Remote Sensing and Global Climate Change

I started the preparation of this set of lectures by looking up the two words “weather” and “climate” in the Concise Oxford Dictionary. I found:

WEATHER: Atmospheric conditions prevailing at a place and time, combination, produced by heat or cold, clearness or cloudiness, dryness or moisture, wind or calm, high or low pressure and electrical state, of local air and sky.

To understand global environment change, it is essential to understand and to document how the Earth works as a system. The scientific success of this understanding depends on the integration and management of numerous data sources. Unlike many scientific conundrums of the past, this one will not require the discovery of new laws of nature: the climate works according to the established rules of physics, chemistry and biology. What is needed is the data to make clear what is going on where. The continuous monitoring of the environment and the investigation of both natural and man-induced changes present a very complex and fascinating challenge. No one nation has the resources to generate the comprehensive understanding required, and many international, collaborative programmes have been established to investigate phenomena and processes on a global scale and to coordinate work carried out on a local scale.

The 20th century is approaching its end under conditions of the intensification of a number of global environmental problems (see “References”). Mankind has entered a phase in its development when the prediction of the great Russian scientist V. I. Vernadsky that human activities become geological-scale forcings capable of changing the world by bringing it to the verge of ecological catastrophe has become quite realistic. This is what determines the absolute urgency of global ecology problems and the need to develop a political concept of priorities in human values, especially in connection with the Second UN Conference on Environment and Development held in June 1992 in Brazil.

Any satellite system is constituted in general of two main components, namely the Space Segment and the Ground Segment. The Space Segment is basically the spacecraft itself, one or more, and its payload: it includes all the hardware and software tools to make the satellite(s) operate as planned and commanded by the ground control centre, from the trusters to perform the orbit and attitude manoeuvres to all the equipment to execute embarked instrument operations and to ensure the data transmission to the ground. The Ground Segment encompasses all the ground facilities required to manage and control the space segment, and to carry out all the operations related to the satellite data handling: data acquisition, processing, archiving, retrieval and distribution; consequently it also includes the user interfaces and services.

The increasing concern for our environment has promoted interest in using remote sensing of the Earth’s surface and atmosphere and microwave measurements from space are particularly important because they can provide all-weather monitoring on a global, day and night basis.

In Chapter 1 we discussed many natural phenomena and human activities which affect the weather and the climate. It would be an interesting and useful scientific activity if we were able to model the climate, taking into account all the natural phenomena, and predict the future behaviour of the climate in the absence of any effects of human intervention. Given the present public concern, it would be particularly useful if we were then able to model the effects of human activities, as well as the natural phenomena, and make reliable predictions of the effects of human activities on the future climate. Unfortunately, the system is so complicated, our present knowledge is somewhat uneven and our historical knowledge is very little indeed. However, in spite of all the difficulties, a great deal of effort has gone into climate modelling in recent years and some very useful results have been obtained. Given what was said in sections 3 and 4 of chapter 1 about weather forecasting, the difficulties of producing weather forecasts for more than a few days ahead, and the whole idea of chaos versus determinism, it is perhaps both surprising and encouraging that some success in climate modelling can be achieved. There are various books on climate modelling, but the most up-to-date text at present is the volume edited by Trenberth (1992); it contains a comprehensive list of references to earlier work.

The basic ideas of numerical weather prediction (NWP) are introduced and it is shown that due to the complexity of the physical systems we are trying to model, frequent and accurate observations of the global atmospheric state are required. In particular, the need to supplement so called conventional observations with remotely-sensed (satellite based) measurements is demonstrated. The true nature of satellite observations will be discussed, highlighting the important fact that they are not direct measurements of the quantities that we require in current NWP systems. The problem of converting the measurements (radiances) to useful quantities will be introduced and a number of retrieval approaches described. The variational approach to the NWP analysis problem will be introduced; this makes the explicit conversion of satellite measurements to products unnecessary. The potential benefits and extensions of such an approach are described. Finally, a number of satellite observation systems currently being investigated for future operational uses are described.

