Alan Grainger

Difficulties in tracking the long-term global trend in tropical forest area

The long-term trend in tropical forest area receives less scrutiny than the tropical deforestation rate. We show that constructing a reliable trend is difficult and evidence for decline is unclear, within the limits of errors involved in making global estimates. A time series for all tropical forest area, using data from Forest Resources Assessments (FRAs) of the United Nations Food and Agriculture Organization, is dominated by three successively corrected declining trends. Inconsistencies between these trends raise questions about their reliability, especially because differences seem to result as much from errors as from changes in statistical design and use of new data. A second time series for tropical moist forest area shows no apparent decline. The latter may be masked by the errors involved, but a “forest return” effect may also be operating, in which forest regeneration in some areas offsets deforestation (but not biodiversity loss) elsewhere. A better monitoring program is needed to give a more reliable trend. Scientists who use FRA data should check how the accuracy of their findings depends on errors in the data.

Biodiversity and REDD at Copenhagen

Reducing carbon emissions through slowing deforestation can benefit biodiversity best if countries implement sensible policies.

Rates of deforestation in the humid tropics: estimates and measurements

for the 1980s, are evaluated using an original set of criteria and the uncertainty is shown to result from lack of attention to remote sensing measurements and overconfidence in the use of expert judgement. Estimates of the total area of tropical moist forest are also unreliable, despite the increased use of remote sensing data in their derivation. A continuous satellite-based global monitoring system is needed to obtain better estimates and the institutional requirements for this are discussed.

The extent of forest in dryland biomes

Mapping the worlds dry forests The extent of forest area in dryland habitats, which occupy more than 40% of Earths land surface, is uncertain compared with that in other biomes. Bastin et al. provide a global estimate of forest extent in drylands, calculated from high-resolution satellite images covering more than 200,000 plots. Forests in drylands are much more extensive than previously reported and cover a total area similar to that of tropical rainforests or boreal forests. This increases estimates of global forest cover by at least 9%, a finding that will be important in estimating the terrestrial carbon sink. Science, this issue p. 635 Previously unreported forest areas in drylands increase the current estimate of global forest cover by at least 9%. Dryland biomes cover two-fifths of Earth’s land surface, but their forest area is poorly known. Here, we report an estimate of global forest extent in dryland biomes, based on analyzing more than 210,000 0.5-hectare sample plots through a photo-interpretation approach using large databases of satellite imagery at (i) very high spatial resolution and (ii) very high temporal resolution, which are available through the Google Earth platform. We show that in 2015, 1327 million hectares of drylands had more than 10% tree-cover, and 1079 million hectares comprised forest. Our estimate is 40 to 47% higher than previous estimates, corresponding to 467 million hectares of forest that have never been reported before. This increases current estimates of global forest cover by at least 9%.

Conceptual issues related to carbon sequestration: Uncertainty and time

Abstract Global climate change is about uncertainty related to ecological and economic processes, and political responses. It is about fairness and income distribution among nations, both now and in the future. It is a dynamic problem that involves national carbon transition functions, damage functions and discount rates. These issues form the basis of the current paper, which examines them from a conceptual point of view.

Desertification, and climate change: the case for greater convergence

Poor knowledge of links between desertification and globalclimate change is limiting funding from the Global Environment Facility foranti-desertification projects and realization of synergies between theConvention to Combat Desertification (CCD) and the FrameworkConvention on Climate Change (FCCC). Greater convergence betweenresearch in the two fields could overcome these limitations, improve ourknowledge of desertification, and benefit four areas of global climate changestudies: mitigation assessment; accounting for land cover change in thecarbon budget; land surface-atmosphere interactions; and climate changeimpact forecasting. Convergence would be assisted if desertification weretreated more as a special case in dry areas of the global process of landdegradation, and stimulated by: (a) closer cooperation between the FCCCand CCD; (b) better informal networking between desertification and globalclimate change scientists, e.g. within the framework of theIntergovernmental Panel on Climate Change (IPCC). Both strategies wouldbe facilitated if the FCCC and CCD requested the IPCC to provide ascientific framework for realizing the synergies between them.

Biodiversity Conservation: Uncertainty in predictions of extinction risk/Effects of changes in climate and land use/Climate change and extinction risk (reply).

Thomas et al. reply — We reconsider our estimates of climate-related extinction in the light of three questions raised by Thuiller et al., Buckley and Roughgarden and Harte et al.. We are able to confirm our original conclusion that climate change represents a major threat to terrestrial species.

An Evaluation of the FAO Tropical Forest Resource Assessment, 1990

The UN Food and Agriculture Organizations latest assessment of tropical forest resources in 1990, gives unreliable estimates of forest areas and deforestation rates in the humid tropics. Estimates for tropical rain forest are too low and statistics in the report cannot be used to derive estimates for tropical moist forest. Comparability with previous assessments is limited; the map used to divide tropical forest cover into different forest types is flawed; and the reliability of many estimates is further constrained by being based on outdated national surveys and modelling adjustments rather than a special pantropical remote sensing survey.

Uncertainty in the construction of global knowledge of tropical forests

Knowledge of tropical forest change remains uncertain, affecting our ability to produce accurate estimates of globally aggregated parameters to support clear global statements about ‘the tropical forests’. This paper reviews current methods for constructing global knowledge of changes in tropical forest area, carbon density, biodiversity and ecosystem services. It finds a deficiency in formal institutions for global measurement and constructing global knowledge. In their absence, informal institutions have proliferated, increasing the spread of estimates. This is exacerbated by dependence on inaccurate official statistics, which has limited construction of knowledge about forest area change through modelling. Employing the new concept of the Knowledge Exchange Chain shows the interdependence of different disciplines in constructing composite information. Limitations linked to compartmentalization and scale are present, as predicted by the ‘post-normal hypothesis’. Disciplinary compartmentalization has impeded construction of information about forest carbon and biodiversity change. There is growth in interdisciplinary research into modelling forest change and estimating carbon emissions using remote sensing data, but not in studying biodiversity. Continuing uncertainty has implications for implementing the Reduced Emissions from Deforestation and Degradation (REDD) scheme. Uncertainty could be reduced by expanding formal scientific institutions, e.g. by establishing an operational scientific global forest monitoring system, and devising formal generic rules for constructing global environmental knowledge.

Measuring the planet to fill terrestrial data gaps

How a growing world population can sustain itself on Planet Earth has been the focus of much research. Norman Borlaug (1), the “father of the Green Revolution,” argued that one answer is to intensify agriculture to produce more food on the same area of land. But what he promoted as a policy strategy, others have called the “Borlaug hypothesis,” aiming to add it to sustainability science theory and test it empirically. In this issue of PNAS Rudel et al. (2) follow earlier national studies (3) by testing the hypothesis at global scale. They demonstrate the existence of the general phenomenon of “land sparing,” by showing that cropland area has increased more slowly than population since 1970. However, they find relatively little evidence that intensification has gone further, by shrinking cropland and generating surplus “spared land,” and much of this evidence is linked to changes in trade patterns. Yet their article will prove important for introducing the concept of spared land into the literature, inspiring more research, and stimulating debate about how land sparing relates to existing theory. This commentary focuses on a key challenge they identify (the measurement of land sparing) and wider monitoring issues raised by it. It refers mainly to developing countries.