Sensitivity and uncertainty of modelled terrestrial net primary productivity to doubled C02 and associated climate change for a relatively large perturbed physics ensemble

摘要

Net primary productivity (NPP) is often modelled explicitly in general circulation models (GCMs) utilising process models that may include plant photosynthesis, respiration, allocation of photosynthates, phenology, mortality and competition between plant functional types. It is an important measure for understanding the role of terrestrial vegetation in the global carbon cycle, and useful for gaining insights into the large-scale, integrated effects of climate and atmospheric changes on potential plant productivity and associated impacts, i.e. food security and carbon cycle feedbacks. However, there are simplifications and uncertainties in GCM projections of future climate change, as well as further uncertainties involved in modelling the associatedterrestrial vegetation responses. In particular, it is important to highlight that many GCM simulations, including the ones used in this study, do not model nutrient limitation, even though primary plant nutrients, e.g. nitrogen and phosphorus, are keylimiting factors on plant productivity. Here, we examine sensitivities and uncertainties in large(global)-scaIe modelled NPP to climate and atmospheric carbon dioxide concentration [CO_2], utilising a relatively large perturbed physics ensemble (PPE) ofsimulations generated from the HadSM3 GCM under equilibrium doubling of pre-industrial atmospheric [CO_2]. We also exploit the ensemble design to highlight the relative importance of two, often opposing, forcings on NPP: (i) plant physiological responsesto CO_2, termed 'Phys'; and (ii) plant responses to physical drivers of climate, termed 'Rad'. It is important to note that this is a sensitivity study that provides useful guidance on the relative importance of the Rad and Phys drivers and their uncertainties. The results cannot be considered quantitatively realistic, particularly because the equilibrium experimental design and lack of nutrient limitation in the model are important limitations that prevent such interpretation. We find that doubled [CO_2] and associated climate changes ultimately increase potential global average NPP by 57%, from 0.293 kgcm~(-2)yr~(-1) (-36 PgCyr~(-1)) to 0.460 kg cm~(-2) yr~(-1) (~57 PgCyr~(-1)). Spatially, the largest decreases (—0.45 kg cm~(-2) yr~(-1)) occur across the north-east of South America in association with the largest decreases in precipitation. The largest increases (up to -0.75 kg cm~(-2) yr~(-1)) occur across tropical Africa and Indonesia, where NPP is already high, and both temperature and precipitation increase under doubled [CO_2 ]. In most regions where NPP shows an increase the changes are significantly larger than the ensemble standard deviation, indicating that increases in global NPP under doubled [CO_2 ] are reasonably robust. However, insome regions, particularly north-eastern South America and Central America, where NPP decreases are projected, the standard deviation across the ensemble is larger than the average NPP change, indicating that even the sign of the NPP sensitivity to doubled [CO_2] and climate is uncertain. These uncertainties are shown to be highly dependent on the relative sensitivities of NPP to the Phys and Rad forcings.