百年尺度地球系统模式模拟的陆地生态系统碳通量对CO2浓度升高和气候变化的响应

利用了加拿大地球系统模式CanE SM2(Canadian Earth System Model of the CCCma)的结果,针对百年尺度大气CO_2浓度升高和气候变化如何影响陆地生态系统碳通量这一问题,分析了1850—1989年间陆地生态系统碳通量趋势对二者响应,以及与关键气候系统变量的关系。结果表明,140年间,当仅仅考虑CO_2浓度升高影响时,陆地生态系统净初级生产力(NPP)增加了117.1 gC m~(-2)a~(-1),土壤呼吸(Rh)增加了98.4 gC m~(-2)a~(-1),净生态系统生产力(NEP)平均增加了18.7 gC m~(-2)a~(-1)。相同情景下,全球陆地生态系统的NPP呈显著增加的线性趋势(约为0.30 PgC/a~2),Rh同样呈显著增加线性趋势(约为0.25 PgC/a~2)。仅仅考虑气候变化单独影响时,NPP平均减少了19.3 gC/m~2,土壤呼吸减少了8.5 gC/m~2,NEP减少了10.8 gC/m~2。在此情景下,整个陆地生态系统的NPP线性变化趋势约为-0.07 PgC/a~2(P0.05),Rh线性变化趋势约为-0.04 PgC/a~2(P0.05)。综合二者的影响,前者是决定陆地生态系统碳通量变化幅度和空间分布的最重要影响因子,其影响明显大于气候变化。值得注意的是,CanE SM2并没有考虑氮素的限制作用,所以CO_2浓度升高对植被的助长作用可能被高估。此外,气候变化的贡献也不容忽视,特别是在亚马逊流域,由于当温度升高、降水和土壤湿度减少,NPP和Rh均呈显著减少趋势。

The 100-year scale response of terrestrial ecosystem carbon fluxes to climate- carbon cycle caused by increasing atmospheric CO2 concentration using an Earth System Model

Using data from CanESM2, the second generation Earth System Model of the Canadian Centre for Climate Modelling and Analysis (CCCma), we analyze the impact of two atmospheric processes-elevated atmospheric CO₂ concentration and climate change through temperature and precipitation-on spatiotemporal change of the terrestrial ecosystem during the period 1850-1989. The results show that the elevated atmospheric CO₂ concentrations enhance the carbon fluxes, with increases of 117.1 gC m^-2 a^-1 for net primary production (NPP), 98.4 gC m^-2 a^-1 for soil respiration (Rh), and 18.7 gC m^-2 a^-1 for net ecosystem production (NEP). Increased linear trends of NPP and Rh of about 0.30 and 0.25 PgC/a², respectively, occur across the whole terrestrial ecosystem, whereas for climate change impact, NPP has a reduced trend of -19.3 gC m^-2 a^-1. The soil respiration reduces by -8.5 gC m^-2 and NEP varies by about -10.8 gC m^-2. For the whole terrestrial ecosystem, linear decreased trends of NPP and Rh are approximately -0.07 PgC/a² and -0.04 PgC/a² (P<0.05). Regions show large differences of NPP distribution in response to climate change. Low latitudes and the Southern Hemisphere exhibit decreased NPP, while NPP is somewhat increased in the mid and high latitudes of the Northern Hemisphere. Enhanced vegetation growth due to the lengthened growing season associated with global warming is probably responsible for such an increase. The response of Rh to warming is consistent with that of NPP. The magnitude of increased Rh is larger than that of NPP in the high latitudes of the Northern Hemisphere. Permafrost soils in these high latitudes, which contain an enormous quantity of organic carbon, may melt with the increasing temperature, which is expected to cause increased Rh due to more dissolved organic carbon. Change in atmospheric CO₂ concentration is a dominant driving factor in the spatiotemporal pattern of carbon fluxes of the terrestrial ecosystem, and its impact significantly supersedes the effects of climate change. It should be noted that the model neglects the impact of nitrogen limitation, and thus the effects of elevated CO₂ on carbon fluxes might be overestimated. In addition, the contribution of climate change is not negligible, particularly in the Amazon basin, because reduction in precipitation and soil moisture can result in decreases in NPP and Rh. In this region, the model estimates that, in response to both rising CO₂ concentration and climate change, the total NPP and Rh decrease by approximately -1.8 gC m^-2 a^-2 and 1.6 gC m^-2 a^-2, respectively. NEP also shows a decreased trend, but most areas of NEP change are not statistically significant at the 5% level. In this region, NEP change is closely related to the variation of temperature, precipitation and soil moisture, and the change correlates more to soil moisture than the other two variables. This indicates that drought is key factor driving NEP changes in this region. The terrestrial carbon fluxes are also driven by multiple factors, e.g. radiation, and the processes involved are complicated. Land use and the effect of aerosol are not considered in this paper. These factors should be incorporated into longer-term simulations to investigate the mechanisms involved in the response of the terrestrial carbon fluxes to CO₂ concentration and climate change.