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Satellite Observations in Support of Climate Challenges
Counting Carbon
The Big Thaw
Sea Level Rise
Water Security
Land Surface Change
Energy Resource Management

Water is essential for of all life on Earth. It is the only known substance that can exist naturally as a gas, liquid and solid within the relatively small range of air temperatures and pressures found on the Earth’s surface. Furthermore, the chemical properties of water make it the best natural solvent and a widely used medium for waste disposal and waste dilution.

In all, the Earth’s water content is about 1.39 billion cubic kilometres and the vast bulk of it, about 96.5%, is in the global oceans. Approximately 1.7% is stored in the polar ice caps, glaciers, and permanent snow, and another 1.7% is stored in groundwater, lakes, rivers, streams, and soil.

Finally, a thousandth of 1% exists as water vapour in the Earth’s atmosphere. Of all this water present on our planet, only 2.5% is fresh, and only 0.007% is readily available to people via rivers, lakes, and reservoirs. Fresh water is a finite and vulnerable resource, essential to sustain life, economic development and the environment, and management of this resource is expected to emerge as one of the greatest challenges facing mankind during the 21st century.

Fresh water availability and use, as well as the conservation of aquatic resources, are key to human well-being. The quantity and quality of surface and groundwater resources, and life-supporting ecosystem services are being jeopardised by the impacts of population growth, rural to urban migration, increasing wealth and resource consumption, and climate change. If present trends continue, 1.8 billion people will be living in countries or regions with absolute water scarcity by 2025, and two thirds of the world population could be subject to water stress.

Humans currently appropriate more than half of accessible freshwater run-off, and this amount is expected to increase significantly in the coming decades. 70% of the water currently withdrawn from all freshwater resources is used for agriculture. With the world’s population set to increase significantly by 2050, the additional food required to feed future generations will put further pressure on fresh water resources. Future management of freshwater resources will be complicated by the uncertainties in rainfall patterns introduced by climate change, with observations and models suggesting increased frequency and intensity of both extreme precipitation and drought events – depending on the region.
Global precipitation 1979–2006. (Credit: GPCP)

Global distribution of the world’s water.

The Stern Review on the Economics of Climate Change warned that global warming will have severe impacts often mediated through water:

— melting glaciers will initially increase flood risk and then significantly reduce water supplies in some areas;

— declining crop yields due to a lack of water, especially in Africa, could leave hundreds of millions without the ability to produce or purchase sufficient food;

— rising sea levels will result in tens to hundreds of millions more people flooded each year with warming of 3 or 4°C. By the middle of the century, 200 million people may become permanently displaced due to rising sea levels, increased flooding, and more intense droughts.

The Water Cycle
The combination of increased scarcity of global water resources and increased uncertainties in the Earth’s water cycle has added urgency to the need to improve predictions of rainfall and water resources. This requires development of an integrated water cycle observing system and extension of our understanding of the physical basis of the climate system driven by the water cycle.

The Water Cycle.

Because water continually evaporates, condenses, and precipitates, with average global evaporation essentially equalling global precipitation, the total amount of water vapour in the atmosphere remains approximately the same over time. This movement of water, in a continuous circulation from the ocean to the atmosphere to the land and back again to the ocean is termed the global water cycle. It is at the heart of the Earth’s climate system, affecting every physical, chemical, and ecological component. Amongst the highest priorities in Earth science and environmental policy issues confronting society are the potential changes in global water cycle due to climate change. Climate changes may profoundly affect atmospheric water vapour concentrations, clouds and precipitation patterns, as well as the atmosphere’s energy budget which drives the winds and the storm patterns. Many uncertainties remain, however, as illustrated by the inconsistent results given by current climate models regarding the future distribution of precipitation.

Better predictions of water cycle behaviour are needed for:

— monitoring climate variability and change;

— effective water management;

— sustainable development of the world’s water resources, requiring knowledge of trends and long-term projections of the intensity of the global water cycle;

— improved weather forecasts and monthly to seasonal climate predictions, including mitigation against drought and flood. As the global water cycle is relatively complex, long-term observational datasets are needed to characterise its behaviour as a function of several key parameters. These parameters include:

— global precipitation;

— surface temperature and salinity of continental water resources;

— atmospheric water vapour and temperature;

— sea surface temperature (as a significant factor that often markedly influences rainfall patterns, as in the El Niño). Coupled with wind and air temperatures it also provides a measure of air-sea fluxes;

— ocean salinity. If measured with sufficient spatial and temporal resolution, this would aid estimate of precipitation over the ocean and be important in helping to support climate model development;

— soil moisture;

— the amount of water stored in snow, glaciers and ice sheets.