Earth-orbiting satellites have provided a wealth of data which has spawned a revolution in the sciences of meteorology and climatology (Rao et a1. 1990). Through the use of satellites we are able to monitor many aspects of the surface and atmosphere of the Earth. Meteorological satellites have enhanced our understanding of the synoptic processes and now form a routine part of weather information which is distributed to the general public. These satellites have also provided increased understanding of many smaller-scale processes which were not resolved by the surface synoptic network. Aside from these obvious implications for meteorology, satellites have broader implications for the study of large-scale climate dynamics (Ohringet al 1989). Satellites also provide essential information for climate modelling. As global climate models include increasingly complex treatments of the land surface, detailed information on soil moisture distribution, snow cover etc. are required for model validation. Satellite data on clouds provide an important method for evaluating model dynamics together with measured top of the atmosphere radiative fluxes. To enhance our understanding of the climate system, satellite techniques must address many areas of climate research. Some topical examples might be (i) the dynamics of drought (e.g. El Niňo), (ii) the monitoring of deforestation, (iii) the monitoring of stratospheric ozone concentrations and the delicate chemical balance of the Antarctic stratosphere and (iv) the potential microphysical changes induced in clouds by the injection of anthropogenic and natural aerosols (such as those produced by volcanoes or dimethyl sulphide (DMS) released from the oceans (Twomey et al 1984 and Somerville and Remer 1984, Charlson et al. 1987).

The climate of the Earth depends on the balance between the energy absorbed from solar radiation and outgoing radiation emitted in the form of long-wave radiation. Any factor that can perturb this balance and thus potentially alter the climate is called a radiative forcing agent. This balance can be affected by a number of factors such as a change in the output of solar radiation, changes in greenhouse gas concentrations and an increase in aerosol loading in the lower atmosphere.

The H₂O molecule is one of the fundamental building blocks of the Earth’s climate system. A remarkable feature of our planet is the existence of significant amounts of water in solid, liquid and gaseous form. Water is continually recycled between these three phases of the hydrological cycle, all of which play important roles in the climate system. By far the largest mass is contained in the oceans, followed by the ice caps, ground water, lakes and rivers and finally the atmosphere.

The methods for laser remote sensing of the atmospheric temperature are based on the temperature dependence of the interaction of the light with the atmospheric gases. A group of methods is based on the temperature dependence of some characteristics of different type elastic and nonelastic scattering, e.g. the line bandwidth of the resonance (Blamont et aI. 1972) or Rayleigh (Fiocco et al. 1971) scattering, the envelope of the rotational Raman scattering (RRS) spectrum (Cooney 1972, Kobayasi et al. 1974, Cohen et al. 1976, Arshinov et al. 1983, 1988, Mitev and Nitsolov 1983, Kobayashi and Taira 1990) or the shape of the RRS spectral lines (Armstrong 1974, 1975), the shape of the Rayleigh-Brillouin scattering spectrum (Shimizu et al. 1986), etc. The temperature dependance of the light absorption in the atmosphere is also used as a basis of some DIAL methods to measure the atmospheric temperature and pressure (Mason 1975, Kalshoven et al. 1981, Korb et al. 1989).

This chapter will review those physical processes in snow and ice which are important to climate change, and will describe how remote sensing techniques are used to map the cryosphere and hence to allow the magnitude and direction of climatic change in the polar regions to be estimated. The review begins with a brief overview of the structure and morphology of ice and snow and the geophysical processes in which ice and snow are involved. We then consider the physics of interaction between electromagnetic waves and a snow or ice surface and the remote sensing techniques commonly used to study ice and snow (including acoustic as well as electromagnetic methods). The greenhouse effect is described with reference to the cryospheric processes which cause the rate of warming to be enhanced in north polar regions. We point out the importance of ice and snow processes in amplifying the rate of ground level warming, and how ice and snow parameters can be used as early indicators of climatic change in action. We go on to consider in detail how remote sensing techniques are used to monitor four key parameters. These are (1) the contribution of melting glaciers and ice sheets to the rate of global sea level rise; (2) the evidence for a retreat of sea ice in the polar regions; (3) the problem of the synoptic mapping of sea ice thickness, and the evidence for thinning of sea ice; (4) the evidence for a retreat of seasonal snowpack in the northern hemisphere.

Over the last decade, the world’s attention has become increasingly focussed on the issue of climate change, and in particular change deemed due to man’s activities. The impact of human activity is felt throughout the world, but is superimposed on a background of complex natural variability. In the polar regions, however, and particularly in Antarctica, some anthropogenic impacts stand out more clearly. Analysis of historical levels of atmospheric carbon dioxide from ice coring, for example, shows a clear increase correlated with industrial development over the last two hundred years.

The NASA Space Shuttle Earth Observations Database is a valuable source of data for research and reconstruction of Earth’s recent environmental history and thus for assessment of the human impact on global Earth processes. This data source, although having the longest length-of-record of any space-derived global change database, has not been fully exploited by scientists studying global changes. With the inception of NASA’s Mission to Planet Earth programme there is a need to integrate these important data into global change studies.

