74 research outputs found
Factors controlling the bifurcation structure of sea ice retreat
The contrast in surface albedo between sea ice and open ocean suggests the possibility of an unstable climate state flanked by two separate stable climate states. Previous studies using idealized single-column models and comprehensive climate models have considered the possibility of abrupt thresholds during sea ice retreat associated with such multiple states, and they have produced a wide range of results. When the climate is warmed such that the summer minimum Arctic sea ice cover reaches zero, some models smoothly transition to seasonally ice-free conditions, others discontinuously transition to seasonally ice-free conditions, and others discontinuously transition to annually ice-free conditions. Among the models that simulate a continuous transition to seasonally ice-free conditions, further warming causes some to smoothly lose the remaining wintertime-only sea ice cover and others to discontinuously lose it. Here, we use a toy model representing the essential physics of thermodynamic sea ice in a single column to investigate the factors controlling which of these scenarios occurs. All of the scenarios are shown to be possible in the toy model when the parameters are varied, and physical mechanisms giving rise to each scenario are investigated. We find that parameter shifts that make ice thicker or open ocean warmer under a given climate forcing make models less prone to stable seasonally ice-free conditions and more prone to bistability and hence bifurcations. The results are used to interpret differences in simulated sea ice stability in comprehensive climate models
Geographic muting of changes in the Arctic sea ice cover
The seasonal cycle in Arctic sea ice extent is asymmetric. Its amplitude has grown in recent decades as the ice has retreated more rapidly in summer than in winter. These seasonal disparities have typically been attributed to different physical factors operating during different seasons. Here we show instead that the seasonal asymmetries in Arctic sea ice extent are a geometric consequence of the distribution of continents. Coastlines block southward ice extension during winter, thereby muting changes in ice extent, but they have relatively little effect at the time of summer minimum extent. We suggest that the latitude of the Arctic sea ice edge, averaged zonally over locations where it is free to migrate, is the most readily interpretable quantity to describe the Northern Hemisphere sea ice cover. We find that the zonal-mean sea ice edge latitude during the 1978–present era of satellite measurements has been following an approximately sinusoidal seasonal cycle that has been migrating northward at an approximately annually constant rate of 8 km/year. These results suggest a change in perspective of the most critical quantities for understanding changes in Arctic sea ice
Sea ice trends in climate models only accurate in runs with biased global warming
Observations indicate that the Arctic sea ice cover is rapidly retreating
while the Antarctic sea ice cover is steadily expanding. State-of-the-art
climate models, by contrast, typically simulate a moderate decrease in both the
Arctic and Antarctic sea ice covers. However, in each hemisphere there is a
small subset of model simulations that have sea ice trends similar to the
observations. Based on this, a number of recent studies have suggested that the
models are consistent with the observations in each hemisphere when simulated
internal climate variability is taken into account. Here we examine sea ice
changes during 1979-2013 in simulations from the most recent Coupled Model
Intercomparison Project (CMIP5) as well as the Community Earth System Model
Large Ensemble (CESM-LE), drawing on previous work that found a close
relationship in climate models between global-mean surface temperature and sea
ice extent. We find that all of the simulations with 1979-2013 Arctic sea ice
retreat as fast as observed have considerably more global warming than
observations during this time period. Using two separate methods to estimate
the sea ice retreat that would occur under the observed level of global warming
in each simulation in both ensembles, we find that simulated Arctic sea ice
retreat as fast as observed would occur less than 1% of the time. This implies
that the models are not consistent with the observations. In the Antarctic, we
find that simulated sea ice expansion as fast as observed typically corresponds
with too little global warming, although these results are more equivocal. We
show that because of this, the simulations do not capture the observed
asymmetry between Arctic and Antarctic sea ice trends. This suggests that the
models may be getting the right sea ice trends for the wrong reasons in both
polar regions
Faster Arctic sea ice retreat in CMIP5 than in CMIP3 due to volcanoes
The downward trend in Arctic sea ice extent is one of the most dramatic
signals of climate change during recent decades. Comprehensive climate models
have struggled to reproduce this, typically simulating a slower rate of sea ice
