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.