The Little Ice Age was a cooling period that occurred between the 14th and 19th centuries. Temperatures were lower than average in many parts of the world during this time, resulting in a variety of climatic changes. While the impact of the Little Ice Age has been well documented in regions such as Europe and North America, its impact on the tropical Pacific has been less well understood until recently.
El Nio is now officially underway. The climate phenomenon, which originates in the tropical Pacific and occurs in a few-year intervals, will shape weather across the globe for the next year or more and give rise to a variety of climatic extremes.
El Nio-like conditions can also occur on longer time scales, such as decades or centuries. This has recently been demonstrated by an international research team led by Ana Prohaska of the University of Copenhagen and Dirk Sachse of the German Research Centre for Geosciences (GFZ). Their analysis of biomarkers (organic molecules or molecular fossils from vascular plants) in the sediments of a lake in the Philippines reveals an unusually dry period in the region between 1600 and 1900 A.D.
The results have now been published in the journal Communications Earth and Environment. They show how important the understanding of past dynamics of the tropical Pacific ocean-atmosphere climate is for the improvement of climate models and the prediction of future climate changes.
Although there is increasing evidence that sudden climatic changes have occurred in the past, current climate models cannot reproduce such abrupt shifts in the mean state in the tropical Pacific. This demonstrates how little we know about the underlying mechanisms.
Dirk Sachse
Background: The El Niño phenomenon
The equatorial Pacific’s climate is notable for its east-west asymmetry, with warmer surface waters in the west and colder surface waters in the east. Surface water is driven westward by Easterly winds, allowing equatorial upwelling to bring cooler water to the eastern side. In today’s climate, this asymmetry breaks down, resulting in El Nio conditions that occur at irregular intervals of a few years and last for 9-12 months.
Sea surface temperatures along the equator rise during an El Nio event, from the Peruvian coast to the central Pacific. The southeast trade wind becomes significantly weaker, and light westerly winds may develop. An exceptional dryness takes hold in the western equatorial Pacific, which is otherwise characterised by abundant precipitation, whereas the otherwise dry eastern edges of the Pacific may experience heavy rainfall.
El Nio is expected to bring record-breaking high temperatures and a variety of extreme climate events such as droughts, floods, and wildfires across the globe, affecting the lives and well-being of millions of people.
El Niño-like phenomena on longer time scales
While El Nio is an interannual climate phenomenon, the tropical Pacific climate system can exhibit El Nio-like behaviour on longer time scales of decades and centuries, which is linked to the east-west gradient of sea surface temperatures in the Pacific. Such behaviour has recently been demonstrated by a team led by Ana Prohaska, assistant professor at the University of Copenhagen and formerly a visiting scientist at the GFZ, and Dirk Sachse, working group leader in GFZ Section 4.6 “Geomorphology” and director of Topic 5 “Landscapes of the Future” of the Helmholtz research programme “Changing Earth — Sustaining our Future,” in the journal Communications Earth and Environment.
They describe such a pronounced shift to El Niño-like conditions in the second half of the Little Ice Age, lasting from about 1630 to 1900 A.D. What is particularly remarkable is the short period of only one generation within which conditions changed for a period of more than 200 years.
Climate insights into the past through studies of sediment cores
The researchers examined sediment cores collected in 2013 from Lake Bulusan in the northern Philippines. The sediment sequence sheds light on the past 1,400 years of climatic evolution in a region that is currently heavily influenced by El Nio events. The researchers focused on the composition of stable hydrogen isotopes in leaf wax biomarkers (Dwax). These are molecular fossils derived from the protective surface layer of vascular plant leaves. The analysis provides information about water supply throughout their lives.
Drier conditions in the western Pacific during the Little Ice Age
The study finds a significant increase in Dwax values between 1600 and 1650 A.D., indicating a shift towards drier conditions in the western tropical Pacific during the second half of the Little Ice Age. The researchers attribute this shift to a shift in the tropical Pacific Ocean’s mean state caused by zonal gradients, or east-west differences in sea surface temperature.
The significance of the current study for climate predictions
Ana Prohaska, lead author of the study, emphasises the importance of this research: “Our study provides compelling evidence for the intricate relationship between zonal gradients in sea surface temperature and hydrological patterns in the tropical Pacific. Understanding the nature and pace of past changes in the tropical Pacific climate system is critical for predicting future climate change and its potential impact on this vulnerable region.”
“Although there is increasing evidence that sudden climatic changes have occurred in the past,” says Dirk Sachse of the GFZ, “current climate models cannot reproduce such abrupt shifts in the mean state in the tropical Pacific. This demonstrates how little we know about the underlying mechanisms. A better understanding of the drivers and consequences of the complex dynamics of the tropical Pacific mean state is critical in the context of anthropogenic climate change. The incorporation of palaeoclimatological data into modern climate models is critical for this.”
