The oxygen isotopic composition of atmospheric CO₂ can help constrain local- to global-scale biophysical processes and partition measured net ecosystem CO₂ fluxes into gross fluxes. Although current models still lack key features controlling gross ecosystem CO¹⁸O fluxes, considerable improvements have been achieved in the last four years. In this study we examine the influence on atmospheric CO¹⁸O of 1) a delayed seasonal cycle in soil water isotopes (relative to rain water) and 2) a new one-way flux model of night-time leaf respiration [Cernusak et al., 2004]. The latter covaries with enhanced night-time stomatal conductance, for which evidence arose recently [e.g. Snyder et al., 2003].

A time series transect has been established in subantarctic surface water off the south east coast of New Zealand.  The 60 km long transect extends from the coast (45-46.20^oS 170-43.20^oE) to a station at 45-50.00^oS 171-30.00^oE. and sea surface temperature, salinity and pCO₂ have been measured bi-monthly since 1998 . SST, pCO₂ and pH of the subantarctic surface water show seasonal cycles that can be fitted with simple harmonic curves.  Temperature has a mean value of 10.4^oC, with an amplitude of 2.1^oC, the maximum occurring in late summer.  pCO₂ has a mean value of 360 matm, an amplitude of 10 matm, the maximum occurring in early spring.  The phase of the pCO₂ and temperature curves are offset by 158 days, indicating that change in sea water temperature is not the major factor affecting pCO₂ in this area.  The relative effects of temperature, biological utilization and air-sea gas exchange on the seasonal change in pCO₂ are determined using a simple model.  The model results reproduce the timing of the observed pCO₂, however the amplitude of the changes is not well reproduced.

A major challenge in reaching a better understanding of global change is the integration of global carbon observations at different scales, made in the atmosphere, ocean and terrestrial domains.  This is essential to optimize efforts supporting national, regional and international policy related to the global carbon cycle.  The partners of the Integrated Global Observing Strategy (IGOS-P) representing all players in carbon cycle research and monitoring recognised this and produced, with the help of an international panels of experts, published theme reports on the Carbon Cycle (IGCO) and on Atmospheric Chemistry (IGACO).  These themes contain recommendations on how to more effectively coordinate and fill gaps in global Earth observations. 

An inverse modeling system has been developed based on the Bayesian principle for estimating the carbon fluxes of the 48 regions globally and 28 regions over North America in monthly steps for 2003 using CO₂ concentration measurements at 95 atmospheric baseline stations and with regularization using remote sensing-based surface flux field. Preliminary inversion results of global carbon flux and a carbon flux field over North America have been obtained.

We have developed a carbon flux inversion method for using a mesoscale meteorological model (CSU-RAMS) within a Maximum Likelihood Ensemble Filter (MLEF, Zupanski 2005; Zupanski and Zupanski 2005). The MLEF is a variant of the Ensemble Kalman Filter, and is used to optimize model state variables and parameters based on continuous observations of CO₂ mixing ratio. The method does not require the development of a model adjoint, but rather relies on transformation of variables to efficiently obtain estimates of fluxes with uncertainties and dynamical model error from an ensemble of forward model simulations. We demonstrate this method using a mesoscale simulation of weather, transport, and the surface carbon budget over the continental USA during the summer. The estimation procedure decomposes the total surface flux into photosynthesis and respiration (which are assumed to be modeled correctly to first order), plus an unknown but time-invariant fractional error in each.  These residuals are estimated for each model grid cell over a moving window in time, allowing atmospheric observations to be integrated over sufficient time to obtain constraint. Model error can also be estimated by this procedure, and the method can be extended to larger domains and longer integrations.

CO₂ and methane are important greenhouse gases, both contributing in increasing amounts towards positive radiative forcing. It is hence important to gain maximum understanding of the carbon cycle in the atmosphere, and the scale of carbon trace gas sources and sinks, not only globally but also on a more regional level. The Orbiting Carbon Observatory (OCO) satellite, scheduled for launch in 2008, is designed for dedicated global mapping of CO₂. In order to investigate the usefulness of a variety of methods, including retrievals from satellite mapping, some preliminary inverse modelling using a Bayesian synthesis technique is performed using pseudodata generated to represent possible future measurement regimes. This study will focus on the ability of in-situ measurements within Australia to reduce the uncertainties in Australian continental CO₂ flux estimates. The specific measurements investigated include a Ghan railway transect between Adelaide (34.9°S, 138.6°E) and Darwin (12.5°S, 130.9°E), and a number of continuous permanent sites. The reduction in flux uncertainties from additional measurements compared to a background inversion is examined, from which it is concluded that measuring on the Ghan railway is potentially worthwhile for reducing uncertainties associated with flux estimates.

Measurements of CO₂ concentration are carried out on a weekly basis since 1992 on the island of Lampedusa (35.5°N, 12.6°E), in the Mediterranean. Measurements are based at the Station for Climate Observations, which rests on a rocky plateau (45 m asl) on the North-Eastern coast of the island, and are made with a NDIR analyzer. World Meteorological Organization (WMO) reference standards are used for calibrations.  Continuous measurements were started in 1998; they were interrupted in early 2003, and activated again in 2005. The continuous observations show evidence of a small daily cycle (amplitude < ±1 ppm) only during the months of June, July, and August. Mean annual cycles derived from weekly flask measurements show a dependency on the wind origin: the annual cycle and the annual CO₂ mean are smaller for winds originating from the Southern sectors, than for winds from Northern sectors. The continuous measurements were combined with daily backward airmass trajectories to identify the dependency of the CO₂ amount on the airmass origin. Trajectories provided by National Oceanic and Atmospheric Administration / Air Resources Laboratory (Hysplit) are used. During winter, low CO₂ is generally connected to Southern/South-Eastern airmasses. In summer airmasses from North often display lower CO₂ content, due to the influence of the European sink.

An accurate knowledge of the thermodynamics of the carbonic acid system in seawater is crucial to our understanding of the behavior of carbon dioxide in seawater. In particular, this knowledge is needed whenever a particular property needs to be calculated from measurements of other related properties; e.g., the estimation of the partial pressure of CO2 in air that is in equilibrium with a sample of sea water, p(CO2), from measurements of the total dissolved inorganic carbon, CT, and of the total alkalinity, AT, of a water sample. This calculation is particularly important for ocean models, which transport CT and AT, but which need to calculate p(CO2) at the sea surface so as to represent air-sea exchange processes. Numerous determinations of dissociation constants for carbon dioxide in seawater media have been published over the years. In each case the authors have recommended “best” values for the dissociation constants, and often the constants are represented in these papers by interpolating equations or tables. Furthermore, a number of investigators have attempted to assess the thermodynamic consistency of the various published values for these dissociation constants with analytical measurements made on natural seawater. Despite all this work, the results of these efforts are, as yet, not conclusive. I shall present a review of the situation and will try to provide a clear description of the magnitude of the problems, their possible sources, and their importance to understanding the behavior of CO2 in seawater.

A one-dimensional (1d) physical-biological-chemical model was developed and tested by Antoine and Morel [1995, AM95 hereafter], with the aim of assessing upper ocean carbon fluxes. This model was specifically designed to be driven by satellite data, and it was used to evaluate the upper ocean CO₂ fluxes at station P in the NE Pacific. Another validation of this model has been carried out at the DYFAMED station (NW Mediterranean), where time series of biological and physical observations are available. This validation is a first step before the basin-scale application to the Mediterranean Sea, as presented here for the period 1998-2000.