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Manifestations of climate change are often shown as gradual changes in physical or biogeochemical properties1. Components of the climate system, however, can show stepwise shifts from one regime to another, as a nonlinear response of the system to a changing forcing2. Here we show that the Arctic sea ice regime shifted in 2007 from thicker and deformed to thinner and more uniform ice cover. Continuous sea ice monitoring in the Fram Strait over the last three decades revealed the shift. After the shift, the fraction of thick and deformed ice dropped by half and has not recovered to date. The timing of the shift was preceded by a two-step reduction in residence time of sea ice in the Arctic Basin, initiated first in 2005 and followed by 2007. We demonstrate that a simple model describing the stochastic process of dynamic sea ice thickening explains the observed ice thickness changes as a result of the reduced residence time. Our study highlights the long-lasting impact of climate change on the Arctic sea ice through reduced residence time and its connection to the coupled ocean–sea ice processes in the adjacent marginal seas and shelves of the Arctic Ocean.

The Arctic cryosphere is a critically important component of the earth system, affecting the energy balance, atmospheric and ocean circulation, freshwater storage, sea level, the storage, and release of large quantities of greenhouse gases, economy, infrastructure, health, and indigenous and non-indigenous livelihoods, culture and identity. Currently, components of the Arctic cryosphere are subjected to dramatic change due to global warming. The need to document, understand, project, and respond to changes in the cryosphere and their consequences stimulated a comprehensive international assessment called “SWIPA”: Snow, Water, Ice, Permafrost in the Arctic. Some of the extensive key SWIPA chapters have been summarized and made more widely available to a global audience with multi-disciplinary interests in this Special Report of Ambio. In this article, an overview is provided of this Special Report in the context of the more detailed and wider scope of the SWIPA Report. Accelerated changes in major components of the Arctic cryosphere are documented. Evidence of feedback mechanisms between the cryosphere and other parts of the climate system are identified as contributing factors to enhanced Arctic warming while the growing importance of Arctic land-based ice as a contributor to global sea-level rise is quantified. Cryospheric changes will result in multifaceted and cascading effects for people within and beyond the Arctic presenting both challenges and opportunities.

A study was undertaken to examine the activity and composition of the seasonal Baltic Sea land-fast sea-ice biota along a salinity gradient in March 2003 in a coastal location in the SW coast of Finland. Using a multi-variable data set, the less well-known algal and protozoan communities, and algal and bacterial production in relation to the physical and chemical environment were investigated. Also, the first coincident measurements of bacterial production and dissolved organic matter (DOM) in a sea-ice system are reported. Communities in sea ice were clearly autotrophy-dominated with algal biomass representing 79% of the total biomass. Protozoa and rotifers made up 18% of biomass in the ice and bacteria only 3%. Highest biomasses were found in mid-transect bottom ice. Water column assemblages were clearly more heterotrophic: 39% algae, 12% bacteria and 49% for rotifers and protozoa. Few significant correlations existed between DOM and bacterial variables, reflecting the complex origin of ice DOM. Dynamics of dissolved organic carbon, nitrogen and phosphorus (DOC, DON and DOP) were also uncoupled. A functional microbial loop is likely to be present in the studied ice. Existence of an under-ice freshwater plume affects the ecosystem functioning: Under-ice water communities are influenced directly by river-water mixing, whereas the ice system seems to be more independent—the interaction mainly taking place through the formation of active bottom communities.

Horizontal variation of first-year landfast sea ice properties was studied in the Gulf of Finland, the Baltic Sea. Several scales of variation were considered; a number of arrays with core spacings of 0.2, 2 and 20 m were sampled at different stages of the ice season for small-scale patchiness. Spacing between these arrays was from hundreds of meters to kilometers to study mesoscale variability, and once an onshore–offshore 40-km transect was sampled to study regional scale variability. Measured variables included salinity, stable oxygen isotopes (δ18O), chlorophyll a (chl-a), nutrients and dissolved organic carbon. On a large scale, a combination of variations in the under-ice water salinity (ice porosity), nutrient supply and the stage of ice development control the build-up of ice algal biomass. At scales of hundreds of meters to kilometers, there was significant variability in several parameters (salinity, chl-a, snow depth and ice thickness). Analyses of the data from the arrays did not show evidence of significant patchiness at scales

Physical, chemical and biological properties of sea ice and underlying brackish water (0–10 m water depth) were investigated in the Bothnian Bay and at a coastal station at the entrance of the Gulf of Finland in March 2000. Sea-ice texture was analysed and vertical profiles for δ18O, inorganic nutrient concentrations [NO3 –, NO2 –, NH4 +, PO4 3+, Si(OH)4] and chlorophyll a and pheopigment concentrations were obtained with a vertical resolution of 1–10 cm. Biomass estimates of bacteria, protists and metazoans were based on microscopical counts. Due to low ice bulk salinities, sea-ice brine volumes were small with a median value of only 3%. Ice thickness varied between 22 and 27 cm. Refrozen snow and granular ice dominated in the surface parts of the sea ice; columnar ice formed the bottom layers. The total biomass in the sea ice and under-ice water ranged from 2.7 mg C m–2 to 45.1 mg C m–2 and 36.8 mg C m–2 to 3,874.3 mg C m–2, respectively. Mean integrated ice values were dominated by pennate diatoms (33%), followed by centric diatoms (29%), autotrophic flagellates (23%), bacteria (8%) and heterotrophic flagellates (7%). The low integrated biomass of ice-associated metazoans (1%) consumed on average only 1.5% of the algal standing stock per day. The combined data suggest that abiotic factors (particularly nutrient availability as reflected by small brine volumes), rather than the heterotrophic component of the sympagic community, controlled autotrophic biomass in Baltic sea ice during March 2000.