Philippe Ciais

Recent decline in the global land evapotranspiration trend due to limited moisture supply

More than half of the solar energy absorbed by land surfaces is currently used to evaporate water. Climate change is expected to intensify the hydrological cycle and to alter evapotranspiration, with implications for ecosystem services and feedback to regional and global climate. Evapotranspiration changes may already be under way, but direct observational constraints are lacking at the global scale. Until such evidence is available, changes in the water cycle on land—a key diagnostic criterion of the effects of climate change and variability—remain uncertain. Here we provide a data-driven estimate of global land evapotranspiration from 1982 to 2008, compiled using a global monitoring network, meteorological and remote-sensing observations, and a machine-learning algorithm. In addition, we have assessed evapotranspiration variations over the same time period using an ensemble of process-based land-surface models. Our results suggest that global annual evapotranspiration increased on average by 7.1 ± 1.0 millimetres per year per decade from 1982 to 1997. After that, coincident with the last major El Niño event in 1998, the global evapotranspiration increase seems to have ceased until 2008. This change was driven primarily by moisture limitation in the Southern Hemisphere, particularly Africa and Australia. In these regions, microwave satellite observations indicate that soil moisture decreased from 1998 to 2008. Hence, increasing soil-moisture limitations on evapotranspiration largely explain the recent decline of the global land-evapotranspiration trend. Whether the changing behaviour of evapotranspiration is representative of natural climate variability or reflects a more permanent reorganization of the land water cycle is a key question for earth system science.

Negative emissions physically needed to keep global warming below 2 °C

To limit global warming to <2 °C we must reduce the net amount of CO2 we release into the atmosphere, either by producing less CO2 (conventional mitigation) or by capturing more CO2 (negative emissions). Here, using state-of-the-art carbon-climate models, we quantify the trade-off between these two options in RCP2.6: an Intergovernmental Panel on Climate Change scenario likely to limit global warming below 2 °C. In our best-case illustrative assumption of conventional mitigation, negative emissions of 0.5-3 Gt C (gigatonnes of carbon) per year and storage capacity of 50-250 Gt C are required. In our worst case, those requirements are 7-11 Gt C per year and 1,000-1,600 Gt C, respectively. Because these figures have not been shown to be feasible, we conclude that development of negative emission technologies should be accelerated, but also that conventional mitigation must remain a substantial part of any climate policy aiming at the 2-°C target.

Integrating Global Models of Terrestrial Primary Productivity

The global integration of net primary productivity (NPP) is one of the major achievements of carbon cycle research over the past years. Models of various types, levels of complexity, and comprehensiveness have been developed with the aim of predicting not only NPP, but also other carbon and sometimes water fluxes exchanged between the biosphere and the atmosphere. In the context of understanding the future behavior of terrestrial productivity during the ongoing anthropic perturbation of the carbon and nutrient cycles, it is fundamental to rely on models that are validated against independent global data sets. In that respect, satellite measurements of normalized difference vegetation index (NDVI) offer an unsurpassed tool to monitor the seasonal, interannual, and decadal variability of the vegetation. It is reasonable to believe that longer time series of NDVI, as well as better sensors, will refine the constraints brought on by NDVI. Additionally, measurements of atmospheric carbon dioxide and related tracers provide indirect estimates of the terrestrial carbon fluxes. At present, inversions of the atmospheric measurements are performant to infer the carbon dioxide budget as a function of latitude, and to a certain extent, at the continental level. Improving carbon dioxide monitoring over the interior of the continents (using aircraft vertical profiles) will help better estimate the spatial patterns of the biospheric fluxes.

Los posibles escenarios energéticos con vistas al cumplimiento del Acuerdo de París

Fallas Henriquez, A., Molina-Murillo, S. 2017. Methodological proposal to quantify and to compensate the agroecosystem services generated by the good agricultural practices of small-farmers. Ecosistemas 26(3): 89-102. Doi.: 10.7818/ECOS.2017.26-3.11 A diversified and less intensive management of agricultural systems produces agroecosystem services that are often not estimated nor compensated in developing countries. The main objective of this study was to create a tool that allows quantification of agroecosystemic services (SAgro) generated in agroforestry peasant farms, and additionally estimate compensation values. Following an initial test in 2013 on 10 farms, an important bibliographical review was carried out, the consultation of experts, and the evaluation in 2015 of 50 farms at the national level, most of them members of the National Agroforestry Union (UNAFOR) of Costa Rica. As a result, a tool was developed to measure in an agile, flexible and economical way the ecosystem services generated by good agricultural practices produced by small producers (peasants) both for their agricultural system as well as the forest system. We also found that the size of farms is not necessarily the determining factor for the production of agroecosystem services, but rather the type of management practices and investments made. In this process, it was estimated that with an average compensation of US

Towards robust regional estimates of CO2 sources and sinks using atmospheric transport models

271.6 per agricultural hectare per year, the production or maintenance of agroecosystem services could be encouraged through good agricultural practices.