| 英文摘要: | The statistical association between temperature and greenhouse gases over glacial cycles is well documented1, but causality behind this correlation remains difficult to extract directly from the data. A time lag of CO2 behind Antarctic temperature—originally thought to hint at a driving role for temperature2, 3—is absent4, 5 at the last deglaciation, but recently confirmed at the last ice age inception6 and the end of the earlier termination II (ref. 7). We show that such variable time lags are typical for complex nonlinear systems such as the climate, prohibiting straightforward use of correlation lags to infer causation. However, an insight from dynamical systems theory8 now allows us to circumvent the classical challenges of unravelling causation from multivariate time series. We build on this insight to demonstrate directly from ice-core data that, over glacial–interglacial timescales, climate dynamics are largely driven by internal Earth system mechanisms, including a marked positive feedback effect from temperature variability on greenhouse-gas concentrations. Earth system models9 have been an effective, albeit indirect, way to quantify causality in the climate system. The effects of CO2 and other greenhouse gases (GHGs) on Earth’s temperature are relatively well understood, but estimates of the effect of temperature variability on GHG dynamics remain uncertain10, 11, 12. Quantifying the actual strength of this effect is challenging, because it involves a plethora of mechanisms that are difficult to measure and sometimes oppose each other. For instance, increased photosynthesis at higher CO2 levels implies a negative feedback, whereas enhanced plant and soil respiration at higher temperatures leads to carbon release and a positive feedback13. A warmer climate may induce the release of CO2, CH4 and N2O from terrestrial ecosystems, especially in polar regions14. Furthermore, at higher temperatures, marine CaCO3 neutralization of anthropogenic CO2 decreases15, and methane is released from hydrate storages below the sea floor, which may amplify global warming16. Overall, higher global temperatures are believed to cause a net increase in atmospheric concentrations of GHGs, implying a positive feedback in warming10, 11, 17, 18, 19. However, given the complexity of the mechanisms and models, uncertainty over the feedback effect remains large. This issue raises the question if there are more direct, model-independent estimates of the feedback effect based on the strikingly parallel dynamics of temperature and GHGs over the Pleistocene ice ages (Fig. 1a). Data-based approaches for unravelling the causation operating behind this correlation have hitherto largely focused on phase lags between past climate data sets3, but these lags vary over time. A slight lead of Antarctic temperature over CO2 variations has been argued to point to temperature as a driver of CO2 changes2. However, more recent studies cast doubt on the existence of a significant time lag of CO2 behind either Antarctic4 or global5 temperature at the last glacial termination, with variations in methane and temperature seeming nearly synchronous at the Bølling transition20. Meanwhile, the latest data on an earlier termination7 and inception6 show periods of significant time lags between CO2 and Antarctic temperature. A simple moving-window scan of optimal time displacement for correlation (Supplementary Fig. 1c) supports the emerging view that the time lag of CO2 behind temperature as recorded in the Vostok ice core1 has varied widely over the past 400 kyr. Although errors in dating may contribute to such variation, detailed recent studies6, 7, 21 confirm that these lags do vary substantially over time.
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