NCEO NC International: Constraining Coupled Carbon & Water Cycle Processes with Earth Observation [CPEO]

The carbon and water cycles are intrinsically linked, and yet there remain large discrepancies in climate models around this coupling. Differences in the approach to modelling the response of stomatal conductance to atmospheric water vapour, and the stress on photosynthesis due to lack of available soil water, lead to significant discrepancies in the predicted response of carbon uptake by the terrestrial biosphere to hydrological changes. Understanding this is critically important for better understanding the impact of climate change on vegetation productivity and, in turn, the control of vegetation on regional climate. Working internationally and collaboratively will enable the global community to progress faster towards meeting these significant challenges; this will be enabled by a new network supported by major international scientific initiatives such as FutureEarth.

In the last decade or so there has been a significant increase in the breadth of information available from Earth Observation satellites, providing new ways of observing water and carbon processes in the Earth system. Examples with specific importance to the CPEO program include Solar Induced Fluorescence (SIF), land-atmosphere fluxes of carbonyl sulfide (COS) and vegetation optical depth (VOD); NCEO is expert in the derivation and exploitation of such data sets. SIF is the only direct observation of the process of photosynthesis available from EO missions and there is strong evidence that it tracks water stress on photosynthesis closely. Carbonyl sulfide acts as a tracer for both the water and carbon cycles. It is absorbed by plants during photosynthesis at a rate proportional to CO2, but is not respired. VOD is retrieved in the same processing as the ESA Climate Change Initiative (CCI) soil moisture data. It provides a combined measure of how much vegetation there is and how much water it holds. Crucially, unlike SIF and COS there exists a long time record of these data, which allows us to observe climate time-scale processes. The CPEO programme will work using these, and other EO data to address questions around the coupling of water and carbon in the terrestrial biosphere with the explicit purpose of improving process representation in climate and Earth system models. Progress will be considerably enhanced by some close working with international collaborators enabled by a new network linking key scientists across datasets, modelling and data assimilation.

Geographically, we will focus on North America and tropical Africa. North America, and the USA in particular, is home to data collecting networks such as NEON (the National Ecological Observatory Network) and Ameriflux which continuously monitor variables relevant to the theme of our research. These will provide an objective truth to compare our EO data and model outputs to. Areas of the USA have experienced high levels of climate variability in recent decades (for example, droughts in California) which will allow us observe how the carbon cycle, and our EO measurements, respond when the hydrological cycle is experiencing extremes. Whereas the USA provides a significant opportunity to test and develop our data products and modelling, tropical Africa is far less well understood in terms of the water and carbon cycles and is an area where climate models tend to disagree. We will apply our techniques in Africa to gain insight into climate scale processes and use these to confront climate model predictions. NCEO has a strong track record of work across Africa, including examining trace gas fluxes, understanding feedbacks between soil moisture and rainfall, and developing an Africa wide data assimilation system for JULES that efficiently ingests EO data, including SIF. We will build on this experience to deliver new understanding about the likely impacts of climate change on the continent and work, via our dedicated UK GEO/CEOS Office, to realise the benefits of this to organisations in Africa.