Precipitation
Observations of annual total precipitation reveal precipitation changes in cities on decadal timescales. Natural climate variability can greatly influence annual precipitation (more so than temperature). Changes in the inter-annual variability of precipitation can also provide insight into how the climate is changing. For example, in New York City, while annual precipitation has only increased slightly in the past century, inter-annual variability has become more pronounced. In addition, analyzing trends in monthly rainfall can also be useful, as many cities experience two distinct seasons, such as the monsoons in Delhi, India. Data on shorter timescales, such as daily and weekly, can be used in extreme events analysis of droughts, floods, and intense precipitation rates. Again, with limited data records for some cities, obtaining climate data on these small timescales can be difficult.
Precipitation trends for the 12 cities reflect the regional nature of precipitation changes with climate change, relative to the largely homogenous changes expected in temperature. Of the 12 cities looked at in the topic, half saw increasing trends in annual precipitation. Over the past century the most rapid rate of precipitation increase has occurred in Sao Paulo (+29 mm per decade), and the largest decrease has occurred in Harare (-21 mm per decade).
For the cities where observed trends in annual precipitation showed increasing (decreasing) precipitation, the trends do not reveal whether the wetter (drier) conditions are caused by more (less) frequent heavy rainfall events or more (less) persistent lighter rainfall. Also, the direction of trends for extreme precipitation events, such as days where precipitation is greater than 50 mm, will not necessarily correspond to the trend in annual precipitation. For example, a city could experience a decreasing trend in annual precipitation, but have an increase in short-duration intense precipitation events.
As was the case with temperature, there are multidecadal fluctuations in observed precipitation trends specific to each city. This is especially true for this climate variable given the large inter-annual variability of precipitation. Cities with increasing annual precipitation over the time period still may have pronounced periods of drought, while those experiencing a drying trend may still have years with higher than average rainfall.
Figure 3.12: Observed precipitation trends from 1938 to 2005 in Toronto, Canada: rain above; snow below. Showfall trend is significant at the 95% level.
Observed precipitation trend in Toronto, Canada
Toronto is one city selected for this topic that receives precipitation in the form of snow. While total precipitation has been increasing over the past century in the city, most of the increase has been in the form of more rainfall. Snowfall in Toronto has been on the decline, leading to a decrease in the percentage of total precipitation falling as snow. The trends for observed annual rainfall and snowfall in Toronto are presented in Figure 3.12, with rainfall and snowfall shown in green and blue respectively. Rainfall has increased at a rate of 10 mm per decade while snowfall has decreased at a rate of 4.3 cm per decade between 1938 and 2008.
To some extent, trends in precipitation for a given city depend on annual mean temperatures. In a warmer climate, the atmosphere can hold more moisture, allowing for more precipitation. But higher temperatures also can alter the type of precipitation that will fall. For Toronto, warmer temperatures have allowed for more precipitation; in the spring, precipitation that once fell as snow is now falling as rain. This trend is not evident in many of the colder parts of Canada. In colder areas the increases in total annual precipitation (like those observed in Toronto) have included increases in snowfall, especially during the winter months. Temperatures in these areas of the country have warmed enough to allow for more precipitation but have not crossed the threshold for snow to become rain (Zhang et al., 2000).
Sea level rise
Sea level rise analysis was not performed for all of the coastal cities due to limited data sets. However, coastal cities are extremely vulnerable to rising sea levels, since approximately 35 percent of world population lives within 100 km of the coast (Hachadoorian et al., 2011; L. Hachadoorian, pers. comm.).
Sorsogon City, Philippines, responding to climate change
Sorsogon City is one of 120 cities in the Philippines in the Asia Pacific region. It has a land area of 313 square kilometers with a population of 151,454 (as of 2007) growing at a rate of 1.78 percent annually. Its economy is based mainly on agriculture, fishing, trade, and services. It is the capital and the administrative, commercial, and educational center of Sorsogon Province.
