At https://phys.org/news/2024-09-tree-reveal-jet-stream-extreme.html … here we have a post that claims tree ring data reveals the jet stream has shaped extreme weather in Europe for centuries. This sounds a bit like HH Lamb writing in the mid to late 20th century and I suspect there is a lot of truth in that position. However, tree rings are not thermometers. Mike Baillie, one of the palaeoecologists/dendrochronologists who put together the Belfast tree ring sequence, said tree rings reflect the environment at the time the trees were living. They did not specifically record temperatures – although he also thought low growth tree ring clusters did so. In the first place they marked an environmental event of some kind, such as a volcanic eruption shooting lots of gas and debris into the upper atmosphere, and the low growth was due to a cool summer. We might add wet to that as very often they appear to coincide with very wet weather in NW Europe – but dry and arid weather in the Near East and Egypt etc. How annual tree rings can be dissected in order to reflect temperature in between the low growth events is an ongoing research project. Having said that we need to understand the jet stream responds to atmospheric conditions. This is why it can move up and down. So, establishing when the jet stream has moved in the past will tell us more than temperature. It will move as a result of volcanic activity, as an example. Or, the earth’s orbit encountering a heavy meteor flux.
We should note the researchers seem to be over one of the targets. Valerie Trouet noticed something that turned to curiosity and then into a major scientific discovery, we are told. She had noticed when the Sun had moved behind clouds , or during an overcast sky, people in Europe tended to reach for something warm to put on – such as a sweater [otherwise a jumper or pullover]. At the same time there was warm and sunny weather in southern Europe – which is why many northerners spend their holiday, or vacation, in Spain, Italy or Greece. As a dendrochronologist her job is the study of tree rings in order to ascertain what climate was doing in the past. Are tree rings the key to understanding wet and cool summers in northern Europe? Together with an international team they set out to see if periods of wet and cool weather could be used to plot movements of the jet stream, leading to a paper in Nature journal – https://doi.org/10.1038/s41586-024-07985-x … ‘Jet stream controls on European climate and agriculture since 1300CE‘ …
It was of course very wet in the early 14th century AD – leading up to the Black Death pandemic. So, just for that, this will make interesting reading.
Jet streams are concentrated bands of wind in the upper atmosphere that travel around the globe in both the northern and southern hemispheres. Jet streams, it seems, largely determine the summer climate in Europe – and it does so in a seesaw pattern researchers call a dipole. When the jet stream is in an extreme northern location we get cooler and wetter conditions over the British Isles, for example, and warmer and drier conditions over the Mediterranean and Balkans. Hotter conditions over the Balkans appear to cause more moisture than normal to evaporate from the Mediterranean Sea and rain down furthern north. Conversely, when the jet stream migrates further south it drags warmer and drier air over the British Isles and pushes cooler temperatures and more moisture towards eastern Europe. By using tree ring samples from across Europe as proxies for temperature the research team was able to reconstruct jet stream variations over the last 700 years. The interesting bit now – do they correspond with what historical sources tell us about the climate back then. The team went on to find past patterns of the jet stream reflected on a societal level, recorded in historical documents. There are centuries long records of grape harvests, grain prices, and epidemics. In comparing the records with jet stream reconstruction the team discovered that the climate dipole created by the jet stream has influenced European society over the centuries. Epidemics happened more frequently in the British Isles when the jet stream was further north, Trouet says, because summers were wet and cold and people stayed indoors. The conditions were more likely to spread diseases. The black death coincided with a period when the jet stream was at an extreme northern position.
At https://phys.org/news/2024-09-ice-age-clues-advanced-climate.html … the Late Glacial Maximum peaked around 20,000 years ago. The period was marked by extensive glaciation in NW Europe and NE North America. Dramatic changes in climate reshaped earth’s oceans, we are told – as well as landscapes and ecosystems. We are then informed that looking at the Ice Age closely it may provide crucial insights into future El Nino weather events. On the other hand, it may not as we don’t know why the Arctic was in that configuration during the Late Glacial Maximum. It seems that in studying Pacific ocean sediments they determined that during LGM the incidence of El Nino’s decreased – which means, we are assured, that as the planet warms they will become more prevalent. El Nino events have been kind to alarmists in the past. They have served to up global temperature measurements when a big one homes in. I don’t know if this study will raise the temperature amongst the general population – but no doubt it is one of those things that will be used to browbeat us into the Net Zero agenda. The LGM is really a great big unknown and has never been fully investigated – geographically for starters.