The seasonal movement of the zero-degree isotherm across the Southern Ocean and Antarctic Peninsula drives major changes in the physical and biological processes around maritime Antarctica. These include spatial and temporal shifts in precipitation phase, snow accumulation and melt, thawing and freezing of the active layer of the permafrost, glacier mass balance variations, sea ice mass balance and changes in physiological processes of biodiversity. Here, we characterize the historical seasonal southward movement of the monthly near-surface zero-degree isotherm latitude (ZIL), and quantify the velocity of migration in the context of climate change using climate reanalyses and projections. From 1957 to 2020, the ZIL exhibited a significant southward shift of 16.8 km decade(-1) around Antarctica and of 23.8 km decade(-1) in the Antarctic Peninsula, substantially faster than the global mean velocity of temperature change of 4.2 km decade(-1), with only a small fraction being attributed to the Southern Annular Mode (SAM). CMIP6 models reproduce the trends observed from 1957 to 2014 and predict a further southward migration around Antarctica of 24 +/- 12 km decade(-1) and 50 +/- 19 km decade(-1) under the SSP2-4.5 and SSP5-8.5 scenarios, respectively. The southward migration of the ZIL is expected to have major impacts on the cryosphere, especially on the precipitation phase, snow accumulation and in peripheral glaciers of the Antarctic Peninsula, with more uncertain changes on permafrost, ice sheets and shelves, and sea ice. Longer periods of temperatures above 0 degrees C threshold will extend active biological periods in terrestrial ecosystems and will reduce the extent of oceanic ice cover, changing phenologies as well as areas of productivity in marine ecosystems, especially those located on the sea ice edge.

Rising atmospheric CO2 is intensifying climate change but it is also driving global and particularly polar greening. However, most blue carbon sinks (that held by marine organisms) are shrinking, which is important as these are hotspots of genuine carbon sequestration. Polar blue carbon increases with losses of marine ice over high latitude continental shelf areas. Marine ice (sea ice, ice shelf and glacier retreat) losses generate a valuable negative feedback on climate change. Blue carbon change with sea ice and ice shelf losses has been estimated, but not how blue carbon responds to glacier retreat along fjords. We derive a testable estimate of glacier retreat driven blue carbon gains by investigating three fjords in the West Antarctic Peninsula (WAP). We started by multiplying ~40 year mean glacier retreat rates by the number of retreating WAP fjords and their time of exposure. We multiplied this area by regional zoobenthic carbon means from existing datasets to suggest that WAP fjords generate 3,130 tonnes of new zoobenthic carbon per year (t zC/year) and sequester >780 t zC/year. We tested this by capture and analysis of 204 high resolution seabed images along emerging WAP fjords. Biota within these images were identified to density per 13 functional groups. Mean stored carbon per individual was assigned from literature values to give a stored zoobenthic Carbon per area, which was multiplied up by area of fjord exposed over time, which increased the estimate to 4,536 t zC/year. The purpose of this study was to establish a testable estimate of blue carbon change caused by glacier retreat along Antarctic fjords and thus to establish its relative importance compared to polar and other carbon sinks.

The aerosol optical depths (AODs) in the wavelength range 380-875 nm and black carbon (BC) mass concentrations were estimated over the tropical Indian Ocean and in the Indian Ocean sector of Southern Ocean, between 14 degrees N and 53 degrees S, during December 2011-February 2012, onboard the Ocean Research Vessel (ORV) Sagar Nidhi. The data were analysed to understand the spectral variability, micro-physical characteristics of aerosols and the associated radiative forcing. Concurrent MODIS-derived chlorophyll a (Chl-a) and sea-surface temperature (SST) provided ancillary data used to understand the variability of biomass in association with fronts and the possible role of phytoplankton as a source of aerosols. AODs and their spectral dependencies were distinctly different north and south of the Inter-Tropical Convergence Zone (ITCZ). North of 11 degrees S (the northern limit of ITCZ), the spectral distribution of AOD followed Angstrom turbidity formule (Junge power law function), while it deviated from such a distribution south of 16 degrees S (southern boundary of ITCZ). At the southern limit of the ITCZ and beyond, the spectral variation of AOD showed a peak around 440 nm, the amplitude of which was highest at similar to 43 degrees S, the axis of the subtropical front (STF) with the highest Chl-a concentration (0.35 mu g l(-1)) in the region. To understand the role of Chl-a in increasing AOD at 440 nm, AOD at this wavelength was estimated using Optical properties of Aerosols and Clouds (OPAC) model. The anomalies between the measured and model-estimated (difference between the measured and estimated AOD values at 440 nm) AOD(440) were correlated with Chl-a concentrations. A very high and significant association with coefficient of determination (R-2=0.80) indicates the contribution of Chl-a as a source of aerosols in this part of the ocean. On the basis of the measured aerosol properties, the study area was divided into three zones; Zone 1 comprising of the area between 10 degrees N and 11 degrees S; Zone 2 from 16 degrees S to 53 degrees S; and Zone 3 from 52 degrees S to 24 degrees S during the return leg. BC mass concentration was in the range 520 ng m(-3) to 2535 ng m(-3) in Zone 1, while it was extremely low in the other zones (ranging from 49.3 to 264.4 ng m(-3) in Zone 2 and from 61.6 ng m(-3) to 303.3 ng m(-3) in Zone 3). The atmospheric direct-short wave radiative forcing (DRSF), estimated using a radiative transfer model (Santa Barbara DISORT Atmospheric Radiative Transfer - SBDART), was in the range 4.72-27.62 wm(-2) north of 16 degrees S, and 4.80-6.25 wm(-2) south of 16 degrees S. (C) 2015 Elsevier Ltd. All rights reserved.

Extensive measurements of columnar aerosol optical depth (AOD), composite (M-T) and black carbon aerosol mass (M-B) concentrations were made over the tropical Indian and Southern Oceans as a part of the Pilot Expedition to the Southern Ocean during the boreal winter. The AOD, M-T and M-B show large latitudinal gradient towards south up to ITCZ. Beyond ITCZ, up to 56 degrees S, AOD and M-B show very low and steady values. However M-T shows large variations in the Southern Ocean due to the enhanced production of sea salt aerosols associated with high sea surface winds. The short wave aerosol radiative forcing at the surface over north of equator is in the range - 10 to -23 W m(-2), whereas that over the Southern Ocean was in the range -4 to -5 W m(-2). The corresponding atmospheric forcing was in the range of 6-13 W m(-2) and 0.8-1.4 W m(-2). This large north south change in the aerosol radiative forcing has important implications to the meridional circulation and hence to climate.

The Antarctic climate system varies on timescales from orbital, through millennial to sub-annual, and is closely coupled to other parts of the global climate system. We review these variations from the perspective of the geological and glaciological records and the recent historical period from which we have instrumental data (similar to the last 50 years). We consider their consequences for the biosphere, and show how the latest numerical models project changes into the future, taking into account human actions in the form of the release of greenhouse gases and chlorofluorocarbons into the atmosphere. In doing so, we provide an essential Southern Hemisphere companion to the Arctic Climate Impact Assessment.