How will Earth's vegetation cover respond to climate change, and how does this compare with changes associated with human land use? Modelling studies reveal how little we still know, and act as a clarion call for further work.
Vegetation and soil take up and release large amounts of carbon dioxide, and are thus key players in the climate system. Writing in Global Biogeochemical Cycles, Davies-Barnard et al.1 describe results from an Earth-system model that incorporates a dynamic component representing vegetation and the associated carbon cycle. They used this to investigate how change of vegetation in response to global warming and increasing atmospheric CO2 levels compares, in terms of area and carbon uptake, with the effects of human land use — particularly deforestation and reforestation — over the coming decades. In total, the projected changes strongly depend on the type and location of future land-use change, and on the magnitude of climate change.
In most parts of the world, humans have greatly altered the type of vegetation that dominates the landscape. Further large changes in land cover are expected as demand for food, timber and biofuels grows, and as the climate warms. Knowledge about the location of dominant vegetation cover in the future is needed for many reasons. Enhanced vegetation growth and the expansion of vegetation cover into new regions owing to climate change takes up CO2 from the atmosphere, whereas large amounts of this greenhouse gas are lost from vegetation and soil following deforestation. Changes in vegetation cover also alter the way in which incoming solar radiation is reflected or absorbed at the land surface, and how it subsequently warms the surface and leads to evaporation and transpiration of water. Taken together, vegetation and soil influence climate change globally and regionally.
Despite this, only the most recent (fifth) report by the Intergovernmental Panel on Climate Change (IPCC) has simultaneously considered the effects of land-use change and of dynamically changing vegetation and soil processes — at least in a few simulations2 of future climate change using Earth-system models. By contrast, stand-alone vegetation models have been used for some years to assess the combined effects of natural vegetation dynamics and deforestation2. However, climate scientists have rarely attempted to systematically disentangle the two, especially with respect to the area covered.
In their modelling study, Davies-Barnard and colleagues show how three scenarios that consider both land-use change and climate change lead to substantial regional discrepancies in where, and by how much, vegetation cover expands or decreases (Fig. 1). The model shows that climate-change effects are larger in boreal forest than in tropical forest, and become more important towards the end of the twenty-first century. In fact, the one outcome 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 CO2 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 projections2, but also how simulated vegetation cover responds to climate and atmospheric CO2 changes4. 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 for5,6. We do not know the degree to which Davies-Barnard and colleagues' 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.
Even more interesting is how much the study's findings depend on the envisaged future world. In the fifth IPCC report, four 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 report7. 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 development8,9,10. 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 CO2 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.