Papers on primary production and climate change

This is a list of papers on terrestrial net primary production and climate change. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 Through 2009 – Zhao & Running (2010) “Terrestrial net primary production (NPP) quantifies the amount of atmospheric carbon fixed by plants and accumulated as biomass. Previous studies have shown that climate constraints were relaxing with increasing temperature and solar radiation, allowing an upward trend in NPP from 1982 through 1999. The past decade (2000 to 2009) has been the warmest since instrumental measurements began, which could imply continued increases in NPP; however, our estimates suggest a reduction in the global NPP of 0.55 petagrams of carbon. Large-scale droughts have reduced regional NPP, and a drying trend in the Southern Hemisphere has decreased NPP in that area, counteracting the increased NPP over the Northern Hemisphere. A continued decline in NPP would not only weaken the terrestrial carbon sink, but it would also intensify future competition between food demand and proposed biofuel production.” Science 20 August 2010: Vol. 329. no. 5994, pp. 940 – 943, DOI: 10.1126/science.1192666.

Global potential net primary production predicted from vegetation class, precipitation, and temperature – Del Grosso et al. (2008) “Net primary production (NPP), the difference between CO2 fixed by photosynthesis and CO2 lost to autotrophic respiration, is one of the most important components of the carbon cycle. Our goal was to develop a simple regression model to estimate global NPP using climate and land cover data. Approximately 5600 global data points with observed mean annual NPP, land cover class, precipitation, and temperature were compiled. Precipitation was better correlated with NPP than temperature, and it explained much more of the variability in mean annual NPP for grass- or shrub-dominated systems (r² = 0.68) than for tree-dominated systems (r² = 0.39). For a given precipitation level, tree-dominated systems had significantly higher NPP (~100–150 g C·m^-2·yr^-1) than non-tree-dominated systems. Consequently, previous empirical models developed to predict NPP based on precipitation and temperature (e.g., the Miami model) tended to overestimate NPP for non-tree-dominated systems. Our new model developed at the National Center for Ecological Analysis and Synthesis (the NCEAS model) predicts NPP for tree-dominated systems based on precipitation and temperature; but for non-tree-dominated systems NPP is solely a function of precipitation because including a temperature function increased model error for these systems. Lower NPP in non-tree-dominated systems is likely related to decreased water and nutrient use efficiency and higher nutrient loss rates from more frequent fire disturbances. Late 20th century aboveground and total NPP for global potential native vegetation using the NCEAS model are estimated to be ~28 Pg and ~46 Pg C/yr, respectively. The NCEAS model estimated an ~13% increase in global total NPP for potential vegetation from 1901 to 2000 based on changing precipitation and temperature patterns.” Ecology, 89:2117—2126, doi:10.1890/07-0850.1. [Full text]

Europe-wide reduction in primary productivity caused by the heat and drought in 2003 – Ciais et al. (2005) “Future climate warming is expected to enhance plant growth in temperate ecosystems and to increase carbon sequestration. But although severe regional heatwaves may become more frequent in a changing climate, their impact on terrestrial carbon cycling is unclear. Here we report measurements of ecosystem carbon dioxide fluxes, remotely sensed radiation absorbed by plants, and country-level crop yields taken during the European heatwave in 2003. We use a terrestrial biosphere simulation model to assess continental-scale changes in primary productivity during 2003, and their consequences for the net carbon balance. We estimate a 30 per cent reduction in gross primary productivity over Europe, which resulted in a strong anomalous net source of carbon dioxide (0.5 Pg C yr^-1) to the atmosphere and reversed the effect of four years of net ecosystem carbon sequestration. Our results suggest that productivity reduction in eastern and western Europe can be explained by rainfall deficit and extreme summer heat, respectively. We also find that ecosystem respiration decreased together with gross primary productivity, rather than accelerating with the temperature rise. Model results, corroborated by historical records of crop yields, suggest that such a reduction in Europe’s primary productivity is unprecedented during the last century. An increase in future drought events could turn temperate ecosystems into carbon sources, contributing to positive carbon-climate feedbacks already anticipated in the tropics and at high latitudes.” Nature 437, 529-533 (22 September 2005) | doi:10.1038/nature03972. [Full text]

