Detection/attribution of Land Use Land Cover Change on global and regional climate
by B.J.J.M. van den Hurk (KNMI), N. de Noblet (IPSL), A. Pitman (UNSW), J.P. Boisier (IPSL), S. Seneviratne (ETH)
Land use changes give rise to series of processes that lead to systematic effects on both local/regional and global climate (Pielke et al, 2011). On a local/regional scale, changes in the radiative properties (albedo), turbulent heat exchanges, water availability, biochemical and trace gases cycles result from the conversion from an ecosystem (e.g. forest) into another that has different functioning (e.g. crop or pasture). Those changes impact on e.g. ambient air temperature, convection, and atmospheric composition. On a global scale, historical conversion into agriculture affects Net Primary Productivity (NPP), and therefore the storage reservoirs of carbon. Agriculture has therefore altered the global carbon cycle (Bondeau et al, 2007) which in turn modifies the atmospheric CO2 concentration and thereby, potentially, the global climate. In addition, land use changes have altered the vulnerability of societies to climate impacts, for instance by enhanced population and economical values residing in river flood plains (Te Linde et al, 2011).
Recently a systematic modelling experiment was conducted involving seven state-of-the-art climate models exploring the relative effects of land use change and greenhouse gas concentrations on regional and global temperature, precipitation and surface fluxes (Pitman et al, 2009). This study revealed a wide variety in land use change implementation strategies and their subsequent responses. However, a general feature is that the biogeophysical land use change effects explored have a local/regional rather than a global character, and that albedo increases associated with deforestation generally lead to local temperature reductions. However, model-specific feedbacks and responses point at a series of relevant mechanisms that play a role: changes in the green vegetation season length, or secondary cloud responses may lead to changes in the local surface energy balance that have contrasting effects on near surface temperature (de Noblet-Ducoudré et al, 2011). Although the model simulations have demonstrated a significant impact of LULCC on local/regional climate, it remains a difficult task to evaluate a) those impacts, and b) the realism of the patterns through which they occur. It is indeed difficult to disentangle, from observations, the sole LULCC impacts. Dedicated detection/attribution methodologies should be used more thoroughly to address this issue.
The generation of a large ensemble of global climate model projections in the context of the 5th Climate Modelling Intercomparison Project (CMIP5) will be used as a starting point to systematically explore the land use change effects on global and regional climate, taking into account both its biogeophysical and its biogeochemical impacts. To achieve this, we will need to run complementary simulations. Under the umbrella of LUCID such a parallel project is being planned and will be launched soon . As many of the direct and indirect effects of land use change on climate act on a regional scale, further explorations of mechanisms and uncertainties using regional climate model experiments is a worthwhile research strategy.