Aerosol Properties
Essential Climate Variables: Aerosol Properties

Aerosols are tiny particles suspended in the air. The majority are derived from natural phenomena such as volcanic eruptions, but it is estimated that some 10–20% are generated by human activities such as burning of fossil fuels. The majority of aerosols form a thin haze in the lower atmosphere and are regularly washed out by precipitation. The remainder are found in the stratosphere where they can remain for many months or years. Scientists have yet to quantify accurately the relative impacts on climate of natural aerosols and those of human origin, so there is still uncertainty whether aerosols are warming or cooling the Earth. Predicting the rate and nature of future climate change requires this clarification of the processes involved.

As a consequence, the IPCC identifies further information on aerosols as a priority, highlighting a particular need for additional systematic, integrated and sustained observations which include the spatial distribution of greenhouse gases and aerosols. The Integrated Global Atmospheric Chemistry Observations (IGACO) Theme of the former IGOS Partnership aimed to provide a framework ensuring continuity and spatial comprehensiveness of the full spectrum of atmospheric chemistry observations, including the monitoring of atmospheric composition parameters related to climate change and environmental conditions. The IGACO Theme Report was finalised in May 2004 and provides a comprehensive overview of current and future satellite measurements for tropospheric and stratospheric aerosols. The report states, in particular, that “satellite observations of aerosol optical properties have progressed to a point where they range from pre-operational to operational, although there are demonstration-mode instruments on a number of research satellites”.

Reliable information on aerosols is also required by applications outside the study of the climate system. For example, accurate and timely warnings of the presence of airborne dust and ash – such as that arising from desert dust clouds and volcanic eruptions – are important to the safety of airline operations. A worldwide volcanic ash monitoring system, which is dependent on satellite observations, is in place to provide real-time advice to pilots.

Measuring the distribution of aerosols through the depth of the atmosphere is technically difficult, particularly in the troposphere.

Previously, techniques using instruments such as AVHRR and ATSR were limited to producing estimates of vertically-integrated total amounts, mainly over oceanic regions.

Measurements over land are difficult (due to persistent cloud cover and the high value, and variability, of land surface reflectance), but the new generation of multi-directional or polarimetric instruments – such as AATSR, MISR and APS (planned for JPSS but recently demanifested) – can provide detailed information. Today, MODIS, MERIS, MISR, and POLDER-P offer better optical depth at different frequencies, enabling aerosol particle sizes, particularly over oceans, to be inferred. The development of active instruments such as ATLID and ALADIN, and laser altimeter sensors, including GLAS on ICESat, should yield much improved measurement capability. Since April 2006, CALIPSO has flown a 3-channel lidar (designed specifically to provide vertical profiles) and passive instruments, orbiting in formation with Aqua, Aura, PARASOL and CloudSat to obtain coincident observations of radiative fluxes and atmospheric state. This comprehensive set of measurements is essential for accurate quantification of global aerosol and cloud radiative effects.

Limb-sounding instruments such as ACE-FTS, SCIAMACHY, and GOMOS principally provide data on the upper troposphere and stratosphere with high vertical resolution, but horizontal resolution is relatively poor (typically of the order of a few hundred km).

Current, long-term climatologies are based upon AVHRR/3 on the NOAA and MetOp series of low Earth orbit satellites. These observations will continue to provide estimates of total column aerosol amounts over the ocean. AVHRR/3 will be replaced by a more capable visible and infrared imager, called VIIRS, on the JPSS series of satellites, starting with the preparatory NPP mission launched in October 2011. VIIRS is designed to acquire high resolution atmospheric imagery and generate a variety of applied products, including some that give information on atmospheric aerosols.

The CEOS response to the GCOS Implementation Plan recognised that no operational aerosol instruments measuring particle composition and size/shape have been yet been flown and efforts should be made to rectify this. It encouraged re-planning of the aerosol measurements envisaged by APS/JPSS and consideration of operational active sensing lidar (such as CALIPSO). CEOS committed to pursue the following action: “CEOS agencies will participate in re-planning the APS instrument removed from the planned payload of [JPSS]”.

Atmospheric Temperature Fields
Essential Climate Variables: Upper Air Temperature

With humidity, atmospheric temperature profile data are a core requirement for weather forecasting and are coordinated within the framework of CGMS (The Coordination Group for Meteorological Satellites). The data are used for numerical weather prediction (NWP), for monitoring inter-annual global temperature changes, for identifying correlations between atmospheric parameters and climatic behaviour, and for validating global models of the atmosphere.