A Continuous Satellite-Derived Measure of Global Terrestrial Primary Production – Running et al. (2004) “Until recently, continuous monitoring of global vegetation productivity has not been possible because of technological limitations. This article introduces a new satellite-driven monitor of the global biosphere that regularly computes daily gross primary production (GPP) and annual net primary production (NPP) at 1-kilometer (km) resolution over 109,782,756 km² of vegetated land surface. We summarize the history of global NPP science, as well as the derivation of this calculation, and current data production activity. The first data on NPP from the EOS (Earth Observing System) MODIS (Moderate Resolution Imaging Spectroradiometer) sensor are presented with different types of validation. We offer examples of how this new type of data set can serve ecological science, land management, and environmental policy. To enhance the use of these data by nonspecialists, we are now producing monthly anomaly maps for GPP and annual NPP that compare the current value with an 18-year average value for each pixel, clearly identifying regions where vegetation growth is higher or lower than normal.” BioScience, June 2004, Vol. 54, No. 6, Pages 547–560, doi:10.1641/0006-3568(2004)054[0547:ACSMOG]2.0.CO;2. [Full text]

Climate-Driven Increases in Global Terrestrial Net Primary Production from 1982 to 1999 – Nemani et al. (2003) “Recent climatic changes have enhanced plant growth in northern mid-latitudes and high latitudes. However, a comprehensive analysis of the impact of global climatic changes on vegetation productivity has not before been expressed in the context of variable limiting factors to plant growth. We present a global investigation of vegetation responses to climatic changes by analyzing 18 years (1982 to 1999) of both climatic data and satellite observations of vegetation activity. Our results indicate that global changes in climate have eased several critical climatic constraints to plant growth, such that net primary production increased 6% (3.4 petagrams of carbon over 18 years) globally. The largest increase was in tropical ecosystems. Amazon rain forests accounted for 42% of the global increase in net primary production, owing mainly to decreased cloud cover and the resulting increase in solar radiation.” Science 6 June 2003: Vol. 300. no. 5625, pp. 1560 – 1563, DOI: 10.1126/science.1082750. [Full text]

Terrestrial net primary production estimates for 0.5° grid cells from field observations—a contribution to global biogeochemical modeling – Zhang et al. (2003) “Net Primary Production (NPP) is an important component of the carbon cycle and, among the pools and fluxes that make up the cycle, it is one of the steps that are most accessible to field measurement. While easier than some other steps to measure, direct measurement of NPP is tedious and not practical for large areas and so models are generally used to study the carbon cycle at a global scale. Nevertheless these models require field measurements of NPP for parameterization, calibration and validation. Most NPP data are for relatively small field plots that cannot represent the 0.5° × 0.5° grid cells that are commonly used in global scale models. Furthermore, technical difficulties generally restrict NPP measurements to aboveground parts and sometimes do not even include all components of aboveground NPP. Thus direct inter-comparison between field data obtained in different studies or comparison of these results with coarse resolution model outputs can be misleading. We summarize and present a series of methods that were used by original authors to estimate NPP and how and what we have done to prepare a consistent data set of NPP for 0.5 °grid cells for a range of biomes from these studies. The methods used for estimation of NPP include: (i) aggregation of fine-scale (plot or stand-level) vegetation inventory data to larger grid cells, (ii) mapping of grid cells and area weighting of field NPP observations in each mapped class, (iii) direct correlation of extensive data sets of ground measurements with remotely sensed spectral vegetation indices, (iv) local modeling of NPP using key independent variables, for which maps are available at the scale of the grid cell, and (v) regression analysis to link productivity with controlling environmental variables. For a few grid cells whose NPP were obtained for multiple years, temporal analysis was conducted. The grid cells are grouped to the biome level and are compared with existing compilations of field NPP and the results of the Miami potential NPP model. Mean NPP was similar to the well-known compilation of Whittaker and Likens, except for temperate evergreen needle-leaved forest, woodland, and shrubland. The grid cell datasets are a contribution to the International Geosphere-Biosphere Programme (IGBP) Data and Information System (DIS) Global Primary Production Data Initiative (GPPDI). The full dataset currently contains 3654 cells (including replicate measurements) developed from 15 studies representing NPP in croplands, sparse vegetation, shrub lands, grasslands, and forests worldwide. An edited subset consists of 2335 cells in which outliers were removed and all replicate measurements were averaged for each unique geographical location. Most of the data incorporated into GPPDI were wholly or partly developed by participants in the GPPDI, in addition to the present authors. These studies are gathered together here to provide a consistent account of the grid cell component of GPPDI and an analysis of the entire data set. The datasets have been deposited in an IGBP-DIS GPPDI database ( http://daacl.esd.ornl.gov/npq/GPPDI/Combined_GPPDI_des.html).” Global Change Biology, Volume 9, Number 1, January 2003 , pp. 46-64(19), DOI: 10.1046/j.1365-2486.2003.00534.x.

