Tuesday, June 30, 2009

What Does Climate Change Mean for Prince Albert?

- Sue Milton-Dean and Richard Dean -

There is strong evidence that the global climate is growing warmer and changing in other ways. Emissions of carbon dioxide, from burning of fossil fuels such as petrol, diesel and coal, felling and burning of forests, and ploughing of large tracts of land, form a gas blanket that functions like a greenhouse, reducing heat loss at night. Glaciers and iceberg are melting and most northern hemisphere weather stations show a constant increase in mean daily temperature over the past century.

Predictions for South Africa, and in particularly the Western Cape, are that the temperature will increase, annual rainfall decrease, rain seasonality in the west switch from winter to summer, and that extreme events – storms, droughts, floods – will become more frequent. On the basis of rainfall data for the past 12 decades, is there any evidence that these predictions are being realised here in Prince Albert – and if so what are the implications for farming, tourism and municipal budgets?

Arid areas throughout the world have boom or bust weather patterns. Weather data averaged over a month or a year may tell a very different story from daily or hourly patterns. This idea was well captured in an article about Australian deserts entitled “Where creeks run dry or ten foot high”. Similarly, the average daily flow of the Sandrivier is a meaningless statistic because this river may be in spate for a few hours annually but dry for the rest of year. So we will discuss annual and monthly rainfall patterns as well as individual rainfall events.

Annual rainfall data from the South African Weather Service shows no evidence for a drying climate in Prince Albert. Instead there may be a long-term cycle of wet and dry periods. The 40 year period 1878-1907 had above-average rainfall and was followed by 40 dry years (1928-1967) and then a wetter period from 1978-2007. The numbers of years within a 20-years period with exceptionally high (>300 mm) or low (<100 mm) total annual rainfall are indicated above and below the graph. The past 20 years have had more highs than lows.

Prince Albert does not have strongly seasonal rainfall and this appears to have been the case for a long time. In every 20 year period for the past 108 years, autumn was the season with the highest rainfall, followed by summer. We expect a lower mean monthly rainfall in spring and winter. So we must accept that neither total annual rainfall nor rainfall seasonality have changed over the past century.

But what about extreme events? “Extreme” is difficult to define, but only 10 months of the past 1440 months have received more than 100 mm of rain. As Table 1 shows, five of these very wet months were in the past 28 years. Rainfall runs rapidly off land in semi-desert areas such as this because there is little vegetation cover to slow it down. Large quantities of rainfall, while welcome, can also be damaging.

Runoff water carrying rocks and branches increases in forces and momentum, as small streams join larger ones. Downpours can cause spectacular and expensive damage, particularly to human built infrastructure. Among the most memorable are the wash away of at least six bridges in the Meiringspoort in 1996, and the bridge over the Sandrivier in 1998.

A few days of heavy rain on the Swartberg (1-3 August 2006) washed away the bridge over the Dorpsrivier, and a single cloudburst (60 mm) on 5 March 2009 caused almost irreparable damage to the 110 year old dry stone walls of the Swartberg Pass as well as flooding houses and dumping mud and rubble in the streets of the village. Damage was particularly severe in parts of North End where little provision had been made to channel water off the roads.
An increase in cloudbursts, hail, or drought that damage infrastructure, houses and crops will increase costs to the municipality and divisional council, pushing up rates and transport costs, while decreasing farm and tourism revenue.

Preparedness is the best way to deal with unpredictability, so it is very reassuring to see drainage culverts being built along the main road from Prince Albert to the N1. Special care will be needed to manage increased runoff water on the Swartberg Pass to minimise damage to the road, walls and downhill vegetation. Training in the rare skills of arts of cambering and stone-masonry will be needed to conserve this heritage asset for use by locals and tourists.

Additional precautions could include removal of alien trees such as gums (Eucalyptus), poplar (Populus) and salt cedar (Tamarix) from drainage lines to prevent uprooted trees damming up water and damaging bridges, and relocation or protection of the garbage dump to prevent floods washing toxic materials and litter into the Dorpsriver.

In conclusion, while there is little evidence that Prince Albert is becoming more arid or that rainfall seasonality is changing, there is some support for the idea that global climate changes are increasing the severity of rain storms. With good planning, an increase in damage to roads, bridges, the Pass and other infrastructure can be minimised.

Sue Milton-Dean and Richard Dean of Renu-Karoo Veld Restoration cc made this presentation on climate change on World Environment Day, 5 June, at the Sydwell Williams Centre.

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