tolerance of uncertainty and broad programs to absorb the stresses that this environment might face. The scale of the context is important in this view: events are viewed as regional stresses rather than local. The intent for a resilient urban environment is to keep many options open, emphasize diversity in the environment, and perform long-range planning that accounts for external systemic shocks. Historic buildings Future-proofing of designated historic structures adds a level of complexity to the concepts of future-proofing in other industries as described above. All interventions on historic structures must comply with the Secretary's Standards for the Treatment of Historic Properties. The degree of compliance and the Standard selected may vary depending on jurisdiction, type of intervention, significance of the