Terrestrial net primary productivity – A brief history and a new worldwide database – Scurlock & Olson (2002) “Consistent data on terrestrial net primary productivity (NPP) are urgently needed to constrain model estimates of carbon fluxes and hence to refine our understanding of ecosystem responses to climate change. The NPP data have been collected in a coordinated manner for the past 30 years, but comprehensive summaries are rare. We report on the development and availability of a global NPP database that is suitable for modeling of the terrestrial carbon cycle at global and regional scales, for validation of remote sensing data, and for other applications. These data were obtained from the literature on ecophysiological field work and from detailed consultation with the scientific community. Data on NPP, biomass, and associated environmental variables are now publicly available for 53 detailed study sites, of which more than half have data for belowground biomass or biomass dynamics. Aboveground NPP ranges from 35 to 2320 g m^–2a^–1 (dry matter) and total NPP from 182 to 3538 g m^–2a^–1. Well-known but previously unobtainable compilations of data, such as the “Osnabrück Data Set” and the International Biological Program (IBP) Woodlands Data Set, are also incorporated in this database. Preliminary exploration of relationships between NPP and mean annual precipitation and temperature suggests that the new 53-site data collection, as well as the Osnabrück and IBP data, are all consistent with the historic “Miami” statistical model. These data are available from the Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC) for biogeochemical dynamics (see http://www.daac.ornl.gov/NPP/).” Environ. Rev. 10(2): 91–109 (2002), doi:10.1139/a02-002.

Net primary production in tropical forests: an evaluation and synthesis of existing field data – Clark et al. (2001) “Information on net primary production in tropical forests is needed for the development of realistic global carbon budgets, for projecting how these ecosystems will be affected by climatic and atmospheric changes, and for evaluating eddy covariance measurements of tropical forest carbon flux. However, a review of the database commonly used to address these issues shows that it has serious flaws. In this paper we synthesize the data in the primary literature on NPP in old-growth tropical forests to produce a consistent data set on NPP for these forests. Studies in this biome have addressed only a few NPP components, all aboveground. Given the limited scope of the direct field measurements, we sought relationships in the existing data that allow estimation of unmeasured aspects of production from those that are more easily assessed. We found a predictive relationship between annual litterfall and aboveground biomass increment. For 39 diverse tropical forest sites, we then developed consistent, documented estimates of the upper and lower bounds around total NPP to serve as benchmarks for calibrating and validating biogeochemical models with respect to this biome. We developed these estimates based on existing field measurements, current understanding of aboveground consumption and biogenic volatile organic carbon emissions, and our judgment that belowground production is bounded by the range 0.2–1.2 × ANPP (aboveground NPP). Across this broad spectrum of tropical forests (dry to wet, lowland to montane, nutrient-rich to nutrient-poor soils), our estimates of lower and upper bounds on total NPP range from 1.7 to 11.8 Mg C·ha^-1·yr^-1 (lower bounds) and from 3.1 to 21.7 Mg C·ha^-1·yr^-1 (upper bounds). We also showed that two relationships that have been used for estimating NPP (the Bray-Gorham relationship based on leaf litterfall and the Miami model based on temperature or precipitation) are not valid for the tropical forest biome.” Ecological Applications, 11:371—384, doi:10.1890/1051-0761(2001)011[0371:NPPITF]2.0.CO;2. [Full text]

