Hydroclimatic study reveals natural variations in Earth’s tilt affect precipitation and humidity

A research team led by the Leibniz Institute for Baltic Sea Research in Warnemünde (IOW) has analysed 50,000 years of hydroclimate in the mid-latitudes of the Southeast Pacific using specific moisture-related indicators in marine sediment cores. They found that natural variations in the Earth’s orbital parameters exert a decisive influence.

Understanding the causes of past changes in Earth’s moisture and precipitation is essential to better assess future hydroclimatic changes on the planet through improved modeling. One area that researchers around the world are focusing on is hydroclimate, the long-term set of weather phenomena in a region that determine the amount of precipitation and moisture. After all, the Intergovernmental Panel on Climate Change (IPCC) states unequivocally: as climate change progresses, the risk of extreme hydroclimatic events, whether droughts or heavy rainfall, increases.

“Understanding the hydroclimate of a region or modelling future scenarios is far from a simple task and involves large uncertainties, as it is the result of an extraordinarily complex interaction of many factors,” says Jerome Kaiser of the IOW. “Analyzing changes in the Earth’s climate in the distant past can help to recognize patterns and thus identify important influencing factors.”

The expert in paleoceanography and paleoclimate is the lead author of a study in Nature Communicationsnow published in collaboration with researchers from the Alfred Wegener Institute, the Helmholtz Centre for Polar and Marine Research, MARUM – Centre for Marine Environmental Science at the University of Bremen and two Chilean universities, the University of Magallanes in Punta Arenas and the University of Chile in Santiago.

The study provides insight into the paleoclimatic past by analyzing several sediment cores from the Southeast Pacific, recovered from water depths between 850 and 3,300 meters on the continental slope off the northern and southern coast of Chile.

“Marine sediments, which have been deposited over thousands of years in layers that can be dated quite well, are excellent archives from which we can reconstruct past environmental conditions on Earth using certain indicators, called environmental proxies,” Kaiser says.

The cores used in the current study reflect a period of about 50,000 years. The research team focused primarily on the content of deuterium, a naturally occurring hydrogen isotope, in the leaf waxes of land plants, which are deposited in marine sediments.

“We know that different levels of deuterium tell us a lot about precipitation conditions in a region: about the amount and intensity of precipitation, and even about the origin of the moisture from which the precipitation formed,” Kaiser says.

The results show clear patterns regarding moisture sources and precipitation amounts in the mid-latitude hydroclimate of the Southeast Pacific: while in southern Chile, rainfall was mainly brought by subantarctic westerlies, precipitation in mid-latitude Chile also came from the subtropics. The amount and origin of precipitation from these sources in both regions, however, are subject to significant fluctuations over millennia.

“It was particularly interesting for us to see that fluctuations in the amount and intensity of precipitation follow distinct time cycles, which only became visible thanks to the long period represented by the sediment cores: in central Chile, the cycle duration is 23,000 years, while in southern Chile it is 41,000 years,” Kaiser emphasizes.

These temporal patterns correspond very well to the temporal cycles of natural changes in the Earth’s orbit around the Sun: during a phenomenon known as “precession,” which corresponds to the shorter precipitation cycle in central Chile, the Earth’s axis undergoes a cone-shaped rotation and thus changes the orientation of the planet relative to the Sun.

In addition, the Earth’s axis also changes tilt within the planet, which is called the “Earth’s axis tilt phenomenon” and which also affects the planet’s position relative to the Sun. This phenomenon is related to the longer cycle of precipitation in southern Chile.

“These two orbital phenomena influence the intensity of solar radiation in different regions by changing the tilt of the planet. And this has consequences for the winds that transport moisture and rain,” explains Kaiser. The hypothesis that the Earth’s orbital variability has consequences for the climate has long been put forward and taken into account in regional climate models, continues the paleoclimatology expert.

“However, based on the results of deuterium measurements, our study provides concrete evidence that the hydroclimate of Chile’s mid-latitudes is largely controlled by orbital parameters. And hydroclimatic extremes in south-central Chile, such as the very high precipitation levels during the last glacial period and the pronounced drought of the early Holocene, can also be plausibly explained by orbital changes,” Kaiser says.

The Warnemünde researcher goes even further in his conclusions.

“Extreme hydroclimatic events cannot be attributed solely to natural changes in the Earth’s obliquity. But to properly recognize the signal of anthropogenic climate change impacts, we need to better understand the fluctuations, which are subject to natural influences, and also take into account that natural and anthropogenic fluctuations can add up in terms of impact.

“This naturally also applies to northern and central Europe, where the Earth’s variable orbit also has a climatic impact.”