retreat than has been observed. However, this bias has been widely noted to
have decreased in models participating in the most recent phase of the Coupled
Model Intercomparison Project (CMIP5) compared with the previous generation of
models (CMIP3). Here we examine simulations from both CMIP3 and CMIP5. We find
that simulated historical sea ice trends are influenced by volcanic forcing,
which was included in all of the CMIP5 models but in only about half of the
CMIP3 models. The volcanic forcing causes temporary simulated cooling in the
1980s and 1990s, which contributes to raising the simulated 1979-2013
global-mean surface temperature trends to values substantially larger than
observed. We show that this warming bias is accompanied by an enhanced rate of
Arctic sea ice retreat and hence a simulated sea ice trend that is closer to
the observed value, which is consistent with previous findings of an
approximately linear relationship between sea ice extent and global-mean
surface temperature. We find that both generations of climate models simulate
Arctic sea ice that is substantially less sensitive to global warming than has
been observed. The results imply that the much of the difference in Arctic sea
ice trends between CMIP3 and CMIP5 occurred due to the inclusion of volcanic
forcing, rather than improved sea ice physics or model resolution.Comment: revised submission to Journal of Climat
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Rain driven by receding ice sheets as a cause of past climate change
The Younger Dryas cold period, which interrupted the transition from the last ice age to modern conditions in Greenland, is one of the most dramatic incidents of abrupt climate change reconstructed from paleoclimate proxy records. Changes in the Atlantic Ocean overturning circulation in response to freshwater fluxes from melting ice are frequently invoked to explain this and other past climate changes. Here we propose an alternative mechanism in which the receding glacial ice sheets cause the atmospheric circulation to enter a regime with greater net precipitation in the North Atlantic region. This leads to a significant reduction in ocean overturning circulation, causing an increase in sea ice extent and hence colder temperatures. Positive feedbacks associated with sea ice amplify the cooling. We support the proposed mechanism with the results of a state-of-the-art global climate model. Our results suggest that the atmospheric precipitation response to receding glacial ice sheets could have contributed to the Younger Dryas cooling, as well as to other past climate changes involving the ocean overturning circulation
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The Effect of Milankovitch Variations in Insolation on Equatorial Seasonality
Although the sun crosses the equator 2 times per year at the equinoxes, at times in the past the equatorial insolation has had only one maximum and one minimum throughout the seasonal cycle because of Milankovitch orbital variations. Here a state-of-the-art coupled atmosphere–ocean general circulation model is used to study the effect of such insolation forcing on equatorial surface properties, including air and sea temperature, salinity, winds, and currents. It is shown that the equatorial seasonality is altered according to the insolation with, for example, either maximum sea surface temperature (SST) close to the vernal equinox and minimum SST close to the autumnal equinox or vice versa. The results may have important implications for understanding tropical climate as well as for the interpretation of proxy data collected from equatorial regions
The Importance of Ice Vertical Resolution for Snowball Climate and Deglaciation
Sea ice schemes with a few vertical levels are typically used to simulate the thermodynamic evolution of sea ice in global climate models. Here it is shown that these schemes overestimate the magnitude of the diurnal surface temperature cycle by a factor of 2–3 when they are used to simulate tropical ice in a Snowball earth event. This could strongly influence our understanding of Snowball termination, which occurs in global climate models when the midday surface temperature in the tropics reaches the melting point. A hierarchy of models is used to show that accurate simulation of surface temperature variation on a given time scale requires that a sea ice model resolve the e-folding depth to which a periodic signal on that time scale penetrates. This is used to suggest modifications to the sea ice schemes used in global climate models that would allow more accurate simulation of Snowball deglaciation
Eisenman Receives 2012 Cryosphere Young Investigator Award: Response
I am honored to receive this award from the Cryosphere Focus Group and glad for the opportunity it gives to acknowledge some of the people who have contributed to my scientific growth.
Ian Eisenman received the 2012 Cryosphere Young Investigator Award at the 2012 AGU Fall Meeting, held 3–7 December in San Francisco, Calif. The award is for “a significant contribution to cryospheric science and technology.
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