The IPY-CARE project - background

Overall objective:

To explore, quantify and model Arctic climate change, its interaction with the climate in lower latitudes and its impact on Arctic marine ecosystem, and to assess the socio-economic consequences for Europe

Specific objectives:

1. To determine the processes responsible for the past and present variability and changes in the Arctic climate system and to improve their representation in regional and global climate models
2. To understand the degree to which recent variability and changes in the Arctic climate system, e.g., shrinking sea-ice cover, thawing permafrost and increased methane emission, are of natural or anthropogenic origin
3. To understand and quantify the response of marine biological processes to climate change and their effects on Arctic marine ecosystems and the air-sea CO2 fluxes and to improve their representation in ecosystem models and inclusion in global climate models
4. To quantify the Arctic freshwater budget and its linkages to the global thermohaline circulation (THC) and climate, and to assess its potential in causing rapid climate change, sea-level change and sequestration of CO2
5. To improve capabilities to predict Arctic climate on decadal and longer time scales and design optimal components of an integrated monitoring and forecasting system
6. To assess the impact of climate change in the Arctic on the THC, marine ecosystems and fisheries, transportation, offshore industry and oil and gas production, coastal infrastructures, and on climate in Europe

State-of-the-art

A consensus from coupled atmosphere-ocean modelling studies of increasing greenhouse-gas (GHGs) scenarios is that anthropogenic global warming will be enhanced in the northern high latitudes, due to complex feedback mechanisms in the atmosphere-ocean-ice system.

The predicted warming in the Arctic over the next 50 years is ~3-4 C or more than twice the global average. This suggests that the Arctic may be where the most rapid and dramatic changes (e.g., a shrinking sea ice cover and a glacial meltdown) may take place during the 21st century. Recent synthesis reviews of fragmentary observational evidence taken together provide a reasonably coherent portrait of Arctic and boreal change, indicating that the last 2-3 decades have experienced unusual warming over northern Eurasia and North America, reduced Arctic sea ice, marked changes in Arctic Ocean hydrography, reduced glaciers and snow cover, increased runoff into the Arctic, increased tree growth in northern Eurasia, reduced tundra areas and thawing permafrost.

There have clearly been significant advancements from observations and models in recent years; however there remain many key uncertainties concerning the fundamental processes underlying Arctic climate-system changes in the past, present and future and how to model them, the spatial-temporal characteristics of Arctic climate change and the bio-geophysical and socio-economic consequences, including their role for Europe:

• Many of the fundamental processes of Arctic climate dynamics are not fully understood. For example, the Arctic warming in the recent decades has coincided with decadal-scale anomalies in atmospheric phenomena such as the North Atlantic Oscillation (NAO), which affects temperature, precipitation and storminess in Europe including the Mediterranean region. The NAO appears as part of the broader Arctic Oscillation (AO) - the Northern Annular Mode (NAM) of atmospheric circulation variability that involves changes in the strength of the atmospheric polar vortex, characterized by strong troposphere-stratosphere coupling. There is evidence of other modes of atmosphere-ocean-ice variability in the Arctic, but neither their spatial-temporal characteristics nor linkages to the NAO/AO and teleconnections to tropical and extra-tropical regions have been determined. Moreover, mesoscale processes and their feedbacks (e.g., those governing fluxes at the atmosphere-ocean-ice interface) are neither well quantified nor well represented in models.
• The degree to which the recent Arctic changes are an enhanced GHG signal or natural variability is uncertain, as observational and modelling studies indicate that the Arctic climate undergoes large variations on decadal to multi-decadal time scales Most notable is the pronounced early 20th-century warming25 between ~1920 and the 1940, a marked cooling from ~1950-1970 and an ongoing warming trend that started around 1970. These events have not been restricted to the Arctic, but the strength of the warming has been by far most pronounced in the Arctic region, as the climate models predict. A new study suggests strongly that while the early 20th-century warming was a natural mode of the climate system, the present warming is likely influenced by the ongoing anthropogenic GHG warming. It has been speculated that the recent NAO/AO trend may itself be anthropogenically forced, possibly through some teleconnection - for example, tropical convective forcing initiated by sea-surface temperature (SST) anomalies in the Indian and Pacific oceans. However, the present understanding is limited regarding the linkages between GHGs, the NAO/AO and low-frequency oscillations in the Arctic-subpolar North Atlantic climate system.
• Arctic marine ecosystems are subject to intense and complex variability caused by effects of ocean-atmosphere fluctuations such as the NAO and other environmental factors, e.g., ice cover and irradiance with consequences for primary and secondary production. Presently, there is low predictability of the processes coupling the 3 sub-systems: sea ice, water column and seafloor. Moreover, a change in the climate system including a retreat of the sea-ice cover will likely affect the biological production, also affecting the driving forces of the air-sea carbon and sulphur fluxes. Furthermore, a shift in the ventilation of deep and intermediate waters will change the transport and mixing of CO2 from the surface to the deep waters, contributing to the sequestration of anthropogenic CO2 - to an unknown extent.
• Arctic Ocean conditions are affected by large-scale climate variability and change (e.g., NAO/AO) and the Arctic Ocean itself plays a major role concerning the surface energy and freshwater budgets, and it exports a significant amount of freshwater into the North Atlantic into areas of deep water formation such as the Greenland, Irminger and Labrador Seas, from where a significant part of the global oceanic THC is driven. Therefore, we need to improve our understanding of the large-scale circulation in the Arctic Ocean and the Fram Strait as well as dense water convection on the continental slopes. Moreover, in order to understand, parameterise and predict large-scale variability in the Arctic Ocean and its impact on climate, one need to take explicitly into account a number of typical meso- and small-scale processes that dictate how properties (e.g., heat, salt, momentum, nutrients, tracers and pollutants) are transferred throughout the entire Arctic marine ecosystem.
• Paleo-environmental evidence from marine and glaciological data has revealed that the strength of the THC during the de-glacial and the present warm period ("Holocene") was highly variable. Even during the last 8000-9000 years, which had been previously assumed to be climatically stable, severe climatic variations on decadal to millennial time scales have been found, the causes of which remain uncertain. These variations, including the termination of the last glacial period, imply fundamental changes in the THC, primarily in the Nordic Seas. Presently, leads and lags of changes and events between the Arctic and North Atlantic oceans cannot be substantiated. Therefore, it is critical to identify the nature and strength of these linkages in order to understand the dynamics and the risk of rapid changes.
• Arctic sea ice is predicted by some coupled models to be reduced by 80% during summer at the end of this century, while during winter the now-seasonally ice-covered Barents Sea will be ice free. There is a range of important potential bio-geophysical consequences (e.g., changes in precipitation) - and associated socio-economic impacts - of a shrinking ice cover. Therefore, it is essential that we improve our abilities to model and assess these changes and their impacts in this century.