Global land processes such as land use, deforestation, soil and land degradation, erosion, desertification, soil salinisation, long-range dust transport, hydrological changes, and stream and reservoir sedimentation have been extensively documented during the more than 50 Space Shuttle missions flown during the past twelve years, as well as by similar baseline data acquired during the even earlier NASA manned spaceflights extending to 1961. These observations provide a unique perpective on our planetary habitat and add understanding and critically important early data points to our models of land use dynamics and their ecological implications.

This paper discusses the Space Shuttle Earth Observations database, the imaging systems utilised during Space Shuttle missions and provides a case study of an application project conducted utilising these image datasets.

The motion of the Earth’s atmosphere is driven by differential heating. The reservoir of the available potential energy, which describes the difference between the actual state of the atmosphere and a hypothetical atmosphere at rest, is estimated to be about 55 x 10⁵ Jm⁻² as a mean value around the globe. This reservoir feeds the reservoir of kinetic energy (15 x 10⁵ Jm⁻²), from where the energy dissipates into heat with a rate of 2.3 Wm⁻². As these reservoirs are fairly constant in a climatological sense, the production rate of available potential energy and the transformation rate into kinetic energy has also to be 2.3 Wm⁻². In traditional weather forecast models it was not necessary to put much effort into a precise description of the differential heating, the main source of available potential energy, because the time to exhaust the reservoirs mentioned above is given in months.

Global changes are the product of complex processes that occur within the Earth system and, as far as mankind is concerned, have an impact on the habitability of the Planet. Global change related activities therefore aim at understanding, modeling and monitoring the relations between Earth (as a whole), anthropogenic influence and external forcing (mainly solar input and solid Earth process). The main contribution to global change comes from the coupling between biogeochemical processes and the physical climate system: exchange of energy-mass-momentum between ocean/land and atmosphere, atmospheric chemistry and transformation of energy, for instance from radiative to non-radiative form (biosphere).

International programs like IGBP (the International Geosphere-Biosphere Program) use NOAA satellite data to calculate the (normalised difference vegetation index) to monitor the growth of arid or semi-arid regions of planet Earth. Therefore well calibrated AVHRR data of channel- 1 and -2 are needed with errors less than 5%. The aim of the work is to show the impact of different calibration coefficients on the calculation of Albedo from channel-1 and -2 NOAA-11 AVHRR data. In addition, the sensor degradation was monitored and a radiative transfer model was applied to remove atmospheric effects.

Various previous chapters have been concerned with several aspects of the oceanic factor in global climate change and it is not intended to repeat that material in this chapter. This chapter, therefore, will concentrate on practical applications of satellite data to marine science in the general context of global climate change.

The rapid development of satellite oceanography since the launch of satellites designed to carry marine observation radiometers has provided a valuable tool to study the oceans and its processes in a new light. Sensors operating in the visible, infrared or microwave part of the electromagnetic spectrum represent the only means of providing synoptic observations of four basic sea surface properties i.e. 1) colour, 2) temperature, 3) height and 4) roughness (Allan 1991). The interpretation of these four basic properties has formed the basis for the determination of phytoplankton, turbidity, algal blooms, sea surface temperature, ocean fronts, geostrophic currents, wind stress and wave fields of the oceans.

SEAWATCH THAILAND is a complete marine environmental monitoring and forecasting system which integrates data collection, data analysis, environmental modelling and forecasting with an advanced computerised system for the distribution of marine information and forecasts to interested operators and/or authorities. This three-year project is being implemented under close co-operation between the National Research Council of Thailand (NRCT), OCEANOR, the Oceanographic Company of Norway, and various other parties, including the Harbour Department, the Meteorological Department, the Port Authority of Thailand, the Naval Hydrographie Department, the Department of Fisheries, the Petroleum Authority of Thailand, the Marine Police Division, Chulalongkorn University, Kasetsart University, Prince of Songkla University, Burapha University, etc.

The most important ecological problem of the St. Petersburg Region is that of Lake Ladoga. Its basic aspect is the continuously deteriorating water quality in Lake Ladoga and the entire system of the Lake Ladoga, the Neva River, the Neva Mouth and the eastern part of the Gulf of Finland. This deterioration is a consequence of human economic activity in the lake’s basin (Voropayeva and Rumyantsev 1991).

During the course of the summer school, Professor Kondratyev suggested that we might hold a panel session or round table discussion on politics and global climate change. This chapter comprises a summary of the discussion on this subject.