Models show natural swings in the Earth’s climate contribute to Arctic sea ice loss
Arctic sea ice loss in the last 37 years is not due to humans alone.
By Anne L. Stark, LLNL
New research by a Lawrence Livermore National Laboratory (LLNL) scientist and collaborators show that Arctic sea ice loss is enhanced by natural climate fluctuations such as El Niños and La Niñas. With manmade greenhouse gases on top of the natural climate variability, the decrease in sea ice is even more severe than climate models originally estimated.
Using a series of climate models, the team used a “fingerprint” method to estimate the impact of natural climate variability. Natural swings in the Earth’s climate contribute to about 40 percent to 50 percent of the observed multi-decadal decline in Arctic sea ice.
“Internal variability can enhance or mute changes in climate due to greenhouse gas emissions. In this case, internal variability has tended to enhance Arctic sea ice loss,” said Stephen Po-Chedley, an LLNL climate scientist and a co-author on a paper appearing in the Nov. 5 edition of Nature Geoscience.
As it turns out, observations of sea ice loss were larger than models predicted. Sea ice loss since 1979 has increased due to natural variability; observations show more Arctic sea ice loss than the climate models average.
“It is important to note that individual runs do show large changes in sea ice that are comparable to observed sea ice changes,” Po-Chedley said. “In these simulations, like in the real world, Arctic sea ice loss was enhanced by natural climate variability.
“When natural variability is taken into account, Arctic sea ice loss is quite similar across models and observations.”
According to NASA, the planet has been shedding sea ice at an average annual rate of 13,500 square miles (35,000 square kilometers) since 1979, the equivalent of losing an area of sea ice larger than the state of Maryland every year.
Model simulations (or “runs”) exhibit a range of sea ice trends. Depending on the timing of natural fluctuations, individual model runs can exhibit greater or smaller-than-average loss. Similarly, both natural variability and greenhouse gas changes contribute to the observed sea ice loss.
“This study helps to quantify the degree to which natural and anthropogenic factors contributed to Arctic sea ice loss over the last few decades,” Po-Chedley said.
The team found that enhanced ridging over the Arctic Ocean promotes warming and moistening in the lower troposphere (the lowest layer of Earth’s atmosphere where nearly all weather conditions take place), which in turn, leads to accelerated sea ice loss. Arctic sea ice decline may be important to rainfall in California. Previous research has suggested that Arctic sea ice loss can exacerbate droughts over California.
Other institutions contributing to the work include University of California, Santa Barbara, University of Washington, National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center, Princeton University and the Geophysical Fluid Dynamics Laboratory.
The paper (paywalled): https://www.nature.com/articles/s41561-018-0256-8
Fingerprints of internal drivers of Arctic sea ice loss in observations and model simulations
The relative contribution and physical drivers of internal variability in recent Arctic sea ice loss remain open questions, leaving up for debate whether global climate models used for climate projection lack sufficient sensitivity in the Arctic to climate forcing. Here, through analysis of large ensembles of fully coupled climate model simulations with historical radiative forcing, we present an important internal mechanism arising from low-frequency Arctic atmospheric variability in models that can cause substantial summer sea ice melting in addition to that due to anthropogenic forcing. This simulated internal variability shows a strong similarity to the observed Arctic atmospheric change in the past 37 years. Through a fingerprint pattern matching method, we estimate that this internal variability contributes to about 40–50% of observed multi-decadal decline in Arctic sea ice. Our study also suggests that global climate models may not actually underestimate sea ice sensitivities in the Arctic, but have trouble fully replicating an observed linkage between the Arctic and lower latitudes in recent decades. Further improvements in simulating the observed Arctic–global linkage are thus necessary before the Arctic’s sensitivity to global warming in models can be quantified with confidence.