Giant see-saw of monsoon rains detected

The regional monsoon has huge effects on agriculture and hence on the livelihoods of half of the world’s population, including India and Indonesia. Understanding how seasonal periods of rainfall in the Northern and Southern Hemispheres of our planet are linked is important for assessing possible long-distance effects of climate change.

“We’ve been surprised by how clearly the ups and downs of precipitation in East Asia on the one hand and Australia on the other hand are opposed, it’s really a giant see-saw,” says Deniz Eroglu from the Potsdam Institute for Climate Impact Research (PIK) and University of São Paulo, lead author of the study to be published in Nature Communications. “We needed to cut through the data clutter to detect this pattern. While our method of sophisticated statistical time series data analysis might seem quite complicated, our findings have substantial real-world consequences.”

Data from caves in China and Australia – these countries are most affected

Australia, just like China, heavily depends on the monsoon summer rains. “Both countries have experienced drier and wetter periods in the past. For instance the Northwest of Australia’s tourism and agriculture industries are vulnerable to flooding and bush fires, so any change from the current precipitation pattern will have huge impacts for the people living here,” says co-author Thomas Stemler from the University of Western Australia.

“However, this is an issue way beyond the region – in fact, the East-Asian-Indonesian-Australian summer monsoon provides a heat source that drives global circulation of airstreams during what is the winter season in the US, Russia and Europe. It will be exciting to investigate how the see-saw we found may be affecting these far-away parts of the world.”

Since there are no direct records of monsoon dynamics over past millennia, the scientists need to work with indirect data evidence. “Dripstones in ancient caves are amazing witnesses of the Earth’s past. Since they’re growing just fractions of a millimeter per year, we can see changes in the chemical composition over time from one layer to another,” says co-author Norbert Marwan from PIK who himself explored various caves, for instance in India. For the new study, the team used data from Dongge Cave in Southern China and cave KNI-51 in Northwestern Australia.

“Though it might seem challenging to climb into the caves to access the dripstones,” says Marwan, “the real challenge is to decrypt the information they carry – analyzing thousands of isotope samples and attributing them to specific climatic conditions. For this, we need sophisticated statistics.” A key partner in this process has been the Institute for Geology, Mineralogy & Geophysics at Ruhr-Universität Bochum.

Mechanics of the heavens are a driver, but human-made warming can change the dynamics

“The monsoon see-saw is likely driven by factors humans cannot influence, including the tilt of our Earth’s axis and solar activity, so that’s celestial mechanics,” says co-author Jürgen Kurths, co-chair of PIK’s research domain Transdisciplinary Concepts and Methods.

“However, disturbing circulation and precipitation patterns is something we unfortunately can do and already are doing by emitting greenhouse gases and thereby warming our planet. Understanding the natural East-Asian-Australian monsoon variability will help us to better understand certain human-caused climate risks in the future.”

Article: Deniz Eroglu, Fiona H. McRobie, Ibrahim Ozken, Thomas Stemler, Karl-Heinz Wyrwoll, Sebastian F. M. Breitenbach, Norbert Marwan, Jürgen Kurths (2016): See-saw relationship of the Holocene East Asian-Australian summer monsoon. Nature Communications [10.1038/NCOMMS12929]

Weblink to the article once it is published: http://dx.doi.org/10.1038/NCOMMS12929

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