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Radiative Heating of an Ice‐Free Arctic Ocean

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Abstract and Figures

During recent decades, there has been dramatic Arctic sea ice retreat. This has reduced the top‐of‐atmosphere albedo, adding more solar energy to the climate system. There is substantial uncertainty regarding how much ice retreat and associated solar heating will occur in the future. This is relevant to future climate projections, including the timescale for reaching global warming stabilization targets. Here we use satellite observations to estimate the amount of solar energy that would be added in the worst‐case scenario of a complete disappearance of Arctic sea ice throughout the sunlit part of the year. Assuming constant cloudiness, we calculate a global radiative heating of 0.71 W/m² relative to the 1979 baseline state. This is equivalent to the effect of one trillion tons of CO2 emissions. These results suggest that the additional heating due to complete Arctic sea ice loss would hasten global warming by an estimated 25 years.
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Radiative Heating of an Ice-Free Arctic Ocean
Kristina Pistone1, Ian Eisenman2, and Veerabhadran Ramanathan2
1Bay Area Environmental Research Institute, NASA Ames Research Center, Moffett Field, CA, USA, 2Scripps
Institution of Oceanography, University of California San Diego, La Jolla, CA, USA
Abstract During recent decades, there has been dramatic Arctic sea ice retreat. This has reduced the
top-of-atmosphere albedo, adding more solar energy to the climate system. There is substantial uncertainty
regarding how much ice retreat and associated solar heating will occur in the future. This is relevant to
future climate projections, including the timescale for reaching global warming stabilization targets. Here
we use satellite observations to estimate the amount of solar energy that would be added in the worst-case
scenario of a complete disappearance of Arctic sea ice throughout the sunlit part of the year. Assuming
constant cloudiness, we calculate a global radiative heating of 0.71 W/m2relative to the 1979 baseline state.
This is equivalent to the effect of one trillion tons of CO2emissions. These results suggest that the
additional heating due to complete Arctic sea ice loss would hasten global warming by an estimated
25 years.
1. Introduction
Much recent work has investigated the conditions under which global warming is limited to 2 or 1.5C,
which are the targets included in the Paris Climate Accord. Recent analyses suggest that these targets are
becoming increasingly difficult to achieve (Millar et al., 2017). With continuing unchecked emissions of
climate warming pollutants, global warming is projected to have a 50% probability of exceeding the 2C
threshold by 2050 (Xu & Ramanathan, 2017).
Climate models suggest a wide range of levels of sea ice retreat associated with 2C of global warming,
including the possibility that the Arctic Ocean becomes ice free at the end of each summer (Jahn, 2018;
Screen & Williamson, 2017; Sigmond et al., 2018) yet with some ice still remaining in the midsummer sea-
son when solar insolation peaks. The disappearance of sea ice alters the Earth's energy balance because a
low-albedo open ocean surface typically absorbs approximately 6 times more solar radiation than a surface
covered with sea ice and snow, which has a substantially higher albedo (e.g., Perovich, 1998). This can play
a substantial role in the energy balance of the climate system. Satellite observations suggest that the albedo
changes associated with the decline of Arctic sea ice during 1979–2011 contributed a global-mean increase
in solar heating of 0.21 W/m2, which is a quarter as large as the direct radiative forcing from rising CO2
concentrations during the same period (Pistone et al., 2014).
In this study we consider the additional solar heating associated with the extreme worst-case scenario in
which a complete disappearance of Arctic sea ice throughout the sunlit part of the year occurs far earlier
than in climate model projections.
2. Climate Model Projections of Arctic Sea Ice Loss
Comprehensive global climate models differ widely in terms of the level of global warming at which the
Arctic becomes ice free. Results from the suite of climate models included in the Coupled Model Intercom-
parison Project Phase 5 (CMIP5) are shown in Figure 1. Although these simulations do not typically reach
annually ice-free conditions, the level of global warming at which the simulated Arctic Ocean would be
annually ice free can be estimated from the simulations. Previous studies have shown that the annual-mean
Arctic sea ice area follows an approximately linear relationship with global-mean temperature (Gregory
et al., 2002; Rosenblum & Eisenman, 2017; Winton, 2011) and similarly with cumulative carbon dioxide
emissions (Notz & Stroeve, 2016). The sea ice sensitivity to global warming remains fairly constant in each
model as the simulated climate changes. Based on the sea ice cover in the 1979 baseline state (Figure 1a)
Key Points:
• The complete disappearance of
Arctic sea ice would contribute an
additional solar radiative heating of
0.71 W/m2to the planet
• This is equivalent to the radiative
forcing from one trillion tons of CO2
• The added solar heating from
complete Arctic sea ice loss would be
an order of magnitude larger in the
month of May than in the month of
Supporting Information:
• Supporting Information S1
Correspondence to:
K. Pistone,
Pistone, K., Eisenman, I., &
Ramanathan, V. (2019). Radiative
heating of an ice-free arctic ocean.
Geophysical Research Letters,46,
Received 21 MAR 2019
Accepted 15 JUN 2019
Accepted article online 20 JUN 2019
Published online 10 JUL 2019
©2019. American Geophysical Union.
All Rights Reserved.
... The AA is seasonally variable, reaching its maximum values in winter and a minimum in summer (Johannessen et al, 2016;Lesins et al 2012). It has been shown that the Arctic influences the climate all over the world (Overland et al 2011;Pistone et al 2019;Screen 2013;Tang et al 2013). The associated sea level rise, particularly from melting of the Greenland Ice Sheet, is also of concern for many coastal areas. ...
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