Since the beginning of the Industrial Revolution in the mid-18th century, humans have largely altered the environment and to some extent the climate system. Their activities such as the combustion of fossil fuels, burning of biomass and continued growth of industries has led to an astronomical increase and concentration of aerosols, greenhouse gases (GHGs) and chlorofluorocarbons (CFCs) in the atmosphere. Furthermore, agricultural practices which lead to changes in land use patterns especially deforestation for large- and small-scale agriculture have replaced forests’ biodiversity with increasingly monoculture crop systems. All these human activities have affected the physical and biological properties of the earth’s surface with a cascading effect on the local, regional and global climate.
However, it is important to keep in mind that in the past, our planet has naturally gone through cycles of warming and cooling, but the changes seen today are happening at a much faster scale than would be expected in a natural cycle. This has led to the emergence of other hypotheses which propose to explain all or most of the observed increase in global temperature including: That the warming is within the range of natural variation, the warming is a consequence of coming out of a prior cool period as the little ice age, and that the warming is primarily a result of variances in solar irradiance, possibly via modulation of cloud cover. For instance, (Oreskes, 2004) said the observed warming actually reflects Urban Heat Island, as most readings are done in heavily populated areas which have been expanding over the last few decades with growing population.
Agriculture – crops, livestock, fisheries and forestry – contributes about 31 per cent of the total emissions of GHGs in the atmosphere. Carbon dioxide, the most important anthropogenic GHG, is released from deforestation when trees are cut down and allowed to decay thereby oxidizing the stored carbon which is then released in the atmosphere. Cultivating the soils after deforesting (land use change) facilitates the oxidation of some organic matter in the upper layer and releases it into the atmosphere. Methane, the second major GHG gas, is released from rice agriculture, conversion of wetlands and livestock farming. Nitrous oxide originates mainly from soils and the oceans. Nitrous oxide from humans in the agriculture sector originates from fertilizer use.
The GHG Effect
The natural greenhouse effect is necessary to support life in our planet. Without global warming occurring naturally, the earth’s surface would be about 35o C cooler on average. The climate change problem is being caused by what scientists call the enhanced GHG effect. This is where over the past 200 years, human activities such as burning of fossil fuels and destruction of forests (especially tropical) has caused massive concentrations of heat-trapping GHGs in our atmosphere. With more of these gases in the atmosphere, more infrared radiation is re-radiated back to earth’s surface as heat when some of the incoming solar radiation is absorbed by the earth’s surface and radiated back into the atmosphere in the form of infrared radiation. This is causing what is referred to in the scientific community as climate change.