Q: What is the contribution of the Arctic sea-ice to the cooling of the planet, or, conversely, what would be the impact on global warming if the Arctic sea-ice disappeared?
A: We can answer this question in many ways, from an energy balance perspective, calculating the amount of energy that the sea-ice can reflect to space, when compared to the open ocean, or calculating the equivalent amount of carbon dioxide in the atmosphere required to trap the same amount energy in the Earth system. Finally, we can also look at what research found when simulating the impact of the Arctic sea-ice vs. the human activities’ induced warming of the planet.
Global Warming
Global warming can be described as the excessive trapping of heat, causing an imbalance between incoming and outgoing energy, due primarily to greenhouse gases dispersed in the atmosphere. This imbalance has been measured by GCOS* at around 0.87 ± 0.12 W/m2 and quickly growing* to over 1 W/m2 when averaged across the surface of the Earth.
If we take a reference value of 1 W/m2, for the surface of the Earth (around 512 million km2) and over an entire year, this will translate into the following amount of energy:
Earth Energy Imbalance = 512 x 10^12 m2 x 1 W/m2 x 365 x 24h
= 4,485,120 TWh / year
* GCOS - Earth’s energy imbalance
* G. Loeb et al. - Increase in Earth's heating rate
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GCOS estimated that over the period 1960-2018 the total excess heat was nearly 100 million TWh (358 ZJ). To go back to some sort of equilibrium, where the Earth stops accumulating extra heat, we would need to go back from the current 420 ppm to around 350 ppm* of atmospheric CO2.
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Energy Consumption
If we consider that the energy consumption for human activities over 1 year is currently in the region of 25,000 TWh, we can see that our planet is absorbing and storing an excess of energy around 180 times the energy that we produce and consume.
Solar Irradiance over the Arctic Ocean
A major role in the Earth Energy Balance is played by sea-ice, with its ability to reflect a large part of the solar radiations. The rapid loss of sea-ice in Arctic regions is having an immediate impact on the energy balance, therefore there’s an immense value in preserving and restoring sea-ice, to reduce energy absorption and slow down climate warming.
We have calculated the annual solar insolation at different Northern latitudes* considering:
the average atmospheric reflection (with clear sky)
the average cloud cover of Arctic regions* (81%)
Note that in recent times the reference values for solar irradiation and atmospheric absorptance have been re-estimated with increased accuracy*.
We have then calculated the reduction in absorption of the average sea-ice surface compared to the open ocean, respectively with albedo of 0.75 and 0.06, over a full year.
* Solarpy - Solar radiation model based on Duffie & Beckman "Solar energy thermal processes" (1974
* Atsumu Ohmura 2012 - Present status and variations in the Arctic energy balance
Daily insolation assuming clear sky (Wh/day)
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If we take a mean reduced absorption of 25 W/m2 and apply this to an area of 7 million km2 which is what we believe we can preserve or restore to full-year sea-ice cover in the Arctic, we obtain the following:
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Sea Ice Reduced Absorption = 7 x 10^12 m2 x 25 W/m2 x 365 x 24h
= 1,540,000 TWh / year.
Which corresponds to over 34% of the Earth Energy Imbalance.
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We can read this in two ways:
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If we continue to lose Arctic sea-ice, we are going to see a massive acceleration of climate warming, as the Earth Energy Imbalance is going to grow significantly. But on the positive side, we have identified a clear way to counteract the warming of the planet, by investing in the preservation and restoration of Arctic sea-ice.
CO2 Atmosphere Warming Offset
Impact of CO2 on global temperatures
The response in temperature increase to the cumulative carbon emissions has been found* to be linear, over long periods of time, within a confidence range of 1.0C - 2.1C (5th and 95th percentile) and best estimate of 1.5C, per trillion tons of carbon (corresponding to 3,700 Gt CO2)
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If we use the best estimate (1.5C per 3,700 Gt CO2), we can calculate that:
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The impact of 1 tCO2 is 405 x 10-15 C.
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It takes approximately 2,467 Gt CO2 to cause an increase in temperature of 1C.
Considering the following*:
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The weight of the atmosphere is 5.14 x 10^15 tonnes (5140 trillion tonnes)
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Air’s specific heat capacity at constant pressure is 1006 J/kg C.
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We can calculate that it takes 1,436,344 TWh to increase the atmosphere temperature by 1C.
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Therefore, we can calculate that:
Atmosphere Warming Effect of CO2 = 582 TWh / GtCO2
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* H. Matthews, N. Gillett, P. Stott and K. Zickfeld, “The proportionality of global warming to cumulative carbon emissions,” Nature, vol. 459, no. 7248, p. 829, 2009.
