Coastal Floods and Climate Change

In the previous post, we looked at the impacts of climate change on river floods. So in this post, we will turn our attention towards another category of floods – coastal floods. Across the globe, numerous coastal cities and towns are susceptible to such floods that create massive destruction and havoc each time they hit. For example, when Hurrican Sandy made landfall last year, large portions of Lower Manhattan was flooded with seawater as a record storm surge of 4.15m swept into New York City. A year after the floods, New York City is still trying to rebuild homes, buildings and transport systems that have been damaged, while many business and building owners are still battling to make insurance claims or gain compensation from the government. Meanwhile, other low-lying regions like the Maldives and Ganges Delta are under the threat of being inundated as sea levels continue to rise in the 21^st century.

Seaside Heights, New Jersey flooded after Hurricane Sandy made landfall. Source: AP

Although these coastal cities and towns are vulnerable to these devastating floods, they are ironically home to large concentrations of population. 13 out of 20 of the most populated cities in the world are coastal cities, including Shanghai, Guangzhou and Jakarta (Hanson et al. 2011). Moreover, many of these cities are also major national and regional economic hubs as they house key ports responsible for handling a large proportion of global seaborne trade. Therefore, the impacts of climate change on the risk coastal flooding have major social and economic consequences on these societies.

Sea level rise

One of the main causes of coastal inundation is the rise in sea levels. Sea level rise is caused by the combined effects of (1) the thermal expansion of seawater due to ocean warming; and (2) input of water from land ice melt and land water reservoirs.

From 1880 to 2009, it is estimated that sea level rise is about 210mm (Church and White 2011). Tide gauge measurements since the 19^th century has shown that sea level rose by an average of 1.7 ± 0.3mm/year since the 1950s. As seen from Fig.1, however, from the early 1990s onwards, the mean rate of sea level rise increased to 3.3 ± 0.4mm/year (Nicholls and Cazenave 2010). Evidently, sea level rise has accelerated in recent years. Moreover, it is critical to note that since the early 1990s, global average sea level rose at a rate near the upper end of the sea level projections for both the Intergovernmental Panel on Climate Change’s 3^rd and 4^th Assessment Reports (Church and White 2011).

Fig.1 Global mean sea level trend from the late 18th to early 21st century. The red curve is based on the tide gauge measurements while the black curve is based on the high-precision altimetry record from 1993 to 2009. As can be seen, there is a close match between tide gauge and altimetry records during that period suggesting that tide gauge records are reliable. Source: Nicholls and Cazenave (2010). 

However, it is important to note that sea level is not rising uniformly across the world as seen in Fig.2. Local factors can either amplify or increase the effects of eustatic sea level rise. These factors include the isostatic rebound of the Earth’s crustal due to the unloading of ice (as evident over Scotland), the subsidence of land due to the withdrawal of groundwater as well as the subsidence of deltaic regions due to the shortage of sediment supply caused by upstream damming. Nonetheless, based on Fig.1, it still seems that there is still a larger proportion of regions that have experienced sea level rise between 1992 and 2009, compared to regions that have experienced a decline in sea level for that period.

Fig.2 Regional sea level trends based on satellite altimetry records from 1992 to 2009. Source: Nicholls and Cazenave (2010).

Modelling studies carried out by Jevrejeva et al. (2012) have shown that sea level rise ranges from 0.57 to 1.10m by 2100, with the maximum rate of sea level rise reaching 17mm/year by 2100 under the RCP8.5 pathway (Fig.3). Moreover, rising sea levels will also elevate storm surges brought about by cyclonic activity, which is another cause of coastal floods (Brecht et al. 2012). Therefore, the impacts of climate change on the risk of coastal floods cannot be underestimated.

Fig.3 Sea level projections till 2100 with RCP scenarios: red - RCP3PD, blue - RCP4.5, green - RCP6.0 and black - RCP8.5. The shaded regions of similar color around the projections represent the upper (95%) and lower (5%) confidence levels. Source: Jevrejeva et al. 2012.  

Coastal flood risks

Two recent reports have attempted to quantify the losses associated with the exposure of people and assets to coastal flood risks under future sea level rise, socio-economic changes and subsidence of land. 

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Population exposure

Currently, the total number of people currently exposed to coastal flood risks is about 38.5 million, which is about 0.6% of the global population. Hanson et al. (2011) estimates that this figure will increase by more than threefold to around 150 million people for the period 2070-2080 due to population growth, sea level rise and subsidence of the coastal areas. Asia will be most vulnerable to coastal floods due to the high population density along the coast. The Asian cities that are within the top 20 cities with the largest population exposure to coastal flood risks in the period 2070-2080 include Mumbia, Guangzhou, Shanghai, Ho Ching Minh City, Bangkok and Tokyo (Fig.4). 

