Arctic report card 2021: rapid and pronounced warming continues to drive the evolution of the Arctic environment

About Arctic Report Card 2021

The Arctic Report Card (hereafter ‘ARC’) has been issued annually since 2006. It is a timely and peer-reviewed source for clear, reliable, and concise environmental information on the current state of different components of the Arctic environmental system relative to historical records. The ARC is intended for a wide audience interested in the Arctic environment and science, including scientists, teachers, students, decision-makers, policymakers, and the general public.

ARC 2021 contains 14 essay contributions prepared by an international team of 111 authors from 12 different countries. As in previous years, independent peer review of ARC 2021 was organized by the Arctic Monitoring and Assessment Programme (AMAP) of the Arctic Council. ARC is classified as a NOAA Technical Report and is archived within the NOAA Library Institutional Repository.

ARC 2021 is organized into three sections: Vital SignsOther Indicators, and Frostbites. The Vital Signs section is for annual updates on seven recurring topics: Surface Air Temperature; Terrestrial Snow Cover; Greenland Ice Sheet; Sea Ice; Sea Surface Temperature; Arctic Ocean Primary Productivity; and Tundra Greenness. The Other Indicators section is for topics that are updated every 2-4 years, many of which have appeared in previous ARCs. The Frostbites section is for reports on new and newsworthy items, describing emerging issues, and addressing topics that relate to long-term observations in the Arctic. People occasionally ask questions such as “How are essay topics selected?” or “Why is topic X not in the Arctic Report Card?” The short answer is that each ARC strives to include some recurrent topics as well as new topics, and thus covers many subjects over a period of years. In this way the ARC achieves a comprehensiveness over time that is not possible in any given year. A complete list of topics covered since the first publication of the ARC is available at the Report Card Archive.


Arctic essays: ocean acidifcation


  • Recent work has shown that the Arctic Ocean is acidifying faster than the global ocean, but with high spatial variability.
  • A growing body of research indicates that acidification in the Arctic Ocean could have implications for the Arctic ecosystem, including influences on algae, zooplankton, and fish.
  • Cutting-edge tools like computational models are increasing our capacity to understand patterns, trends, and impacts of ocean acidification in the Arctic region.

The uptake of anthropogenic carbon dioxide (CO2) causes a cascade of chemical reactions that decreases ocean pH and carbonate ion concentrations, a process known as ocean acidification (OA). While OA is a global process, some of the fastest rates of ocean acidification around the world have been observed in the Arctic Ocean (e.g., Qi et al. 2017, 2020). These extremely rapid rates of acidification reflect the Arctic’s natural vulnerability to changes in pH, caused by cold temperatures, naturally higher baseline CO2 concentrations resulting from global circulation processes, seasonal processes that rapidly concentrate CO2 in some water masses, as well as unique land-sea interactions and hydrological mechanisms (circumpolar perspective broadly reviewed by AMAP 2018). Surface waters in some parts of the Arctic Ocean are already undersaturated with respect to some biologically important calcium carbonate minerals (e.g., aragonite and calcite) and most regions of the Arctic are likely to become corrosive (able to dissolve biologically important carbonate minerals) by the end of the century (AMAP 2018). These changes could have serious implications for the regional ecosystem, including detrimental impacts on local wildlife, cultural assets and practices, and subsistence resources.

Robust sampling programs that prioritize the collection of OA data (e.g., pH, partial pressure of CO2, dissolved inorganic carbon (DIC), and total alkalinity (TA)) are extremely difficult to implement in the Arctic. The coastal sub-Arctic seas exhibit a highly dynamic spatial and temporal variability as the underlying biogeochemistry is impacted by a range of land, ocean, and atmosphere processes. Accordingly, mature OA monitoring systems must be highly resolved in both space and time to provide adequate information for decision support. Given the expansive area, the remote geographic location, and harsh winters, traditional monitoring tools are also challenging to deploy consistently in the Arctic region, although some of these time series are starting to mature (e.g., Beaufort Gyre: Zhang et al. 2020; Canadian Archipelago: Beaupré-Laperrière et al. 2020; Eurasian Basin: Ulfsbo et al. 2018; Fram Strait: Chierici et al. 2019; Svalbard: Jones et al. 2021).