Upper air temperatures are a key dataset for detection and attribution of tropospheric and stratospheric climate change, measured both by radiosondes and satellite instruments. Temperatures measured by high-quality radiosondes are an important reference against which satellite-based measurements can be calibrated. Upper air temperatures are important for separating the various possible causes of global change, and are vital for the validation of climate models.

Infrared (HIRS) and microwave sounders (MSU and AMSU) have been providing atmospheric profiles for almost thirty years. The microwave data in particular have become key elements of the historical climate record and equivalent measurements need to be continued into the future to sustain a long-term record. The MSU radiance record is a primary resource for this, providing essential coverage over the oceans and data for comparison and combination with radiosonde data over land.

For global NWP, polar satellites provide information on temperature with global coverage, good horizontal resolution and acceptable accuracy, but improvements in vertical resolution are needed. Performance in cloudy areas has been poor, but the microwave measurements such as AMSU have provided substantial improvements. As in the case of humidity profiles, the Aqua, MetOp, NOAA and JPSS missions offer comparable improvements in vertical resolution for measuring atmospheric temperature (using AIRS+, AMSU-A, CrIS, HIRS, IASI, MSU).

For regional NWP, polar orbiting satellites provide information on temperature with acceptable accuracy and good horizontal resolution, but with marginal temporal frequency and vertical resolution for mesoscale prediction. Advanced radiometers or interferometers planned for future satellites should improve on the vertical resolution and accuracy of current radiometers. Geostationary satellites provide frequent radiance data, but their use over land is hindered because of the difficulty in estimating surface emissivity. In nowcasting, the temperature and humidity fields are particularly useful for determining atmospheric stability for predicting precipitation type, the amount of frozen precipitation, and convective storms. As with humidity profiles, nowcasting predictions using atmospheric temperature data benefit from hourly geostationary infrared soundings (such as from the GOES and MSG series – with these missions now capable of providing such data at 15 minute intervals).

The combination of the HIRS/3 and AMSU instruments on the NOAA and MetOp series allows improved information, sufficient to infer temperature within several thick layers in the vertical. On the MetOp series, IASI is used with other instruments to deliver very precise sounding capacity.

IASI data assimilation has significantly improved NWP forecasts. CrIS on the JPSS series, which will replace HIRS, is designed to enable retrievals of atmospheric temperature profiles at 1K accuracy for 1 km layers in the troposphere. The GRAS instrument on MetOp provides temperature information of high accuracy and vertical resolution in the stratosphere and upper troposphere (helping to improve analyses around the tropopause) using a GPS radio occultation (RO) technique. Its information will thus be complementary to that provided by the passive sounding instruments on MetOp. China’s FY-2 series of satellites (FY-2D, E & F), features improved measurements from October 2004 with the addition of new spectral channels to their IVISSR instrument.

GPS radio occultation (RO) measurements provide high vertical resolution profiles of atmospheric refractive index that relate directly to upper air temperatures. They provide independent observations that can be utilised to calibrate all other data. Instruments are being flown on multiple low Earth orbiting satellites (such as SAC-C and the COSMIC constellation). Systems need to be developed for real-time data exchange and use, implemented into operational meteorological data streams. Plans also need to be made to ensure future RO instruments and platforms, including on operational meteorological satellites.

In response to the GCOS IP, CEOS undertook to ensure continuity of GPS RO measurements with, at a minimum, the spatial and temporal coverage established by COSMIC by 2011. CEOS will also continue efforts to exploit the complementary aspects of radiometric and geometric upper air determinations of temperature and moisture.

Water Vapour
Essential Climate Variables: Water Vapour

The observations for water vapour (atmospheric humidity) are a core requirement for weather forecasting and are largely dealt with in the framework of the Coordinating Group for Meteorological Satellites (CGMS).

A wide range of sensors is available to measure column water vapour – microwave imagers like SSM/I and traditional imagers like AVHRR or MERIS on LEO platforms, and GOES and SEVIRI on GEO platforms. Vertical profiles are provided by microwave sounders like SSM/T2, AMSU-B, HIRS/4 and MHS, by hyperspectral infrared sounders like IASI and AIRS, or by radio-occultation observations provided by GPSMET or GRASS on MetOp. These data are supplemented by instruments on Aqua (AIRS+, AMSU-A), Aura (HiRDLS, MLS, TES), and the FY-3 series (MWHS), amongst others.

