The processes of natural climate change are a complex of seemingly chaotic, interactive and dynamic oscillating systems interplaying and overriding each other on short, mid and long term cycles. Embedded within these often poorly understood natural cycles are a number of anthropogenic influences - this paper will consider one of these influences: deforestation - and suggest an immediate and relatively cheap solution to its environmental impact.
There is no doubt that over the last century there have been measurable anthropogenic effects on local environments and climates due to, in part, deforestation and other vegetative land clearances for agriculture, grazing, mining, urbanisation, and so on. In the second half of the 20th century, the Earth’s forest cover dropped from 162 million to 98 million hectares due to deforestation. This has resulted in habitat destruction, species losses, changed albedos (surface reflectivity) and significant alterations to the water cycle, including adverse impacts on soil moisture content.
Land clearing decreases precipitation and soil moisture allowing incoming solar radiation to significantly heat the ground surface, relative to its pre-vegetated shaded condition, with resultant rises in surface air temperatures. An explanation has been found for what has been otherwise antedotally observed - that deforestation reduces precipitation.
The March 4, 2005 the Sydney Morning Herald reported that Professor Ann Henderson-Sellers, Director of Environment at the Australian Nuclear Science and Technology Organisation, and Dr Kendal McGuffie, from the University of Technology, Sydney, had carried out a study in the Amazon Basin, an area extensively clearfelled for lumber, grazing and some cropping. They discovered that by comparing the ratio of the heavy molecules found in rain and tracing its movement through the water cycle, and by knowing that the heavier molecules were slower to evaporate from streams and groundwater but were readily transpired by plants, that since the 1970s the ratio of the heavy molecules found in precipitation over the Amazon and the Andes had declined significantly.
Professor Henderson-Sellers said that “the only possible explanation was that they were no longer being returned to the atmosphere to fall again as rain because the vegetation was disappearing. With many trees now gone and the forest degraded, the moisture that reaches the Andes has clearly lost the heavy isotopes that used to be recycled so effectively.” She added that, “forests played a vital role in keeping the heavy molecules, and their far more common relatives [regular molecules of H2O], moving through the water cycle”.
The effects of deforestation on the water cycle and precipitation is of enormous importance in its impact on local climate variability, and has a direct impact on agricultural production and food security. Clear away the forests and woodlands, or overstock the rangelands and, over a relatively short period of time, the landscape dries out with corresponding rises in extreme temperatures, and reductions in relative humidity and precipitation due, in part, to changes in soil moisture content and subsequent evaporation. The amount of soil moisture is one of the most important controls of surface temperatures over land.
It has been said that the extended drought that has affected the Murray Darling Basin this first decade of the 21st century is, in part, a result of higher temperatures driven by climate change leading to more evaporation and drier catchments. Though it seems counterintuitive this notion is incorrect. The physics of evaporation is such that with low soil moisture, as occurs during droughts or after deforestation, nearly all incoming solar radiation that reaches the ground heats the surface with resultant rises in near surface air temperatures. When soil moisture content is higher some of the sun’s radiation is used in evaporation with resultant reductions in surface heating, hence lower temperatures, a process described as evaporative cooling. In other words, higher temperatures are due to the lack of evaporation, not a cause of significantly higher evaporation.
Another impact of deforestation and land clearing is changes in albedo or surface reflectivity. The easternmost part of the Western Australian wheatbelt has less rainfall than the uncleared wilderness country lying immediately to its east, even though there is an overall natural trend of rainfall to decrease from the west to the east. Land clearing has altered the albedo of the ground surface by replacing relatively dark native vegetation with relatively light cereal crops, resulting in less heat absorption by incoming solar radiation and consequently less low-level turbulence as that heat is re-radiated. Low-level turbulence is an integral part of the rainmaking process; a process also affected by the removal the surface roughage (the tree layer) that provides transpired moisture, which also assists in the creation of low-level atmospheric turbulence.
The solution to the effects of deforestation and land clearing is simple and direct, and could be implemented immediately, with results apparent within a year or two, and very obvious changes apparent within five years - revegetate parts of the landscape through natural regeneration. Natural regeneration simply involves the ceasing of land use activity on certain parcels of land so allowing Nature to naturally revegetate that land. A requirement for this to occur is that remnant endemic species still exist in the area to be naturally regenerated; if this is not the case then an initial seeding or replanting regime of endemic native species may be required.
Suitable parcels of land for natural regeneration would include stream and drainage lines, groundwater intake beds, steeply sloping upper catchment valleys, and wetlands and their surrounds. It is not the intention that natural regeneration occur on arable farming land, thus threatening food security, nor that it be viewed as a carbon sink, in order to ameliorate atmospheric carbon dioxide levels, a result of which is to drive up farmland prices for virtually no appreciable benefit - natural regeneration is primarily about maintaining the integrity of the water cycle.
This approach would be a “no regret” policy decision that would have numerous benefits. Natural regeneration allows for more water to be held in the landscape thus increasing soil moisture, an important variable in mitigating temperature extremes. As well, natural regeneration: reduces sheet and gully erosion; soil nutrient loss; the drying effect of desiccating winds and hence wind erosion; increases evapotranspiration; increases the near-ground air turbulence thus assisting the rain-making process; reduces the level of the water table with resultant reductions in soil salinity; and mitigates surface runoff so increasing groundwater and soil throughflows, with a resultant slowing down of land-based water cycle throughputs.
Natural regeneration also provides shade for stock as well as creating wildlife corridors that could perhaps be connected, over time, with existing stands of vegetation on farms and nearby reserves, including road reserves and former stock routes. Natural regeneration requires virtually no energy inputs and is therefore very cheap to implement.