THE GREENHOUSE EFFECT:
At the beginning of the 21st century, terms such as the ‘greenhouse effect’, ‘greenhouse gases’ and ‘greenhouse warming’ are printed or spoken thousands of times a week in the context of climate change caused by human activities. It is necessary to learn the basic science behind these terms, and then to review what is known about the human impact on the composition of the atmosphere since the dawn of the industrial age, commonly put (in this context) at around AD 1750.
WHAT DETERMINES THE EARTH’S GLOBAL MEAN SURFACE TEMPERATURE (GMST)?
The Sun is the ultimate source of energy for the Earth’s climate. A planet such as the Earth will have a stable temperature if there is a balance between the rate at which energy comes in from the Sun and the rate at which it is returned to space by the planet. If the two rates fail to match, the planet will either warm-up or cool down until a balance is restored. Thus, it is appropriate, to begin with, a review of this global balancing act. The important aspect is that the energy flows to and from space are in the form of radiation – or to be more precise, electromagnetic radiation.
ALTERING THE ENVIRONMENT
Most people would point to the built environment, the creation of towns and cities and their industrial base, before moving on to the pollution that goes with a modern lifestyle. If you thought about the clearing of tropical rainforests, you might have appreciated that the temperate forests have also largely gone to make way for agricultural land. You might have global warming and the extinction of species down on your list too. Humans are not unique in affecting their environment. Many large mammals make very noticeable changes to their environment. For example, the passage of a herd of elephants through an area can be as obvious as the visit of a plague of locusts. Not only does their feeding behaviour destroy woodland, but other species are destroyed as they trample around their feeding area. Like hippopotamuses, which enjoy a watery wallow and thereby transform clear pools into areas of ‘mud, glorious muds’. All species alter their environment to some extent, and you may be able to think of examples of this, particularly if you own pets.
HOW HUMANS ARE RESPONSIBLE?
There are several ways in which humans have altered ecosystems, that have led to the decline of species. We will leave to one side any major interference such as felling forests to provide land for agricultural and urban development and instead begin by looking at examples where we have eroded or eradicated stocks of species. This has notably been a consequence of the over-exploitation of food species (prey items). Predators do not normally eliminate their prey, but the use of sophisticated tools, such as guns and fishing gear, has led to our being over-successful as hunters. The disappearance of the dodo must surely be the best-known example, but it is not an isolated occurrence. For example, around 100 species of bird are known to have become extinct since 1600.
POLLUTION AND LOSS OF BIODIVERSITY
Unfortunately, halting the disappearance of species cannot be achieved simply by measures such as putting fences around special habitats and asking people not to pick the flowers or disturb the breeding birds. Many species are vanishing because of pollution. There are various environment (air, water, or land) contaminants, which are likely to be injurious to human, animal, or plant life. For example, Sulphur dioxide and oxides of nitrogen have direct effects on health, particularly on the health of those people who have respiratory diseases. There is an indirect effect too because they are responsible for the deposition of acid rain that damages many organisms, and this could potentially include humans, through the workings of food webs.
