IPCC climate models do not capture Arctic sea ice drift acceleration:
Consequences in terms of projected sea ice thinning and decline
2011, Rampal et al., Journal of Geophysical Research
IPCC climate models underestimate the decrease of the Arctic sea ice extent. The recent Arctic sea ice decline is also characterized by a rapid thinning and by an increase of sea ice kinematics (velocities and deformation rates), with both processes being coupled through positive feedbacks. In this study we show that IPCC climate models underestimate the observed thinning trend by a factor of almost 4 on average and fail to capture the associated accelerated motion. The coupling between the ice state (thickness and concentration) and ice velocity is unexpectedly weak in most models. In particular, sea ice drifts faster during the months when it is thick and packed than when it is thin, contrary to what is observed; also models with larger long-term thinning trends do not show higher drift acceleration. This weak coupling behavior (1) suggests that the positive feedbacks mentioned above are underestimated and (2) can partly explain the models' underestimation of the recent sea ice area, thickness, and velocity trends. Due partly to this weak coupling, ice export does not play an important role in the simulated negative balance of Arctic sea ice mass between 1950 and 2050. If we assume a positive trend on ice speeds at straits equivalent to the one observed since 1979 within the Arctic basin, first-order estimations give shrinking and thinning trends that become significantly closer to the observations.
Sea ice state and kinematics have been associated with profound changes in recent decades. Arctic sea ice shrinkage is accompanied by a strong thinning seen in ice draft as well as ICESat ice freeboard data. Between 1980 and 2008 the net average mean thickness has decreased by 1.75 m in winter and 1.65 m in summer [Kwok and Rothrock, 2009]. Taking into account the thickness uncertainties, this average thinning corresponds to a trend of 16.5(±7.0)% per decade over the same period, which can be essentially explained by a drastic reduction of the perennial ice cover [Kwok et al., 2009]. At the same time, Arctic sea ice kinematics have undergone large changes [Hakkinen et al., 2008; Rampal et al., 2009]. From buoy data, Rampal et al.  showed that the sea ice drift speed has increased at a rate
of 9.0(±1.9)% per decade on average within the Arctic basin from 1979 to 2007, whereas the average deformation rate has increased at a rate larger than 50(±10)% per decade over the same period.
In summary, IPCC AR4 models strongly underestimate Arctic sea ice thinning, in a way even more spectacular than they do for the extent decline [Stroeve et al., 2007]. As the observed thinning is essentially the result of a reduction of the perennial sea ice cover [Kwok et al., 2009; Nghiem et al., 2007], one may wonder to what extent this underestimation of the thinning would simply result from the underestimation of the perennial decline. It was shown that, in terms of summer minimum extent and declining trend, recent years are about 30 to 40 years ahead of the ensemble mean forecast [Stroeve et al., 2007].
Most models underestimate by half on average the mean sea ice speed estimated from buoy data (Figure 3).
This means that IPCC AR4 models do not capture, or at least strongly underestimate, the acceleration of sea ice drift in recent decades.
A trend of +9% per decade imposed on the ice speeds at the gates increases significantly the ensemble mean trend on mean ice thickness of models, which becomes about −13.4% per decade. The previous gap existing between modeled and observed trends is then reduced by 80%.
Note that these simple calculations do not take into account the strengthening of the positive albedo feedback loop in summer through increasing fracturing, lead opening and ice concentration decline (see above). Therefore, although necessarily oversimplified, we believe that our calculation illustrates at least qualitatively the impact of inaccurate modeled kinematics onto present and future projections of sea ice thinning and mass balance.
We analyzed the evolution of average Arctic sea ice thickness and drift speed as modeled by 13 IPCC‐AR4 climate models from 1950 to 2050, and compared the trends to observations over the period 1979–2008. The main conclusions are:
1. Models underestimate the observed thinning trend by almost a factor 4 on average, i.e., in a way even more spectacular than they do for the sea ice extent decline. This underestimation of the thinning trend cannot be explained entirely by an underestimation of the decline of the perennial, thicker, ice‐covered portion of the Arctic.
2. Models do not capture the acceleration of Arctic sea ice drift observed during the last decades.
3. An unexpectedly weak coupling between the ice state (thickness and concentration) and kinematics characterizes these IPCC simulations: for most models, ice drifts faster during the months it is thicker, in contradiction with observations, and models that show a stronger long‐term thinning trend do not necessarily accelerate more.
4. The simulated ice velocities across export gates (essentially, Fram Strait) show no significant long‐term trend, with the consequence that sea ice volume fluxes from the Arctic basin are decreasing with time in absolute value, which is in disagreement with observations. Moreover, the relative percentage of simulated Arctic sea ice exported outward each year is remarkably constant, which means that in the models the ice export plays no role on the Arctic sea ice negative mass balance.
5. The IPCC deficiencies in reproducing the recent kinematics evolution is likely to have a significant impact on the simulated sea ice mass balance in the Arctic. As suggested by our simplistic evaluation, a (imposed) positive trend of 12% per decade on sea ice speeds at fluxgates, i.e., a value close to the observed drift acceleration within the Arctic basin,would significantly reduce the mismatch between modeled and observed sea ice area declining trends. Such accelerated export would also help the modeled ice cover thinning trend to be much closer to the observations.
Therefore, this strong underestimation of sea ice thinning and drift acceleration in models would imply that former projections for an ice‐free summer in the Arctic by 2100, based only on the comparison of simulated and observed sea ice extent reduction rates [Arzel et al., 2006; Boé et al., 2009b], are too conservative. This is reinforced by the fact that the current thinning trend is more than 40 years ahead of the ensemble mean forecast (Figure 1), whereas the fraction of the ocean covered by ice is highly sensitive to ice thickness changes [Lindsay et al., 2009].