Aim of ACAS

This project aims to determine and quantify the main processes responsible for the observed rapid Arctic climate change. New observations and unique modeling capabilities will provide insights on the sensitivity and fate of the future Arctic climate. Our goal is to answer the question:
Are we on an irreversible path to an ice-free summer Arctic?


Climate change is an inherently global problem but the largest changes are manifested regionally. In particular, the climate in the Arctic is changing dramatically faster than anywhere else on Earth. The prospect of an ice-free summer Arctic has serious implications for life north of the Arctic Circle and it has been suggested that it may also affect weather patterns at lower latitudes. The societal and economic effects are global.

Observations are essential to deepen process understanding and to develop, improve and assess models, however, the existing observational record for the Arctic Ocean is very limited. Within ACAS we have unique possibilities to improve on this, using platforms such as the world-class icebreaker Oden (top left), and the research station at Ny Ålesund located at a main gateway to the Arctic (top right). Combined use of these observational assets and novel computer modeling allows us to sharpen climate predictions for this region and to provide the scientifically-based advice needed by society to develop mitigation and adaptation strategies.

Photo: Michael Tjernström

Decreasing sea ice is observed in all seasons but is largest in late summer when the sea-ice extent has its annual minimum. Over the past 35 years, the minimum sea-ice area has dropped by ~12% per decade, driven by the rise in temperature (Figure 1). This decrease is projected to continue until essentially all summer ice is lost sometime during the second half of this century, at the current rate of climate change (IPCC 2013).

The ACAS tenet

Our leading idea is to target the surface-energy budget and the primarily atmospheric and surface processes central to it, assuming that improved projections of Arctic climate necessitate a better understanding of the main processes the that contributes to sea-ice melt and freeze. This includes the effects of transport events of warm and moist air into the Arctic, the formation of clouds and the interaction between the sea-ice surface and the atmosphere and effects on the upper ocean.

Read more about ACAS on Polarforskningsportalen (in Swedish).

Project Organization

The project activities evolves around three scientific work packages and two supporting infrastructures, for modelling and observations, with science questions designed to promote interaction across all the parts.

WP1: Large-scale atmospheric interactions and natural variability

Team Leader: Rodrigo Caballero
Co-Leads: Annica Ekman and Johan Nilsson

This WP will examine interactions between the global atmospheric circulation and Arctic cli-mate change, focusing on the atmospheric transport of energy, moisture and aerosols from lower latitudes to the Arctic and on the role of global natural climate variability in driving de-cadal-scale fluctuations in the Arctic.



WP2: Clouds, aerosols, and aerosol/cloud interactions

Team Leaders: Annica Ekman and Paul Zeiger
Co-Leads: Radovan Krejci and Michael Tjernström

This WP will identify key processes and parameters governing the “life cycle” of Arctic clouds, examine how the clouds are affected by changes in these variables and the feedback on the Arctic energy budget.

WP3: Surface energy budget

Team Leader: Gunilla Svensson
Co-Leads: Ilona Riipinen & Johan Nilsson

This WP will explore the importance of the sea ice and related feedbacks in an Arctic with diminished summer sea ice, and explore strategies for coupled modeling. The presence of the perennial sea ice in the present system sets the research of the Arctic climate system apart from the rest of the Earth. The climate effects of sea ice are dynamic, changing the atmosphere–ocean momentum exchange, the albedo and thermodynamic processes coupling the ocean and atmos-phere. The ice cover is also sensitive to changes in forcing and exhibits large interannual vari-ability, while integrating energy fluxes over time.

IO1: Numerical modelling
Team Leader: Ilona Riipinen
Co-Leads: Gunilla Svensson & Rodrigo Caballero

The overarching modeling philosophy is to develop and apply a family of models with a con-sistent set of descriptions of important processes governing the Arctic climate system, across different scales. Particular foci will be to facilitate the flow of data, ideas and knowledge be-tween: 1) models at different scale; 2) models and observational data, and; 3) the three work packages.

IO2: Observations
Team Lead: Michael Tjernström
Co-Leads: Hans-Christen Hansson, Radovan Krejci

We will carefully invest in observations critical for the science we propose but not currently available: A new observatory on the Swedish icebreaker Oden and at the Ny Ålesund long-term observatory. These are infrastructures where several of the co-PIs have experience and long-term presence. Data management will be handled within the Bolin Centre database framework.