All of these are being improved as technology allows. In broad terms the challenges are to improve vertical resolution of observations and temporal sampling, to overcome cloud problems and improve the ability to process sounding data over land. For instance, JPSS will feature the combination of the CrIS interferometer and ATMS sounder to derive accurate water vapour profiles.

The 3-dimensional field of humidity is a key variable for global and regional weather prediction (NWP) models that are used to produce short- and medium-range forecasts of the state of the troposphere and lower stratosphere. Polar satellites provide information on tropospheric humidity with global coverage, good horizontal resolution and acceptable accuracy, but with poor vertical resolution.

In the case of observations for regional NWP models, polar and (mainly) geostationary satellites provide estimates of total column water vapour accurate to within 10–20%. Enough information is collected to infer moisture concentration within several thick layers vertically, with good horizontal resolution. Vertical resolution is marginal for mesoscale prediction, and the infrared information is available only for cloud-free fields of view. Despite this coarse vertical resolution, the high temporal resolution of the geostationary satellite observations allows derivation of products like the instability index for convective initiation, which is used for nowcasting applications.

Until recently, performance in cloudy areas was poor, but the new microwave measurements from AMSU offer substantial improvements. Geostationary infrared soundings (e.g. by the GOES sounders and SEVIRI on MSG) are also helping to expand coverage in some regions by making measurements on repeat timescales of fifteen minutes to one hour, thus creating more cloud-free observations.

Over oceans, coverage is currently supplemented by information on total column water vapour from microwave imagers.

Satellite sounding data are difficult to use over land, but progress in data interpretation is expected in the near future. Recent research has shown that the GPS-based radio occultation (RO) technique also has the potential to provide, in the middle to lower troposphere, high resolution profiles of atmospheric refractivity, combining the effects of temperature and water vapour in this region of the atmosphere.

In response to the GCOS IP, CEOS undertook to ensure continuity by 2011 of GPS RO measurements with, at a minimum, the spatial and temporal coverage established by COSMIC. CEOS will also continue efforts to exploit the complementary aspects of radiometric and geometric determinations of temperature and moisture in the upper air.

Atmospheric Winds
Essential Climate Variables: Upper Air Winds

Measurements of atmospheric winds are of primary importance to weather forecasting, and as a variable in the study of global climate change. Upper air wind speed and direction is a basic element of the climate system that influences many other variables.

Horizontal wind may be inferred by motion vectors or by humidity and ozone tracers in geostationary imagery. Substantial information can be derived by these methods but quality control is difficult and vertical resolution is poor. Planned instruments for geostationary satellites promise improved information, but the limited vertical resolution and the problems of accurate height assignment of winds will remain areas to be improved.

For global NWP models, wind profile information – mostly over land – is available mainly from radiosondes. Satellite Doppler wind lidar technology is being developed to provide line-of-sight wind profiles of acceptable coverage and vertical resolution, but thick cloud is a limitation. Geostationary imagers offer wind profile information by cloud tracking, or through tracking of highly-resolved features in the water vapour channels in cloud-free areas. Coverage may be supplemented in future by tracking ozone features in satellite imagery. Regional NWP models also rely heavily on radiosondes (over land) and aircraft (over ocean and over the poles) for atmospheric wind profile measurements, but they would benefit from improved satellite data.

At present, geostationary multi-channel visible and infrared imagers, such as INSAT, SEVIRI and VISSR, are used to measure cloud and water vapour motion vectors from which tropospheric wind estimates may be derived. Atmospheric motion vectors generated from the global ring of geostationary imagers provide improved data in terms of coverage, spatial and temporal resolution, and accuracy of both wind vectors and height assignment. Though valuable, because they offer wind information in areas of the world where otherwise there would be none, atmospheric wind vectors are single level observations which are only available where there are suitable image features to be tracked. Geostationary satellite measurements have been recently supplemented by the addition of water vapour wind motions from polar orbiters (MODIS). Plans need to be made to continue the polar orbiting wind measurements.

In the longer term, laser instruments such as Doppler lidars offer the promise of directly measuring clear air winds and winds within optically thin aerosol and cloud layers. Although such active instruments will provide a global coverage of vertically resolved, highly accurate measurements, the coverage offered by polar missions, such as that planned for the research-oriented ALADIN, is limited to measurements twice a day along the satellite line of sight.

