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  Geotimes - July 2007 - Geophenomena

Watching Our Stone Heritage Crumble

Anne Bolen
The degradation of the bluestone and sarsen rocks at Stonehenge from freeze-thaw weathering, salt encrustation and growth of lichens and algae is being exacerbated by the changing climate.

Stonehenge in England. The Tower of London. Torre del Oro in Spain. Egypt’s Great Pyramid of Giza. Mount Rushmore in South Dakota. These iconic sites of human culture may someday be only a memory, as weathering is putting them all in danger of crumbling into piles of stone rubble before our eyes. New research is aiming to figure out just how much historical sites will degrade over the next century as weather patterns continue to evolve.

Rain, sunlight, freezing and thawing, and chemical and other processes leave their mark on rock, breaking it down into smaller and smaller pieces until it eventually becomes soil. Stone — rock used in construction — usually decays faster than its uncut counterpart, says Bernard Smith, a geomorphologist at Queen’s University Belfast in Northern Ireland. He runs The Limestone Project, a team of researchers from Queen’s University Belfast, City University in London and Oxford University, who joined together in 2005 to study and develop new methods to detect catastrophic decay of limestone structures. Stone degradation can be accelerated by weather, atmospheric pollution and poor maintenance, he says, marring a structure’s exterior until architectural detail, or even an entire cultural site, is lost.

Natural weathering rates of rock are fairly well-defined, says Richard Livingston, a researcher at the U.S. National Institute of Standards and Technology. Geologists can figure out roughly how fast an unaltered boulder of granite or marble will weather, he says. But a number of different factors go into weathering rates of cut stone, Smith says, making those rates more difficult to predict. Before they can figure out how fast a stone building will deteriorate, researchers need to know what stresses and environmental conditions the stone was exposed to prior to construction, such as whether it was quarried from deep belowground or from the surface, and whether the stone has been conserved at some point since the original construction.

About 50 years ago, Smith says, “people started to become increasingly concerned that large parts of our built heritage were crumbling away,” mainly because of increases in acid rain. Researchers began measuring and modeling stone degradation to estimate rates of decay, particularly where structures were centuries old. In London, for example, thousand-year-old limestone buildings were turning black with soot and gypsum crystals that precipitated in the stone as chemicals in the rain reacted with the stone. Now, he says, those rates are changing, thanks to climate change.

Alfred Kärcher GmbH & Co. KG
Recently, lichen, algae, moss and other organics that could damage the underlying rock were blasted off Mount Rushmore in South Dakota by Alfred Kärcher GmbH & Co. KG, using high-pressure water cleaning systems.

Researchers with the Noah’s Ark Project, based at the University of East Anglia’s School of Environmental Sciences in the United Kingdom, and coordinated by ISAC in Italy, are studying just how global climate change will impact the built heritage and cultural landscapes of the world, especially in Europe. During the past three years, a group of atmospheric researchers from all over Europe has been working to determine which factors will most affect building materials, how serious a problem climate change could pose to our cultural heritage, and if any mitigation or adaptation strategies can be adopted, says Carlota Grossi-Sampedro, a University of East Anglia researcher participating in the project. “We wanted to know how temperature or humidity changes, for example, will affect different building materials, including stone, bricks and mortar,” she says. “We found that there are going to be a lot of changes.”

Temperatures across much of Europe could rise as much as 3 to 5 degrees Celsius (about 5 to 9 degrees Fahrenheit) by 2100, says Peter Brimblecombe, a University of East Anglia atmospheric chemist and head of the Noah’s Ark Project. Climate models the team used also predict that much of the continent will be wetter in the winter and drier in the summer, Brimblecombe says. Although these increases in temperature and precipitation are relatively small, even “a small increase in temperature can mean a big change,” if it means crossing the temperature or humidity threshold at which a material is best preserved, he says. Freeze-thaw weathering and salt weathering both occur when temperatures and humidity cross these thresholds. As moisture from precipitation, dew, fog or humidity wets a stone, salt from the water builds up in its pores. If the stone dries, the salt crystallizes, expands and fractures the stone.

Dropping humidity levels in Europe in the summertime, even by just a small percent, means that more locations will be at or near their thresholds more often, allowing stone degradation to occur more frequently, Brimblecombe says. The same effect occurs in the wintertime as humidity levels rise. So, “it’s not the humidity change itself, but the number of times a stone transitions” from wet to dry that impacts the stone, he says. Models suggest that a drier climate will “increase salt weathering dramatically all across Europe — a three- to four-fold increase over the next century alone,” Brimblecombe says.

