Heat waves becoming more prominent in urban areas, research reveals

Prolonged periods of extreme heat increased significantly between 1973 and 2012 in almost half of the urban areas the researchers analyzed. Credit: Ucla

The frequency of heat waves has increased dramatically over the past 40 years, and the trend appears to be growing faster in urban areas than in less-populated areas around the world, a new study suggests.

“Our findings suggest that urban areas are experiencing a kind of double whammy — a combination of general climatic warming combined with the heat island effect, wherein human activities and the built environment trap heat, preventing cities from cooling down as fast as rural areas,” said Dennis Lettenmaier, a co-author of the study and a UCLA geography professor. “Everything’s warming up, but the effect is amplified in urban areas.”

Lettenmaier and his co-authors studied 217 urban areas across the globe and found that prolonged periods of extreme heat increased significantly in 48 percent of them between 1973 and 2012.

The results, which were published today in the journal Environmental Research Letters, show that about only 2 percent of those urban areas experienced a significant decline in heat waves. And the change was more dramatic at night: Almost two-thirds of the urban areas showed significant increases in the frequency of extremely hot nights.

“The fact that the trend was so much stronger at night underscores the role of the heat island effect in urban areas,” Lettenmaier said. “You have heat being stored in buildings and in asphalt, concrete and other building materials, and they don’t cool down as quickly as they would outside of the urban area. This effect was likely exacerbated by decreasing wind in most of the urban areas.”

The study is one of the first to focus solely on the extent of extreme weather in urban areas globally and to examine disparities between densely populated and less-densely populated areas.

Lettenmaier collaborated with researchers at the Indian Institute of Technology Gandhinagar, Northeastern University and the University of Washington. The team obtained daily observations for rain, air temperature and wind speed from the National Oceanic and Atmospheric Administration. The researchers identified about 650 urban areas with populations greater than 250,000 and then refined the list to the 217 locales based on the areas’ proximity to weather stations with complete weather records and NOAA data — most were located at airports close to urban areas. Although the researchers would have liked to have more data for urban areas in Africa, Lettenmaier said the report provides as close as possible to a representative sample of changing weather conditions in the world’s cities. 

For each of the locales in the study, the researchers identified extremes for temperature, precipitation and wind, calculated heat and cold waves, and pinpointed individual extremely hot days and nights.

The study defined heat waves as periods in which the daily maximum temperature was hotter than 99 percent of days for the four-decade period and in which those temperatures were sustained for a consecutive period of six or more days. (The median length of heat waves was eight days.) It found that the average number of heat waves per year increased by over 50 percent during the period.

Of the five years with the largest number of heat waves, four were the most recent years for which data was available: 2009, 2010, 2011 and 2012. Urban areas in South America experienced the greatest increase in frequency of heat waves, followed in order by those in Africa, Europe, India and North America.

Researchers also found other striking examples of climate change within urban settings. Sixty percent experienced a significant decline in extreme windy days, 17 percent experienced a significant increase in daily precipitation extremes, and 10 percent experienced a significant increase in maximum annual precipitation.

“Urban areas make up a relatively small part of the global land area, but over half the world’s populations now live in them, so the trend is troublesome,” said lead author Vimal Mishra, an assistant professor of civil engineering at IIT Gandhinagar. “The combination of higher temperatures and lower wind in particular is not a good combination for human health and well-being. This should concern everyone.”

The increase in precipitation could damage cities’ infrastructure, which could also mean large economic losses, Mishra said.

Using a separate data set of 142 pairs of urban and non-urban areas, the researchers found that the frequency of heat waves grew 56 percent more quickly in urban settings than in surrounding areas that were less populated. Urban areas experienced 60 percent fewer extremely windy days than non-urban areas.

“In urban areas, buildings are disrupting the air flow, which affects not only the immediate area of buildings, but apparently the larger regional wind fields,” Lettenmaier said. “The reduction in wind may well be exacerbating the heat island effect.” 

Source: UCLA

Archaeologists reveal layout of medieval city at Old Sarum

Old Sarum -- aerial photo. Credit: Image courtesy of University of Southampton

Archaeologists from the University of Southampton have revealed for the first time the plan of a network of buildings in a once thriving medieval city at the historic site of Old Sarum, near Salisbury.

