Uncertain future for vegetation cover

Abstract

future anthropogenic emission scenarios (known as representative concentration pathways) were each realized by a different integrated assessment model, which combines knowledge about aspects of climate change and economics into a single framework. The uncertainties associated with projections of land-use change are therefore unknown, even though different outcomes of land-use change are feasible for each of the scenarios. However, the uncertainties in land-use change-in terms of the total area, location and direction of change-will need to be considered to develop land-based policies for mitigating and adapting to the effects of climate change. Scientists are addressing this issue by developing a broader range of land-use change projections, using different integrated assessment models, for each of the representative concentration pathways used in the IPCC report 7. In addition, projections from global and regional models of land-use change that are conceptually different from integrated assessment models are emerging or are under development 8-10. We will soon be able to test how components of the future carbon cycle and the climate, and of many other crucial ecosystem properties, will alter when a range of CO 2 levels and climate changes are combined with various land-use-change scenarios. This will help us to answer the overarching question of how to share a finite resource: the land. ■ Almut Arneth is in the Figure 1 | Simulations of future forest cover. Davies-Barnard et al. 1 have used a computational model to investigate how the change of vegetation cover in response to global warming and increasing atmospheric CO 2 levels compares with the effects of land use (deforestation and reforestation) over the coming decades. The graphs depict changes in the percentage of the global land area covered by forest in 2100, using three different scenarios of climate change and land use; results from each scenario are shown in a different colour. The results differ greatly for each scenario. (Adapted from ref. 1.) Land-use response Global forest change (%) 4 3 2 1 0-1-2 Climate-induced response Net response SYNTHETIC BIOLOGY Ribosomal ties that bind The ribosome is the cellular complex of proteins and RNA molecules that synthesizes proteins. An artificial ribosome in which the two main subunits are tethered together creates opportunities for engineering this process. T o engineer a system is to demonstrate a mastery of physical understanding. Mechanical engineers harness a deep understanding of fundamental physics to design new motors. Similarly, biologists are using the current explosion in information about molecular structure and function to engineer biological systems. The ribosome-the macro molecular complex containing RNAs and proteins that translates the genetic code-represents one of nature's most sophisticated machines. Engineering ribosomes would en able experimental manipulation of protein synthesis and provide deeper insights into cellular and molecular biology. On page 119 of this issue, Orelle et al. 1 describe drastic, but simple, engineering of functional ribosomes, in which two separate subunits are linked as one. that emerges from all three scenarios is the poleward expansion of boreal forest, a finding that has also been reported in previous work (see ref. 3, for example). By contrast, tropical forests are more affected by land-use change than are boreal ones, and the effects become evident in the next few decades, but the direction and speed of change depends greatly on the scenario-for example, the ratio of the land area adopted for crop and pasture lands to the area of reforestation. Thus, a complex picture emerges in which changes in vegetation cover depend on the speed of vegetation's response to human-induced forcing, whether warming and higher atmospheric CO 2 levels stimulate the expansion of forest cover, and the relative size of areas of deforestation and reforestation. To complicate matters further, the magnitude and direction of vegetation-area change and ecosystem carbon changes are not proportional to each other. The regional differences associated with each scenario count, and not just because of their effects on climate. Changes in land cover will affect species and habitat diversity , but also water supplies, food provision, air quality and other services that society derives from ecosystems. A better understanding of where and when we can expect land-cover changes is therefore needed to develop sustainable land-management strategies. As Davies-Barnard and co-workers note, there are several caveats to their analysis, some of which relate to the vegetation and carbon-cycle model used. In their study, the nitrogen and carbon cycles do not interact; such a lack of interaction can affect not only future carbon-cycle projections 2 , but also how simulated vegetation cover responds to climate and atmospheric CO 2 changes 4. Furthermore, the representation of croplands is highly simplified in their model, and does not consider crop-management practices that are known to affect the carbon content of soil. Another caveat is that forest-management practices, the dynamics of forest regrowth and tree-age distributions are not accounted for in the authors' model, but these are important for carbon cycling in ecosystems. And only net land-use changes-the net area that undergoes a change from one time period to the next-are considered, even though the accuracy of estimates of total land-use change and carbon-cycle calculations can be substantially improved when the more-detailed, multidirectional changes that occur within a region are accounted for 5,6. We do not know the degree to which Davies-Barnard and col-leagues' results would be affected if all of these caveats were explicitly addressed. Their study will therefore stimulate and challenge scientists to account for land-use and land-cover change much more realistically than is done at present.