In August 2008 the city launched a Climate Change Initiative, championed by the new mayor. Until then the popular perception was that climate change is a global and national issue requiring limited action from the local government. A series of briefings for decision-makers and local leaders was conducted to enhance basic understanding of climate change and the important role of local government. This resulted in an expressed commitment from decision-makers in developing their city’s climate change profile and defining responsive local actions.
Various city stakeholders worked together with the local government in the conduct of a participatory vulnerability and adaptation (V&A) assessment. Using climate change projections and risk assessments from national government agencies and private research institutions, the city government developed its local vulnerability assumptions. To assess local impacts, the city gathered and analyzed its own recorded observations. These were further substantiated by local people’s accounts of their personal experiences. During city consultations, residents recounted how typhoons and storm surges over the past decade had become stronger and more destructive. These records and personal accounts were recorded as evidence of climate change impacts through community risk mapping. Using hand-drawn maps, local people graphically described the changes in the reach of tidal flooding and identified the areas gradually lost due to sea level rise and erosion. This participatory exercise promoted ownership by the locals of the assessment process and results, and increased their awareness of climate change impacts. Moreover, the process empowered the people to work together with the local government in finding practical solutions that they can personally act on.
As noted in the city’s climate change profile, the city was badly hit by two super-typhoons in 2006, causing widespread devastation within a two-month interval and leaving in their wake a total of 27,101 families affected and 10,070 totally damaged houses (Box Figure 3.2). The first typhoon, in just 5 hours, caused damage to public infrastructure estimated at 208 million pesos or US$4.3 million. The city is projected to experience more cases of prolonged monsoon rains resulting in total rainfall exceeding 2,800 to 3,500 mm per year.
The V&A assessment revealed that the city’s geographical location and previous stresses make it sensitive to changes in extremes – such as tropical cyclones, storm surges, and extreme rainfall/flooding – and changes in means – such as increased temperature, increased precipitation, and sea level rise. With sea level rise projected to accelerate, the city’s built-up areas situated near the coast present the highest vulnerability to climate change impacts because they have the highest concentration of people, especially informal settlers, living in inadequate structures in danger zones. These areas are also the hub for economic activities (accounting for 60 percent of the economy) and the location of basic lifelines such as water, electricity, and basic service facilities. Around 36 percent of the total population, or 55,000 people, are vulnerable to flooding. Over 35,000 people from nine coastal villages are threatened by sea level rise and storm surge, of whom 22,000 are women.
Box Figure 3.2: Section of seawall in Beacon District, Sorgoson, partially destroyed by a 2006 storm. Many of Sorgoson’s informal settlements are just behind the seawall.
Knowing these climate change vulnerabilities (areas, population, economic activities, policy gaps), the city government is now engaging local communities and the private sector in climate change adaptation planning. Using tools from UN-HABITAT’s Sustainable Cities Programme, the local government conducted multisector city consultations that resulted in the identification of four priority "quick-win" responses to increase people’s resilience to climate change, namely: (i) improving settlements and basic infrastructure, (ii) enhancing livelihoods, (iii) developing climate and disaster risk management systems, and (iv) improving environmental management and climate change mitigation actions. Issue working groups composed of representatives from people’s organizations, NGOs, private sector, and LGU were organized to develop the action agenda per "quick-win" area and ensure its implementation.
So far the following important lessons have been learned: 1. There is a need to promote and advocate awareness on climate change among the general public and stakeholders through various media and community activities. This would broaden/establish partnerships among the private, public, academic, civil society, and neighbourhood associations for convergence of efforts on adaptation and mitigation.
2. The city government’s capacity must be developed to make it more responsive to increase its resilience to climate change impacts. A framework must be developed to help and guide the city in integrating climate change considerations in the land use and development plans. A stronger link with national climate change programs is critical especially in enhancing building code and land use planning parameters.
3. The city needs to learn from good practices by other cities. It should also share its own experience in engaging various stakeholders in defining a collective climate change action.
4. It is crucial for the business sector to play a vital role in providing green building technology development and in promoting risk-resilient communities through the use of appropriate and innovative technologies in housing and infrastructure development.
