Cause, conseguenze e strategie di mitigazione Proponiamo il primo di una serie di articoli in cui affronteremo l’attuale problema dei mutamenti climatici. Presentiamo il documento redatto, votato e pubblicato dall’Ipcc - Comitato intergovernativo sui cambiamenti climatici - che illustra la sintesi delle ricerche svolte su questo tema rilevante.

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Climate Change 2007: The Physical Science Basis.

A climatological mean distribution for the surface water pCO2 over the global oceans in non-El Nino conditions has been constructed with spatial resolution of 4° (latitude) ×5° (longitude) for a reference year 2000 based upon about 3 million measurements of surface water pCO2 obtained from 1970 to 2007. The database used for this study is about 3 times larger than the 0.94 million used for our earlier paper [Takahashi et al., 2002. Global sea–air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects. Deep-Sea Res. II, 49, 1601–1622]. A time-trend analysis using deseasonalized surface water pCO2 data in portions of the North Atlantic, North and South Pacific and Southern Oceans (which cover about 27% of the global ocean areas) indicates that the surface water pCO2 over these oceanic areas has increased on average at a mean rate of 1.5 μatm y−1 with basin-specific rates varying between 1.2±0.5 and 2.1±0.4 μatm y−1. A global ocean database for a single reference year 2000 is assembled using this mean rate for correcting observations made in different years to the reference year. The observations made during El Nino periods in the equatorial Pacific and those made in coastal zones are excluded from the database.

Seasonal changes in the surface water pCO2 and the sea-air pCO2 difference over four climatic zones in the Atlantic, Pacific, Indian and Southern Oceans are presented. Over the Southern Ocean seasonal ice zone, the seasonality is complex. Although it cannot be thoroughly documented due to the limited extent of observations, seasonal changes in pCO2 are approximated by using the data for under-ice waters during austral winter and those for the marginal ice and ice-free zones.

The net air–sea CO2 flux is estimated using the sea–air pCO2 difference and the air–sea gas transfer rate that is parameterized as a function of (wind speed)2 with a scaling factor of 0.26. This is estimated by inverting the bomb 14C data using Ocean General Circulation models and the 1979–2005 NCEP-DOE AMIP-II Reanalysis (R-2) wind speed data. The equatorial Pacific (14°N–14°S) is the major source for atmospheric CO2, emitting about +0.48 Pg-C y−1, and the temperate oceans between 14° and 50° in the both hemispheres are the major sink zones with an uptake flux of −0.70 Pg-C y−1 for the northern and −1.05 Pg-C y−1 for the southern zone. The high-latitude North Atlantic, including the Nordic Seas and portion of the Arctic Sea, is the most intense CO2 sink area on the basis of per unit area, with a mean of −2.5 tons-C month−1 km−2. This is due to the combination of the low pCO2 in seawater and high gas exchange rates. In the ice-free zone of the Southern Ocean (50°–62°S), the mean annual flux is small (−0.06 Pg-C y−1) because of a cancellation of the summer uptake CO2 flux with the winter release of CO2 caused by deepwater upwelling. The annual mean for the contemporary net CO2 uptake flux over the global oceans is estimated to be −1.6±0.9 Pg-C y−1, which includes an undersampling correction to the direct estimate of −1.4±0.7 Pg-C y−1. Taking the pre-industrial steady-state ocean source of 0.4±0.2 Pg-C y−1 into account, the total ocean uptake flux including the anthropogenic CO2 is estimated to be −2.0±1.0 Pg-C y−1 in 2000.

Climatological mean and decadal change in surface ocean pCO2, and net sea–air CO2 flux over the global oceans

[1] From 1953 to 2006, Arctic sea ice extent at the end of the melt season in September has declined sharply. All models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) show declining Arctic ice cover over this period. However, depending on the time window for analysis, none or very few individual model simulations show trends comparable to observations. If the multi-model ensemble mean time series provides a true representation of forced change by greenhouse gas (GHG) loading, 33–38% of the observed September trend from 1953–2006 is externally forced, growing to 47–57% from 1979–2006. Given evidence that as a group, the models underestimate the GHG response, the externally forced component may be larger. While both observed and modeled Antarctic winter trends are small, comparisons for summer are confounded by generally poor model performance.

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Arctic sea ice decline: Faster than forecast

The sequence of extreme September sea ice extent minima over the past decade suggests acceleration in the response of the Arctic sea ice cover to external forcing, hastening the ongoing transition towards a seasonally open Arctic Ocean. This reflects several mutually supporting processes. Because of the extensive open water in recent Septembers, ice cover in the following spring is increasingly dominated by thin, first-year ice (ice formed during the previous autumn and winter) that is vulnerable to melting out in summer. Thinner ice in spring in turn fosters a stronger summer ice-albedo feedback through earlier formation of open water areas. A thin ice cover is also more vulnerable to strong summer retreat under anomalous atmospheric forcing. Finally, general warming of the Arctic has reduced the likelihood of cold years that could bring about temporary recovery of the ice cover. Events leading to the September ice extent minima of recent years exemplify these processes.