Hyperspectral observations are needed to improve the vertical resolution of atmospheric motion vectors derived from geostationary satellite observations – especially in clear areas. The first opportunity for these observations may be the IRS payload on EUMETSAT’s MTG-S-1 mission.

CEOS identified two actions in response to the GCOS requirements:

— to commit to reprocessing the geostationary satellite data for use in reanalysis projects before the end of the decade;

— to identify options for continuing improvements to wind determinations demonstrated by MODIS and to be demonstrated with ALADIN on the ADM-Aeolus mission.

Cloud Type, Amount and Cloud Top
Temperature
Essential Climate Variables: Cloud Properties

The study of clouds, their location and characteristics, plays a key role in the understanding of climate change. Low, thick clouds primarily reflect solar radiation and cool the surface of the Earth. High, thin clouds primarily transmit incoming solar radiation, but at the same time they trap some of the outgoing infrared radiation emitted by the Earth and radiate it back downward, thereby warming the surface. The Earth’s climate system constantly adjusts in a way that tends toward maintaining a balance between the energy that reaches the Earth from the Sun and the energy that is reradiated from Earth into space. This process is known as Earth’s ‘radiation budget’. The components of the Earth System that are important to the radiation budget are the planet’s surface, atmosphere and clouds.

The IPCC points out that even the most advanced climate models cannot yet simulate all aspects of climate, and that there are particular uncertainties associated with clouds and their interaction with radiation and aerosols.

Weather forecasters are able to draw on a range of satellite data on clouds in improving models and in making forecasts. For both global and regional NWP models, satellite instruments offer detailed information on cloud coverage, type, growth and motion. The coverage is global from polar orbiting satellites and (with the exception of high latitudes) geostationary satellites. Infrared imagers and sounders can provide information on cloud cover and cloud top height with good horizontal and temporal resolution. Hyperspectral observations in the 14 mm band are ideal to derive accurate cloud top height information. For example, observations in the oxygen A band by SCIAMACHY, MERIS (Envisat) and GOME-2 (MetOp) are used to derive cloud top pressure in an independent way. By using observations in the NIR part of the spectrum, for example from AVHRR observations, bulk cloud properties such as liquid water content can be derived.

Passive microwave imagers and sounders (SSM/I, AMSU/B, MHS) give information on cloud liquid water, cloud ice and precipitation. Microwave information is valuable for regional mesoscale models which have sophisticated parameterisation of cloud physics. In the context of nowcasting and very short range forecasting, meteorological satellite data are well suited to monitoring the rapid development of precipitation-generating systems in space and time.

In the field of climate research, the MODIS and MISR spectroradiometers on the Terra mission are enabling viewing of cloud features at higher resolutions than were previously available. MODIS measurements allow more precise determination of the contribution which clouds make to the greenhouse warming of the Earth. MISR is observing angles at which sunlight is reflected from clouds. These observations are critical in support of new research on the radiative properties of clouds. Also on the Terra mission, the ASTER radiometer, which measures visible and infrared wavelengths, complements the other instruments by providing high resolution views of specific targets of interest.

For weather forecasting, satellite instruments will continue to offer a wealth of useful information on clouds. On polar orbiting missions, HIRS, AMSU-A, MHS and IASI offer improved information on clouds. Geostationary imagers and sounders (on MSG, GOES, Elektro-L, INSAT, MTSAT and FY-3 series) will contribute to retrieval of information about cloud cover, cloud top temperature, cloud top pressure and cloud type, and will be close to meeting regional NWP modelling needs for these variables.

Retrievals will not only comprise the temperature and moisture profiles, but also fractional cloud cover, cloud top height, cloud top pressure, surface temperature and surface emissivity from both infrared and microwave soundings.

The increased use of imagery data to determine cloud amount will improve the performance and the number of retrieved profiles. In general, IASI will increase sounding performance to a level very significant for global and regional NWP. On the JPSS series of satellites, parameters that may be derived from VIIRS will include cloud cover.

The WCRP International Satellite Cloud Climatology Project (ISCCP) has developed a continuous data record of infrared and visible radiances since 1983, utilising both geostationary and low Earth orbiting meteorological satellite data. A range of products have been derived, but unfortunately the record suffers from inhomogeneities. Reprocessing the data to account for orbital drift and other issues has helped reduce uncertainties in the observations.

The active satellite instruments on board CloudSat, CALIPSO and EarthCARE will be crucial for the validation of cloud parameters observed by passive instruments, in particular cloud top height and type. EarthCARE will provide new insights by observing with lidar, radar, multi-spectral imager and a broad-band radiometer in synergy.

