The primary environmental science challenge in the early 21st century is to improve our understanding of how global changes are affecting the Earth System, and the feedbacks within this system, in order that human societies can assess their likely impacts and adopt ways to mitigate and adapt to them. Deeply embedded in the functioning of the Earth system is the carbon cycle, which consists of intermeshed processes by which carbon is exchanged between the atmosphere, land and ocean (Figure 1). Quantifying this global-scale cycle is fundamental to understanding many of the dramatic changes taking place on Earth because of its close connection with both fossil fuel combustion and land use change. These are the two most significant drivers of global change, leading to increases in atmospheric CO2 and the associated global warming (IPCC, 2007). 5)

Terrestrial processes play a crucial role in the carbon cycle through the carbon uptake and respiration associated with plant growth, emissions due to the disturbance of natural processes (e.g. wildfires) and anthropogenic land use change. There is strong evidence that over the last 50 years the terrestrial biosphere has acted as a net carbon sink, removing from the atmosphere approximately one-third of the CO2 emitted in the process of fossil fuel combustion (Canadell et al., 2007).6) However, the status, dynamics and evolution of the terrestrial biosphere are the least well understood and most uncertain elements of the carbon cycle.

Consequently, the UN Framework Convention on Climate Change (UNFCCC) has identified biomass as an Essential Climate Variable that is needed to reduce uncertainties in our knowledge of the climate system (GCOS, 2004). 7)While global observation programs for most terrestrial ECVs (Essential Climate Variables) are advanced or evolving, there is currently no such effort for biomass (Houghton et al., 2009). 8) In addition, the sequestration of carbon in forest biomass is the only mechanism for mitigating climate change recognized under the Kyoto Protocol, other than reduced emissions.

Scientific Objectives: The Biomass mission will address a fundamental gap in our understanding of the land component of the Earth system, which is the status and the dynamics of Earth's forests, as represented by the distribution of forest biomass and its changes. With accurate, frequent and global information on these forest properties at a spatial scale of 200 m, it will be possible to address a range of critical issues with far-reaching scientific and societal consequences. In particular, the Biomass mission will help to:

- reduce the large uncertainties in the carbon flux due to changes in land use

- provide scientific support for international treaties, agreements and programs such as the UN's REDD (Reducing Emissions from Deforestation and Forest Degradation in Developing Countries) program

- improve understanding and predictions of landscape-scale carbon dynamics

- provide observations to initialize and test the land element of Earth system models

- provide key information for forest resources management and ecosystem services.

The Biomass mission will explore Earth's surface for the first time at the P-band wavelength, making observations that could have a wide range of as yet unforeseen applications, such as for mapping subsurface geological features in deserts in support of palaeohydrological studies and in ice sheets, and the surface topography of areas covered by dense vegetation.

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Background: The primary environmental science challenge in the early 21st century is to improve our understanding of how global changes are affecting the Earth System, and the feedbacks within this system, in order that human societies can assess their likely impacts and adopt ways to mitigate and adapt to them. Deeply embedded in the functioning of the Earth system is the carbon cycle, which consists of intermeshed processes by which carbon is exchanged between the atmosphere, land and ocean (Figure 1). Quantifying this global-scale cycle is fundamental to understanding many of the dramatic changes taking place on Earth because of its close connection with both fossil fuel combustion and land use change. These are the two most significant drivers of global change, leading to increases in atmospheric CO2 and the associated global warming (IPCC, 2007). 5)

Terrestrial processes play a crucial role in the carbon cycle through the carbon uptake and respiration associated with plant growth, emissions due to the disturbance of natural processes (e.g. wildfires) and anthropogenic land use change. There is strong evidence that over the last 50 years the terrestrial biosphere has acted as a net carbon sink, removing from the atmosphere approximately one-third of the CO2 emitted in the process of fossil fuel combustion (Canadell et al., 2007).6) However, the status, dynamics and evolution of the terrestrial biosphere are the least well understood and most uncertain elements of the carbon cycle.

Consequently, the UN Framework Convention on Climate Change (UNFCCC) has identified biomass as an Essential Climate Variable that is needed to reduce uncertainties in our knowledge of the climate system (GCOS, 2004). 7)While global observation programs for most terrestrial ECVs (Essential Climate Variables) are advanced or evolving, there is currently no such effort for biomass (Houghton et al., 2009). 8) In addition, the sequestration of carbon in forest biomass is the only mechanism for mitigating climate change recognized under the Kyoto Protocol, other than reduced emissions.

Scientific Objectives: The Biomass mission will address a fundamental gap in our understanding of the land component of the Earth system, which is the status and the dynamics of Earth's forests, as represented by the distribution of forest biomass and its changes. With accurate, frequent and global information on these forest properties at a spatial scale of 200 m, it will be possible to address a range of critical issues with far-reaching scientific and societal consequences. In particular, the Biomass mission will help to:

- reduce the large uncertainties in the carbon flux due to changes in land use

- provide scientific support for international treaties, agreements and programs such as the UN's REDD (Reducing Emissions from Deforestation and Forest Degradation in Developing Countries) program

- improve understanding and predictions of landscape-scale carbon dynamics

- provide observations to initialize and test the land element of Earth system models

- provide key information for forest resources management and ecosystem services.

The Biomass mission will explore Earth's surface for the first time at the P-band wavelength, making observations that could have a wide range of as yet unforeseen applications, such as for mapping subsurface geological features in deserts in support of palaeohydrological studies and in ice sheets, and the surface topography of areas covered by dense vegetation.

In May 2013, ESA's Earth Observation Program Board selected the Biomass mission as its 7th Earth Explorer mission following the review of three candidate concepts. Earth Explorers are research missions dedicated to specific aspects of our Earth environment whilst demonstrating new technology in space. Earth Explorer missions focus on the atmosphere, biosphere, hydrosphere, cryosphere and the Earth's interior with the overall emphasis on learning more about the interactions between these components and the impact that human activity is having on natural Earth processes. The Biomass mission concept is set to become the next in a series of satellites developed to further our understanding of Earth. The mission aims to take measurements of forest biomass to assess terrestrial carbon stocks and fluxes for a better understanding of the carbon cycle.