Tiistai 14.9.2021 klo 16:11 - Mikko Nikinmaa
When it snows in Texas or Spain, the climate denialists rejoice saying that clearly climate change is only scare tactics of the climate believers. Hitherto it has been difficult to give explanations as to why climate change causes the cold spells in the south, heavy rains in Central Europe and extreme heat spells in the Arctic. However, two recent articles clarify the situation significantly, the ones by Osman et al in PNAS (PNAS September 21, 2021 118 (38) e2104105118; https://doi.org/10.1073/pnas.2104105118) and Cohen et al in Science (Science 3 Sep 2021, Vol 373, Issue 6559, pp. 1116-1121; DOI: 10.1126/science.abi9167). Below I try to summarize my understanding of the findings presented.
Much of the weather in North America and Europe is affected by North Atlantic Jetstream. The rains and heat spells depend on its position. With increasing mean temperature of the arctic it appears that North Atlantic Jetstream undulates more and its position shifts northwards as Arctic temperature increases. Southward undulations mean that cold air from the Arctic reaches more southern latitudes than earlier and northward undulations that southern hot air reaches north. Presently, changes in the general position of the North Atlantic Jetstream and its undulations are so variable that consistent weather changes do not occur – cold summer can follow a hot one and dry autumn a wet one. However, it can be predicted that by 2060 the jetstream has moved generally so much north that Southern Europe becomes consistently drier and Central Europe and Scandinavia wetter than currently.
The Arctic temperature increases more rapidly than temperatures elsewhere. This is called Arctic Amplification (AA). As a result of this general AA, it appears that mid-latitudes experience cooling of winters, as the still cold arctic air is able to stream to southern latitudes. This is the result of Stratospheric Polar Vortex stretching southwards, which may, in my opinion, coincide with the southward undulations of the North Atlantic Jetstream (where NAJ occurs). However, these are clearly independent in continental North America and Asia (where there is no NAJ). So, cold weather in southern areas can be a consequence of climate change.
In any case the predictability of weather patterns decreases markedly with climate change. As a consequence, agricultural production all over the world may be reduced, leading to increased problems for the mankind.