Reduced High-Latitude Land Seasonality in Climates with Very High Carbon Dioxide

Matthew Henry aDepartment of Mathematics, University of Exeter, Exeter, United Kingdom

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Geoffrey K. Vallis aDepartment of Mathematics, University of Exeter, Exeter, United Kingdom

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Abstract

Observations of warm past climates and projections of future climate change show that the Arctic warms more than the global mean, particularly during winter months. Previous work has attributed this reduced Arctic land seasonality to the effects of sea ice or clouds. In this paper, we show that the reduced Arctic land seasonality is a robust consequence of the relatively small surface heat capacity of land and the nonlinearity of the temperature dependence of surface longwave emission, without recourse to other processes or feedbacks. We use a general circulation model (GCM) with no clouds or sea ice and a simple representation of land. In the annual mean, the equator-to-pole surface temperature gradient falls with increasing CO2, but this is only a near-surface phenomenon and is not caused by the change in total meridional heat transport, which is virtually unaltered. The high-latitude land has about twice as much warming in winter than in summer, whereas high-latitude ocean has very little seasonality in warming. A surface energy balance model shows how the combination of the smaller surface heat capacity of land and the nonlinearity of the temperature dependence of surface longwave emission gives rise to the reduced seasonality of the land surface. The increase in evaporation over land also leads to winter amplification of warming over land, although amplification still occurs without it. While changes in clouds, sea ice, and ocean heat transport undoubtedly play a role in high-latitude warming, these results show that enhanced land surface temperature warming in winter can happen in their absence for robust reasons.

Significance Statement

As we add greenhouse gases to the atmosphere, Earth’s surface gets warmer, and this is especially pronounced in the Arctic in winter. For the current and near-future climate, this is at least in part due to the melting of sea ice. However, as time progresses all the sea ice melts, and even after that climate models show enhanced polar warming, with most of the warming occurring over Arctic land in winter. Moreover, fossils indicate that the very warm climates of the past (some 50 million years ago for example) had exceptionally warm Arctic winters. Previous work has attributed this reduced seasonality over Arctic land to the effects of sea ice or clouds. Here, we identify a robust mechanism, based on the smaller heat capacity of land and the fact that cold bodies need to warm more to reach a given increase in radiation, as to why Arctic land should have a reduced seasonality in very warm climates. The mechanism depends on neither sea ice nor clouds.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Matthew Henry, m.henry@exeter.ac.uk

Abstract

Observations of warm past climates and projections of future climate change show that the Arctic warms more than the global mean, particularly during winter months. Previous work has attributed this reduced Arctic land seasonality to the effects of sea ice or clouds. In this paper, we show that the reduced Arctic land seasonality is a robust consequence of the relatively small surface heat capacity of land and the nonlinearity of the temperature dependence of surface longwave emission, without recourse to other processes or feedbacks. We use a general circulation model (GCM) with no clouds or sea ice and a simple representation of land. In the annual mean, the equator-to-pole surface temperature gradient falls with increasing CO2, but this is only a near-surface phenomenon and is not caused by the change in total meridional heat transport, which is virtually unaltered. The high-latitude land has about twice as much warming in winter than in summer, whereas high-latitude ocean has very little seasonality in warming. A surface energy balance model shows how the combination of the smaller surface heat capacity of land and the nonlinearity of the temperature dependence of surface longwave emission gives rise to the reduced seasonality of the land surface. The increase in evaporation over land also leads to winter amplification of warming over land, although amplification still occurs without it. While changes in clouds, sea ice, and ocean heat transport undoubtedly play a role in high-latitude warming, these results show that enhanced land surface temperature warming in winter can happen in their absence for robust reasons.

Significance Statement

As we add greenhouse gases to the atmosphere, Earth’s surface gets warmer, and this is especially pronounced in the Arctic in winter. For the current and near-future climate, this is at least in part due to the melting of sea ice. However, as time progresses all the sea ice melts, and even after that climate models show enhanced polar warming, with most of the warming occurring over Arctic land in winter. Moreover, fossils indicate that the very warm climates of the past (some 50 million years ago for example) had exceptionally warm Arctic winters. Previous work has attributed this reduced seasonality over Arctic land to the effects of sea ice or clouds. Here, we identify a robust mechanism, based on the smaller heat capacity of land and the fact that cold bodies need to warm more to reach a given increase in radiation, as to why Arctic land should have a reduced seasonality in very warm climates. The mechanism depends on neither sea ice nor clouds.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Matthew Henry, m.henry@exeter.ac.uk
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