What are we to say to our anxious friends when, from every media outlet, they are bombarded by stories of human influence on climate change. Let us turn to a source which is authoritative and generally accepted by most of those who are concerned. The Intergovernmental Panel on Climate Change (IPCC) was set up by the World Meteorological Organization and the United Nations Environment Programme. Here is the part of its 2021 report on ‘Human Influence on Modes of Climate Variability’. Since the report as written is virtually incomprehensible, interleaved are translations of the individual paragraphs.
It is very likely that human influence has contributed to the observed trend towards the positive phase of the Southern Annular Mode (SAM) since the 1970s and to the associated strengthening and southward shift of the Southern Hemispheric extratropical jet in austral summer. The influence of ozone forcing on the SAM trend has been small since the early 2000s compared to earlier decades, contributing to a weaker SAM trend observed over 2000-2019 (medium confidence). Climate models reproduce the summertime SAM trend well, with CMIP6 models outperforming CMIP5 models (medium confidence). By contrast, the cause of the Northern Annular Mode (NAM) trend towards its positive phase since the 1960s and associated northward shifts of the Northern Hemispheric extratropical jet and storm track in boreal winter is not well understood. Models reproduce the observed spatial features and variance of the SAM and NAM very well (high confidence).
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