Despite these advances, we do not have a synoptic understanding of OA across the pan-Arctic system. Accordingly, computational models grounded in observable data have emerged as a useful tool to help explore spatial-temporal variability due to their much finer spatial and temporal resolution. Using these outputs, researchers are better able to explore the intensity, duration, and extent of ecosystem exposure to OA processes. In recent years, regional and global modeling studies have been used to explore both long- and short-term aspects of OA in the Arctic (e.g., Bering Sea: Pilcher et al. 2019; pan-Arctic, Terhaar et al. 2020), as well as the processes leading to these trends that are notoriously difficult to observe (e.g., pan-Arctic sea-ice related impacts: Mortenson et al. 2020). However, there is substantial regional and seasonal variability especially where land processes can influence OA, highlighting potential problems with interpolating sparse measurements (e.g., Chierici et al. 2019; Jones et al. 2021). Better regional to local climate projections may provide key improvements. Model studies continue to be refined and will likely form a pivotal part of future Arctic OA research.

As the observational record of OA in the Arctic continues to grow, research on the possible impact of OA on Arctic ecosystems continues to progress both in the laboratory and in the field (Fig. 1). The primary concern is that the short food web linkages so characteristic of the Arctic may lead to widespread impacts of OA across the ecosystem, creating both winners and losers. This is evident at the very base of the food chain: for example, OA negatively affects the calcification of some Arctic phytoplankton (pan-Arctic: Ardyna and Arrigo 2020) and may shift the community toward smaller species (western Arctic: Sugie et al. 2020). Some primary producers may experience little impact; research syntheses indicate that OA likely has a limited effect on sea ice algae, given that the biogeochemistry of the ice matrix itself naturally undergoes extreme fluctuations that result in evolutionary resilience (central Arctic: Torstensson et al. 2021).

Onboard laboratory setup for collection and filtering of Arctic seawater samples
Fig. 1. Onboard laboratory setup for collection and filtering of Arctic seawater samples. Discrete sampling remains critical to understanding ocean chemistry. Photo by J. N. Cross.

At the zooplankton trophic level, the quintessential species for detrimental OA impacts is the pteropod (sea snail). These organisms are extremely sensitive to ocean pH and are often used around the world as indicators that can inform OA conditions. Both laboratory and field observations have shown that pteropod responses to OA include reduced juvenile survival, reduced shell growth and condition, as well as costly metabolic regulation. Arctic population connectivity and morphological characteristics of pteropods is a growing area of research. For example, recent studies of natural populations indicate a high occurrence of severe shell dissolution in the Bering Sea, Amundsen Gulf, and Svalbard margin (Niemi et al. 2021; Bednaršek et al. 2021; Anglada-Ortiz et al. 2021, respectively). While pteropods are an important biological indicator, research on other organisms specific to Arctic ecosystems will also support regional relevance. For example, fish show sensitivities to OA, including important species such as Arctic cod (e.g., western Arctic cod populations: Steiner et al. 2019; eastern Arctic cod: Hänsel et al. 2020). However, key questions remain to fully understand the mechanisms that produce individual and population-level responses to OA. Across species (fish, benthic and pelagic invertebrates) repair, adaptability, and associated tolerance have been linked to resource availability (e.g., Niemi et al. 2021; Hänsel et al. 2020; Duarte et al. 2020; Goethel et al. 2017), indicating the importance of a holistic ecosystem approach to understand OA biological responses (Fig. 2).

Mudryk L., Chereque A. E., Derksen C., Luojus K. & Decharme B., 2021. Arctic report card 2021: rapid and pronounced warming continues to drive the evolution of the Arctic environment. In: Moon T. A., Druckenmiller M. L. & Thoman R. L. (Eds.). Terrestrial snow cover. Report.

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