Fig.4 Map showing the top 20 cities for exposed populations under the FAC scenario for the period 2070-2080. The FAC scenario takes into account population and economic growth, natural subsidence/uplift, global sea level rise and potential human-induced subsidence. As can be seen, there is a disproportional number of Asian cities within the top 20 cities. Source: Hanson et al. 2011.  

Assets exposure

A recent report by Hallegatte et al. (2013) assessed the average annual flood losses in 136 coastal port cities and estimated that at present, the aggregated average annual flood losses (AAL) in these cities is about US$6 billion per year. Guangzhou tops the list with an AAL of US$687million, which is about 1.32% of the city’s GDP (Fig.5). In fact, most of the losses are concentrated within a small number of cities, with Guangzhou, Miami, New York and New Orleans accounting for 43% of the global losses.

Fig.5 The top 20 cities where the economic average annual losses with respect to local GDP are the largest in 2005. Source: Hallegatte et al. (2013).

The report suggested that future losses associated with coastal floods will increase dramatically under sea level rise, socio-economic changes and subsidence of land. They projected that if no adaptations were made to maintain or reduce the flood risks, the projected increase in average losses will rise to more than US$1 trillion per year. On the other hand, if adaptations were carried out, the losses will be lower, reaching between US$60 and 63 billion per year.

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Given that coastal flood risks will inevitably increase in future due to the combined effects of sea level rise, population growth in coastal areas as well other isostatic effects such as subsidence of land, coastal cities must attempt to adapt to the increase in coastal flood risks. Failure to adapt will result in massive losses both in terms of deaths and economic losses. Moreover these studies have not factored in other causes of coastal flooding including storm surges and tsunamis. Hence, future coastal risks might be even larger than what has been projected. However, Hallegatte et al. (2013) highlighted a grim reality that even though improvements in flood defence infrastructure can help to reduce the risk levels and reduce the number of floods, but when an extremely large flood overwhelms these infrastructure, the resultant losses will still increase. Therefore, there is a limit to these flood defence infrastructure and effort must be channelled to other forms of adaptations as well. This includes effective land use planning, early warning systems, flood evacuation plans and post-disaster relief response, which would help communities be more resilient towards coastal floods. 

Floods and Climate Change

Credits: Getty images

Now that we have spent quite a fair bit of time discussing the links between climate change and hurricanes, let’s focus the spotlight on floods! A point to note is that there are many types of floods, including riverine floods (overtopping of river banks), coastal floods (usually caused by rising sea levels and storm surges), urban floods (could be caused by overflowing of storm drainage systems), etc. In order for the discussions to be more focused, we shall just look at rain-fed river floods in this post and coastal floods in the following.

First, let’s start off with the Disaster Bites for the Colorado Floods 2013: 

Disaster Bites: Colorado Floods 2013

Video credits: ABC News

• Began September 9 2013
• Boulder county, Colorado was the worst hit out of the 14 counties affected by the floods
• At least 8 lives were lost and thousands of homes destroyed
• More than half a year’s worth of rain fell within three days
• 1 in 1000 year storm event, 1 in 100 year flood event
• High rainfall due to an active southwest monsoon and a presistent broad area of low pressure at upper levels of the atmosphere. This low-pressure area helped pull the moisture out of the tropics and into Colorado.
• Moisture was forced up the Rocky Mountains by the southwesterly winds to form orographic rain.
• Floods were exacerbated by the long-term drought in the Colorado River basin, which hardened the soil and reduced the infiltration capacity of the ground. 

More information:

http://www.huffingtonpost.com/2013/09/14/colorado-flooding-climate-change_n_3926284.html

http://news.nationalgeographic.co.uk/news/2013/09/130913-colorado-flood-boulder-climate-change-drought-fires/

http://www.cnbc.com/id/101039317

As seen above, one of the primary causes of the Colorado floods was the massive amount of rainfall that hit the region within a short period of time. Boulder County’s total 3-day (10^th to 12^th September) rainfall was 12.30 inches (312.42mm). This has far exceeded the highest recorded rainfall in the city for any month since records started in 1897. The previous record rainfall was 9.59 inches (243.59mm) back in May 1995. Could this be part of a trend of more frequent intense precipitation events associated with climate change, which are causing more frequent extreme floods?

Theory

In theory, climate warming will result in the intensification of the hydrological cycle according to the Clausius-Clapeyron relation that suggests that the atmosphere’s water holding capacity increases with temperature (Fig.1). Hence, as the air gets warmer, the increased moisture in the atmosphere will favour heavier precipitation events. Modelling studies such as those done by Stephen and Ellis (2008) suggest that precipitation would increase by 1-3%/K. Given that precipitation is one of the key drivers of river floods, an increase in precipitation, ceteris paribus, would lead to increased likelihood of such floods (Kundzewicz et al. 2010).

