Earth’s energy balance
If one were to monitor from space the quantum of incoming and outgoing radiation; and average it over centuries – both radiations would be approximately in balance, keeping the earth in radiative equilibrium in a long term sense. But the spectral quality would certainly shift from a predominantly visible spectrum to infrared because of its thermal interaction with the climate system involving solid earth, atmosphere, oceans and biological elements. Radiation measured at the earth’s surface would however not be balanced as energy would be consumed in driving atmospheric and oceanic movements and for fixation in plants. This energy would, of course, finally dissipate as heat and be converted to outgoing infrared radiation to space to foot the bill of overall earth’s energy balance.
On an annual or sub decadal scale the top of the atmosphere balance does not exist and there is net deficit or excess which is the heat actually stored or released from the deep ocean reservoir. This heat predominantly forces climate variability.
Earth’s surface, clouds and aerosols are principal reflectors of solar energy. Clouds are highly variable in their global distribution and have the effect of cutting down radiation by reflecting part of it back to space, thereby causing cooling. Clouds have another effect of trapping the outgoing infrared terrestrial radiation and thereby warming the earth’s surface. But the clouds themselves depend on the extent of surface heating, which determines the rate of evaporation thereby being both a cause and effect of radiation. This makes it difficult to pinpoint their role in future climate change. Aerosols reflect radiation in same way as clouds and cause surface cooling – but some aerosols partly absorb solar radiation during daytime thereby allowing the atmosphere to gain heat. Thus aerosols that reflect solar radiation have a mixed role to play. Any trend in these, either caused directly as in the case of aerosols or indirectly as in case of clouds, cannot be treated as obvious indicators of global climate change.
Atmospheric compositions of trace gases have a bearing on atmospheric radiation balance. Normally only about 7 to 10 per cent of solar energy – most of it in the infrared range of the spectrum is absorbed by water vapour, carbon dioxide and ozone. The rest reaches the ground to heat it up during day. A part of this heat is stowed away into the upper layers of the soil and to a greater extent in the upper 100 metres or so of the oceans, to be released during night. The remaining is returned to the atmosphere by convection and terrestrial radiation at infrared wave lengths. Any change in the atmospheric composition alters the balance and is likely to have significant climate impacts. Some of the impacts that have anthropogenic origins are:
- Aerosol absorption of solar radiation
- Greenhouse gas absorption of terrestrial radiation
- Changes in composition of chemically reactive gases that effect aerosol and greenhouse gas concentrations.
New Environmental Issues | Some recent concerns
The problem of greenhouse gases and its increase in the atmosphere is well documented. However, the debate concerning global warming is not related to the initial warming, which is inevitable, but the later response in terms of added moisture (which is also a greenhouse gas and must increase with progressive warming), sequestration by oceans, response from changes in cloud cover – all unknown with an increasing order of uncertainty. While the greenhouse gas debate is fuelled by power generating technology, there are some recent developments, which merit attention and awareness.
Atmospheric Brown Cloud
An international field experiment in the waters of the tropical Indian Ocean in the late 1990s (the Indian Ocean Experiment – INDOEX) focused on the regional effects of aerosols on climate. INDOEX showed that radiation trapping within the atmosphere is much larger than at the top of the atmosphere (TOA). For example, for the surface, atmosphere and TOA, aerosol radiative forcings for the tropical Indian region have been estimated at -14±3, 14±3 and 0±2 Wm-2 respectively – considerably higher than the corresponding global average values for CO2 forcing of 1, 1.6 and 2.6 Wm-2. Positive forcing implies a gain in radiation energy leading to warming of the atmosphere, while negative forcing implies cooling at the surface. This would lead to a lower rate of temperature drop with height in the tropical Indian region as compared to other regions, presumably due to the aerosols emanating from biomass burning, forest fires, bio and fossil fuel use, which can be independently detected either from satellites or ground based measurements. The differential manifestation of the effect of aerosol forcing over land and oceans may be seen in Figure 1, which gets accentuated when we consider carbon aerosols. The land-sea contrast drives monsoon type of systems and is thus vitally important in meteorology.
The INDOEX was followed up by a global survey of such effects and found to be associated with all downwind sites of carbon soot producing pollution – be it in different magnitudes of concentration. Thus the phenomenon was termed as the Atmospheric Brown Cloud (ABC). Brown and not gray because it absorbs radiation.
The ABC is suspected of stabilising the lower atmosphere thereby cutting down on vertical mixing, depriving cloud systems of adequate moisture supply for heavy rainfall. The special experiment planned during the Asian Monsoon Year in 2010 by the international community will look into the multiple roles of small sized anthropogenic carbonaceous aerosols in altering atmospheric stability, in lowering rain forming efficiency of clouds and also their special roles in acting like an elevated heat pump over the sub Himalayan region to cause early monsoon activity. Interestingly there are already findings that say that small aerosols of the monsoon region are actually adsorbed on larger dust particles thereby reducing their numbers below the level where they possibly cannot significantly alter rain forming mechanisms but can still cause elevated heating.
Self sustaining haze
A carbonaceous composition of aerosols should theoretically lead to increased pollution not just due to increasing emissions but also due to reduced convective mixing in the lower atmosphere, inducing a self sustaining mechanism especially in winters. Although there isn’t observational evidence to show that such sustained episodes of haze was never there in the past, yet the hypothesis is worth testing by other means than simply circumstantial. Another aspect of reduced vertical mixing could be ground heating in summer which even though coincides with geographical distribution of brown cloud in recent years needs to be tested as there has been an interannual variation in the heat wave occurrences that should at least be partly caused by aerosol variability.
Fog is another phenomenon, which is ascribed to pollution. Earlier it was alleged to be the direct cause but in recent times with substantial number of years without the occurrence of dense fog, it is surmised that pollution alone cannot cause dense fog – you need favourable conditions which can easily have a decadal variability. Persistent haze is however due to pollution. The lingering doubt which still prevails is that the lowered mixing due to brown haze might simply make the atmosphere that much more vulnerable.
New Environmental Issues | Strategic and tactical approaches to mitigation
It is important to conduct scientific experiments to prove with added certainty the causative mechanisms of our observed environmental trends and their likely consequences. Since technology, lifestyles – economics in general, is all settled around the present day practices, it will need conviction to mitigate existing problems. As a strategy, issues revolving around health impacts should be dealt with first. Such a tactical move would in many ways contribute to the long term mitigation of the adverse effects of climate change – devising fundamental and strategic measures in the wake of new technological developments.