Hydrology and Climate Change Through the Holocene: Flooding

A changing hydrological regime is of chief importance to human societies. Unstable global hydrological cycles can influence food production, settlement location, disease transportation, ecosystem services, and, fundamentally, lifestyles. A shift in one direction of the continuum from arid lands to a moist environment is perhaps one of the most striking consequences of a changing climate. Throughout the Holocene we have seen hydrological changes across the globe. In low latitudes there was a tendency for low latitude regions to experience a drier system from the mid-Holocene onwards, with a weakening of summer monsoon systems (Oldfield, 2005). Of course, temperature plays a key role in regulating moisture regimes. There is significant evidence from a variety of sources including pollen and fossil-based evidence from lakes, faunal distribution, and archaeological records that these changes even influenced the Sahara desert (deMenocal andTierney, 2012). Check out these snap shots that summarise some of the evidence and changes observed in this region through the Holocene. The Sahara was ‘well vegetated with widespread freshwater lakes and abundant water-loving animals.... and extensively settled by human populations’:

At the present day environmental managers, government advisors and scientists need to understand exactly how river systems will respond to a changing climate. Many of the world’s major cities have been built around river systems that will be subject to change and could threaten normal activities associated in the region. For many people, the river is still one of the most important natural resources for harvesting food, transportation, water resources and many other things.

Whilst climate change may threaten shifts in the hydrological regime, morphological alterations are just as important. A changing climate will have implications for sedimentation, erosion and other river processes. Macklin and Lewin (2003) present an 11.5 ka yr record of river response to climate change for the UK. The results are based on radiocarbon dated alluvial units up to 2002, separated by alluvial environment – channel deposits, fills in abandoned channels, floodplain surfaces and flood basins. 14 major flood episodes spanning ~10 ka yr were identified:

Having identified these episodes, there were compared to proxy climate record from Britain, Europe and North America:

The authors state that ‘the emerging evidence for an increase in the frequency of severe floods in major world rivers during the twentieth century is therefore entirely consistent with the Holocene fluvial sedimentary record where river systems appear to reflect global climate changes. The unusually long and complete British record also demonstrates that changes in land cover, principally the conversion of woodland to agricultural grassland, have resulted in a step change in river basin sensitivity to variations in climate. Such sensitivity might otherwise be absent from humid temperate environments. This has very important implications for assessing and mitigating the impact of increasing severe flooding. In small and medium sized river basins land use is likely to play a key role in either moderating or amplifying the climatic signal. In Britain river basin managers and planners now have the advantage of hindsight provided by the Holocene alluvial record and can hopefully avoid mistakes made by their prehistoric farming ancestors.’