Fig.1 As temperature increases, the vapor pressure increases exponentially. Source: Ohlone College

Observed Trends

Temperature and Precipitation

Hansen et al. (2012) has shown that the distribution of seasonal mean temperature anomalies has shifted towards higher temperatures, especially in summer, likening it to the ‘loading of the climate dice’. They suggest that the chances of unusually warm seasons have greatly increased in the past 30 years. According to the Clausius-Clapeyron relation, the warmer temperatures ought to result in heavier precipitation. Indeed, the IPCC AR4 Report (Trenberth et al. 2007) highlighted that there had been increases in the frequency of heavy precipitation events over the second half of the 20^th century over many land areas, particularly in many regions of North America.

Runoff and floods

There have been a number of flood events in recent years where the river flow records have been unprecedented. Kundzewicz et al. (2010) highlighted several examples including the 2002 flood in Central and Eastern Europe where the Vltava River exceeded a flow rate of 5000m³/s for the first time in the last 175 years; in fact the flow rate has never reached 2500m³/s in the 60 years. However, global analyses of runoff trends for the 20^th century (Bates et al. 2008) have concluded that there is great variation in annual runoff over different regions, with the high latitudes and large parts of the USA experiencing an increase in runoff and southern Europe, West Africa and southernmost South America experiencing a decline in runoff. The same report pointed out that observed changes in runoff might not be consistent with changes in precipitation due to the competing effects of evaporation, effect of human interventions such as dam construction as well as poor data quality for some rivers.

Interestingly though, two recent reports have attempted to draw the links between anthropogenic greenhouse gas emissions and the hydrological cycle. First, modelling results by Min et al. (2011) suggest that anthropogenic greenhouse gas emissions have contributed to the observed intensification of heavy precipitation events over approximately two-thirds of the Northern Hemisphere land area. Second, Pall et al. (2011) concluded that their modelling studies show that anthropogenic greenhouse gas emissions had increased the risk of occurrence of floods in England and Wales in autumn 2000. These studies are part of a growing number of studies, known as attribution science, that are attempting to attribute specific climate and weather phenomenon to anthropogenic climate change.

Projections

With the projected changes in temperature and precipitation under climate change, it is expected that river discharge and flood risk would likely change as well. Modelling done by Hibarayashi et al. (2013) using 11 AOGCMs participating in the CMIP5 suggest that for the projected period of 2071-2100, flood frequency increases across large areas of South and Southeast Asia, Northeast Eurasia, eastern and low-latitude Africa and South America. Meanwhile, areas like northern and eastern Europe, Central Asia, central North America and southern South America see a decrease in flood frequency (Fig.2).

Fig.2 Projected return period of the 1971-2000 100-year flood projected onto the 2071-2100 period for 29 selected river basins under RCP 8.5. A) Basin map of 29 selected rivers. The color of each basin represents the multimodel median return period at basin outlets. B) The height of the grey box indicates the interquartile range (75th-25th percentile) and the solid line represents the median value. The dashed line represents the maximum and minimum return periods. Directions of change in return period and model consistency are indicated. Source: Hibarayashi et al. 2013

Nonetheless, floods are complex phenomena and we need to consider other factors that may amplify or diminish flood risks including changes in land cover such as deforestation as well as the alteration of flow regimes by activities including reservoir impoundment. Moreover, although not covered in this discussion, changes in atmospheric circulation associated with climate change such as ENSO may also have implications on flood frequency and extent. Hence, a flat-rate statement on the change in flood risk in future is hard to be made. However, this does not mean that land use planners can put off thinking of ways to minimise the exposure of people and assets to flood impacts until studies produce more concrete results, for we never know when another 1 in 1000 storm event or 1 in 100 flood event might hit us again. 

Disaster Bites: Super Typhoon Haiyan

Just as we wrap up on our discussion of hurricanes, South East Asia had just been recently struck by the massive typhoon Haiyan. My deepest condolences go out to those who have been affected by this monstrous storm.

Children holding signs asking for help along the highway. Credits: Reuters

• Category 5 typhoon
•  Made landfall on November 7 2013 in the Philippines at Guiuan, Eastern Samar province.
• Brought about sustained winds of 235km/h, with one minute sustained winds of 315km/h. This makes it the strongest typhoon to ever make landfall.
• Storm surge reached 15m; rainfall hit 400mm
• President Aquino estimated that the death toll is around 2500 although the previous estimate was at 10000.
• About 600000 were made homeless by the disaster.
•  Initial estimates of economic losses amount to $15 billion.
• Looting has become a problem amongst the chaos as well.
• Rescue efforts have been hampered by the damage to roads and airports.
• Struck northern Vietnam on November 10 as a severe tropical storm and subsequently hit southern China as well.

