Takakiyo Nakazawa

Saturation of the Southern Ocean CO2 Sink Due to Recent Climate Change

Based on observed atmospheric carbon dioxide (CO2) concentration and an inverse method, we estimate that the Southern Ocean sink of CO2 has weakened between 1981 and 2004 by 0.08 petagrams of carbon per year per decade relative to the trend expected from the large increase in atmospheric CO2. We attribute this weakening to the observed increase in Southern Ocean winds resulting from human activities, which is projected to continue in the future. Consequences include a reduction of the efficiency of the Southern Ocean sink of CO2 in the short term (about 25 years) and possibly a higher level of stabilization of atmospheric CO2 on a multicentury time scale.

Weak Northern and Strong Tropical Land Carbon Uptake from Vertical Profiles of Atmospheric CO2

Measurements of midday vertical atmospheric CO2 distributions reveal annual-mean vertical CO2 gradients that are inconsistent with atmospheric models that estimate a large transfer of terrestrial carbon from tropical to northern latitudes. The three models that most closely reproduce the observed annual-mean vertical CO2 gradients estimate weaker northern uptake of –1.5 petagrams of carbon per year (Pg C year–1) and weaker tropical emission of +0.1 Pg C year–1 compared with previous consensus estimates of –2.4 and +1.8 Pg C year–1, respectively. This suggests that northern terrestrial uptake of industrial CO2 emissions plays a smaller role than previously thought and that, after subtracting land-use emissions, tropical ecosystems may currently be strong sinks for CO2.

Northern Hemisphere forcing of climatic cycles in Antarctica over the past 360,000 years.

The Milankovitch theory of climate change proposes that glacial–interglacial cycles are driven by changes in summer insolation at high northern latitudes. The timing of climate change in the Southern Hemisphere at glacial–interglacial transitions (which are known as terminations) relative to variations in summer insolation in the Northern Hemisphere is an important test of this hypothesis. So far, it has only been possible to apply this test to the most recent termination, because the dating uncertainty associated with older terminations is too large to allow phase relationships to be determined. Here we present a new chronology of Antarctic climate change over the past 360,000u2009years that is based on the ratio of oxygen to nitrogen molecules in air trapped in the Dome Fuji and Vostok ice cores. This ratio is a proxy for local summer insolation, and thus allows the chronology to be constructed by orbital tuning without the need to assume a lag between a climate record and an orbital parameter. The accuracy of the chronology allows us to examine the phase relationships between climate records from the ice cores and changes in insolation. Our results indicate that orbital-scale Antarctic climate change lags Northern Hemisphere insolation by a few millennia, and that the increases in Antarctic temperature and atmospheric carbon dioxide concentration during the last four terminations occurred within the rising phase of Northern Hemisphere summer insolation. These results support the Milankovitch theory that Northern Hemisphere summer insolation triggered the last four deglaciations.

Long-term changes of methane and hydrogen in the stratosphere in the period 1978–2003 and their impact on the abundance of stratospheric water vapor

[1]xa0The long-term changes of the stratospheric mixing ratio of CH4 over the period of 1978–2003 are derived from balloon-borne data of H2, CH4 and N2O. The data were obtained by collecting whole air samples and subsequent gas chromatographic analyses. To eliminate the short-term variability attributed to dynamical processes, the N2O mixing ratio is used as a proxy for altitude. A correlation analysis for the individual years is applied and the CH4 mixing ratios are interpolated to four different levels of N2O, corresponding to altitudes of approximately 17, 23, 26 and 30 km at midlatitudes. For the investigated period of 1978 to 2003 we find increases at the four levels of 207 ± 32 ppb, 159 ± 21 ppb, 140 ± 34 ppb and 111 ± 60 ppb, respectively. The CH4 trend has slowed down in recent years and is best fitted by a second-order polynomial. The increase of CH4 can account for only 25–34% of the increase in stratospheric H2O of 1%/yr over the last decades as derived from previous studies. The simultaneously measured time series of stratospheric H2 mixing ratios shows that the contribution of stratospheric H2 to the H2O trend in the period 1988–2003 is minor.

Global-scale transport of carbon dioxide in the troposphere

[1]xa0An atmospheric transport model was used to examine the roles of variously scaled atmospheric transport processes (Lagrangian mean motions, large-scale eddies, and parameterized vertical diffusion and convective transport) in the spatiotemporal distributions of tropospheric carbon dioxide (CO2). The mean and eddy transports were analyzed using the mass-weighted isentropic zonal mean. We found several differences in the dominant transport processes for tropospheric CO2 distributions between the extratropics of both hemispheres and the tropics. (1) In the northern extratropics in boreal autumn to spring, CO2 emitted by anthropogenic and biospheric sources is uplifted and dispersed through quasi-isentropic eddy mixing associated with baroclinic waves and accumulates in the extratropical low-isentropic troposphere (in “cold pocket” below 300K). (2) High-CO2 air is transported from the northern extratropics into the tropics through low-level mean meridional flow. It is uplifted together with CO2 emitted by tropical vegetation through deep convection and diabatic eddies in the tropics during boreal winter to spring. (3) During summer at the northern midlatitudes, the low mixing ratio of CO2 produced by biospheric uptake is uplifted into the upper troposphere by convection and is strongly isolated from the lower latitudes. (4) The CO2 emitted in the Northern Hemisphere and the tropics is transported into the Southern Hemisphere via the tropical upper troposphere due to eddy mixing during boreal winter to spring and mean divergent flow of Hadley circulation during boreal summer. (5) In the Southern Hemisphere, an upward gradient of CO2 forms by upper tropospheric southward advection during boreal spring-autumn.