The same situation occurs with temperatures as they cross the freezing threshold. Water seeps into pores of building stones. When temperatures drop below freezing, the water turns to ice and expands, causing minute cracks in the stone. The more often a stone freezes and thaws, the more the stone fractures. That same process ultimately brought down New Hampshire’s famed “Old Man of the Mountain” (see Geotimes, June 2004). This physical weathering process is currently one of the big problems across much of Europe. But with temperatures across much of the continent set to rise over the next century, much of temperate Europe will see a “significantly reduced incidence” of freezing in the next 100 years, Brimblecombe says. Certain locations, such as Scandinavia and Greenland and parts of mainland Europe at high elevations, will see a lot more freeze-thaw fracturing, but it will not be as big a factor across most of Europe as it is now, he says.

Courtesy of the Noah’s Ark Project
Salt crystals can often show up as dark spots on stone, as seen on St. Maria del Fiore Cathedral in Florence, Italy. Air pollution also darkens building surfaces, so that they absorb more heat, which contributes to their deterioration.

Still, Brimblecombe says, salt weathering is going to increase enough to more than make up for the decline in freeze-thaw weathering. Furthermore, the Noah’s Ark team found that other weathering processes will also likely accelerate as the climate changes. Predicted increases in wind velocities, especially in coastal areas where the winds carry a lot of salt, are projected to accelerate deterioration of buildings. Any increases in solar radiation and precipitation will also accelerate deterioration. Increases in pollutants, such as carbon dioxide, sulfur dioxide or nitrogen oxides also accelerate weathering. Increasing storm intensities, as predicted by the models, will also cause accelerated weathering rates, as floods and hurricanes tear through historic areas. And increasing temperatures and moisture can cause biological activities, such as algal and fungal growth, to skyrocket. This is already being seen in Northern Ireland and England, Smith says, as buildings are “going green.” The Tower of London, for example, now has a green sheen as algae have colonized the building.

The Noah’s Ark team, Brimble-combe points out, has primarily looked at the big picture — general changes predicted across a wide swath of Europe — and has published a Web-based atlas of climate change risks and vulnerabilities that heritage managers can use to form conservation plans. “We didn’t look at particular buildings, but focused on regional changes in the response of a few materials,” he says. That research needs to happen, he says, as stone types vary in how fast they decay. Porous limestone, for example, weathers far faster than marble or granite in humid environments, as water is able to more easily infiltrate the structure of the stone and react with the material. Furthermore, all the sides of a building do not weather at the same rate, as they may be exposed to different forces, such as stronger winds, or more rain and sunlight.

The problems stone buildings face across the world vary, Smith says. In a rapidly industrializing city like Rio de Janeiro, Brazil, for example, acid rain is still a problem, disintegrating limestone buildings and building black gypsum crusts on the buildings. In Budapest, Hungary, which is just starting to improve its air quality, stone is still decaying because of acid rain, but the situation is getting better, he says. In Belfast and London, where acid rain is less of an issue than 20 years ago, Smith says, the structures are reacting to warmer, wetter, longer winters by growing algae and precipitating more salts. The stones in the buildings are getting wet all the way through, thus depositing salts deep into the stone rather than just on the surface. In limestone, “if a stone starts to wear back, it can keep wearing back until it’s gone — it can be catastrophic, causing whole stones to crumble in the façade of a building,” he says.

Just telling heritage managers and conservationists about the problems they are going to face isn’t enough, Brimblecombe says. But other than continual maintenance, such as clearing building gutters before heavy rains, there are not a lot of solutions, he says. “We may have to rethink conservation measures, too,” Smith adds, such as treating the outsides of buildings with water repellants or chemicals that restrict biological activity, or blasting away salts and biological growth, as was done on Mount Rushmore in South Dakota. Once stones start to decay, “dressing back” (shaving off the outer layers of the stone that salt contaminated) or replacing the stone entirely are common options in Europe, he says.

Smith’s team is working with engineers on another option, however. They want to develop a “smart building” that has sensors built into its stone that measure wetness, alkalinity and acidity, and temperature. “These smart materials would tell you when a building is getting wet or growing salts, so that you can act before damage occurs,” Smith says. Such sensors would be placed strategically throughout a new building. But such technologies won’t be available in the near future, he recognizes. In the meantime, simply observing the changes in our built heritage and doing what can be done to prevent serious damage to it as the climate changes and our buildings age are crucial to its preservation for future generations.

Megan Sever

"Revisiting the Fall of the Old Man of the Mountain," Geotimes, June 2004 

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