Primary Production of the Biosphere: Integrating Terrestrial and Oceanic Components – Field et al. (1998) “Integrating conceptually similar models of the growth of marine and terrestrial primary producers yielded an estimated global net primary production (NPP) of 104.9 petagrams of carbon per year, with roughly equal contributions from land and oceans. Approaches based on satellite indices of absorbed solar radiation indicate marked heterogeneity in NPP for both land and oceans, reflecting the influence of physical and ecological processes. The spatial and temporal distributions of ocean NPP are consistent with primary limitation by light, nutrients, and temperature. On land, water limitation imposes additional constraints. On land and ocean, progressive changes in NPP can result in altered carbon storage, although contrasts in mechanisms of carbon storage and rates of organic matter turnover result in a range of relations between carbon storage and changes in NPP.” Science 10 July 1998: Vol. 281. no. 5374, pp. 237 – 240, DOI: 10.1126/science.281.5374.237. [Full text]

Global Primary Production: A Remote Sensing Approach – Prince & Goward (1995) “A new model of global primary production (GLObal Production Efficiency Model, GLO-PEM), based on the production efficiency concept, is described. GLO-PEM is the first attempt to model both global net and gross primary production using the production efficiency approach and is unique in that it uses satellite data to measure both absorption of photosynthetically active radiation (APAR) and also the environmental variables that affect the utilization of APAR in primary production. The use of satellite measurements gives global, repetitive, spatially contiguous and time specific observations of the actual vegetation. GLO-PEM is based on physiological principles, in particular the amount of carbon fixed per unit absorbed photosynthetically active radiation ( epsilon ) is modelled rather than fitted using field observations. GLO-PEM is illustrated with the first available year (1987) of the 8 x 8 km resolution NOAA/NASA AVHRR land Pathfinder data set. The global net primary production, respiration and epsilon values obtained indicate that even the rather simple AVHRR provides a wealth of information relevant to biospheric monitoring. The algorithms and results presented indicate that there are significant possibilities of inferring biological and environmental variables using multispectral techniques that need to be explored if the new generation of satellite remote sensing systems is to be exploited productively.” Journal of Biogeography, Vol. 22, No. 4/5, Terrestrial Ecosystem Interactions with Global Change, Volume 2 (Jul. – Sep., 1995), pp. 815-835.

Terrestrial biogeochemical cycles: Global estimates with remote sensing – Schimel (1995) “The carbon and nitrogen cycles are crucial for understanding the changing Earth system, influencing atmospheric concentrations of greenhouse gases, primary productivity of the biosphere, and biogenic emissions of reactive trace species. The carbon budget of the terrestrial biosphere has attracted special attention because of its role in atmospheric changes in carbon dioxide. The terrestrial biosphere influences atmospheric CO₂ through three main modes: First, large, nearly balanced fluxes of CO₂ in photosynthesis and respiration exhibit a degree of interannual variability which can influence atmospheric CO₂, at least on annual to decadal time scales. Second, land use changes release C0₂ to the atmosphere. Third, poorly understood processes are likely resulting in enhanced uptake of CO₂ in certain ecosystems, acting as a sink in the global carbon cycle. This sink may result from forest demographics, atmospheric N deposition, or direct CO₂ fertilization, or some synergistic combination of those processes. Global estimates of terrestrial carbon cycle components requires the use of remote observations; however, the appropriate remote sensing strategies are quite different for the various components.” Remote Sensing of Environment, Volume 51, Issue 1, January 1995, Pages 49-56, doi:10.1016/0034-4257(94)00064-T.

Global net primary production: Combining ecology and remote sensing – Field et al. (1995) “Terrestrial net primary production (NPP) is sensitive to a number of controls, including aspects of climate, topography, soils, plant and microbial characteristics, disturbance, and anthropogenic impacts. Yet, at least at the global scale, models based on very different types and numbers of parameters yield similar results. Part of the reason for this is that the major NPP controls influence each other, resulting, under current conditions, in broad correlations among controls. NPP models that include richer suites of controlling parameters should be more sensitive to conditions that disrupt the broad correlations, but the current paucity of global data limits the power of complex models. Improved data sets will facilitate applications of complex models, but many of the critical data are very difficult to produce, especially for applications dealing with the past or future. It may be possible to overcome some of the challenges of data availability through increased understanding and modeling of ecological processes that adjust plant physiology and architecture in relation to resources. The CASA (Carnegie, Stanford, Ames Approach) model introduced by Potter et al. (1993) and expanded here uses a combination of ecological principles, satellite data, and surface data to predict terrestrial NPP on a monthly time step. CASA calculates NPP as a product of absorbed photosynthetically active radiation, APAR, and an efficiency of radiation use, ε. The underlying postulate is that some of the limitations on NPP appear in each. CASA estimates annual terrestrial NPP to be 48 Pg and the maximum efficiency of PAR utilization (ε*) to be 0.39 g C MJ⁻¹ PAR. Spatial and temporal variation in APAR is more than fivefold greater than variation in ε.” Remote Sensing of Environment, Volume 51, Issue 1, January 1995, Pages 74-88,doi:10.1016/0034-4257(94)00066-V.

