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1072 results in Geography: general interest

Page 13 of 54

  • 4 - Is bush encroachment caused by global change?

  • from Section 2 - Consequences of a changing climate for the Southern African environment
  • Bob Scholes, Mary Scholes, Mike Lucas
  • Book:
    Climate Change
    Published by:
    Wits University Press
    Published online:
    20 April 2018
    Print publication:
    31 December 2015, pp 63-65

  • Summary

    Rising carbon dioxide (CO2) concentration in the atmosphere probably contributes to the observed increase in woody cover in savannas and the invasion of trees into grasslands, but it is unlikely to be the sole cause. Historical and current changes in grazing by wildlife and domestic livestock have also played a role in bush encroachment, as have changes in the fire regime.

    When European settlers began raising cattle in South Africa around 100 years ago in open grassy landscapes with few trees, they found that the density of woody plants suddenly increased. Historical photographs show that areas now covered in bush were once grassland areas largely devoid of trees. This phenomenon, called bush encroachment in South Africa, has also been observed in Australia, Texas, Argentina and many other places. It is a big problem for cattle farmers because trees compete with grass, reducing the feed available for cattle and other grazers. Up to a fifth of the land in South Africa (12–20 million hectares) is thought to be affected by bush encroachment. Bush encroachment has historically been blamed on sustained heavy grazing in fenced-offareas, which disadvantages grass relative to trees. Sharp contrasts, easily observable across fence lines, show that some farms (or camps within farms) are more affected than others. Therefore, the direct cause of bush encroachment must be related to land management rather than climate or atmosphere, which pay no heed to fences. However, it is likely that atmospheric and climatic change are contributing or predisposing factors to bush encroachment.

    Periodic natural fires used to keep the trees in check. When most of the grass, the main fuel for fire, is eaten by herbivores, hot, frequent fires are not possible. The formerly suppressed tree seedlings are then able to shoot up, out of the range of grazers and flames, and come to dominate the grasses. This further suppresses grass growth and the ecosystem becomes stuck in a bush-encroached state. Ranchers frequently extinguish natural fires to preserve their grazing, and have added to the problem by hunting out browsers such as kudu, giraffe and elephants that formerly ate the trees. It is speculated that, in the pre-colonial era, indigenous grazers and the cattle maintained by pastoralists did not lead to bush encroachment because the herds were migratory and fires were not controlled.

  • 8 - How do climate and air pollution interact to affect human health?

  • from Section 3 - Consequences of a changing climate for society
  • Bob Scholes, Mary Scholes, Mike Lucas
  • Book:
    Climate Change
    Published by:
    Wits University Press
    Published online:
    20 April 2018
    Print publication:
    31 December 2015, pp 131-132

  • Summary

    Some of the most dangerous pollutants are also greenhouse gases. Reducing air pollution has health benefits and also reduces climate change and vice versa.

    Direct effects of climate change on human health include cardiovascular disease resulting from increasing heat stress, and the increased spread of some insect-transmitted and water-borne diseases. Changes in atmospheric composition, partly linked to climate change, alter the duration and intensity of ultraviolet (UV) radiation, increasing the risk of certain types of skin cancers, including dangerous melanomas. South Africa has high ultraviolet levels and incidences of skin cancer, partly because much of the country is above 1 000 m in altitude and the thinner atmosphere at those levels provides less protection.

    Weather and climate play an important role in air quality. Reducing the emissions of long-lived greenhouse gases such as carbon dioxide (CO2) also generally leads to lower pollutant emissions of all sorts. The direct impact of low air quality on human health mostly involves respiratory allergies and diseases. Certain types of air pollution get worse under hotter, more humid conditions. Indirect but interactive effects of climate change and air pollution include increased human exposure to pollen (due to altered growing seasons), moulds (from extreme or more frequent precipitation), dust (from land laid bare by droughts) and smoke (from veld burning). On the positive side, action taken to reduce climate change by restricting emissions from vehicles and from fossil-fuel-burning power stations has the side benefit of reducing sulphur and nitrogen oxide emissions, which are harmful to humans and ecosystems.

    Two of the biggest air quality problems in South Africa are the impacts of ozone (O3) and particulate matter (PM). Ozone near Earth's surface (tropospheric ozone, not to be confused with high-altitude stratospheric ozone which acts as an ultraviolet shield) is a powerful but short-lived greenhouse gas and an aggressive pollutant because it is highly reactive with other substances. It forms from the interaction of other pollutants, articularly nitrogen oxides and volatile organic compounds, in the presence of sunlight. Since tropospheric ozone does not last very long in the atmosphere, high concentrations of ozone tend to be quite local.

    Tropospheric ozone usually coexists with other hazardous pollutants such as carbon monoxide, sulphur dioxide and PM of different sizes.

