Satellite remote sensing provides a unique way of monitoring the complex and dynamic processes that occur in the atmosphere. Since the future of the human race is critically dependent on the long term variability of the atmosphere, great efforts are being made to understand the many processes involved. In response to this, researchers develop models of the atmosphere as a mechanism whereby chemical reactions and physical changes in the atmosphere can be placed in context within the overall Earth system.
Such models require large amounts of data describing the spatial and temporal variability of the Earth's atmosphere at different locations and altitudes around the globe, taking account of diurnal, seasonal and longer-term cycles. Sources, reservoirs and sinks of critical trace gases all need to be described. Satellite remote sensing provides a powerful set of techniques for acquiring these data to sustain models of the atmosphere, especially where information is required on a global scale and within a short time span.
Many of the factors affecting the global environment are related to changes in the chemical composition of the atmosphere. The results of these changes include: the enhanced greenhouse effect, increase in the levels of ultraviolet-B radiation reaching the Earth's surface, acidification and reduced transparency. The atmosphere is very dynamic, both in terms of chemical composition and associated radiative properties, and also in the way it transports materials around the globe, providing a link between land and ocean. The key role of the atmosphere in the maintenance of the Earth's environment emphasises the need to conduct research, to understand properly the processes involved and to monitor long-term changes.
As a result of man's activities, which have become progressively more significant over the last century, large quantities of carbon, chlorine, nitrogen and sulphur compounds have been injected into the atmosphere and are disrupting the natural equilibrium which had become established. Whilst long-term change has always been a feature of the atmosphere, it has become apparent that it is the increased rate of change, brought about by man's activities, which is having such a potentially detrimental effect on the Earth's system.
The reduction in stratospheric ozone concentrations over Europe since 1960 is the direct result of the use of ozone depleting chemicals such as refrigerants, industrial cleaners, foaming agents and those in fire extinguishers. Conversely, pollution at the Earth's surface has led to increased levels of tropospheric ozone, particularly over industrial areas, with consequent threats to human health. No other chemical in the troposphere has a concentration which is so close to being toxic.
The greenhouse effect, shown in figure 1.10 below, concerns the warming of the troposphere by increasing concentrations of the so-called greenhouse gases (carbon dioxide, methane, nitrous oxide, ozone and others). This warming occurs because the greenhouse gases are transparent to incoming solar radiation, but absorb infrared radiation from the Earth that would otherwise escape from the atmosphere into space. The greenhouse gases then re-radiate some of this heat back towards the surface of the Earth. The rise in carbon dioxide as a result of industrialisation is primarily responsible for the enhanced greenhouse gas effect. Current carbon dioxide levels are more than double pre-industrial levels and are the focus of international efforts to reduce emissions and offset the consequences of changed climate patterns, sea level rise, effects on hydrology, threats to ecosystems and land degradation.
Figure 1.11 - Figure 1.10 The greenhouse effect
Many studies of the effect of greenhouse gases on the climate have and are being carried out. An effective doubling of carbon dioxide concentrations is now predicted for 2030, which is expected to produce an estimated temperature rise of between 1.5° and 4.5°C, but with considerable variations in the rate of warming in different regions. The situation is highly complex due to mechanisms whereby, for example, an increase of sulphur dioxide in the atmosphere, through industrialisation, reduces the greenhouse effect because of an increase in the atmosphere's reflectivity.
While predictions continue to be refined, the overall objective remains as that set out in Article 2 of the United Nations Framework Convention on Climate Change (UNFCCC), which calls for the stabilisation of greenhouse gas concentrations at a level that prevents dangerous anthropogenic interference with the climate system, and in a time frame that allows ecosystems to adapt naturally.
