There are many who will not like this recent paper published in Nature Communications on principle as it talks of the hiatus in global temperatures for the past 20 years or so, that the Little Ice Age was global in extent, and that climate models cannot account for the observations we already have let alone make adequate predictions about what will happen in the future. It also makes what has happened in the past 50 years seem a little less unusual. This is however an interesting paper that deserves wide consideration, but as it doesn’t tow what some regard as a “party-line” it will probably get few mentions in the media.
Researchers working at the Australian National University Research School of Earth Sciences have discovered century-scale patterns in Pacific rainfall and temperature, and linked them with global climate changes in the past 2000 years. These past El Niño (ENSO) oscillations in the Pacific Ocean may have amplified global climate fluctuations for hundreds of years at a time. Member of the team Alena Kimbrough says, “We’ve shown ENSO is an important part of the climate system that has influenced global temperatures and rainfall over the past millennium…Our findings, together with climate model simulations, highlight the likelihood that century-scale variations in tropical Pacific climate modes can significantly modulate radiatively forced shifts in global temperature.”
The team measured trace elements and stable isotopes in stalagmites from the Indonesian island of Flores and used them to reconstruct ancient rainfall, comparing them to records from East Asia and the central-eastern equatorial Pacific. They found that northern hemisphere warming and droughts between the years 950 and 1250 corresponded to an El Niño-like state in the Pacific, which switched to a La Niña-like pattern during a cold period between 1350 and 1900. They found periods of predominantly El Niño-like patterns for several hundred years that alternate with La Niña patterns, impacting on global climate over the last 2000 years. Climate models cannot reproduce this.
“Our results highlight significant discrepancies between the proxy records and model simulations for the past millennium. Critically, these discrepancies coincide with century-scale anomalies in the strength of the Pacific Walker Circulation. We cannot rule out the possibility that some of the low-frequency Pacific variability was a forced response to variable solar intensity and changing teleconnections to higher latitudes that are not simulated by the models, or that non-climatic processes have influenced the proxies…the paleodata-model mismatch supports the possibility that unforced, low-frequency internal climate variability (that is difficult for models to simulate) was responsible for at least some of the global temperature change of the past millennium.”
The researchers say that the La Nina-like pattern is thought to be a factor contributing to the recent so-called ‘warming hiatus’ and earlier twentieth century cool and warm decades. “Therefore, our analysis of multicentury hydroclimate variability suggests that projections of tropical rainfall patterns, and global temperature extremes, will remain uncertain until paleoclimate records and models consistently capture the lower-frequency variability, and associated feedbacks, in the tropical Pacific.” Lead author Dr Michael Griffiths from William Paterson University, in the United States, added, “Until we can model this lower-frequency behaviour in the tropical Pacific, one can only speculate on how the warming will play out over the next few decades.”
Thus we have another natural climatic change mechanism that is relevant to how we asses the climatic changes that have occurred in the past century and in recent decades.