Global climate change and terrestrial net primary production – Melillo et al. (1993) “A process-based model was used to estimate global patterns of net primary production and soil nitrogen cycling for contemporary climate conditions and current atmospheric C0₂ concentration. Over half of the global annual net primary production was estimated to occur in the tropics, with most of the production attributable to tropical evergreen forest. The effects of C0₂ doubling and associated climate changes were also explored. The responses in tropical and dry temperate ecosystems were dominated by C0₂, but those in northern and moist temperate ecosystems reflected the effects of temperature on nitrogen availability.” Nature 363, 234 – 240 (20 May 1993); doi:10.1038/363234a0. [Full text]

Potential Net Primary Productivity in South America: Application of a Global Model – Raich et al. (1991) “We use a mechanistically based ecosystem simulation model to describe and analyze the spatial and temporal patterns of terrestrial net primary productivity (NPP) in South America. The Terrestrial Ecosystem Model (TEM) is designed to predict major carbon and nitrogen fluxes and pool sizes in terrestrial ecosystems at continental to global scales. Information from intensively studies field sites is used in combination with continental—scale information on climate, soils, and vegetation to estimate NPP in each of 5888 non—wetland, 0.5° latitude °0.5° longitude grid cells in South America, at monthly time steps. Preliminary analyses are presented for the scenario of natural vegetation throughout the continent, as a prelude to evaluating human impacts on terrestrial NPP. The potential annual NPP of South America is estimated to be 12.5 Pg/yr of carbon (26.3 Pg/yr of organic matter) in a non—wetland area of 17.0 x 10⁶ km². More than 50% of this production occurs in the tropical and subtropical evergreen forest region. Six independent model runs, each based on an independently derived set of model parameters, generated mean annual NPP estimates for the tropical evergreen forest region ranging from 900 to 1510 g m^-2 yr^-1 of carbon, with an overall mean of 1170 g m^-2 yr^-1. Coefficients of variation in estimated annual NPP averaged 20% for any specific location in the evergreen forests, which is probably within the confidence limits of extant NPP measurements. Predicted rates of mean annual NPP in other types of vegetation ranged from 95 g m^-2 yr^-1 in arid shrublands to 930 g m^-2 yr^-1 in savannas, and were within the ranges measured in empirical studies. The spatial distribution of predicted NPP was directly compared with estimates made using the Miami mode of Lieth (1975). Overall, TEM predictions were ~10% lower than those of the Miami model, but the two models agreed closely on the spatial patterns of NPP in south America. Unlike previous models, however, TEM estimates NPP monthly, allowing for the evaluation of seasonal phenomena. This is an important step toward integration of ecosystem models with remotely sensed information, global climate models, and atmospheric transport models, all of which are evaluated at comparable spatial and temporal scales. Seasonal patterns of NPP in South America are correlated with moisture availability in most vegetation types, but are strongly influenced by seasonal differences in cloudiness in the tropical evergreen forests. On an annual basis, moisture availability was the factor that was correlated most strongly with annual NPP in South America, but differences were again observed among vegetation types. These results allow for the investigation and analysis of climatic controls over NPP at continental scales, within and among vegetation types, and within years. Further model validation is needed. Nevertheless, the ability to investigate NPP—environment interactions with a high spatial and temporal resolution at continental scales should prove useful if not essential for rigorous analysis of the potential effects of global climate changes on terrestrial ecosystems.” Ecological Applications. 1:399—429, doi:10.2307/1941899. [Full text]