In large parts of the world, the collection and dissemination of water-related information has been in decline in recent years. In order to strengthen cooperation amongst countries in gathering the necessary information, the WMO, in association with the World Bank, established the World Hydrological Cycle Observing System (WHYCOS) in 1993. WHYCOS is based on a global network of reference stations which transmit hydrological and meteorological data in near real-time, via satellites, to national and regional centres. A number of international scientific research programmes have been developed to address the key challenges relating to the global water cycle – most notably under the auspices of the World Climate Research Programme and its Global Energy and Water Cycle Experiment (GEWEX).

The main forum for coordination of the supporting observation programmes, including those of the satellite and in situ measurement communities, is the Integrated Global Water Cycle Observations Theme (IGWCO) – formerly of the IGOS Partnership and now within the GEO framework. IGWCO provides a framework for guiding international decisions regarding priorities and strategies for the maintenance and enhancement of water cycle observations so that they will support the most important applications and science goals, including the provision of systematic observations of trends in key hydrologic variables.

The Role of Earth Observation Satellites
Earth observation satellites play a major role in the provision of information for the study and monitoring of the water cycle and represent an important element of the observation strategy defined within IGWCO. The first element of this is the CEOP project (now known as the Coordinated Energy and water cycle Observation Project), which is taking advantage of the simultaneous, long-term operation of European, Japanese and U.S. satellites to generate new integrated data sets of the water cycle. This is taking advantage of the simultaneous, long-term operation of European, Japanese and U.S. satellites to generate new integrated data sets of the water cycle.
SMOS provides soil moisture and ocean salinity measurements.

Atmospheric temperature and water vapour data have been provided by polar orbiting meteorological satellites for decades – provided by USA (NOAA series) and more recently Europe (EUMETSAT’s MetOp series), as well as China and Russia. Recent advances using high resolution infrared soundings (IASI) or radio occultation techniques (which look at the interaction of radio signals with the atmosphere to derive characteristics of the atmosphere) and the Global Positioning Satellite signal (e.g. by the COSMIC satellite constellations and GRAS on MetOp) have further augmented the contribution from space.

Sea surface temperature measurements are also provided by the operational meteorological satellites, by ERS and Envisat (ATSR and AATSR), and by the Terra and Aqua missions (MODIS). Ocean wind measurements are also provided by these missions, as well as by NASA’s QuikSCAT and EUMETSAT’s ASCAT (on MetOp) which acquires all-weather, high resolution measurements of near-surface winds over most of the global oceans on a daily basis.

Precipitation is clearly a key parameter in the water cycle. Traditionally visible/infrared images from geostationary meteorological satellites, such as GOES, GMS and Meteosat, provided the best source of information from spacecraft, with indirect, but frequent, estimates of rainfall derived from measurements of cloud top temperature. These data are used in the WCRP’s GEWEX Global Precipitation Climatology Project (GPCP), which has provided monthly mean precipitation data from 1979 up to the present. Precipitation systems tend to be somewhat random in character and also evolve very rapidly, especially during the summer in convection regimes. Within a single storm, it is not uncommon for precipitation amounts to vary widely over a very small area. Also, in any given area, the amount of precipitation can vary significantly over a short time span. All of these factors make precipitation difficult to quantify. Reliable ground-based precipitation measurements are difficult to obtain over regional and global scales because more than 70% of the Earth’s surface is covered by water; and many countries are not equipped with precision rain-measuring sensors (i.e. rain gauges and/or radars). The only practical way to obtain useful regional and global precipitation measurements is from the vantage point of a space-based remote sensing instrument.

The advent of the Tropical Rainfall Mapping Mission (TRMM of NASA/JAXA) in 1997 provided a breakthrough in the provision of 3D information on rainfall structure and characteristics. TRMM was the first satellite dedicated to rainfall measurement, and carries a weather radar. Now in its 13th year, the TRMM mission has provided a wealth of knowledge on severe tropical storms such as hurricanes and short-duration climate shifts such as El Niño. Such active sensors have proved themselves to be an essential tool for the measurement of precipitation.

