There is a new paper
Stroeve, J., M. M. Holland, W. Meier, T. Scambos, and M. Serreze (2007), Arctic sea ice decline: Faster than forecast, Geophys. Res. Lett., 34, L09501, doi:10.1029/2007GL029703
which discusses the decline in Arctic sea ice.
The abstract reads,
“From 1953 to 2006, Arctic sea ice extent at the end of the melt season in September has declined sharply. All models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) show declining Arctic ice cover over this period. However, depending on the time window for analysis, none or very few individual model simulations show trends comparable to observations. If the multi-model ensemble mean time series provides a true representation of forced change by greenhouse gas (GHG) loading, 33â38% of the observed September trend from 1953â2006 is externally forced, growing to 47â57% from 1979â2006. Given evidence that as a group, the models underestimate the GHG response, the externally forced component may be larger. While both observed and modeled Antarctic winter trends are small, comparisons for summer are confounded by generally poor model performance.”
Excerpts from the paper reads,
“From 1953â2006, Arctic sea ice extent at the end of the summer melt season in September has declined at a rate of â7.8%/decade. Over the period of modern satellite observations (1979â2006) the trend is even larger (â9.1% per decade). Trends for March (the climatological maximum ice extent), while much smaller, are also downward, at â1.8% and â2.9%/decade over these two time periods.
Although it is tempting to attribute these statistically significant (99% level) trends to GHG loading, the observed sea ice record has strong imprints of natural variability. An overall rise in SATs over the Arctic Ocean is consistent with ice loss [Comiso, 2003], but rates of change depend strongly on season, the time period analyzed, as well as the data set employed [Serreze and Francis, 2006]. Variability in the Northern Annular Mode (NAM) and other atmospheric patterns has played a role through impacts on ice circulation [e.g., Rigor and Wallace, 2004], as have changes in oceanic heat transport [Polykov et al., 2005; Shimada et al., 2006]. However, a role of GHG loading finds strong support in the recent study of Zhang and Walsh . They show that from 1979â1999 the multi-model mean annual trend from models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) is downward, as are trends from most individual models.
This paper makes three points: (1) if the IPCC AR4 multi-model mean time series properly reflect the response to GHG loading, then both natural variability and forced change have been strong players in the observed September and March trends, with the latter becoming more dominant during 1979â2006; (2) given evidence that that the IPCC models as a group are too conservative regarding their GHG response, the GHG imprint may be larger; and (3) there is more consistency between models and observations regarding much smaller sea ice trends in the Antarctic.”
“To summarize, there is qualitative agreement between observations and models regarding an overall decline in September ice extent. This points to an imprint of GHG loading [Zhang and Walsh, 2006]. Since both observed and modeled September trends have become larger in more recent years, it appears that GHG imprints are growing. Simulations run with pre-industrial GHG concentrations do not produce the magnitude of September trends just discussed.”
“It is useful at this point to turn briefly to the Antarctic. In contrast to the Arctic, Antarctic ice extent has shown little change. The observed September (end of austral winter) trend from 1973â2006 is essentially zero. The corresponding March trend is â1.7 Â± â2.3%/decade, but given the high variability in the Antarctic March extent, the trend is not statistically significant…… While one might argue that the large scatter in the modeled March trends (â6.5%/decade to 0.1%/decade) is broadly consistent with the insignificant observed trend, only the 5 of the 15 models passed the initial performance screening described earlier. The appropriate conclusion is that there are strong shortcomings in the ability of most models to simulate March Antarctic ice extent.”
A final excerpt is
“The Arctic has often been viewed as a region where the effects of GHG loading will be manifested early on, especially through loss of sea ice. The sensitivity of this region may well be greater than the models suggest.”
There are several issues with respect to this otherwise excellent study:
1. The authors neglected to discuss the role of black carbon deposition on reducing the albedo of the sea ice, and thus promoting melting in the summer.
In terms of black carbon deposition, as reported on page 38 in the 2005 National Research Council Report Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties
âDeposition of BC aerosols over snow-covered areas can result in changes to the surface albedo (Chylek et al. 1983). Further reductions in albedo occur due to the enhanced melting that accompanies the heating of absorbing soot particles in snow. Chylek et al. (1983) estimate this enhancement to be up to a factor of ten in the rate of melting. Recent model results indicate radiative forcings of +0.3 W mâ2 in the Northern Hemisphere associated with albedo effects of soot on snow and ice (Hansen and Nazarenko 2004).â?
2. The authors neglected to attribute any of the arctic sea ice melt to warming due to ozone.
âNASA scientists have found that a major form of global air pollution involved in summertime âsmogâ? has also played a significant role in warming the Arcticâ¦â¦According to this new research, ozone was responsible for one-third to half of the observed warming trend in the Arctic during winter and spring. Ozone is transported from the industrialized countries in the Northern Hemisphere to the Arctic quite efficiently during these seasons. â?
(see and see).
3. The authors neglected to discuss the role of human caused (from land degradation) dust deposition on Arctic sea ice.
At the recent NASA Land Cover/Land Use Change meeting at the University of Maryland, Irinia Sokolik of Georgia Tech reported on significant warming associated with dust deposition in the higher northern latitudes. As soon as her powerpoint talk is posted at the website http://lcluc.nasa.gov, Climate Science will report on this newly recognized positive radiative forcing. Her conclusion in her talk is that dust deposition is at least as important as black carbon deposition as a positive radiative forcing.
4. The authors inadequately discussed the problems with the multi-decadal global model predictions in the Antarctic.
This inadequacy was recently reported in a press release on February 15 2007 entitled “Antarctic Temperatures Disagree With Climate Model Predictionsâ?. That news release (which will be discussed on Climate Science next week) includes the statement that
“A new report on climate over the worldâs southernmost continent shows that temperatures during the late 20th century did not climb as had been predicted by many global climate models.”
These studies document that the well-mixed greenhouse gases are just one of a set of positive radiative forcings in the Arctic. The human-caused attribution of the melt of the sea ice solely to the well-mixed greenhouse gases is not scientifically demonstrated.
Finally, the presentation of forecasts of sea ice melt for the coming years provides a valuable benchmark to validate the predictive skill of the multi-decadal global models. An interesting policy question is what would society and government do differently in terms of energy and environmental policy if the sea ice melt stopped or reversed (e.g. see and see).
The policymakers should at least consider this possibility.