Geophysical Research Letters
Arctic Sea Ice in a 1.5
C Warmer World
Anne Laura Niederdrenk
and Dirk Notz
Max Planck Institute for Meteorology, Hamburg, Germany
We examine the seasonal cycle of Arctic sea ice in scenarios with limited future global
warming. To do so, we analyze two sets of observational records that cover the observational uncertainty
of Arctic sea ice loss per degree of global warming. The observations are combined with 100 simulations of
historical and future climate evolution from the Max Planck Institute Earth System Model Grand Ensemble.
Based on the high-sensitivity observations, we ﬁnd that Arctic September sea ice is lost with low probability
10%) for global warming of
C above preindustrial levels and with very high probability (
for global warming of
C above preindustrial levels. For the low-sensitivity observations, September sea
ice is extremely unlikely to disappear for
C warming (
1%) and has low likelihood (
disappear even for
C global warming. For March, both observational records suggest a loss of 15% to
20% of Arctic sea ice area for 1.5
C global warming.
Arctic sea ice has been declining rapidly in the past few decades, with the strongest decrease observed dur-
ing late summer (e.g., Cavalieri & Parkinson, 2012; Simmonds, 2015; Stroeve et al., 2012). As the observed
loss of sea ice is tightly coupled to increasing global-mean air temperature (Gregory et al., 2002; Li et al.,
2013; Mahlstein & Knutti, 2012; Notz & Stroeve, 2015; Ridley et al., 2012; Winton, 2011) and thus to cumula-
tive anthropogenic CO
emissions (Notz & Stroeve, 2016), the loss of sea ice can only be stopped if global
warming is limited below a certain threshold. To examine the response of the Arctic sea ice cover to such a
threshold, we here estimate the seasonal cycle of Arctic sea ice resulting from a limitation of global warm-
C and to
C above preindustrial levels, as called for by the Paris Agreement of all Parties of
the United Nations Framework Convention on Climate Change (United Nations Framework Convention on
Climate Change, 2015).
Our analysis has two overarching aims: First,we want to estimate the mean seasonal cycle of sea ice for a given
maximum level of global warming. And second, we want to quantify possible variations around this mean
seasonal cycle caused by internal variability. These two aims can only be reached by a combination of the
observational record of Arctic sea ice decline with large-scale climate-model simulations. This is because the
observational record allows us to estimate the sensitivity of Arctic sea ice to a given global-mean warming,
while climate-model simulations allow us to estimate the impact of internal variability. We cannot robustly
infer the sensitivity directly from model simulations as they usually simulate a lower sensitivity of Arctic sea
ice loss per degree of global warming than has been observed (e.g., Mahlstein & Knutti, 2012; Notz & Stroeve,
2016; Rosenblum & Eisenman, 2017). Note that we focus on the Arctic, as there the sea ice evolution is tightly
coupled to atmospheric temperature. Our approach does not work in the Antarctic, where processes related
to sea ice dynamics play a major role for the time evolution of the ice cover.
While previous studies of future sea ice evolution are mainly limited to annual mean or summer months (e.g.,
Mahlstein & Knutti, 2012; Notz & Stroeve, 2016; Rosenblum & Eisenman, 2016; Screen & Williamson, 2017),
we give, for diﬀerent levels of global warming, a complete annual cycle of Arctic sea ice. Using a very large
ensemble of simulations, we further estimate its uncertainty due to internal variability. We also speciﬁcally
address the issue of observational uncertainty.
We start with a brief overview of the model simulations and observational data that we use for our study,
explaining in particular how we use the observational data to recalibrate the sensitivity of the modeled sea ice
evolution. We then present the resulting seasonal cycle of Arctic sea ice for diﬀerent levels of global warming
and provide maps that show the regional likelihood of ice coverage for a speciﬁc global warming.
• We ﬁnd it is unlikely that September
Arctic sea ice vanishes for 1.5
• September sea ice might vanish
C global warming, but
observational uncertainty prevents
a conclusive statement
• Internal variability causes a range of
C to the global warming
magnitude at which September Arctic
sea ice is lost
• Supporting Information S1
A. L. Niederdrenk,
Niederdrenk, A. L., & Notz, D.
(2018). Arctic sea ice in a 1.5
warmer world. Geophysical
Research Letters, 45, 1963–1971.
Received 24 OCT 2017
Accepted 17 JAN 2018
Accepted article online 25 JAN 2018
Published online 20 FEB 2018
©2018. American Geophysical Union.
All Rights Reserved.
NIEDERDRENK AND NOTZ