Cloud Particle Properties and Profile
Essential Climate Variables: Cloud Properties

A key to predicting climate change is to observe and understand the global distribution of clouds, their physical properties – such as thickness and droplet size – and their relationship to regional and global climate. Whether a particular cloud will heat or cool the Earth’s surface depends on the cloud’s radiating temperature – and thus its height – and on its albedo for both visible and infrared radiation, which depends on the number and details of the cloud properties. As clouds interact with radiation at all wavelengths, a multitude of observations can be used to infer cloud properties.

Because clouds change rapidly over short time and space intervals, they are difficult to quantify from low Earth orbits. High temporal sampling provided by geostationary satellites is better suited to monitor rapidly changing conditions, albeit on a regional scale. Full 3D observations of cloud structure is a new capability that has been provided by CloudSat and CALIPSO since 2006 and will eventually be offered by ESA’s planned EarthCARE mission. Together, these missions are capable of measuring the vertical structure of a large fraction of clouds and precipitation, from very thin cirrus clouds to thunderstorms producing heavy precipitation. However, the CALIPSO lidar is unable to penetrate thick clouds and the radar on CloudSat cannot penetrate heavier rain.

Traditionally basic macro- and micro-physical information on the structure of clouds (i.e. determination of whether water or ice particles are present) is being obtained from VIS and IR multi-spectral imagery, such as that provided by MODIS and MISR on Terra in LEO, and GOES and SEVIRI in GEO. These measurements are important for climate purposes as the structure of clouds (particle size and phase) greatly affects their optical properties, and hence their albedo. This has been demonstrated by the WCRP International Satellite Cloud Climatology Project which, since 1983, has provided a record of cloud properties derived from multi-spectral VIS/IR imagery observations that were initially collected for operational meteorological applications.

Together with cloud top temperatures, information on the 3D structure of clouds can be used as a basic tool for the real time surveillance of features such as thunderstorms. Microwave observations provided by instruments such as SSM/I on DMSP and AMSU-A, and MHS on NOAA and EUMETSAT polar platforms, have enhanced capabilities over the VIS and IR multi-spectral observations through their ability to probe the entire cloud and not only the cloud top. However, one limitation of these sensors is their coarse spatial resolution.

Additional phase and cloud particle information is available from polarimetric radiometers such as POLDER and from the polarisation measurement devices of SCIAMACHY on Envisat and GOME-2 on MetOp. As these instruments observe the UV-VIS-NIR part of the spectrum at moderate spectral resolution, very accurate information on macro-physical cloud properties can be obtained. However, for detailed process studies, the users’ requirements for cloud data are unlikely to be met until data from instruments such as ATLID or the cloud profiling radar on EarthCARE become available.

A good example of international cooperation is the multiple satellite constellation comprising CloudSat, Aqua, Aura, CALIPSO and PARASOL, which has flown in orbital formation since April 2006. Its objectives are to gather data needed to evaluate and improve the way clouds are represented in global models, and to develop a more complete knowledge of their poorly understood role in climate change and the cloud-climate feedback. CloudSat maintains a tight formation with CALIPSO, with a goal of overlapping measurement footprints at least 50% of the time. CALIPSO carries a dual-wavelength, polarisation-sensitive lidar that provides high resolution vertical profiles of aerosols and clouds. CloudSat and CALIPSO maintain a somewhat looser formation behind Aqua, which carries a variety of passive microwave, infrared, and optical instruments.

EarthCARE (launch 2013) will fly a cloud/aerosol lidar, cloud radar, multi-channel imager and broad-band radiometer for measuring clouds and aerosols simultaneously with TOA radiances.

In responding to the GCOS IP, CEOS recognised that accurate measurement of cloud properties has proved to be exceedingly difficult. CEOS agreed to support investigations of cloud properties and cloud trends from combined satellite imager and sounder measurements (with horizontal as well as vertical information) using Cloudsat/CALIPSO for validation.

Liquid Water and Precipitation Rate
Essential Climate Variables: Precipitation and Cloud Properties

Water forms one of the most important constituents of the Earth’s atmosphere and is essential for human existence. The global water cycle is at the heart of the Earth’s climate system, and better predictions of its behaviour are needed for monitoring climate variability and change, weather forecasting and sustainable development of the world’s water resources. A better understanding of the current distribution of precipitation, and of how it might be affected by climate change, is vital in support of accurate predictions of regional drought or flooding.