TRMM Image of Hurricane Katrina before it hit New Orleans. The satellite’s 3D look inside the storm provided unique information on the rainfall structure as it approached land.
Microwave-based techniques (utilising either passive remote sensors or weather radars) provide the most accurate measurement of rainfall, especially when integrated with surface observations. An all-microwave constellation of sensors, anchored by a ‘mother ship’ with a weather radar to provide accurate calibration, is necessary for reliable, global coverage of precipitation in all of its liquid and solid forms. This is the measurement philosophy embodied by the Global Precipitation Measurement (GPM) suite of sensors.

Water vapour observations from a geostationary satellite.

GPM aims to provide precipitation measurements on a global basis with sufficient quality, Earth coverage and sampling to improve prediction of the weather, climate and specific components of the global water cycle. GPM aims to ensure a repeat observation cycle of approximately 3 hours.

Recognising the central role of the water cycle to our understanding of the Earth System and climate change, selected space agencies are operating or developing a number of new missions aimed at addressing key water cycle issues. These include Aqua (NASA), CloudSat (NASA), EarthCARE (ESA/JAXA), CryoSat-2 (ESA) and Megha-Tropiques (CNES/ISRO), which will study water cycle and energy exchanges in the tropical belt.

Revolutionary new measurement capabilities – such as the provision of information on soil moisture and ocean salinity – are provided by missions such as SMOS (ESA, from 2009) and will be provided by Aquarius (CONAE/NASA, from 2011). Both soil moisture and ocean salinity are key variables that link the water cycle and climate.

Other areas where satellite data are being used to explore the water cycle include: the GRACE mission and its gravimetric measurements, which are providing information that is being used to quantify groundwater; the use of optical wavelengths to assess plankton and other water-borne materials; and the exploration of radar altimetry to measure water levels in lakes and rivers.

In the context of the CEOS follow-up to the 2002 Johannesburg World Summit on Sustainable Development, the European Space Agency launched the TIGER initiative – focusing on the use of space technology for water resource management in Africa and providing concrete actions to match the Summit Resolutions.

Future Challenges

New technologies for measuring, modelling and organising data on the Earth’s water cycle offer the promise of deeper understanding of water cycle processes and of how management decisions may affect them. Earth observation satellites will provide synoptic, high resolution coverage that is unprecedented in the geophysical sciences. The challenges to be faced in utilisation of these new capabilities include:

— development of new methodologies to exploit existing, long time series of satellite measurements;

— investigating novel approaches to convert satellite measurements into useful parameters that can be applied in scientific models, and that can be inter-compared and inter-calibrated among the different satellite missions;

— development of assimilation methodologies to integrate satellite and in situ observations;

— capacity building, particularly in developing countries, so that those countries in most direct need of water information have the means of access, analysis, and understanding required to derive maximum benefit from the data;

— continuing to collect consistent and accurate data over many years in order to detect the trends necessary for climate change studies;

— succeeding in the technology developments aimed at accurately measuring key parameters from space – including precipitation, soil moisture and ocean salinity.

Thanks to the efforts of the IGOS Global Water Cycle Theme and of GCOS in defining which Essential Climate Variables are required, the observations required to characterise and predict the water cycle are well defined, but remain challenging in some cases. To complement the satellite data, existing ground-based measurement networks and systems must continue operating to obtain current data that can be compared meaningfully with past records.

CEOS Virtual Constellation for Precipitation
CEOS recognises the vital importance of timely and accurate precipitation measurements in support of a broad range of societal needs, including climate studies, weather forecasting (including flood predictions for extreme events), water resource management and agriculture. As a result, CEOS selected a Virtual Constellation for Precipitation as one of four pioneering projects intended to improve international coordination of Earth observation satellite planning in support of common needs. The goals of this Precipitation Constellation are to:

— provide a framework to advocate and facilitate the timely implementation of the Global Precipitation Measurement (GPM) mission and encourage more nations to contribute to the GPM constellation. Although GPM offers impressive new measurement capabilities, the mission period is only 3 years;

— sustain and enhance an accurate global precipitation data record, including a Fundamental Climate Data Record essential for understanding the integrated weather/climate/ecological system, managing freshwater resources, and monitoring and predicting high-impact natural hazard events. This data record should be fit for the purpose specified by GCOS for the monitoring of precipitation as an essential climate variable.

NASA and JAXA are co-leading the development of the GPM mission which is the cornerstone of the Constellation. The Constellation team is in discussion to add further satellite missions developed by France (CNES) and India (ISRO).

Further Information
Water cycle:
Global Precipitation Measurement (GPM) mission:
World Water Forum:




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