Haiyan's track. Credits: CNN

Typhoon Haiyan has been one of the key issues of discussion at the Warsaw Climate Change Conference 2013 that is occurring over these couple of weeks and Yeb Sano, the head of the government’s delegation from the Philippines, made an emotional call for action to resolve the climate talks deadlock. You can catch his speech here:

More information:

http://www.bbc.co.uk/news/world-asia-24863480

http://www.bbc.co.uk/news/world-asia-24894529

http://www.economist.com/news/asia/21589916-one-strongest-storms-ever-recorded-has-devastated-parts-philippines-and-relief

http://www.theguardian.com/environment/2013/nov/11/typhoon-haiyan-philippines-climate-talks

2013: The year of low Atlantic hurricane activity

Despite the doom and gloom, of increasing global tropical cyclone frequency and intensity as the climate warms, predicted by scientists like Kerry Emanuel (as mentioned in the previous post), I recently came across a few other pieces of information that would add a new dimension to the current debate.

2013 Atlantic Hurricane Season

2013 Atlantic hurricane season forecast. Credits: AccuWeather.com

At the start of the year, experts from NOAA predicted that 2013 would likely be an active or extremely active year. They stated that there are 3 climate factors that favour the development of an active Atlantic hurricane season this year:

1. Continuation of the atmospheric climate pattern since 1995 that includes a strong west African monsoon
2. Warm sea surface temperatures (SSTs) over the tropical Atlantic Ocean and Caribbean Sea
3. Absence of El Nino that would otherwise suppress hurricane formation

Yet, thus far, there have only been a total of 12 named storms, of which only 2 are hurricanes (both are short-lived Cat 1 hurricanes i.e. no major hurricanes this season so far). The first hurricane of the season, Hurricane Humberto, came only after the mid-season point on September 11. The Accumulated Cyclone Energy (ACE) developed by NOAA is calculated as the square of the wind speed every 6 hours for every named storm with at least 40mph sustained winds. It is a measure of both tropical cyclone activity and of potential damage. So far, the ACE is 28.55, which is way below the 1981-2010 average of 104 units.

So what happened to the Atlantic hurricane season this year? As highlighted in a post on ClimateCentral.org, researchers have posited that there are a few factors that have led to the low hurricane activity despite the warmer-than-average SSTs over the main developing region and lack of El Nino. Firstly, Brian McNoldy from the University of Miami suggested that the air has been anomalously dry at the lower and higher altitudes and is coupled with large-scale subsidence. Both factors suppress the growth of thunderstorms, which is needed for hurricane development. Secondly, frequent plumes of dry, dusty air coming from the Sahara Desert has also contributed to the dry and sinking air over the Atlantic. Thirdly, Landsea and Klotzbach suggested that there has been a higher-than-average wind shear over the Atlantic that is hindering the intensification of the storms. These factors were missed out from the initial calculations during hurricane forecasting.

Nonetheless, given that there are still a few more days to the end of the Atlantic hurricane season, anything could still happen.

Bigger Picture

The low hurricane activity this year is not restricted to the Atlantic only. In fact, all the ocean basins, other than the North Indian basin, are recording below average ACE values this year. This year also marks the 8^th year since the last major hurricane of Cat 3 strength or greater made landfall in the USA. Although Sandy peaked at Cat 3, it was only a Cat 1 hurricane by the time it made landfall. Moreover, a recent report stated that based on results from climate model simulations, there is a decreasing probability of a steering flow oriented towards the US eastern coast in future under the RCP8.5 pathway (Barnes et al. 2013). In addition, other than the Atlantic Ocean that have been experiencing enhanced hurricane activity associated with the warm phase of the Atlantic Multidecadal Oscillation (AMO) since 1995, the other basins have actually recorded decreasing ACE since 2006 due to the evolution of tropical and North Pacific inter annual and interdedacal climate modes such as ENSO and the Pacific Decadal Oscillation (PDO) (Maue 2011). 

Nonetheless, there is a worry that the lack of major hurricane activity might lead to a sense of complacency among those living in Hurricane Alley. Since Hurricane Wilma in 2005, Florida has not ben hit by a major hurricane. Consequently, the sunshine state has seen a massive influx of people in recent years, many of whom have not had a first-hand encounter with hurricanes. This actually increases the vulnerability of the state to hurricanes and it only takes one hurricane that makes landfall to bring about major devastation to its people.

What all these pieces of information suggest is that the debate may not be as clear-cut as it seems after all. There are still so many factors that can contribute to hurricane and cyclone activity, many of which are still not well understood by scientists.