Mapping Regional Forest Evapotranspiration and Photosynthesis by Coupling Satellite Data with Ecosystem Simulation – Running et al. (1989) “Annual evapotranspiration (ET) and net photosynthesis (PSN) were estimated for a mountainous 28 x 55 km region of predominantly coniferous forests in western Montana. A simple geographic information system integrated topographic, soils, vegetation, and climatic data at a 1.1 -km scale size defined by the satellite sensor pixel size. Leaf area index (LAI) of the forest was estimated with data from the NOAA (National Oceanic and Atmospheric Administration) Advanced Very High Resolution Radiometer (AVHRR). Daily microclimate of each cell was estimated from ground and satellite data and interpolated using MT-CLIM, a mountain microclimate simulator. A forest ecosystem simulation model, FOREST-BGC. was used to calculate ET and PSN daily for each cell. Ranges of estimated LA1 (4-U) ET (25-60 cm/yr), and PSN (9-20 Mg ha^-1 yr^-1) across the landscape follow the trends expected in both magnitude and spatial pattern. These estimates compared well with field measurements of related variables, although absolute validation of these predictions is not now possible at large spatial scales.” Ecology, Vol. 70, No. 4 (Aug., 1989), pp. 1090-1101. [Full text]

Relating seasonal patterns of the AVHRR vegetation index to simulated photosynthesis and transpiration of forests in different climates – Running & Nemani (1988) “Recent research has suggested that the Normalized Difference Vegetation Index (NDVI) calculated from the AVHRR sensor is directly related to photosynthesis (PSN), transpiration (TRAN), and aboveground net primary production (ANPP) of terrestrial vegetation. Weekly NDVI data for 1983–1984 were retrieved for seven sites of diverse climate in North America. The sites were Fairbanks, AK, Seattle, WA, Missoula, MT, Madison, WI, Knoxville, TN, Jacksonville, FL, and Tucson, AZ. Meteorological data from ground stations were retrieved to drive an ecosystem simulation model (FOREST-BGC) calculating daily canopy PSN and TRAN and annual ANPP of a hypothetical forest stand for the corresponding period at each site. Correlations of annual integrated NDVI across all sites for both years were: annual PSN, R² = 0.87; annual TRAN, R² = 0.77; annual ANPP, R² = 0.72. Correlation between weekly NDVI and PSN was variable; with high latitude wet sites, R² = 0.77–0.84. On sites with less seasonal amplitude of NDVI and PSN, or on sites with substantial seasonal water stress correlations ranged from R² = 0.08 to 0.64. Correlations of weekly NDVI with TRAN followed the same pattern as PSN, but were slightly lower. The tendency of raw NDVI data to overpredict PSN and TRAN on water limited sites was partially corrected using an “aridity index” of annual radiation/annual precipitation that could be computed from general climatological data for improving large scale NDVI maps of PSN and TRAN. The spatial subsampling done for the global vegetation index (GVI) precludes following specific study sites through the growing season. We conclude that estimates of vegetation productivity using the GVI should only be done as annual integrations until unsubsampled local area coverage (LAC) NDVI data can be tested against forest PSN, TRAN, and ANPP, measured at shorter time intervals.” Remote Sensing of Environment, Volume 24, Issue 2, March 1988, Pages 347-367, doi:10.1016/0034-4257(88)90034-X. [Full text]

North American vegetation patterns observed with the NOAA-7 advanced very high resolution radiometer – Goward et al. (1985) “Spectral vegetation index measurements derived from remotely sensed observations show great promise as a means to improve knowledge of land vegetation patterns. The daily, global observations acquired by the Advanced Very High Resolution Radiometer, a sensor on the current series of U.S. National Oceanic and Atmospheric Administration meteorological satellites, may be particularly well suited for global studies of vegetation. Preliminary results from analysis of North American observations, extending from April to November 1982, show that the vegetation index patterns observed correspond to the known seasonality of North American natural and cultivated vegetation. Integration of the observations over the growing season produced measurements that are related to net primary productivity patterns of the major North American natural vegetation formations. Regions of intense cultivation were observed as anomalous areas in the integrated growing season measurements. These anomalies can be explained by contrasts between cultivation practices and natural vegetation phenology. Major new information on seasonality, annual extent and interannual variability of vegetation photosynthetic activity at continental and global scales can be derived from these satellite observations.” Plant Ecology, Volume 64, Number 1, 3-14, DOI: 10.1007/BF00033449. [Full text]

Closely related

Science shocker: Drought drives decade-long decline in plant growth – Climate Progress (2010)