Information on liquid water and precipitation rate is used for initialising NWP models. A variety of satellites provide complete global NWP coverage, but they present two major challenges. Firstly, the satellite sensors (such as visible/IR imagers on geostationary weather satellites) typically observe quantities (such as cloud height and cloud top temperature) related to precipitation, so algorithms must be developed to get the best estimates from each particular sensor. Secondly, the mix of available data is constantly changing in space and time.

The new generation of geostationary imagers, available since the start of EUMETSAT’s Meteosat Second Generation, also allows for the observation of cloud liquid water path and particle size at high temporal resolution (15 min).

Microwave imagers and sounders offer information on precipitation of marginal horizontal and temporal resolution, acceptable to marginal accuracy (though validation is difficult). Satellite-borne rain radars (e.g. on TRMM and CloudSat), together with plans for constellations of microwave imagers, offer most potential for improved observations. For regional NWP, no satisfactory precipitation estimates are available from satellites at present, although they are the only potential source of information over the oceans. Geostationary satellites do provide vital information on the location of tropical cyclones.

Increasing amounts of useful microwave data – such as those from the TRMM mission – are becoming available. TRMM was dedicated to studying tropical and sub-tropical rainfall and carried the first spaceborne precipitation radar, JAXA’s PR instrument, and NASA’s TMI microwave imager. Data from PR and TMI have provided new insights into the internal composition of tropical thunderstorms associated with hurricanes. NASA, JAXA and partner agencies plan to continue this collaboration in future to develop the Global Precipitation Measurement (GPM) constellation of satellites that will launch from 2013 onwards. The GPM series will provide global observations of precipitation every three hours to help develop the understanding of the global structure of rainfall and its impact on climate. The CNES-ISRO Megha-Tropiques mission (launched October 2011) will provide further measurements of tropical rainfall; MADRAS, a passive multi-frequency radiometer, will collect data on rain over the oceans.

The 94 GHz cloud radars on CloudSat and (from 2013) EarthCARE provide complimentary information on light precipitation. EarthCARE’s Doppler capability will provide additional detail on sedimentation velocities.

Future coordination of these satellite programmes, as well as the efforts of the in situ measurement community, was addressed by the Integrated Global Water Cycle Observations Theme (IGWCO) of the IGOS Partnership. The first element of IGWCO is a ‘Coordinated Enhanced Observing Period (CEOP)’ which is taking the opportunity of the simultaneous operation of key satellites of Europe, Japan and USA to generate new data sets of the water cycle.

The IGWCO Theme report is available from http://www.earthobservations.org/wa_igwco.shtml. This document represents a comprehensive overview of the state-of-the-art in water cycle observations and formulates recommendations for an international work programme to better understand, monitor and predict water processes.

To meet GCOS IP needs, CEOS agencies will ensure continued improvements to precipitation determinations demonstrated by TRMM and planned by GPM from 2013. JAXA and NASA are leading a CEOS study team to establish the basis for a future Global Precipitation Constellation – building on GPM to incorporate measurements from more countries over an extended period.

Ozone
Essential Climate Variables: Ozone

Ozone (O3) is a relatively unstable molecule, and although it represents only a tiny fraction of the atmosphere, it is crucial for life on Earth. Depending on its location, ozone can protect or harm life on Earth. Most ozone resides in the stratosphere, where it acts as a shield to protect the surface from the Sun’s harmful ultraviolet radiation. In the troposphere, ozone is a harmful pollutant which causes damage to lung tissue and plants. Man-made chemicals and weather conditions over Antarctica combine to deplete stratospheric ozone concentrations during the southern hemisphere’s winter.

The total amount of O3 in the troposphere is estimated to have increased by 36% since 1750, due primarily to anthropogenic emissions of several O3-forming gases.

Satellite instruments have for many years provided data measuring interactions within the atmosphere that affect ozone, and more advanced sensors will soon be in orbit to collect more detailed measurements, increasing knowledge of how human activities are affecting the protective ozone layer.

Total column measurements of ozone have been provided over long periods by NASA’s TOMS and NOAA’s SBUV instruments. Stratospheric ozone profiles have also been measured by instruments such as HALOE and MLS (UARS mission), GOME (ERS-2), and SAGE III (part of the International Space Station payload).

