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We investigated how ozone pollution and climate change/variability have interactively affected net primary productivity (NPP) and net carbon exchange (NCE) across China's forest ecosystem in the past half century. Continental China. Using the dynamic land ecosystem model (DLEM) in conjunction with 10-km-resolution gridded historical data sets (tropospheric O concentrations, climate variability/change, and other environmental factors such as land-cover/land-use change (LCLUC), increasing CO and nitrogen deposition), we conducted nine simulation experiments to: (1) investigate the temporo-spatial patterns of NPP and NCE in China's forest ecosystems from 1961-2005; and (2) quantify the effects of tropospheric O pollution alone or in combination with climate variability and other environmental stresses on forests' NPP and NCE. China's forests acted as a carbon sink during 1961-2005 as a result of the combined effects of O, climate, CO, nitrogen deposition and LCLUC. However, simulated results indicated that elevated O caused a 7.7% decrease in national carbon storage, with O-induced reductions in NCE (Pg C year) ranging from 0.4-43.1% among different forest types. Sensitivity experiments showed that climate change was the dominant factor in controlling changes in temporo-spatial patterns of annual NPP. The combined negative effects of O pollution and climate change on NPP and NCE could be largely offset by the positive fertilization effects of nitrogen deposition and CO. In the future, tropospheric O should be taken into account in order to fully understand the variations of carbon sequestration capacity of forests and assess the vulnerability of forest ecosystems to climate change and air pollution. Reducing air pollution in China is likely to increase the resilience of forests to climate change. This paper offers the first estimate of how prevention of air pollution can help to increase forest productivity and carbon sequestration in China's forested ecosystems. [ABSTRACT FROM AUTHOR]

The historic Paris Agreement calls for limiting global temperature rise to "well below 2 °C." Because of uncertainties in emission scenarios, climate, and carbon cycle feedback, we interpret the Paris Agreement in terms of three climate risk categories and bring in considerations of low-probability (5%) highimpact (LPHI) warming in addition to the central (~50% probability) value. The current risk category of dangerous warming is extended to more categories, which are defined by us here as follows: >1.5 °C as dangerous; >3 °C as catastrophic; and >5 °C as unknown, implying beyond catastrophic, including existential threats. With unchecked emissions, the central warming can reach the dangerous level within three decades, with the LPHI warming becoming catastrophic by 2050. We outline a threelever strategy to limit the central warming below the dangerous level and the LPHI below the catastrophic level, both in the near term (<2050) and in the long term (2100): the carbon neutral (CN) lever to achieve zero net emissions of CO2, the super pollutant (SP) lever to mitigate short-lived climate pollutants, and the carbon extraction and sequestration (CES) lever to thin the atmospheric CO2 blanket. Pulling on both CN and SP levers and bending the emissions curve by 2020 can keep the central warming below dangerous levels. To limit the LPHI warming below dangerous levels, the CES lever must be pulled as well to extract as much as 1 trillion tons of CO2 before 2100 to both limit the preindustrial to 2100 cumulative net CO2 emissions to 2.2 trillion tons and bend the warming curve to a cooling trend. [ABSTRACT FROM AUTHOR]

The three Kyoto flexible mechanisms-emissions trading, the clean development mechanism (CDM), and Joint Implementation (JI)-have always been controversial. Proponents saw the mechanisms as clever tools to ensure environmental outcomes were achieved at least cost. Reducing the costs of compliance, they argued, would make righter environmental targets possible, and certainly more politically feasible. Detractors have argued that the flexible mechanisms commoditize Earth's atmosphere in a manner that will allow dubious projects and the exchange of ‘hot air’ to substitute for serious engagement on climate change. This chapter reviews the Kyoto flexible mechanisms, which will become fully operative during the period 2008 to 2012. The review assesses their progress and success to date, examines the problems that have emerged, and considers suggestions for future developments in climate policy. [ABSTRACT FROM AUTHOR]