The amount of water vapour in the atmosphere is an important component of the Earth's climate system. It varies considerably in response to variations in temperature and relative humidity and acts as an energy carrier, redistributing energy around the planet. Water vapour has a large radiative effect and is the most important greenhouse gas. Water, in the form of clouds, liquid or ice, modifies the radiation reaching the surface and thereby strongly influences the surface energy flux. The role of clouds in the climate system is poorly understood and this undermines the overall validity of modelling and prediction activities. Research into the influence of water vapour and clouds is needed in order that anthropogenic effects can be isolated from long-term natural climate variations. MERIS contributes to this field by providing the column water vapour content over land, oceans, and clouds.
There is evidence to suggest that in recent decades there have been long-term changes in aerosol loading in the stratosphere. For example, amounts of sulphate aerosol in the stratosphere increased significantly in 1991 and 1992 as a result of the 1991 eruption of Mount Pinatubo. GOME data has been analysed to produce estimates of SO2 loading of the atmosphere; for example, from the eruption of the Nyamuragira volcano in Zaire. Whilst there is a good relationship between the degree of aerosol loading and volcanic events, an upward trend has been detected in background levels.
The impact of aerosols on the Earth's radiation budget (see below) is both direct, through scattering and absorption, and indirect, through the modification of cloud properties. In both cases, aerosols in the stratosphere seem to have a cooling effect with regard to the Earth's radiation budget. Sulphate aerosol loading in the mid-latitudes has also been correlated with ozone trends in mid-latitude and polar regions, through a modification of the concentration of gases involved in ozone depletion. However, the extent to which aerosols influence the Earth's climate has been difficult to assess since aerosols vary a great deal in terms of size, shape and chemical composition. Satellite-borne sensors have the potential to improve knowledge of the origin, dynamics and fate of aerosols, through their ability to monitor the whole globe within very short data capture repeat cycles. Critical to the determination of aerosol types is the wavelength dependence of extinction coefficients in the visible and near infrared parts of the spectrum.
The use of spaceborne instruments to measure aerosols in the stratosphere is well established. The SAGE (Stratospheric Aerosol and Gas Experiment) series of instruments has demonstrated the concept and share features with the atmosphere sensors onboard ENVISAT; with GOMOS in particular. Several of the instruments onboard ENVISAT are capable of making aerosol measurements with sufficient spectral coverage to determine size distribution and composition. GOMOS and MIPAS make observations of the distribution and structure of the stratospheric aerosol layers. Moreover, the ability of MIPAS to acquire data perpendicularly to its flight direction, strengthens its ability to record aerosol injections into the stratosphere from volcanic eruptions. SCIAMACHY provides further information about aerosols through its ability to make polarisation measurements, and its large spectral coverage. MERIS has the capacity to evaluate tropospheric aerosol properties including optical thickness and type.
Earth Radiation Budget
Processes in the atmosphere which alter the Earth's radiation budget need to be better understood. To achieve this, it is necessary to monitor certain trace gases and other constituents such as aerosols, whose temporal changes affect the Earth's climate by modifying radiative transfer. Long-term global measurements improve current assessments of changes in the abundance of ClOX, HOX, and NOX which are associated with decreases in stratospheric temperatures through their impact on radiative transfer in the atmosphere. Observations on the extent of radiative cooling of the atmosphere can be obtained from measurements of CO2 and NO in the middle atmosphere.
Of particular interest, in the context of the "greenhouse effect", is the transportation of water vapour from the surface of the Earth into the free troposphere. While climate models have suggested that this is a phenomenon associated with global warming, there is no firm evidence suggesting that the free troposphere is becoming moister and therefore providing the positive feedback necessary to stimulate global warming to the levels being suggested. In terms of radiation budget, water vapour is the most important atmospheric gas in the context of cloud amount, precipitation and evaporation rates. Even small changes in global measurements of cloud albedo have a significant effect on the Earth's radiation budget.
MERIS contributes to this work by providing information on cloud amount, cloud top height, cloud optical thickness, water vapour and cloud albedo, as well as the aerosol information discussed above. Cloud coverage and other parameters, including water/ice discrimination and particle size distribution, are also available from the visible channels on AATSR. The MWR instrument also produces total column measurements of water vapour and liquid water.