Three professors from Wits University predict difficult times ahead for the country in their new book Climate Change: Briefings from South Africa. In these two extracts, Robert Scholes, Mary Scholes and Mike Lucas ask how hot it will get and when we will run out of water.

How hot will it get in South Africa?

A warming trend is already apparent, and it is much higher than the global average rate. Temperatures in the interior of the country could rise by about 3°C by the end of the century if the world greatly and urgently reduces its greenhouse gas emissions, but by up to 6°C if it does not.

The global average air temperature measured near the surface in 2010 has risen by 0.8°C since 1870, when accurate records began and, measured over multi-decade periods, the rate of warming has been accelerating. The rise in air temperature has been unsteady: there is a general upward trend interspersed by some long periods of no change, or even cooling. For instance, in the decade after 2000 there was little overall rise, just as there was little rise in the period 1945 to 1968, but in between were periods of rapid rise.

The annual average air temperature in South Africa has risen by around 1.2°C over the period of accurate records. In the medium term, global warming in the northern hemisphere will generally exceed that in the southern hemisphere because oceans, which dominate the southern hemisphere, warm more slowly than the land.

Despite this, the rate of warming in South Africa is nearly twice the average rate recorded worldwide so far. This is partly because inland regions of South Africa are distant from cooling oceanic influences. It is also because much of South Africa falls within a dry belt. Projections of future warming in southern Africa are a further 3°C to 6°C within the 21st century, but perhaps more later if atmospheric greenhouse gas concentrations remain high. Greatest warming is projected for the western interior from the Northern Cape to southern Angola, particularly in the Kalahari, where temperatures could rise by 5°C to 6°C. Coastal areas will eventually warm by 3°C to 5°C.

A global mean temperature rise of 3°C would be highly damaging (reaching 6°C for parts of South Africa), but probably within the bounds of adaptation. Above this global mean temperature rise, there are serious questions regarding our ability to cope.

Life on Earth has experienced hotter temperatures in the distant past, and will survive in some form, but complex human societies have never faced a climate challenge of that magnitude.

Will South Africa run out of water?

In South Africa, the water that people use for drinking, agriculture and industry ultimately comes from rivers and underground aquifers. How much is available there depends not only on the amount of rainfall, but also on what fraction evaporates and runs off the soil surface.

To provide the same water supply, places that are sunny, hot, dry and windy need more rainfall than places that are cloudy, cool, humid and calm. Evaporation rates throughout southern Africa are projected to increase over the next century as the land warms due to climate change. Increased evaporation results in increased cloud formation and subsequent rainfall – but that rain may not occur where the evaporation occurred. So a modest increase in rainfall could be completely offset by a larger accompanying increase in losses due to evaporation.

Another complicating factor is the effect of future higher atmospheric carbon dioxide concentrations on the amount of water used by plants, which in turn affects the amount of water that reaches the rivers. Slow-growing and seasonal plants use much less water through the year than fast-growing and evergreen plants do. Since many invasive species such as wattles, pines and blue gums fall in the latter category, water flow from catchments dominated by thirsty alien invasive species is substantially reduced, even to the point of rivers drying up.

Water security in relatively dry catchments that become yet drier will decline sharply. For example, in the Cape metropolitan region, water supply is likely to decrease by about 0.3% a year up to 2020, but climate change is predicted to increase demand by about 0.6% a year – leaving a regional shortfall.

Various rainfall projections agree that storm intensity – the average amount of rain falling per hour – is likely to increase. More intense storms allow less time for water to be absorbed into the soil, therefore generating a rush of overland flow that causes erosion.

Stormflow is also hard to capture effectively in dams. Less intense, gentler rainfall that soaks the soil and is gradually released as a steady flow of clean water into the river system is preferable to intense rainfall events.

South Africa’s allocatable water … is already almost fully claimed by agriculture, industry and domestic use, even before considering the effects of climate change.

In the future, every water user taking a bigger share will cut into the allocation of other users: a declining overall supply means everyone gets less.

This extract originally appeared in City Press…