* https://scholarsandrogues.com/2013/05/09/csfe-heat-capacity-air-ocean/
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Validation from other sources:
The IPCC in 2021 estimated that average temperatures would grow to +2.0C (an additional 0.93C) from pre-industrial temperatures, with 50% likelihood once we used our remaining “budget” of 1,350 tCO2 of net equivalent emissions*.
It also estimated that it has taken the emission of 2,390 GtCO2 (median value) to increase the temperature, until 2021, by 1.07C.
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GCOS estimated that over the period 1960-2018 the total excess heat was nearly 100 million TWh (358 ZJ). To go back to some sort of equilibrium, where the Earth stops accumulating extra heat, we would need to go back from the current 420 ppm to around 350 ppm* of atmospheric CO2.
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* * IPCC Climate Change 2021 – The Physical Science Basis – Summary for Policymakers
Both values are broadly in agreement, or at least are within the same ballpark with the long-term historical trend (1451 and 2234 vs. 2467 Gt CO2 per 1C increase) which is a sufficient validation, as here we are talking about much shorter periods of time, and these are all best estimates within large intervals of confidence.
Both values are broadly in agreement, or at least are within the same ballpark with the long-term historical trend (1451 and 2234 vs. 2467 Gt CO2 per 1C increase) which is a sufficient validation, as here we are talking about much shorter periods of time, and these are all best estimates within large intervals of confidence.
Equivalent
sea-ice
Considering a mean reduced power absorption of 25 W/m2 from increased albedo, we can calculate the following:
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The mean reduced energy absorption for 1 m2 of sea-ice is 219 kWh / (m2 year)
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We can also calculate that, historically with an increase (to 2021) of 1.07C and a total EEI of 113,000,000 TWh (GCOS estimate extrapolated to 2018), only 1.36% of EEI has been stored in the atmosphere, which is consistent with the 1 – 2% estimate provided by GCOS, growing to 2% in recent years. Roughly 90% of the EEI is stored in oceans.
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If we use the 2% of the recent trend, we obtain:
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4.38 kWh / (m2 year): the mean reduced energy absorption in the atmosphere that we can attribute to the presence of sea-ice.
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133 m2 of sea-ice area for 1 year to offset the warming effects of 1 tCO2 (582 kWh/tCO2)
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1 GtCO2 can be offset by 132,946 km2 of sea-ice for 1 year.
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1 km2 of sea-ice over 1 year, can offset the warming effects of 7,522 tCO2.
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Atmosphere Cooling Effect Equivalent for Sea-ice = 7,522 tCO2 / (km2 year)
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Atmosphere Cooling Effect Equivalent for 7m km2 of full-year Arctic sea-ice = 52.7 GtCO2
Validation from other sources:
"The albedo change from the loss of the last 4 million km2 of ice will have the same warming effect on the Earth as the last twenty-five years of carbon dioxide emissions."
(p. 4)*.
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709 GtCO2 were emitted from 1991 to 2015.
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This is equated to the "warming contribution" of losing 4 million km2 of all-year-round Arctic sea-ice.
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We obtain an equivalence of 177,250 tCO2 per km2 of sea-ice.
If we average this over 25 years (not declared in the book statement, but implicit)
we obtain:
The warming effect of losing 1 km2 of sea-ice = 7,090 tCO2 / (km2 year)
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* Book "A Farewell to Ice" from Peter Wadhams - 2016
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This is very consistent with the equivalence calculated from the carbon contribution to atmospheric warming.
Loss of Arctic sea-ice vs. Greenhouse gas forcing
An interesting study * found through weather simulations that the response in average temperature increase of losing the Arctic sea-ice and the doubling of atmospheric CO2 (2X) from pre-industrial (PI) times are effectively linearly additive.
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The reduction of Arctic sea-ice from PI to 2X conditions in a PI climate and in a 2X climate results in global average surface air temperature (SAT) increases of 0.6°C and 0.7°C, respectively. Similarly, doubling CO2 concentration from PI to 2X with high and low Arctic sea-ice area increases global average SAT by 2.8°C and 2.7°C, respectively.
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We can therefore conclude that the loss of Arctic sea-ice could contribute for around 24% (0.65°C / 2.75°C) of human induced increase in CO2 concentration, to surface air temperature increases.
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* McCusker, K. E., P. J. Kushner, J. C. Fyfe, M. Sigmond, V. V. Kharin, and C. M. Bitz (2017), Remarkable separability of circulation response to Arctic sea ice loss and greenhouse gas forcing, Geophys. Res. Lett., 44, 7955–7964, doi:10.1002/2017GL074327.