A research team of students and academics carried out a geophysical survey of the ancient monument, scanning ground at the site with state-of-the-art equipment to map the remains of buried structures. They concentrated their survey around the inner and outer baileys of what was once a fortification, with its origins in the Iron Age and the Roman conquest.1

Their investigations reveal the layout of a settlement including structures from the late 11th century, contemporary with the construction of a cathedral and castle. The city was inhabited for over 300 years, but declined in the 13th century with the rise of New Sarum (Salisbury).

The project findings mainly concentrate on the medieval period and highlight:

* A series of massive structures along the southern edge of the outer bailey defensive wall, perhaps suggesting large buildings of a defensive nature.
* An open area of ground behind these large structures, perhaps for mustering resources or people, or as part of a circular route through the city..
* Residential areas in the south east and south west quadrants of the outer bailey alongside the inner bailey ditch.
* Evidence of deposits indicating industrial features, such as kilns or furnaces.
* Features suggesting quarrying at the site after the 1300s and following the city's decline -- indicating a later period of habitation at the site..

Archaeologist Kristian Strutt, Experimental Officer and Director of Archaeological Prospection Services at the University of Southampton, says: "Archaeologists and historians have known for centuries that there was a medieval city at Old Sarum, but until now there has been no proper plan of the site.

"Our survey shows where individual buildings are located and from this we can piece together a detailed picture of the urban plan within the city walls."

The research was conducted as part of the Old Sarum and Stratford-Sub-Castle Archaeological Survey Project, directed by Kristian Strutt and fellow Southampton archaeologists Timothy Sly and Dominic Barker. Old Sarum is under the custodianship of English Heritage, who kindly granted permission for the investigation to take place.

Heather Sebire, Property Curator at English Heritage, comments: "Having the team of archaeologists on site over the summer gave our visitors a chance to find out more about how important historic landscapes are surveyed. The use of modern, non-invasive surveying is a great start to further research at Old Sarum.

"From this work we can surmise much about the site's past and, whilst we can't conclusively date the findings, it adds a new layer to Old Sarum's story. We welcome the chance to find out more about our sites, and look forward to exploring ideas for further research in the future."

The team used a variety of techniques to examine the outer and inner bailey of the site. These included the use of topographic survey methods and geophysical survey techniques -- comprising of magnetometry, earth resistance, ground penetrating radar (GPR) and electric resistivity tomography (ERT) survey.

Kristian Strutt concludes: "Our research so far has shown how the entire outer bailey of the monument was heavily built up in the Middle Ages, representing a substantial urban centre. Results have given us compelling evidence as to the nature of some of the structures. It is clear, however, that there is more non-intrusive work that could be carried out to further expand our understanding of the site."

The team hopes to return to complete the survey of the inner and outer baileys and survey the Romano-British settlement to the south of Old Sarum in Easter 2015. The project fieldwork in 2014 was used as a training season for undergraduate and postgraduate archaeology students at the University, continuing a long tradition of research-led teaching at some of the most impressive archaeological sites in the south of England. Previous fieldwork has been conducted by students at Portchester Castle, Netley Abbey and Bishop's Waltham Palace in Hampshire, and at Bodiam Castle in East Sussex.

Source: University of Southampton

Protect the world's deltas, experts urge

The Atchafalaya River delta meets the Gulf of Mexico. The view is upriver to the northwest. Credit: Photo courtesy A. Belala/U.S. Army Corps of Engineers

Extensive areas of the world's deltas -- which accommodate major cities such as Shanghai, Dhaka and Bangkok -- will be drowned in the next century by rising sea levels, according to a Comment piece in this week's Nature. In the article, Dr. Liviu Giosan, a geologist with the Woods Hole Oceanographic Institution (WHOI), and colleagues call for maintenance efforts to be started now to avert the loss of vast expanses of coastline, and the consequent losses of ecological services, economic and social crises, and large-scale migrations.

The authors state the problems start upstream: deltas are built from sediments deposited at the mouths of rivers, but dams and river engineering have lowered rates of sediment flow. The Nile and the Indus, for example, carry 98 percent and 94 percent less mud respectively than they did 100 years ago. At the coast, rising seas resulting from warmer global temperatures are eroding delta plains, increasing the chance of flooding. Coastal lands lower than a meter in elevation will be inundated within a century.

Lack of quantitative knowledge of basic delta processes is hindering efforts to develop maintenance strategies for deltas, the authors say. At the same time, the role of healthy marshes in coastal processes needs to be more fully understood. Giosan and colleagues call for river sediment flows to be restored, and natural land-building methods to be exploited in delta plains under worldwide monitoring programs coordinated and guided by United Nations committee of experts.

Source: Woods Hole Oceanographic Institution