The influence of climate on terrestrial carbon dioxide fluxes

The concentration of CO2 in the atmosphere ([CO2]) is increasing at only about half the rate expected based on fossil fuel emissions. This "missing sink" is highly variable due primarily to the effects of climate variability on terrestrial CO2 fluxes in the northern hemisphere. Using a series of model simulations, we studied how climate influences inter-annual variability and long-term trends in terrestrial CO2 fluxes. We modeled Net Ecosystem Exchange (NEE) of CO2 from 1958--2002 (45 years) using the Simple Biosphere model, Version 2 (SiB2). As input weather, we used the National Centers for Environmental Prediction (NCEP) reanalysis and the European Centre for Medium-range Weather Forecasts (ECMWF) Reanalysis. To define the Leaf Area Index, we used the Fourier-Adjustment, Solar zenith angle corrected, Interpolated Reconstructed (FASIR) Normalized Difference Vegetation Index (NDVI) dataset. We used correlations, trends, and other statistical techniques to isolate the relationships between NEE and climate.; The simulated NEE reproduces the salient features and magnitude of the measured global CO2 growth rate. The northern hemisphere shows a pattern of alternating positive and negative NEE anomalies that cancel such that the tropics dominate the global simulated NEE inter-annual variability.; Climate influences on NEE have strong regional differences with precipitation dominating in the tropics and temperature in the extra-tropics. In tropical regions with drier soils, precipitation control of photosynthesis (i.e., drought stress) dominates. By contrast, in moist soils, precipitation control of respiration dominates. Due to cancellation and competing effects, no single climate variable controls global or regional NEE inter-annual variability. Globally, precipitation accounts for 44% of NEE variability; followed by Leaf Area Index (23%), soil carbon (12%), and temperature (16%). The influence of ENSO is consistent with that expected for shifting precipitation patterns in the tropics.; The AO strongly influences autumn, winter, and spring NEE through its influence on temperature. Soil retains the AO temperature signal for many months, influencing respiration fluxes well into spring. Seasonally asymmetric NEE trends influence the seasonal amplitude of atmospheric CO2 concentration. Positive AO polarity in winter advances the date of leaf out, increasing the spring drawdown of atmospheric CO2. Positive AO polarity in winter increases temperature and respiration, increasing the winter buildup of atmospheric CO2. The influence of the AO on summer NEE is minimal except for North America in August.; The trend in the winter AO partially explains observed trends towards warmer winters and earlier springs. The timing of spring correlates with the AO where the AO influences temperature (Eurasia and southeast United States). Modeled trends in leaf out, snowmelt, and soil thaw are consistent with observations. The AO shows a statistically significant influence on spring trends in the eastern United States and northern Europe. Seasonally asymmetric trends in NEE can partially explain the observed trend towards larger seasonal amplitudes in [CO2]. The components of the land surface with climate memory (plant buds, snow pack, and soil temperature) integrate the noisy AO input over time to control the transition from winter to spring. In summary, climatic memory is very important in the study of seasonal dynamics and that the winter AO influences the transition from winter to spring.