Temporal variations of the carbon isotopic ratio of atmospheric methane observed at Ny Ålesund, Svalbard from 1996 to 2004

Systematic observations of the atmospheric CH4 mole fraction and its carbon isotope ratio δ 13 CH 4 have been carried out at Ny Alesund, Svalbard (78°55N, 11°56E) since 1991 and 1996, respectively. The CH4 and δ 13 CH 4 showed clear seasonal cycles with respective peak-to-peak amplitudes of 48 ppb and 0.42%o. By comparing the anomalies in the increase rate of the CH4 with those of δ 13 CH 4, it is suggested that the cause of the rapid increase in the CH4 mole fraction observed at Ny Alesund in 1998 could be attributable to an enhanced CH 4 release from wetland and biomass burning.

Variations of atmospheric nitrous oxide concentration in the northern and western Pacific

Atmospheric N2O concentrationwas observed in the Pacific for the period 1991–2006, using commercial container ships sailing between Japan and North America and between Japan and Australia or New Zealand. The N2O concentration showed a secular increase and interannual variations at all sampling locations, but a seasonal cycle was detectable only at northern high latitudes. The annual mean N2O concentration showed little longitudinal variations (within ± 0.3 ppb) in the northern Pacific, but showed a clear north-south gradient of about 0.8 ppb, with higher values in the Northern Hemisphere. The annual mean N2O was also characterized by especially high values at 30◦N due to strong local N2O emissions and by a steep latitudinal decrease from the equator to 20◦S due to the suppression of interhemispheric exchange of air by the South Pacific Convergence Zone. The N2O growth rate showed an interannual variation with a period of about 3 yr (high-values in 1999 and 2000), with a delayed eastward and poleward phase propagation in the northern and western Pacific, respectively. The interannual variations of the N2O growth rate and soil water showed a good correlation, suggesting that the N2O emission from soils have an important causative role in the atmospheric N2O variation.

Ice‐core record of methyl chloride over the last glacial–Holocene climate change

[1]xa0Methyl chloride (CH3Cl) concentration was measured in air trapped in a deep ice core from Dome Fuji, Antarctica covering the last glacial–present interglacial (Holocene) change. The record shows that the CH3Cl concentration was relatively constant, being similar to the present levels, during the pre-industrial Holocene. In contrast, the CH3Cl concentration was significantly high and variable in the last glacial period, possibly due to impurity-related production of CH3Cl in ice sheet. Under the assumption that the production was the sole cause of the excess CH3Cl, the atmospheric CH3Cl concentration during the last glacial was estimated using simultaneously measured calcium data for the ice core to have been enhanced by 30% compared with the pre-industrial Holocene concentration. Because the major sink of CH3Cl was stronger during the last glacial than during the Holocene, the enhancement of CH3Cl during the last glacial was likely due to the glacial period source being enhanced.

Temporal variations of atmospheric carbon dioxide in the southernmost part of Japan

We present analysis of the temporal variation of atmospheric CO2 in the subtropical region of East Asia, obtained aboard a ferry between Ishigaki Island and Hateruma Island, Japan for the period June 1993–April 2005. The annual mean CO2 concentration increases from 360.1 ppmv in 1994 to 378.4 ppmv in 2004, showing an average growth rate of 1.8 ppmv yr-1. The growth rate shows interannual variations with high values duringENSOevents. The average seasonal CO2 cycle reaches the maximum in early April and the minimum in mid-September, with a peak-to-peak amplitude of 8.5 ppmv. Numerical simulations using a three-dimensional atmospheric transport model show interannual variations of the CO2 growth rate similar to the observation, but the amplitude of the seasonal cycle is larger, with maximum concentration appearing earlier than the observation by 1 month. Low CO2 values observed during the spring of 1998 are likely associated with the 1997/1998 ENSO event. A backward trajectory analysis suggests that they were due to changes in atmospheric transport whereby maritime air masses from the Pacific Ocean dominated over polluted air masses from the Asian Continent. Extreme values (either high or low) of CO2 are also occasionally observed. A comparison of backward trajectories of air parcels with CO2 concentration fields calculated using the atmospheric transport model shows that these unusual CO2 concentrations result from the transport of air affected not only by anthropogenic CO2 emissions but also by terrestrial biospheric activities mainly in China.

A New Compact Cryogenic Air Sampler and Its Application in Stratospheric Greenhouse Gas Observation at Syowa Station, Antarctica

Abstract To collect stratospheric air samples for greenhouse gas measurements, a compact cryogenic air sampler has been developed using a cooling device called the Joule–Thomson (J–T) minicooler. The J–T minicooler can produce liquefied neon within 5 s from high pressure neon gas precooled by liquid nitrogen. The sampler employs liquid neon as a refrigerant to solidify or liquefy sampled atmospheric constituents. Laboratory experiments showed that the sampler is capable of collecting about 3 and more than 7 L STP of air at 25 and 120 hPa, respectively, which corresponds to about 25 and 15 km above ground within 240 s, respectively. The new balloon-borne sampling system, which was set up for Antarctic experiments, consists of the compact sampler, a 2-L high pressure neon gas cylinder, pneumatic and solenoid valves, a controller with a GPS receiver, a telemetry transmitter, and batteries. The size of the sampling system is 300 mm width × 300 mm depth × 950 mm height and it weighs about 22 kg (including liqu...