Milestones polari

Troppo bello, lucido, profetico e tremendamente attuale questo lavoretto miliare (nel senso che ha anticipato di miglia e miglia altri lavori e prefigurato la realtà odierna e del futuro prossimo) uscito su JGR il 20 ottobre 1980 e frutto del lavori di due "dissidenti" al GFDL. Sarebbe bello poterlo riscoprire. Magari pure utopico che i soliti ignoti del ghiaccio non governato dal caldo o della temperatura che non c'entra mai nulla (quando aumenta) o della concentrazione di CO2 che _deve_ necessariamente agire allo stesso modo su entrambi i poli, decidessero una volta (almeno una!) di leggersi un intero lavoro e non solo il nome dell'edizione o quello del solito dispaccio per solita interposta persona.


Uno dei due autori del lavoro miliare è lo stesso dissidente che ritroveremo in un' altra puntata delle milestones ed è anche colui il quale (con un altro "eretico") pochi anni prima - facendo girare un modello di simulazione climatica decisamente meno complesso di quelli odierni - già riusciva a descrivere il meccanismo attraverso il quale si sarebbe poi manifestata l'amplificazione artica di questi ultimi 2-3 lustri (ne abbiamo già parlato ad es. qui).

Poche parole aggiuntive, facciamo parlare gli autori mediante abstract (in figura sotto, cliccateci su per ingrandire) e qualche passaggio chiave (in citazione a seguire, la prima dalle conclusioni, la seconda dal cap. 5 dedicato alla risposta termica) impreziositi da qualche immagine tratta dal lavoro:

❝It is shown that the sensitivity of a global climate model to an increase of the CO2 content in the atmosphere has significant seasonal and latitudinal variations. For example, the CO2-induced warming of the surface air is particularly large in high latitudes owing mainly to the poleward retreat of highly reflective snow cover and sea ice. However, the warming over the Antarctic continent is significantly less than the warming over the Arctic Ocean partly because of the smallness of a snow albedo feedback mechanism over Antarctica. Over the Arctic Ocean and its neighborhood, the warming of the surface layer of the model atmosphere is much larger in the winter than in the summer.  It is found that a reduction of sea ice thickness is an important factor responsible for the seasonal asymmetries in the atmospheric warming. Owing to the reduction (or the disappearance) of highly reflective sea ice, the surface albedo reduces significantly, and the net incoming solar radiation increases markedly in summer in response to an increase of the CO2 concentration in the atmosphere. However, the additional solar energy is used either for the melting of sea ice or the warming the mixed layer ocean which has a large heat capacity. Thus the increase of the surface air temperature in summer tums out to be relatively small. However, the additional heat energy, which is absorbed by the ocean during the warm season, delays the appearance of the sea ice or reduces its thickness. This reduces the thermal insulation effect of the sea ice in early winter, when the air-sea temperature difference becomes large, thereby enhancing the warming of the surface atmospheric layer. The winter warming is enhanced further by the stable stratification of the model atmosphere in the winter which confines the warming to the surface layers.

❝The latitude-height distribution of the difference of zonal mean temperature between the 4 x CO2 and 1 x CO2 atmosphere is shown in Figure 14. This figure indicates a general warming of the model troposphere and a cooling of the stratosphere resulting from the quadrupling of the CO2 content. The warming is particularly pronounced in the lowest layers in high altitudes and is relatively small in the tropics, resulting in a general reduction of the meridional temperature gradient in the lower model troposphere. As was discussed by Manabe and Wetheraid [1975], this reduction of meridional temperature gradient results from (1) the poleward retreat of highly reflective snow cover and sea ice and (2) the increase in the poleward latent heat transport. (...) Figure 14 also reveals that the high latitude warming of the northern hemisphere is significantly larger than that of the southern hemisphere. One of the important factors responsible for this interhemisphereic difference in the warming is the difference in the reflected solar radiation between the 4 x CO2 and 1 x CO2 experiment. According to Figure 15, which shows the latitudinal distribution of difference in the net incoming solar radiation at the top of the model atmosphere, the difference in the northern hemisphere increases with increasing latitude polewards of 30°N, whereas in the southern hemisphere the difference has local maximum along the periphery of the Antarctic continent, but it is smaller over the continent. As a whole, the northern hemisphere change of the net incoming solar radiation (or planetary albedo) which is caused by the poleward retreat of highly reflective snow cover and sea ice is significantly larger than the corresponding change in the southern hemisphere.