Since launch in March 2002, GOMOS, MIPAS and SCIAMACHY on ESA’s Envisat mission have provided improved observations of the concentration of ozone and trace gases in the stratosphere. Operation of GOME-2 on EUMETSAT’s MetOp satellites guarantees the continuity of these observations for another decade.

A wide range of instruments dedicated to, or capable of, ozone measurements are planned for the next decade. On the recently launched Aura mission, HIRDLS, OMI and MLS study and monitor atmospheric processes which govern stratospheric and mesopheric ozone, and continue the TOMS record of total ozone measurements. TES on Aura is used to create three-dimensional maps of ozone concentrations in the troposphere. AIRS on Aqua (and, in future, CrIS on NPP/JPSS) also supplies an ozone product that has some application in the lower stratosphere and also can be used to identify regions of stratospheric/tropospheric mixing.

IASI and GOME-2 on the MetOp series have provided information since early 2007 on both total column ozone and vertical profile. The Ozone Profiler on China’s FY-3 series has contributed further data since its launch in 2008.

Though the infrared imagers on the GOES and Meteosat geostationary platforms have limited capabilities to provide vertical information on ozone, they provide total stratospheric ozone amount with a high temporal resolution. This information can be used to depict stratospheric dynamical processes, relevant for NWP applications.

The IGOS theme on Atmospheric Chemistry Observations (IGACO) has developed a strategy for the integrated provision of chemistry observations (and associated meteorological parameters) required to realise the theme’s objectives, including the monitoring of atmospheric composition parameters related to climate change.

The CEOS response to the GCOS IP acknowledged that profiles of ozone were to be addressed by the JPSS OMPS, but that instrument has been removed from the payload manifest. Furthermore, the discontinuation of solar occultation measurements will profoundly impact one of the climate data record pillars of ozone assessments.

Radiation Budget
Essential Climate Variables: Earth Radiation Budget (Including Solar Irradiance)

The Earth’s radiation budget is the balance within the climate system between the energy that reaches the Earth from the Sun and the energy that returns from Earth to space. Satellite measurements offer a unique means of assessing the Earth’s radiation budget. The goal of such measurements is to determine the amount of energy emitted and reflected by the Earth. This is necessary to understand the processes by which the atmosphere, land and oceans transfer energy to achieve global radiative equilibrium, which in turn is necessary to simulate and predict climate.

Systematic observations of the Earth System energy balance components are noted by the IPCC as being of key importance in narrowing the uncertainties associated with the climate system. In addition to these continuous global measurements of the radiation budget, which are necessary both to estimate any long term climatic trends and shorter term variations overlying these trends, measurements on a regional scale are useful to understand better the dynamics of certain events or phenomena and to assess the effect of climate change, for example on agriculture and urban areas.

In general, three types of measurements are currently possible:

— the shortwave and longwave radiation budget at the top of the atmosphere;

— the shortwave radiation budget at the Earth’s surface;

— the total incoming broadband radiation flux.

Since the mid-1960s, NASA has been measuring the net radiation with the ERBE, ACRIM, and CERES sensors. The MISR spectroradiometer (also on Terra with CERES) provides data on the top of the atmosphere, cloud and surface hemispheric albedos, and aerosol opacity. Continuity of Total Solar Irradiance (TSI) measurements was assured by the launch of the SORCE mission at the beginning of 2003, carrying 4 instruments (TIM, SOLSTICE, SIM, XPS) that operate over the 1 nm–2000 nm waveband and measure over 95% of the spectral contribution to TSI. ESA’s EarthCARE will embark a broadband radiometer (BBR) together with instruments providing profile information (ATLID, CPR).

The French-Indian mission Megha-Tropiques (launched October 2011) carries the broadband ScaRaB radiometer, similar to the instrument flown in the mid-1990s on the Russian Meteor satellites, for ERB measurements over the tropical and equatorial regions.

An increasing number of radiation budget measurements are featuring on operational meteorology missions. These include: GERB (operating since September 2002 on Meteosat and measuring shortwave and longwave radiation every 15 minutes from a geostationary orbit); TSIS on JPSS; and continued narrowband information from the HIRS, AVHRR, SEVIRI (top of atmosphere and surface radiative fluxes) and VIIRS instruments.

An important component of the Earth Radiation Budget is the Outgoing Longwave Radiation (OLR). This is calculated from multi-spectral infrared imager observations, such as those from AVHRR or imagers on geostationary platforms.

