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By evaluating historical climate records, observational and projection data, an international team of researchers found a "pushing and triggering" mechanism that has driven the Arctic climate system to a new state, which will likely see consistently increased frequency and intensity of extreme events across all system components—the atmosphere, ocean and cryosphere—this century.

"We know that mean temperatures are rising, and the Arctic is commonly considered an indicator of global changes due to its higher sensitivity to any perturbation of external and internal forcings," says Xiangdong Zhang, research professor at North Carolina State University and senior scientist at the North Carolina Institute for Climate Studies.

"The annual mean warming rate of the Arctic is more than three times the global average—this is known as Arctic amplification," Zhang says. "But no systematic review has been done about the interplay of warmer temperatures with the dynamics of atmosphere, ocean and sea ice in weather and climate extremes around the Arctic." Zhang is the lead author of the study.

The team looked at historical temperature data and the records of extreme events in the Arctic system components, as well as CMIP6 model projections that covered a period from the present-day to the end of the century. Overall, they saw that extreme events—atmosphere and ocean heat waves, heavy precipitation, sea ice loss and ice sheet melt—have consistently occurred across the Arctic climate system with an increased frequency and intensity since the year 2000. The CMIP6, or Coupled Model Intercomparison Project Phase 6, is an international project consisting of modeling centers and groups worldwide.

The findings are in the journal Nature Reviews Earth & Environment.

"We usually think about warming as a gradual, quasi-linear change of temperature over time—it slowly gets warmer everywhere," Zhang says. "But nonlinear changes occur across the entire system. The interplay between warming and the changes in atmospheric, oceanic and sea ice dynamics creates 'pushing and triggering' mechanisms that result in a tipping point for the climate system."

According to the researchers, since the year 2000, these pushing and triggering mechanisms have forced a step change, or sudden shift, in the baseline of the Arctic climate system.

The mechanisms in question include changes in large-scale atmosphere and ocean circulation that enhance poleward atmospheric heat and moisture transport and ocean heat transport into the Arctic. Intense cyclones and blocking high-pressure systems that obstruct the movement of other systems through the upper atmosphere further enhance warming, increasing sea ice and ice sheet temperatures and pushing the Arctic climate system to a tipping point, triggering more extremes.

"Once there is a baseline state change in climate, we also see a change in extreme events," Zhang says.

According to the researchers' analysis, since the year 2000, the probability of atmospheric heat waves has increased by 20%; Atlantic Ocean layer warm events have increased by 76%; sea ice loss events have increased by 83%; and Greenland Ice Sheet melt extent has increased by 68%.

"Prior to the 21st century, these events were rare," Zhang says. "But with continued warming they will become the new norm, and we could see ice-free summers in the Arctic by mid-century. More work must be done to understanding the interplay of multiscale climate drivers in the Arctic, so that we can predict and plan for the future."