As mentioned last week, I want to spend some time looking at a few of the impacts of climate change on natural stone heritage and new build. This paper provides some background on what changes we expect to see in our climate (specifically in the NW UK) based on the most sophisticated projections we have – and outlines (on a conceptual level) the physical, chemical and biological implications for stone in buildings.
Environmental controls on stone decay processes (generally accepted as being governed by fluctuations in temperature and moisture) are changing as a result of changing climate. UKCP09 projections for the 2020s (2010–2039 – i.e. what we are living in now) indicate that over much of the UK seasonality of precipitation will increase. That means that both summer dryness and winter wetness are set to increase (the latter linked to projected precipitation increases in autumn and spring months). If this is so (and current trends suggest that it is), this could increase the time that stone structures remain wet and possibly the depth of moisture penetration in stone (this has now been demonstrated by data collection – wait for a future post).
It appears that building stone in Northern Ireland has already responded through an increased occurrence of algal ‘greening’. Current and projected climatic trends are therefore considered to have aesthetic, physical and chemical implications that are not currently built into our models of sandstone decay (models associated with traditional views on salt weathering, which I will elucidate on another occasion). This is especially true with respect to the role played by deep-seated wetness on sandstone deterioration, and the growth of algal cover on the stone surface. In particular, the research linked to above proposes that algal biofilms may aid moisture retention (by reducing the ability of the stone to dry out, or ‘breathe’) and further facilitate moisture and dissolved salt penetration to depth.
Even whilst the outer surface of a stone block may continue to experience frequent wetting and drying associated with individual precipitation events, the interiors of building blocks may only experience wetting/drying in response to seasonal cycling – i.e. block interiors may remain wet throughout an entire winter. A possible consequence of this is deeper soluble salt penetration into stone blocks, which could cause an initial delay in the onset of surface deterioration (usually brought about by repeated dissolution and crystallisation of salts at the surface), but more rapid and effective retreat of the surface once decay commences as weathering mechanisms tap into a reservoir of deep salt residing in the stone interior.