 |
| Credits: Getty images |
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:
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 (10th to 12th
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 20th 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 20th 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.
