Key observational indicators of climate change in the Arctic, most spanning a 47 year period (1971–2017) demonstrate fundamental changes among nine key elements of the Arctic system. A new study finds that, coherent with increasing air temperature, there is an intensification of the hydrological cycle, evident from increases in humidity, precipitation, river discharge, glacier equilibrium line altitude and land ice wastage. Director at the Nansen Center, Sebastian H. Mernild, has been one of the authors to "Key indicators of Arctic climate change: 1971-2017" published at Environmental Research Letters, ERL. 

Downward trends continue in sea ice thickness (and extent) and spring snow cover extent and duration, while near-surface permafrost continues to warm. Several of the climate indicators exhibit a significant statistical correlation with air temperature or precipitation, reinforcing the notion that increasing air temperatures and precipitation are drivers of major changes in various components of the Arctic system. To progress beyond a presentation of the Arctic physical climate changes, we find a correspondence between air temperature and biophysical indicators such as tundra biomass and identify numerous biophysical disruptions with cascading effects throughout the trophic levels. These include: increased delivery of organic matter and nutrients to Arctic near‐coastal zones; condensed flowering and pollination plant species periods; timing mismatch between plant flowering and pollinators; increased plant vulnerability to insect disturbance; increased shrub biomass; increased ignition of wildfires; increased growing season CO2 uptake, with counterbalancing increases in shoulder season and winter CO2 emissions; increased carbon cycling, regulated by local hydrology and permafrost thaw; conversion between terrestrial and aquatic ecosystems; and shifting animal distribution and demographics. The Arctic biophysical system is now clearly trending away from its 20th Century state and into an unprecedented state, with implications not only within but beyond the Arctic. The indicator time series of this study are freely downloadable at AMAP.no.

Key messages

Arctic air temperature: Arctic annual average air temperatures 1971–2017 increased 2.7 °C, at 2.4 times the rate of the Northern Hemisphere average. The 3.1 °C increase in the cold season (October–May) is the largest by season, 2.8 times the rate of the Northern Hemisphere cold season average. Arctic warm season (June through September) temperatures increased 1.8 °C, 1.7 times the rate of Northern Hemisphere summer.

Alaskan permafrost: New record-high annual average temperatures in the upper 10–20 m of the ground have been observed at many permafrost observatories. At 20 m depth for three North Slope of Alaska sites (West Dock, Deadhorse and Frankiln Bluffs) we find a 2.5 °C permafrost temperature increase in the past 47 years.

Arctic hydroclimatology: Observations from land and coastal stations indicate widespread increases in humidity, low-level clouds, precipitation, rainfall (at the expense of snowfall), river discharge, sedimentation and delivery of organic matter to the Arctic ocean, freshening of the Arctic Ocean, and reductions in snow cover, all of which are controlling factors in Arctic terrestrial and probably marine ecosystems.

Snow cover: Arctic snow cover is responding to multiple environmental drivers and feedbacks (such as warming, increased moisture availability, changing atmospheric circulation, changing vegetation, increased frequency of winter thaws, rain-on-snow events). There is widespread multi-dataset evidence of declining snow cover over the Arctic with the annual duration of snow on the ground shortening by 2 to 4 days per decade with the largest negative trends occurring at high latitudes and elevations consistent with AA of warming and enhanced albedo feedbacks.

Arctic Ocean sea ice: Sea ice extent and volume are continuing their downward trends. The past decade had record-low extent in summer 2012, and it is the lowest decade ever in satellite era beginning in the 1970s. These are unprecedented change in Arctic sea ice, in both the rates and magnitude of change in extent, area, thickness, and spatial distribution. Along with Arctic sea ice decline, there is emerging evidence for a loss of biodiversity in sea-ice habitats.

Arctic land ice: In the 47 year period (1971–2017), the Arctic was the largest global source of sea-level rise contribution, 48% of the global land ice contribution 2003–2010 and 30% of the total sea-level rise since 1992. Temperature effects are dominant in land ice mass balance; precipitation represents a source of either damping or amplifying feedbacks respectively via snow and rain.

Arctic region wildfires: Drier conditions and an increase in maximum air temperatures contribute to increased fire risk. Fire clearly causes dramatic short-term changes in vegetation and ecosystem function. The fire-climate relationship is related to increasing lightning ignition that is shown to correlate with air temperature and precipitation, thus linking Arctic warming with the liklihood for increased fire.

Tundra and terrestrial ecosystems: Arctic greening has been observed across tundra ecosystems over the past 30 years. The increase of Arctic tundra average and maximum NDVI both correlate with Arctic warm season air temperature with high confidence.

Carbon cycling: The changes in the global climate system are already affecting biogeophysical energy exchange and transport within the Arctic. The response of the carbon cycle in northern high latitude regions is influenced by terrestrial carbon exchange and by coupling between the land and ocean, which has worldwide consequences. Importantly, there are substantial organic matter stocks of carbon in the Arctic contained in permafrost and within the methane hydrates that exist beneath both subterranean and subsea permafrost of the Arctic, all of which can affect carbon cycling. Observational data indicate increased tundra ecosystem CO2 uptake during the growing season. Further temperature increase will affect tundra CO2 and CH4 emissions, their ratio being dependent on local hydrology and permafrost thaw.

- The Arctic biophysical system is now clearly trending away from its previous state and into a period of unprecedented change, with implications not only within but also beyond the Arctic. These indicator-based observations also provide a foundation for the research that is needed to address the gaps in knowledge and to support a more integrated understanding of the Arctic region and its role in the global dynamics of the Earth's biogeophysical systems, says Sebastian H. Mernild.

Read more at Environmental Research Letters, ERL.