The aforementioned key uncertainties have been formulated into the over-arching questions and hypotheses that will compel the CARE research program:

• Our contention is that the present paradigm of the NAO/AO coupled atmospheric-ocean-ice mode of variability as the predominant driver of Arctic climate variations through the range of time scales is not satisfactory. We hypothesize that other modes of variability - and their interactions and teleconnections to tropical and extra-tropical regions - are under-appreciated, especially when amplified through dynamical feedback processes into the Arctic ocean-ice-atmosphere system. The pronounced early 20th-century Arctic warming has been theorised to be a natural internal mode of variability, while the recent and ongoing warming has an anthropogenic signal, possibly superposed on - and interacting with - a natural low-frequency oscillation. We hypothesize therefore that the early 20th-century warming is not an analog for the recent and future warming, which will be increasingly anthropogenic. Therefore, the 21st century Arctic represents a new "terra incognita" with an unprecedented state and unknown response; e.g., the summer ice cover may be less stable than presently believed.
• The ocean transpots a great amount of heat (~0.25  1015 W) to the Arctic via the THC, transforming surface and upper-layer incoming warm and salty buoyant waters of subtropical origin into deep outflowing cold and dense waters after gradual mixing with fresh and cold polar waters, to form the main body of the North Atlantic Deep Water (NADW). We hypothesize that most of this heat loss contributes to: (1) brine formation in polynyas on shelves, (2) deep convection and ventilation of the deep ocean, (3) cooling of Atlantic waters intruding the Nordic Seas and the Arctic Ocean, and (4) ocean heat fluxes through the Arctic cold halocline and sea ice. The key questions to be answered here include: What is the relative importance of each of these processes, how do they vary and interact, how sensitive are they to external influences (freshwater input, incoming radiation, anthropogenic impact, etc.) and what are the potential feedbacks affecting climate? Analyses of paleoclimatological proxy data led to the conclusion that ice formation and brine release are the preferential mode for ventilating the deep ocean during glacial ages rather than deep convection driven by cooling during interglacial periods. Does the modern ocean have any preferential mode of ventilation or is it a mixed case?
• Multidecadal to millennial variability of Arctic Ocean ice coverage and Atlantic Water inflow was a consistent feature in the ongoing interglacial period (ca. last 10 000 years). We hypothesize that in the 21st century feedback processes of anthropogenic and natural forcings will drive the Arctic climate system beyond the range of natural variability and may affect critical factors in the Arctic that determine the strength of the THC and the European climate.
• A climate shift from a 'cold/abundant ice' to a 'warm/limited ice' mode will have profound ecological consequences propagating through all trophic levels, as sea-ice dynamics is the prime physical factor driving marine Arctic biology from cellular physiology and biochemistry to food web and habitat structure. We hypothesize that upon warming the relative importance of sea-ice biota, pelagic communities and benthic assemblages will shift from a 'benthos-dominated' to a 'zooplankton-dominated' mode. This will fundamentally change the general pattern of cryo-pelago-benthic fluxes of matter and energy in Arctic seas, as well as the air-sea flux of CO2.
• The fact that the surface atmospheric temperature (SAT) has increased significantly (1.5-2°C) in the Arctic during the last 2-3 decades, that the ice cover has decreased with 3-4% per decade during the same decades and that models indicate that the ice cover will be dramatically reduced in this century requires that new assessment of the impact of these changes must be undertaken using the expected results from this proposed research. We hypothesize that the expected changes in the Arctic climate system will have far-reaching consequences for the freshwater budget and its effect on the THC, significant impact on the ecosystem, fisheries and wildlife; impact on CO2 uptake in the Arctic Ocean; impact on transportation, offshore industry and oil and gas production; impact on indigenous people: and impact on climate in Europe including the Mediterranean region.

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