Atmospheric deposition of mineral dust aerosols supplies the essential nutrient of iron to the ocean.  However, only the readily soluble iron is available to biological uptake while the insoluble iron precipitates to the ocean bottom.  Here we present a global model simulation of Aeolian iron input to the ocean, considering hematite dissolution in mineral dust aerosols catalyzed by nitric and sulfuric acids.  Our model suggests that atmospheric deposition of soluble iron to the oceans is much larger than previous model results in high nitrate low chlorophyll (HNLC) regions. 

Different CO₂ stabilization scenarios and CO₂ emission scenarios have been carried out with an earth system model to investigate feedbacks between future climate change and carbon cycle. The model predicts a sensitivity of 1.6±0.1 K for an increase of 280 ppm in atmospheric CO₂ concentration. The decrease of the thermohaline circulation is predominantly controlled by an enhanced atmospheric moisture transport to high latitudes by global warming. Overall, the simulated effect of atmospheric CO₂ concentration on climate change reduces the total carbon uptake of the ocean and the land is reduced by 24-29%.

Observations suggest the global reflectivity of Earth changed during recent decades. Although there is some ambiguity surrounding these findings, it is clear that, should there be changes in clouds or scattering aerosols, a change in the total solar radiation received at the surface and the fraction of diffuse light could result. Intriguingly, the d¹⁸O of CO₂ time series measured at Mauna Loa shows variability during the 1990s that does not match secular trends in CO₂ concentration or d¹³C. While a decrease in total solar radiation alone would reduce biospheric productivity, an increase in diffuse light can increase productivity, as has been argued for the period following the eruption of Pinatubo. Moreover, since the changes in radiation affect the surface latent energy exchange, the isotopic composition of terrestrial water with which CO₂ interacts (specifically leaf and soil water) will be modified and can thus drive a change in isotopic fluxes.

Is the massive Amazon forest a CO₂ sink, a source or is it in equilibrium?  

There is a large uncertainty in carbon fluxes estimates for the tropics as a whole and in particular for the Amazon region in South America, bringing the attention to the lack of information to call the region a carbon source or sink. The production of scientific consistent and long term data series for the region is a process that has to advance step by step.

Data from long-term measurements of carbon balance in boreal, mid-latitude and tropical ecosystems are used to assess the mechanisms that drive changes in ecosystem carbon balance in response to a changing climate. We find that most model parameterizations overestimate the temperature sensitivity of ecosystem respiration and underestimate the role of soil water balance in controlling respiration and flammability. We conclude that model assessments of climate—carbon feedbacks must carefully simulate regional precipitation, evaporation, evapotranspiration, and water balance, including factors leading to fires (e.g. sources of ignition), in addition to assessing changes in temperature. Covariances among these drivers of ecosystem respiration and vegetation change may be critically important for these simulations.

The carbon cycle has undergone changes from 1998-2003 as a result of extensive droughts.  The CO₂ seasonal amplitude at MLO halted its increase, and the CO₂ growth rate accelerated as a result of a slowing down of the North American carbon sink.  In a series of coupled carbon-climate model experiments, we show a greater probability of drier soils in the 21^st century, especially in the tropics and in mid-latitude summers as temperature-driven evapotranspiration exceed precipitation, and a positive feedback between the carbon cycle and climate. This positive feedback reduces the land and ocean’s capacity to store fossil fuel CO­_2­ and accelerates the warming. A fossil fuel emission accelerating rapidly as the sink capacities decrease leads to further increases in the airborne fraction of fossil fuel CO₂.

Paleo-climate records in ice cores revealed high variability in temperature, atmospheric dust content and carbon dioxide. The longest CO₂ record from the Antarctic ice core of the Vostok station went back in time as far as about 410 kyr BP showing a switch of glacials and interglacials in all those parameters approximately every 100 kyr during the last four glacial cycles with CO₂ varying between 180-300 ppmv [Petit et al., 1999]. New measurements of dust and the isotopic temperature proxy deuterium of the EPICA Dome C (EDC) ice core covered the last 740 kyr, however, revealed glacial cycles of reduced temperature amplitude [EPICA community members, 2004]. These new archives offer the possibility to propose atmospheric CO₂ for the pre-Vostok time span as called for in the EPICA challenge [Wolff et al., 2004]. Here, we contribute to this challenge using a box model of the isotopic carbon cycle [Köhler et al., 2005] based on process understanding previously derived for Termination I. Our results show that major features of the Vostok period are reproduced while prior to Vostok our model predicts significantly smaller amplitudes in CO₂ variations.

Because vegetation growth in the Northern Hemisphere is typically nitrogen-limited, increased nitrogen deposition could have attenuating effect on rising atmospheric CO₂ by stimulating the accumulation of biomass. Given the high carbon to nitrogen ratios and long lifetimes of carbon in wood, a most significant effect of nitrogen fertilization is expected in forests. Forest inventories indicate that the carbon content of northern forests have increased concurrently with increased nitrogen deposition since the 1950s [Spiecker et al., 1996]. In addition, variations in atmospheric CO₂ indicate a globally significant carbon sink in northern mid-latitude forest regions [Schimel et al., 2001]. It is unclear however, whether elevated nitrogen deposition or other factors are the primary cause of carbon sequestration in northern forests. We argue that the elevated nitrogen deposition is unlikely to enhance vegetation carbon sink significantly because of its differentiating effect on the carbon sequestration capacity of uneven aged forests and climatic limitations on carbon sequestration in the Northern Hemisphere. We estimate the potential of forests with lifted nitrogen limitation to decelerate CO₂ concentrations rise in the atmosphere and therefore to mitigate climate warming. We also outline areas of the Northern Hemisphere which are most sensitive to increased nitrogen deposition.