Today, we are fortunate to have a guest post by Anthony Blair Mount. It is titled
Draining Away The Earth’s Coolant
Introduction. Local Tasmanian coastlines show increased erosion which seems to confirm the sea level has risen slightly. The recent arrival of fish from warmer climes indicates that it has also warmed. Thermal expansion probably accounts for most of the rise in sea levels but there could be some contributory agents as discussed below.
Catchment Evaporation. When a year’s rain falls on an undisturbed dense forest catchment, about one quarter wets the canopy and is then evaporated. About half is used by surface evaporation plus transpiration through the vegetation from the soil moisture store. The latent heat required to evaporate these three quarters of the local annual rainfall helps cool both the canopy and the land it shades.
Catchment Run-off. The other quarter of annual rainfall runs-off and is exported from the local catchment via streams, rivers, ponds and lakes to the sea, and on the way it evaporates.
Dense Forest Conversion. On land cleared of dense forest then grassed or cropped, all the rain reaches the ground. Surface evaporation and transpiration from regenerating plants together still use about half of it, so the other half runs off. In this way the catchment’s evaporative cooling is reduced by a third while run-off is doubled. The accompanying removal of both tree-top cooling and canopy shade aggravate this reduction in catchment evaporative cooling. The extra run-off only replaces what is already evaporating from existing water surfaces so cannot add to evaporative cooling, but it may contribute to sea level rise.
Open Forest Conversion. Real but lesser reductions in local cooling and increases in run-off also occur when shorter and less dense forests in lower rainfall areas are cleared and cropped or grassed.
Sealed and Drained Lands. Where natural vegetation is replaced by roads, roofs and runways that are sealed and drained, local cooling by transpiration and evaporation from a soil store are much reduced and most of the drainage water is usually exported to existing water surfaces – including the sea. These reductions in local cooling are additional to the surface warming caused by power stations, transport, urban areas, air conditioners, etc.
CO2 Increases. As surface evaporation plus plant transpiration hardly change with conversion of forests to low vegetation (once established), CO2 absorption may behave similarly. But ploughing aerates the soil which produces extra CO2 and removes nearby plants; and all sealed and drained lands have relatively low CO2 absorption. However glasshouse crops seem happy to absorb 1000ppm CO2 so it is likely this extra local and all other extra CO2 should be quickly absorbed and increases plant growth. Whenever global temperatures rise all land CO2 plant sinks should gain from all water degassing sources.
Summary. Forest clearing and land sealing and draining by humans all decrease local land evaporative cooling and increase local run-off. It is probable that, at world levels, that these two local effects of human land management make some contribution to both land warming and sea level rise.
Bio. Born in Canada, schooled in England, University at Adelaide, Canberra and Hobart (MSc). Started work at Forestry Tasmania’s Maydena Research Station Jan 1957. Invited to address the Tall Timbers Fire Ecology Conference at Tallahassee in 1969 and still active in that field (see www.ffic.com.au for two 2009 broadsheets “Tasmania’s Ancient Bushfire Heritage” and “Managing Tasmania’s Fire Environment”). Lectured at Melbourne Uni Forestry School.
Wrote the Australian Forestry Council’s booklet “Australian Bushfire Research” (1987). Published “The Forest Green and Other Poems” in 2000.
Currently attend the University of the Third Age and regularly Orienteering.
Credentials re posted topic.
Tony Mount (79) is a long retired forest fire ecology researcher and fire manager. He is the author of the Soil Dryness Index (SDI) – see Mount, A.B.(1972) The derivation and testing of a soil dryness index using run-off data. Tasmanian Forestry Commission. Bull. 4.(points & Fahrenheit) and Mount, A.B. (1980) Estimation of evaporative losses from forests; a proven (=tested) simple model with wide applications. Institution of Engineers, Australia, Hydrology and Water Resources Symposium. (mm & Centigrade)
The SDI has been used in Tasmania for some 40 years for fire management and flood forecasting purposes and for fire management in Western Australia and elsewhere.
The SDI was re-calibrated by Melbourne Metropolitan Board of Works to accurately model Melbourne’s run of the river water supply – see Langford, K.J., Duncan, H.P. and Heeps, D.P. (1977) Evaluation and use of a water balance model. Institution of Engineers, Australia, Hydrology Symposium.