The above lessons have become major considerations as the city works on mainstreaming climate change risk management into its local governance processes and implementing local climate change adaptation actions.
Cities in this topic at risk of sea level rise include New York, Sao Paulo, Tokyo, London, Shanghai, Melbourne, and Dakar. For all of these coastal cities, the sea level rise they are experiencing is caused by a combination of global and local factors. While the rate of sea level rise for each city due to global thermal expansion and meltwater from glaciers and ice sheets is the same, city-specific factors include land subsidence and local ocean height. These city-specific terms are necessary not only to determine the local rate of sea level and compare it to other cities and the global trend, but also for sea level projections.
Observed sea level rise in Dakar, Senegal
One city that is at risk of and has been experiencing rising sea levels over the past century is Dakar. Unfortunately, due to limited data availability, a common phenomenon in developing country cities, very little can be said about sea level rise trends. While sea level has been increasing at a rate of 1.5 cm per decade over the 11-year data record, for such short timescales natural variations and cycles can dominate any climate change signal. This example highlights the need for expanded data collection and quality control in many cities.
Extreme events
Extreme events can be defined as climate variables experienced in a limited duration. Temperature extremes include hot days where temperatures exceed a specified threshold, and heat waves-consecutive hot days. Precipitation extremes, which cover varying timescales, include intense/heavy precipitation events and droughts. Coastal storms and tropical cyclones are also types of climate extreme events.
Limited data availability at short timescales constrains analysis of urban extreme events. Extreme events also differ by city; for many inland locations, for example, coastal storm surges can be ignored. Some extreme events occur more frequently in association with certain phases of climate variability patterns such as ENSO. Because some variability patterns are somewhat predictable, there is an opportunity for seasonally forecasting these events, allowing cities to prepare for them in advance.
Hot days in Melbourne, Australia
One city that experiences hot days and heat waves is Melbourne. As defined by the World Meteorological Organization (WMO), hot days have maximum temperature exceeding 35 °C. For Melbourne, the trend from 1900 to 2008 shows no significant increase in the number of hot days, shown in Figure 3.13. The trend in hot days does not reveal a significant increase, even though annual average temperatures have risen significantly over the past century.
Combined with prolonged periods of dry weather, consecutive hot days (heat waves) have the potential to greatly impact cities and their surrounding areas. Specifically for Melbourne, years with above-average hot days, combined with other meteorological conditions, yield an increased threat of wildfires. Although the fires themselves are often in outlying areas away from the city, infrastructure, agriculture, ecosystems, water, and human resources critical to the city’s survival may be impacted. Fires can also directly impact the city by reducing air quality. The most recent extreme warmth in early 2009, the heat wave of 2006, and in the summer of 1983 are all examples of years with increased fire activity and high numbers of hot days.
Figure 3.13: Observed temperature extremes, hot days with temperatures above 35 °C, in Melbourne, Australia.
Drought in Harare, Zimbabwe
Drought is a precipitation extreme event that occurs over longer timescales, ranging from months to years. Unlike other extreme events, droughts lack a formal definition or index that is applicable globally, which makes assessments of their severity, trends in their frequency, and future projections difficult. Because of the varying indices and definitions, a qualitative assessment of drought based on precipitation is appropriate for multi-city analyses.
One city that experiences frequent and prolonged periods of drought is Harare, Zimbabwe. Over the past century, droughts have occurred several times, including 1991/1992, 1994/1995, and 1997/1998. Analysis of precipitation data reveals that precipitation in Harare has been declining over the past century at a rate of -21 mm per decade. In addition, of the ten driest July-June periods between 1938 and 2002, five have been since 1980. These results suggest that drier conditions may be becoming more frequent.
The droughts that occur in Harare have a strong connection to ENSO events. While ENSO is not the only factor that affects droughts in Zimbabwe, using this as one forecast predictor can help the city prepare and issue drought warnings with ample lead-time. However, reliance onjust this predictor can be dangerous, as was the case in 1997/1998. Substantial preparations for a drought were made that year with the prediction of a strong El Nino, yet conditions did not become dry as expected (Dilley, 2000).