The past multi-satellite record of measurements suffers from an absence of absolute calibration. It is recognised that development of absolute, spectrally resolved measurements is needed to provide information on variations in climate forcings and responses, and to calibrate the operational meteorological satellite sensors. In addition, there is the likelihood of a measurements gap after 2020; the recent change in JPSS plans for climate-relevant measurements calls for coordinated re-planning efforts.

In support of the GCOS IP, CEOS aims by 2011 to make absolute, spectrally resolved measurements of radiance emitted and reflected to space by the Earth for information on variations in both climate forcings and responses. CEOS agencies will also participate in re-planning ERBS removed from the planned payload of JPSS-1.

Trace Gases (Excluding Ozone)
Essential Climate Variables: Carbon Dioxide, Methane and Other Greenhouse Gases

Trace gases other than ozone may be divided into three categories:

— greenhouse gases affecting climate change;

— chemically aggressive gases affecting the environment (including the biosphere);

— gases and radicals impacting on the ozone cycle, thereby affecting both climate and environment.

The presence of trace gases in the atmosphere can have a significant effect on global change as well as potentially harmful local effects through increased levels of pollution. The chemical composition of the troposphere, in particular, is changing at an unprecedented rate. Meanwhile, the rate at which pollutants from human activities are being emitted into the troposphere is now thought to exceed that from natural sources (such as volcanic eruptions).

As explained in Part I of this document, the IPCC noted in 2007 that:

— changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation alter the energy balance of the climate system;

— global greenhouse gas emissions due to human activities have grown since pre-industrial times, with an increase of 70% between 1970 and 2004;

— carbon dioxide (CO2) is the most important anthropogenic greenhouse gas. Its annual emissions grew by about 80% between 1970 and 2004.

The IPCC concluded that “most of the observed increase in globally averaged temperatures since the mid-20th century is very likely (over 90% probability) due to the observed increase in anthropogenic (man-made) greenhouse gas concentrations”. They consider that reductions in greenhouse gas emissions and the gases that control their concentration would be necessary to stabilise radiative forcing.

Measurements from satellite sensors have already made an important contribution to the recognition that human activities are modifying the chemical composition of both the stratosphere and the troposphere, even in remote regions.

A variety of instruments provide measurements on the concentration of trace gases. In general, high spectral resolution is required to detect absorption, emission and scattering from individual species. Some instruments offer measurements of column totals, i.e. integrated column measurements, whilst others provide profiles of gas concentration through the atmosphere (usually limited to the upper troposphere and stratosphere, using limb measurements).

To date, the instruments on UARS (operated 1991–2005) have provided the most significant source of data on trace gases and have been vital for studies of stratospheric chlorine chemistry, stratospheric tracer-tracer correlation, tropospheric water vapour, the chemistry of the Arctic lower stratosphere in winter, and tropospheric aircraft exhaust studies

The last few years have seen the arrival of new and significant capabilities, with advanced instruments on Terra (MOPITT, providing global measurements of carbon monoxide and methane in the troposphere), and Envisat (GOMOS, MIPAS and SCIAMACHY, providing profiles of trace gases through the stratosphere and troposphere). AIRS (on Aqua) and IASI (on MetOp) also contribute to such information and their sounder products can help quantify atmospheric mixing and help determine sources and sinks.

On NASA’s Aura mission, HiRDLS, an infrared limb-scanning radiometer, carries out soundings of the upper troposphere, stratosphere and mesosphere to determine concentrations of trace gases, with horizontal and vertical resolutions superior to those previously obtained. On the same mission, MLS measures concentrations of trace gases for their effects on ozone depletion, TES provides a primary input to a database of 3D distribution on global, regional and local scales of gases important to tropospheric chemistry, and OMI continues the TOMS record for atmospheric parameters related to ozone chemistry and climate. JAXA’s GOSAT mission (launched 2009) and NASA’s OCO-2 mission (from 2012) are expected to make significant contributions to observations of trace gases, particularly carbon dioxide and methane.

The IGOS IGACO Theme for observations of atmospheric chemistry has considered all relevant chemical species to interpret properly the observations and intends to monitor the research required to improve understanding of Earth processes so that air quality evolution can be predicted. ESA is considering atmospheric composition missions (such as TRAQ, PREMIER and A-SCOPE) to meet these needs.

The CEOS Response to the GCOS IP cautions that demonstrations of potential future operational measurements are neither complemented by plans for operational implementation nor any R&D follow-on. CEOS agencies will participate in planning, by 2011, the current chemistry missions and those planned for the next 5 to 7 years.