Climate scientists interested in how El Nino/La Nina weather patterns may interact with future global warming are expected to take a close interest in the latest research using tree rings to build a picture of Northland’s climate since Polynesians discovered New Zealand. In kauri trees, El Nino periods are linked to wide tree rings, and La Nina to narrow rings.
A team of researchers today predicted that “New Zealand climate is likely to be more dominated by El Nino/Southern Oscillation (ENSO) related inter-annual variability as the world continues to warm”.
The new work on Northland kauri (Agathis australis) shows that the 20th century was active in the past 500 years in terms of (ENSO) activity, and suggests that New Zealanders may see more of it as global temperatures rise.
“ENSO activity comparable to or elevated above that experienced during the late 20th century is plausible under warmer-than-present conditions,” said the Auckland University, National Institute of Water and Atmospheric Research (NIWA), and Gondwana Tree-Ring Laboratory researchers — who worked with scientists in New Zealand from France, Switzerland, and Australia. The research was published today in Nature Climate Change.
If further research shows ENSO activity has increased the El Nino/La Nina effects on NZ have strengthened as the world has warmed, then people in regions which have seen impacts such as droughts in the past few decades “can expect more of the same”, the team told the SMC. “It is also plausible that as warming (global, not regional) continues we may see elevated impacts, something that planners might want to explore in terms of sensitivity to such changes”
The scientists warned in their paper that activity such as the big El Ninos that began in 1982 and 1997, and the similarly large La Nina triggered in 1973 and 1988 may not be unique in the context of the past 700 years: “We also find evidence that there may have been significant changes in the ENSO tele-connection to the New Zealand region during the 14th and 15th centuries”
Two key periods of interest for scientists are Europe’s warm Medieval Climate Anomaly from about 950 to 1250AD, and the following Little Ice Age (LIA) — summer cold and ice growth in the northern hemisphere which suddenly began over 25 years up to 1300, then stepped up a notch during 1430-1455AD.
Variations in the kauri rings bottomed out around the time of the LIA, then ramped up to the early 20th century, and the researchers told the SMC: “This is critical to our inference that ENSO activity / teleconnection is strongly linked to global warmth … We also know from the NZ glacial record that many glaciers were more extensive during that time than present, which suggests temperatures across the western South Island and Southern Alps were cooler than present during the LIA.”
Many researchers are still working on the details of the LIA in New Zealand, and how it might be similar or different to what happened in Europe. But consideration of whether recent increasing El Nino/La Nina activity represents a human-forced change or perhaps is a return to pre-LIA conditions will require data covering a range of records over thousands of years.
Other projects — involving some of the same scientists who wrote today’s paper — are collecting data from “subfossil” kauri through Northland’s Matakohe Kauri Museum and going back to another icy period in the northern hemisphere, the “Younger Dryas” about 12,000 years ago. Sawmillers have helped provide samples.
The paper’s authors were: Anthony M. Fowler, Gretel Boswijk, Andrew M. Lorrey, Joelle Gergis, Maryann Pirie, Shane P. J. McCloskey, Jonathan G. Palmer and Jan Wunder. They responded to questions from the SMC. :
Has 2007 research on cores from 191 trees at 17 sites and over 423 years — which showed a 50-80 year ENSO cycle pattern repeated seven times since 1580 — been confirmed in the latest paper?
“The earlier paper dealt only with living and recently living trees. This paper adds the archaeological data set (roughly half the sample), refines methods, and compares findings with other ENSO reconstructions. It is a continuation of that research work, with the next step being the addition of swamp kauri going back about 4000 years. The earlier findings have been confirmed …with some refinement. The seven cycles are retained and appear to be robust features”.
What has the latest kauri research shown for the time of the Medieval Climate Anomaly?
“There is some suggestion that ENSO activity in NZ was higher at this time, but results are inconsistent with a couple of other reconstructions”.
What do the kauri records show of the New Zealand experience of the Little Ice Age?
” Kauri variance (the proxy for ENSO activity / teleconnection to our region) bottoms out around the time of the LIA, then ramps up to the early 20th century. This is critical to our inference that ENSO activity / teleconnection is strongly linked to global warmth … We also know from the New Zealand glacial record that many glaciers were more extensive during that time than present, which suggests temperatures across the western South Island and Southern Alps were cooler than present during the LIA”
What is the significance of the perceived increased variance in 14th century ENSO patterns and “collapse ” of this variance in the 15th century?
“The collapse in variance in the 15th century suggests suggests that ENSO activity quietened or (we think more likely) that the teleconnection to NZ weakened. One possibility here is that there was a northward movement of zonal bands which inhibited the ENSO teleconnection. If so, then early (Polynesian) settlers may have found themselves in a rather bleaker environment than when they arrived in the 1300s, but one less prone to the extremes that ENSO can throw up … (this) is probably, at best, an interesting working hypothesis. The changes that have occurred across New Zealand for the last millennium are still being refined, so future work will help to answer this question”.
What are potential implications of a peak in ENSO activity in the middle of the 18th century?
“Consistency across multiple records means that we can be reasonably confident that we have identified an unambiguous ENSO active period. If so, then other researchers have scope for exploring whether ENSO’s biophysical fingerprint has been consistent through time. It is also important as a pre-greenhouse effect baseline … at a minimum this peak would have to be unambiguously exceeded before a clear human influence on ENSO activity could be claimed”.
What possible implications are there for a greater domination of the NZ climate by El Nino-La Nina oscillations in the future?
“If ENSO activity has increased and/or the teleconnection to NZ has strengthened as the world has warmed the clear implication is that we can expect ENSO activity to influence NZ climate and climate-sensitive activities with comparable strength to what we have experienced over the last few decades. So regions sensitive to El Niño and La Niña events can expect more of the same. “It is also plausible that as warming (global not regional) continues we may see elevated impacts, something that planners might want to explore in terms of sensitivity to such changes. If the multi-decadal cyclicity pattern of the last several continues, that impact may be delayed as we may be coming off an activity peak. The impact may be delayed, but much stronger when the next peak arrives”.