En wat zou toch de oorzak voor die verandering van het stromingspatroon zijn?
Ehmm nee, dat is te makkelijk. De auteurs hebben ook gekeken of menselijke invloed verantwoordelijk is voor de veranderingen in het stromingspatroon en de conclusie luidt dat dat niet het geval is. Uit het News and Views commentaar dat het bewuste artikel begeleidt:
"If the circulation changes are caused by anthropogenic greenhouse warming (or other human or natural external forcings such as ozone depletion, aerosol emissions, or solar activity) this pattern of atmospheric change should emerge as a clear signature when averaging together many climate model simulations of this period. Averaging together many simulations effectively cancels out all random internally generated fluctuations seen in individual climate simulations, leaving behind the model response to external forcings, such as increasing greenhouse gases. Using this approach, Ding et al. find that external forcing accounts for very little of the observed circulation changes, and therefore attribute the changes predominantly to internally generated variability. The result is surprising, in that it attributes a multi-decadal atmospheric circulation anomaly to internal variability."
De auteurs van het artikel beweren overigens niet dat de afname van de hoeveelheid ijs op de noordpool niet mede door mensen wordt veroorzaakt, alleen dat de bijdrage een stuk kleiner is dan gedacht.
Het artikel laat wel mooi zien dat ook wetenschappelijk onderzoek dat de invloed van de mens kleiner maakt, gewoon in toptijdschriften kan worden gepubliceerd. Ondanks claims van sceptici dat kartelvorming onder "gevestigde wetenschappers" publicatie van dit soort resultaten zou tegenhouden.
Wetenschap werkt, kortom, zoals ze zou moeten.
Paul
*edit*
Voor de liefhebbers hieronder de tekst van de rest van het News and Views verhaal (opgepikt direct na de laatste zin van bovenstaande aangehaalde tekst:
"Typically, internal variability is most prominent at shorter timescales — from months to years — and accounts for less of the observed variability as one extends to longer and longer timescales. The simulation-averaging approach they use to make this attribution is common, but it relies on the assumptions that the models have been supplied with the correct forcing, and are faithfully replicating the real world response to that forcing — assumptions that are always open to question12. Nonetheless, multi-decadal-scale internal variability does exist, most often relying on the longer-term memory of the ocean. For example, tropical Pacific sea surface temperature (SST) variability has been shown to have a strong connection with atmospheric circulation anomalies over Greenland13 and Arctic sea ice6. Thus, there is a plausible link between tropical SST variability and Arctic sea-ice decline, but firmly establishing this relationship would require further work.
The overall conclusion is that purely internally generated, random variability has had a significant influence on Arctic summer-time sea-ice loss, and Ding et al. quantify this contribution at 40% of the total observed loss since 19794. Robustly establishing that such a large fraction of observed Arctic summer-time sea-ice loss can be attributed to internal variability will need independent observational corroboration. As it stands, the attribution is consistent with model-based studies on the influence of internal variability on Arctic sea-ice trends10, 11. The results of Ding et al. do not call into question whether human-induced warming has led to Arctic sea-ice decline — a wide range of evidence shows that it has3. Rather, the implication is that Arctic sea-ice is less sensitive to human-induced forcing than if one assumes that all loss observed to date is anthropogenically driven.
By establishing that observed internal variability is large, the results of Ding et al.4 provide further evidence that there is not a fundamental inconsistency between observations and climate model projections of Arctic sea-ice decline. However, by the same logic there are implications for the predictability of future Arctic sea-ice changes. If internal variability is as large as estimated, it follows that long-range (multi-decadal) projections of future Arctic sea-ice have an inherent and irreducible uncertainty attached to them. This is consistent, for example, with recent modelling results that suggest that predictions of the first year in which the Arctic summer will be ice-free are irreducibly uncertain to within ±20 years, due to the influence of random internal variability14. Conversely, if the causal link between tropical Pacific SST variability and Arctic sea-ice changes suggested by Ding et al. can be confirmed, there is the possibility that initialized forecasting systems could leverage this relationship to improve the accuracy of Arctic sea-ice forecasts with lead times at the seasonal to decadal timescale."
Quote selectie