Extreme Weather #8 – Trends in Floods
In the last article we looked at trends in extreme rainfall. Now we’ll take a look at what the 6th Assessment Report (AR6), chapter 11, says on floods.
Here’s the plain English version:
The Special Report on Extremes in 2012 and the 5th Assessment Report in 2013 didn’t know whether floods were getting worse globally. There have been lots of studies since but they are still regional and sub-regional so it’s still not possible to measure whether floods are getting worse on a global level.
This isn’t bad news or good news, it’s uncertainty. But if you learnt about climate from the media this may be surprising.
The actual text, from p. 1568, is in the Notes at the end of this article.
For a bit of background, here is the plain English version of the Special Report on Extremes (2012) on floods that no one read:
We can’t find identify significant changes in floods in any region, but there’s lots of measurement uncertainty in many regions. As a result we don’t even know if floods are increasing or decreasing globally.
The actual text from p. 175-176 of SREX is also in the Notes. It’s worth a read.
Given that we lack measured changes in floods, there are two main lines of evidence on trends in floods:
One is from peak streamflow - think rivers and waterways overflowing
The other is not related to rivers, just too much rain hitting a plain or a city and no where for it to go
Here’s the actual text from the introductory section, p. 1567, which is reasonable clear:
Flooded area is difficult to measure or quantify and, for this reason, many of the existing studies on changes in floods focus on streamflow. Thus, this section assesses changes in flow as a proxy for river floods, in addition to some types of flash floods.
Pluvial and urban floods – types of flash floods resulting from the precipitation intensity exceeding the capacity of natural and artificial drainage systems – are directly linked to extreme precipitation.
Because of this link, changes in extreme precipitation are the main proxy for inferring changes in pluvial and urban floods (see also Section 12.4), assuming there is no additional change in the surface condition. Changes in these types of floods are not assessed in this section, but can be inferred from the assessment of changes in heavy precipitation in Section 11.4.
So we don’t have data on global floods so the best we can do is assume that increases in extreme rainfall are causing worse floods.
However, we have been measuring peak streamflow.
So, in the next article we’ll look at what the data says about peak streamflow (proxy for floods from rivers and waterways overflowing). It’s surprising.
Notes
Here is the text of IPCC AR6 (i.e. the recent report) from p. 1568:
The SREX (Seneviratne et al., 2012) assessed low confidence for observed changes in the magnitude or frequency of floods at the global scale. This assessment was confirmed by AR5 (Hartmann et al., 2013).
The SR1.5 (Hoegh-Guldberg et al., 2018) found increases in flood frequency and extreme streamflow in some regions, but decreases in other regions.
While the number of studies on flood trends has increased since AR5, and there were also new analyses after the release of SR1.5 (Berghuijs et al., 2017; Blöschl et al., 2019; Gudmundsson et al., 2019), hydrological literature on observed flood changes is heterogeneous, focusing at regional and sub- regional basin scales, making it difficult to synthesize at the global and sometimes regional scales.
The vast majority of studies focus on river floods using streamflow as a proxy, with limited attention to urban floods. Streamflow measurements are not evenly distributed over space, with gaps in spatial coverage, and their coverage in many regions of Africa, South America, and parts of Asia is poor (e.g., Do et al., 2017), leading to difficulties in detecting long-term changes in floods (Slater and Villarini, 2017).
Here is the text of IPCC SREX (2012) from p. 175-176:
Although changes in flood magnitude/frequency might be expected in regions where temperature change affects precipitation type (i.e., rain/ snow separation), snowmelt, or ice cover (in particular northern high- latitude and polar regions), widespread evidence of such climate-driven changes in floods is not available.
For example, there is no evidence of widespread common trends in the magnitude of floods based on the daily river discharge of 139 Russian gauge stations for the last few to several decades, though a significant shift in spring discharge to earlier dates has been found (Shiklomanov et al., 2007). Lindström and Bergström (2004) noted that it is difficult to conclude that flood levels are increasing from an analysis of runoff trends in Sweden for 1807 to 2002.
In the United States and Canada during the 20th century and in the early 21st century, there is no compelling evidence for climate-driven changes in the magnitude or frequency of floods (Lins and Slack, 1999; Douglas et al., 2000; McCabe and Wolock, 2002; Cunderlik and Ouarda, 2009; Villarini et al., 2009). There are relatively abundant studies on the changes and trends for rivers in Europe such as rivers in Germany and its neighboring regions (Mudelsee et al., 2003; Tu et al., 2005; Yiou et al., 2006; Petrow and Merz, 2009), in the Swiss Alps (Allamano et al., 2009), in France (Renard et al., 2008), in Spain (Benito et al., 2005), and in the United Kingdom (Robson et al., 1998; Hannaford and Marsh, 2008), but a continental-scale assessment of climate-driven changes in the flood magnitude and frequency for Europe is difficult to provide because geographically organized patterns are not seen in the reported changes..
[they continue with similar commentary on other regions]
..The above analysis indicates that research subsequent to the AR4 still does not show clear and widespread evidence of climate-driven observed changes in the magnitude or frequency of floods at the global level based on instrumental records, and there is thus low confidence regarding the magnitude and frequency and even the sign of these changes.
References
Seneviratne et al, 2021: Weather and Climate Extreme Events in a Changing Climate. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
IPCC, 2012: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change [Field, C.B., V. Barros, T.F. Stocker, D. Qin, D.J. Dokken, K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen, M. Tignor, and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, UK, and New York, NY, USA, 582 pp.