Forest fires in Northern Africa. Heatwaves in Italy. And floods in Beijing. The world is hit by extreme weather. Now imagine this situation lasting for over 200 years. Not pleasant. However, this may have been the case in the tropical Pacific from the mid-17th century to beginning of the 20th century, the period corresponding to the second half of the Little Ice Age. During that period, extreme drought plagued the region, a new study of the rainfall in the tropical Pacific suggests.
Ana Prohaska is an Assistant Professor at the Globe Institute at the University of Copenhagen and the leading author of the new study. She and her team had visited the Philippines and using a custom build raft, they collected sediments from the Bulusan Lake. Then they analysed the sediments using a state-of-the-art biomarker method involving plant compounds.
What they saw was surprising.
“The reconstructed rainfall record shows El Niño-like conditions during the Little Ice Age. While sea surface temperature records from previous studies have indicated such conditions, this is the first rainfall record that clearly shows it,” she says.
Ana Prohaska and her team coring sediments on the Bulusan lake. Photo: Department of Environment and Natural Resources/ Aggrupation Of Advocates For Environmental Protection Bulusan.
El Niño is a climate phenomenon where the weakening of important equatorial winds, called ‘the trade winds’, breaks the temperature pattern in the tropical Pacific, and creates extreme weather events. Normally El Niño happens every two to seven years and lasts nine to 12 months. Right at this moment, we are in an El Niño event. That is one of main reasons we see so many major forest fires, floods, and heatwaves all around the world.
“But imagine this lasting for more than 200 years. People in the tropical Pacific likely lived in one long El Niño-like period. And we could see that the shift to this period of extreme climatic conditions happened very quickly, within one generation. People would not have much time to adapt to the extreme weather conditions. It must have been challenging for the societies in the tropical Pacific,” Ana Prohaska says.
She pinpoints that extreme weather events brought about by El Niño can have directly negative impact on human health. For instance, heatwaves can cause heat exhaustion and in more serious cases heart strokes. Drought and flooding can damage crops, changes in sea temperatures can cause fish stocks to plummet and storms and fires can destroy human property and threaten human lives.
“These are not conditions that we would want ever to take place, let alone every year. It's already extremely difficult to cope with them on the frequency of every two to seven years,” she says.
Current climate models do not faithfully mimic El Niño behavior
Most climate models predict that due to climate change El Niño will become more frequent and severe in the future.
“This is bad news. We do not yet know exactly how climate change will interact with the tropical Pacific climate system and cause El Niño to become more frequent and severe. Climate change certainly brings a lot of uncertainty into the projections of El Nino dynamics for the future. So, we need to improve the fundamental understanding of these processes to improve our climate models,” Ana Prohaska says and adds:
“Long-term rainfall records, such as this one, together with other kinds of paleo records, are really important because they provide empirical evidence of what has been happening in the past when major climate changes have taken place.”
This is how the researchers did
The researchers collected sediments from the Lake Bulusan in the Philippines. First, they built a custom coring platform with the help of the park’s management team. They collected five meters of sediments, which consisted of biogenic organic material, namely diatoms and green algae.
They shipped the sediments back to the University of Potsdam and extracted the so called ‘leaf waxes’ which are a complex mixture of fats and oils produced by plants on the surface of their leaves as a protection from water loss.
From this, the researchers could measure the stable hydrogen isotopic signature of specific group of compounds found in leaf waxes called alkanes, which tells them how much rain has fallen when the leaf was still sitting on the tree. In this way, they could create a continuous record of past rainfall that spans the last 1,400 years. To reconstruct past dipterocarp forest vegetation, they further analysed fossil pollen, charcoal, and chemical elemental concentrations.
Lastly, the researchers compared the new rainfall record with the already available modern rainfall and stable isotope data from the region in order to identify the underlying drivers. The analysis showed that the changes in the Bulusan rainfall record links to changes in the east-west gradient of tropical Pacific sea surface temperatures. This allowed them to pinpoint the period of prolonged El Niño-like conditions in the region during the Little Ice Age. Fossil pollen analysis revealed changes in the abundance of individual tree types.
She adds that, we rely on these climate models, among other things, to predict the future climate dynamics, including the occurrence of extreme weather events such as drought, floods, and heatwaves, and therefore help the society to prepare for what is to come.
Prolonged El Niño-like drought conditions negatively affect lowland rainforest
Ana Prohaska and her colleagues also studied how the approximately 250 years of El-Niño conditions affected the lowland dipterocarp forest. This is the main forest type in Southeast Asia, and it used to cover 80 percent of the region, but its geographic range has been reduced to only 30 percent due to extensive logging.
It is one of the most biodiverse forests in the world and it hosts many species of plants and animals found only here.
“A rainforest of course needs rain, so we were curious how prolonged drought conditions in the past spanning decades to centuries impacted this globally important forest,” she says.
“We found that while different types of trees had different responses to it, there was a significant negative relationship between the drought conditions and the abundance of many tree types.”
That includes the dominant tree group in the lowland dipterocarp forests, called Dipterocarps.
“Interestingly, Dipterocarps flower triggered by El Niño events, so they actually need El Niño events to effectively reproduce. But there is a question if the more frequent El Niño will still have a net negative effect even if flowering increases because El Niño brings drought and fire that damage and kill these trees. So, there is this trade-off between tree reproduction and their growth and survival. Our study finds that the overall effect on Dipterocarps is in fact negative,” Ana Prohaska says.
These results have far-reaching implications. Lowland tropical rainforests represent the most productive terrestrial biome and cover approximately 10 percent of Earth’s land surface. As such, they are a key component of the planet’s capacity to combat increasing levels of CO2, for instance, through carbon sequestration.
So, the effect that prolonged droughts associated with El Niño-like conditions have on lowland dipterocarp forests may hinder their ability to store carbon. Other important ecosystem services that tropical rainforest provide to our societies, such as water cycle regulation, erosion control and medicinal resources, may also be negatively affected.
You can read all of “Abrupt change in tropical Pacific climate mean state during the Little Ice Age” and “Long-term ecological responses of a lowland dipterocarp forest to climate changes and nutrient availability” in Nature Communications Earth & Environment and New Phytologist.
Assistant Professor Ana Prohaska
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Journalist and press consultant Liva Polack
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