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The Arctic’s rapidly shrinking sea ice cover: a research synthesis

Linear trends in arctic sea-ice extent over the period 1979 to 2006 are negative in every month. This ice loss is best viewed as a combination of strong natural variability in the coupled ice-ocean-atmosphere system and a growing radiative forcing associated with rising concentrations of atmospheric greenhouse gases, the latter supported by evidence of qualitative consistency between observed trends and those simulated by climate models over the same period. Although the large scatter between individual model simulations leads to much uncertainty as to when a seasonally ice-free Arctic Ocean might be realized, this transition to a new arctic state may be rapid once the ice thins to a more vulnerable state. Loss of the ice cover is expected to affect the Arctic's freshwater system and surface energy budget and could be manifested in middle latitudes as altered patterns of atmospheric circulation and precipitation.

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Perspectives on the Arctic's Shrinking Sea-Ice Cover

The Southern Ocean closes the global overturning circulation and is key to the regulation of carbon and heat, biological production, and sea level. However, the dynamics of the general circulation and upwelling pathways remain poorly understood. Here, a unifying framework is proposed invoking a semi-circumpolar `supergyre' south of the Antarctic circumpolar current: a massive series of &amp;#160;&amp;#8216;leaking&amp;#8217; sub-gyres spanning the Weddell and Ross seas that are connected and maintained via rough topography that acts as scaffolding. The supergyre framework challenges the conventional view of having separate circulation structures in the Weddell and Ross seas and suggests a limited utility for climate applications of idealized models and conventional zonal averaged frameworks. Machine learning was used to reveal areas of coherent driving forces within a vorticity-based analysis. Predictions from the supergyre framework are supported by available observations and could aid observational and modelling efforts of the climatically key region undergoing rapid change. 

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The Southern Ocean supergyre: a unifyingdynamical framework identified by machinelearning

Gradual Warming of the Weddell Sea Deep and Bottom Water

The ARGO system is expected to become the backbone of a global ocean observing system. However, it can not be easily extended into polar oceans, since the floats have to return to the sea surface to be located and to transmit the measured data. Since location and data transmission under the ice is only possible by acoustic means, an observation system in the deep polar oceans requires the combination of different technologies. The envisioned system comprises ice avoiding profiling subsurface floats which are acoustically ranged by RAFOS technology. In a first step they transmit the recorded data only when they are able to reach open water in real-time, in a second phase, they provide delayed mode data transfer during the ice-free season. Acoustic real-time data transmission is planned as final step. Here we report on our efforts to develop an acoustically positioned, ice-sensing floats featuring delayed mode data transfer (iStore).Since 1999 92 autonomous, profiling floats were deployed in the Weddell gyre area. Since 2001 these float have been registered with ARGO, forming a contribution of 61 floats in total. Since 2002, floats use the Ice Sensing Algorithm (ISA), which determines likeliness of ice coverage based on upper ocean temperatures. Of 18 ISA equipped floats 14 resurfaced after the first winter season while 11 of 13 resurfaced after the first and the second winter seasons. All of those surviving the first winter survived the second winter as well. A total of 20 floats are equipped with RAFOS receivers. Sound coverage for the RAFOS navigation system is provided by 3 sound sources installed since 2003 (2 refurbished in 2005) and 3 sources installed in 2005. Sound coverage varies according to season, location and sound source manufacturer, though a value of 400 km can be safely assumed for planning purposes.

Towards an Hybrid Antarctic Float Observation System (HAFOS)

Introduction The recent calving event in the southern Weddell Sea caused a significant southward retrea Ronne Ice Shelf edge (Oerter, this volume). Considered as a dynamical barrier for water m exchange between the continental shelf and the ice shelf cavity (Grosfeld et al., 1997; Mak and Nicholls, 1999) it was speculated during the FRISP Workshop held in Münster (July, 20 whether this retreat might have an impact on shelf water circulation, cross-frontal exchange related ocean-ice shelf interaction.

Eberhard Fahrbach

Papers

The Southern Ocean supergyre: a unifyingdynamical framework identified by machinelearning

Gradual Warming of the Weddell Sea Deep and Bottom Water

Towards an Hybrid Antarctic Float Observation System (HAFOS)

On the influence of ice shelf edge position on circulation and water mass modification