South Africa is likely to become a significantly drier country than it currently is, but with stronger wet weather events, reports Univer- sity of Pretoria’s Geography, Geoinformatics and Meteorology researcher and lead-author of a climate simulation of the Southern African region published in the International Journal of Climatology, Dr Francois Engelbrecht.
The study was sponsored by the water research and development organisation, the Water Research Commission, to determine the significant climatic changes Southern Africa may experience as a result of global climate change and the consequences this could have for South Africa’s water resources.
The study was undertaken because, although a great deal of research has been conducted into global climate change, very few studies have focused on the Southern African region, explains Engelbrecht.
His study places strong empha- sis on how rainfall and circula- tion patterns may change in Southern Africa owing to anthropogenic influence, or the influence of human activities on the environment.
The study examines the effect of increased greenhouse-gas (GHG) emissions on the region’s weather systems.
A climate model simulates climate patterns all over the world. For the purposes of configuring the model, the earth’s surface is divided into a grid and the climate model performs calcu- lations for each grid point. The calculations are carried out for different layers in the vertical, with layers spanning both the troposphere and the stratosphere.
Engelbrecht says that some of the climate change projections obtained from the simulation may already be observed in climate records of the last few decades.
South Africa’s Simulated Climatic Future
The study has simulated a future climate for South Africa that is generally drier than the present day climate.
Engelbrecht explains that in the current climate, most of the sun’s energy falls on the equator where the air rises to the top of the troposphere and moves towards the poles. However, the rotation of the earth means that the heated air coming from the equator does not reach the South Pole and falls back to the ground over South Africa, explaining why most of South Africa has dry winters. This is known as the Hadley Cell circulation.
The report suggests that Hadley Cell circulation is likely to intensify as more of the sun’s energy is trapped by GHGs (after being re-emitted in the form of long-wave radiation by objects in the land-ocean atmosphere system), which, in turn, leads to more air mass sinking over Southern Africa
It may be expected that increased Hadley Cell circulation would push rain systems further south. The model simu- lates a large reduction in rain-fall of up to 30% over the south-western Cape. The region could become drier as rain systems would increasingly follow more southerly tracks over the southern ocean.
However, during summer the strengthening of the high-pressure belt would favour the development of tropical cyclones that tend to make landfall over northern Mozambique from the Indian Ocean, making that region wetter.
The Simulation Model
Engelbrecht says that his study is one of only a few that have modelled the Southern African region’s climate at a high level of detail for two 30-year periods. He explains that performing climate simulations over periods of at least 30 years is necessary because the ocean atmospheric phenomena known as El Niño and La Niña, which are temperature fluctuations in surface waters of the tropical Pacific Ocean and which create dry and wet spells across the globe, can be statistically accounted for over a 30-year period.
Global climate simulations are modelled at a spatial resolution of roughly 3º. Engelbrecht says that this equates to grid points every 300 km on the earth’s surface, or a grid square of 90 000 km2.
He says his team’s research was performed at a higher spatial resolution of 0,5º , or a grid point every 50 km for a grid square of 2 500 km2, over a smaller region (Southern Africa) because this allows the model to simulate the climate for the region in much greater detail. He adds that computational restrictions prevent a simulation of the whole globe at such a high resolution.
Engelbrecht says that the study offers a realistic and reliable model of Southern Africa’s climate.
He says that before the model could be used to simulate future climatic conditions, its ability to simulate current conditions was thoroughly investigated.
The atmospheric gas compo- sition for the period from 1975 to 2005 was fed into the model and the resulting climate simula- tion was then compared with observations of actual climatic conditions during that time.
Once the model’s accuracy had been verified, the estimated atmospheric gas composition for the period from 2070 to 2100, which includes projected GHG emissions, was fed into the model and the model responded accor- dingly, absorbing more solar energy and, therefore, changing climate circulation patterns.
Engelbrecht says the earth’s atmosphere is a physical entity constrained by the laws of physics and, therefore, it cannot behave in a random way. “This model is based on basic physical principals and insights. This is why we have such confidence in it,” says Engelbrecht.
However, Engelbrecht adds that no model is a complete representation of the earth’s climate system in its entirety or complexity as there are many climate processes that are still not fully understood. Also, simulations always work with discreet points rather than with the whole earth, which introduces mathematical errors.
There is also uncertainty about the rate at which GHG emissions will increase in the future, as this depends on decisions humans will make over the next few decades.
Climate simulation does not necessarily require any climate observations, says Engelbrecht. Climate simulation models are based on the laws of physics, particularly the three laws governing the conservation of energy, momentum and mass. The only data required by the mathematical model is the composition of the atmosphere, the gases present in the atmosphere and their concentrations, as well as the amount of solar radiation reaching the atmosphere.
He says that even the solar energy reaching a grid point is determined by the model’s calcu- lations, using the orbit of the earth around sun, the tilt of the earth’s axis at that time of year, the latitude of the grid point and the time of day. The model is also provided with the distribution of the land masses, oceans, vegetation types and related processes on the earth’s surface that influence the atmosphere.
Engelbrecht adds that once the atmospheric compositions are set up, the model responds to the amount of radiation reaching the atmosphere and gradually the global climate patterns are simulated. A global ocean-atmos- phere model would typically attempt to simulate all processes occurring in the ocean-land- atmosphere system, such as the ocean and land surfaces that absorb solar energy and clouds and snow surfaces that reflect energy back into space.
He says the model followed the recognised Intergovernmental Panel on Climate Change’s ‘business as usual’ emissions scenario, in which nations continue to drive their economies and increase their gaseous emissions. Under such a scenario, GHG emissions are predicted to be double what their natural values would have been by 2050. Engelbrecht says that many climatologists believe the ‘business as usual’ scenario to be a very realistic future emissions projection.
Engelbrecht says he is working with a team from the Agri- cultural Research Council to conduct further analysis into how seasonal extreme weather events may change over Southern Africa. He says that a particular question relevant to the Southern African region in this regard is the frequency of tropical cyclones and whether they will become more intense.
He says that most climate change simulations indicate an increase in extreme weather events across the globe in the future as the atmosphere will have more energy to work with owing to an increase in solar energy being trapped by GHGs. This will build more intense weather systems, giving rise to a climate with more extreme wet weather events, such as storms, torrential rain and cyclones.
He says the model predicts an increase in flooding in northern Mozambique. Extreme wet weather events could be expected to become more frequent during spring along the south coast.
However, Engelbrecht says that, on average, the south coast will still be drier in winter, with respect to the current climate, even with increased extreme wet weather events.
He says this highlights an important aspect of climate change analysis, namely that the climate displays a high degree of natural variability – one gets dry and wet seasons – but the frequency and the characteristics of such events will change.
Engelbrecht adds that climate change over Southern Africa may have many effects, including implications for human comfort owing to increasing temperatures, and potential changes in the distribution patterns of numerous pests and diseases within the future climate.