This article focuses on entrepreneurship in economic geography and aims at a systematic investigation of regional variation in knowledge-based entrepreneurial activity. We develop and test a three-phase structural model for regional systems of entrepreneurship after introducing a systems approach to entrepreneurship. The model is built upon the absorptive capacity theory of knowledge spillover entrepreneurship that identifies new knowledge as one source of entrepreneurial opportunities and human capital as the major source of entrepreneurial absorptive capacity. Based on data of US metropolitan areas, we find that entrepreneurial absorptive capacity is a critical driving force for knowledge-based entrepreneurial activity. We also find that high technology and cultural diversity contribute to the vibrancy of regional systems of entrepreneurship.

The ongoing and projected warming in the northern high latitudes (NHL; poleward of 601N) may lead to dramatic changes in the terrestrial carbon cycle. On the one hand, warming and increasing atmospheric CO 2 concentration stimulate vegetation productivity, taking up CO 2 . On the other hand, warming accelerates the decomposition of soil organic matter (SOM), releasing carbon into the atmosphere. Here, the NHL terrestrial carbon storage is investigated based on 10 models from the Coupled Carbon Cycle Climate Model Intercomparison Project. Our analysis suggests that the NHL will be a carbon sink of 0.3 AE 0.3 Pg C yr À1 by 2100. The cumulative land organic carbon storage is modeled to increase by 38 AE 20 Pg C over 1901 levels, of which 17 AE 8 Pg C comes from vegetation (43%) and 21 AE 16 Pg C from the soil (8%). Both CO 2 fertilization and warming enhance vegetation growth in the NHL. Although the intense warming there enhances SOM decomposition, soil organic carbon (SOC) storage continues to increase in the 21st century. This is because higher vegetation productivity leads to more turnover (litterfall) into the soil, a process that has received relatively little attention. However, the projected growth rate of SOC begins to level off after 2060 when SOM decomposition accelerates at high temperature and then catches up with the increasing input from vegetation turnover. Such competing mechanisms may lead to a switch of the NHL SOC pool from a sink to a source after 2100 under more intense warming, but large uncertainty exists due to our incomplete understanding of processes such as the strength of the CO 2 fertilization effect, permafrost, and the role of soil moisture. Unlike the CO 2 fertilization effect that enhances vegetation productivity across the world, global warming increases the productivity at high latitudes but tends to reduce it in the tropics and mid-latitudes. These effects are further enhanced as a result of positive carbon cycle-climate feedbacks due to additional CO 2 and warming.

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[1] The behavior of the coupled carbon cycle and physical climate system in a global warming scenario is studied using an Earth system model including the atmosphere, land, ocean, and the carbon cycle embedded in these components. A fully coupled carbon-climate simulation and several sensitivity runs were conducted for the period of 1860 -2100 with prescribed IPCC-SRES-A1B emission scenario. Results indicate a positive feedback to global warming from the interactive carbon cycle, with an additional increase of 90 ppmv in the atmospheric CO 2 , and 0.6 degree additional warming, thus confirming recent results from the Hadley Centre and IPSL. However, the changes in various carbon pools are more modest, largely due to the multiple limiting factors constraining terrestrial productivity and carbon loss. The large differences among the three models are manifestations of some of the poorly constrained processes such as the global strength of the CO 2 fertilization effect and the turnover time and rates of soil decomposition.