  • 3 - Is it possible to take carbon dioxide out of the atmosphere?

  • from Section 4 - What we can do to avoid and adapt to climate change
  • Bob Scholes, Mary Scholes, Mike Lucas
  • Book:
    Climate Change
    Published by:
    Wits University Press
    Published online:
    20 April 2018
    Print publication:
    31 December 2015, pp 147-149

  • Summary

    Yes it is, but it is slow, expensive, unproven at large scale and in most cases risky. It is better not to add carbon dioxide (CO2) to the atmosphere in the first place.

    Carbon dioxide removal is one of two broad possible approaches to geo-engineering, that is, deliberate, large-scale interventions to cool Earth independently of reducing greenhouse gas emissions. The other main geo-engineering approach is to reduce solar radiation by reflecting it back into space (see Sec. 4, Q5). Both approaches remain speculative, and neither their effectiveness nor their risks have been seriously demonstrated at large scales. Most experts regard them as last-chance options in a climate emergency that cannot otherwise be solved, or where the political will to reduce emissions is found wanting or insufficient.

    Carbon dioxide removal directly tackles one of the main causes of climate change – CO2 accumulation in the atmosphere – but, if implemented, it will take decades or even centuries to be effective. One of the simplest CO2 removal approaches is to plant trees and then ensure that the carbon stored in the wood does not find its way back into the atmosphere. To make an appreciable dent in the excess atmospheric CO2, afforestation would have to be at such a massive scale that food production, biodiversity and global water supplies would be adversely affected (see Sec. 4, Q10). Even if all former forests were restored, global CO2 concentrations would only fall by about 50 parts per million (ppm). Relative to the CO2 increase of about 120 ppm between the 1750s and 2014, this would help, but not nearly enough to solve the problem of global warming.

    About a quarter of all CO2 emissions to the atmosphere are currently removed by the global oceans – but could human intervention encourage the oceans to do more? Phytoplankton production in some oceans is limited by insufficient nutrients. For example, large areas of the Southern Ocean are well supplied with most nutrients, but are short of iron. There and elsewhere, experimental additions of relatively small doses of dissolved iron to the ocean surface have increased phytoplankton growth, which takes up more CO2 due to increased photosynthesis. This process removes CO2 from the atmosphere, although only temporarily. When phytoplankton cells die, they sink and sequester CO2 in the deep ocean (below 1 000 m) for hundreds of years or more.

  • 9 - Do biofuels offer a solution?

  • from Section 4 - What we can do to avoid and adapt to climate change
  • Bob Scholes, Mary Scholes, Mike Lucas
  • Book:
    Climate Change
    Published by:
    Wits University Press
    Published online:
    20 April 2018
    Print publication:
    31 December 2015, pp 162-164

  • Summary

    Currently biofuels hardly make a difference in the big picture, but they may fill an essential niche in future, for instance as aviation fuels. Care will need to be taken to avoid competition for land and water between biofuels and food production.

    Coal, oil and gas started out as plant biomass millions of years ago. Plant biomass is currently the biggest energy source in Africa. Even in coal- and electricity-rich South Africa, many families still burn wood and charcoal. Provided the biomass is sustainably harvested, burning biomass is approximately carbon neutral – in other words, the amount of carbon dioxide (CO2) generated by the fire is the same as the amount originally removed from the atmosphere when the plant grew. Approximately the same amount of CO2 will be removed from the atmosphere when replacement plants grow.

    Modern biomass energy technologies are much more thermally efficient than traditional wood-burning fireplaces, stoves or charcoal kilns. Wood, grass and forestry or agricultural waste can be burned in carefully controlled furnaces, producing energy at a cost quite similar to coal, but without most of the pollution. The problem is that biomass energy is generally spread out all over the landscape, rather than concentrated in a mine, like fossil fuels. The costs of moving it to the power plant can be high. Therefore, efficient use of biomass energy requires a shift from a few enormous power stations to many small power stations.

    One of the most intractable future renewable energy challenges is to provide liquid fuels for ships, trains, vehicles and aeroplanes. Some of these modes of transport can use electricity directly from the grid or stored in batteries. Yet despite great advances in battery technology, their ‘energy density’ (that is, the energy stored per unit mass of the battery) remains too large to be a viable power source for aircraft. Liquid hydrocarbon fuels have a much higher energy density than current batteries. Biomass can be converted to liquid fuel, which can substitute for petrol, diesel and aviation fuel. Biofuels can also be blended with liquid fossil fuels, reducing the overall total carbon footprint. There are several paths to make liquid fuels from biomass:

  • • Oil can be extracted from many species and converted to a diesel fuel suitable for trucks, trains and jets.

  • • Sugars and starches can be fermented to form ethanol, which can be burned in many internal combustion engines. […]

Page 13 of 54