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

Understanding the causes of changing humidity and precipitation in the Earth’s past is crucial for better assessments of the planet’s future hydroclimate changes through improved modeling. One field that climate researchers around the world are focusing on is hydroclimate—i.e. the entirety of all long-term weather phenomena in a region that determine the amount of precipitation and humidity. After all, the Intergovernmental Panel on Climate Change (IPCC) states unequivocally: As climate change progresses, the risk of hydroclimate extremes—both droughts and heavy rainfall events—increases.

“Understanding the hydroclimate of a region or modeling future scenarios is anything but trivial and involves major uncertainties as it is the result of an extraordinarily complex interplay of many factors,” says Jérôme Kaiser from the IOW. “Analyzing changes in the Earth’s climate far back into the past can help to recognize patterns and thus identify important influencing factors.”

The expert in paleoceanography and paleoclimate is lead author on a study in Nature Communications, now published together with researchers from the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, the MARUM—Centre for Marine Environmental Sciences at the University of Bremen and two Chilean universities, the University of Magallanes in Punta Arenas and the Santiago-based University of Chile.

The study provides a glimpse into the paleoclimatic past by analyzing several sediment cores from the South-East Pacific, which were recovered from water depths between 850 and 3,300 meters on the continental slope off the northern and southern Chilean coast.

“Marine sediments, which are 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—so-called environmental proxies,” explains Kaiser.

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

“We know that different deuterium levels say a lot about the precipitation conditions in a region—about the amount and intensity of the precipitation, and even about the origin of the humidity from which the precipitation has formed,” Kaiser explains the approach.

The results show clear patterns for the sources of humidity and the amount of precipitation in the mid-latitude hydroclimate of the southeast Pacific: While in southern Chile the rain was mainly brought by the sub-Antarctic westerly winds, the precipitation in the mid-latitudes of Chile also came from the subtropics. The amount and origin of precipitation from these sources in the two regions, however, is subject to significant fluctuations over the millennia.

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

These temporal patterns correlate very well with temporal cycles of natural changes in the Earth’s orbit around the sun: During a phenomenon known as “precession,” which correlates with the shorter precipitation cycle in central Chile, the Earth’s axis undergoes a cone-shaped rotation and thus changes the planet’s orientation in relation to the sun.

In addition, the Earth’s axis also changes its inclination within the planet, which is known as the “Earth axis tilt phenomenon” and also affects the planet’s positioning towards the sun. It correlates with the longer time cycle of precipitation in southern Chile.

“Both orbital phenomena influence the intensity of the solar radiation in different regions by changing the tilt of the planet. And this in turn has consequences for the winds that transport moisture and rain,” says Kaiser. That the Earth’s orbital variability has climatic consequences has long been hypothesized and taken into account in regional climate models, the paleoclimate expert continues.

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

The Warnemünde-based researcher goes even further in his conclusions.

“It can’t be a matter of blaming extreme hydroclimatic events entirely on natural changes in the Earth’s obliquity. But to correctly 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.

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

Ozone gas is reducing the growth of tropical forests—leaving an estimated 290 million tonnes of carbon uncaptured each year, new research shows.

The ozone layer in the stratosphere shields our planet from harmful ultraviolet radiation—and protecting it is one of the major successes of environmental action.

But ozone at ground level—formed by the combination of pollutants from human activities in the presence of sunlight—interferes with plants’ ability to absorb carbon dioxide. Ozone is also harmful to human health.

The new study, published in the journal Nature Geoscience, calculates that ground-level ozone reduces new yearly growth in tropical forests by 5.1% on average.

The effect is stronger in some regions—with Asia’s tropical forests losing 10.9% of new growth.

Tropical forests are vital “carbon sinks”—capturing and storing carbon dioxide that would otherwise stay in the atmosphere and contribute to global warming.

“Tropical forests play a crucial role in mopping up our carbon dioxide emissions,” said co-lead author Dr. Alexander Cheesman, of James Cook University and the University of Exeter.

“Our study shows that air pollution can jeopardize this critical ecosystem service. We estimate that ozone has prevented the capture of 290 million tonnes of carbon per year since 2000. The resulting cumulative loss equates to a 17% reduction in carbon removal by tropical forests so far this century.”

The researchers ran experiments to measure the ozone susceptibility of various tropical tree species, then incorporated the results into a computer model of global vegetation.

Urbanization, industrialization, burning fossil fuels and fires have led to an increase in “precursor” molecules—such as nitrogen oxides—that form ozone.

“Ozone concentrations across the tropics are projected to rise further due to increased precursor emissions and altered atmospheric chemistry in a warming world,” said co-lead author Dr. Flossie Brown, a recent graduate of the University of Exeter.

“We found that areas of current and future forest restoration—areas critical for the mitigation of climate change—are disproportionately affected by this elevated ozone.

“It is clear that air quality will continue to play an important but often overlooked part in the way forests absorb and store carbon.”

Professor Stephen Sitch, from the University of Exeter, added, “Embracing a future with greater environmental protection would lead to reduced ground-level ozone, thus improved air quality and the additional benefit of enhanced carbon uptake in tropical forests.”

The paper is entitled “Reduced productivity and carbon drawdown of tropical forests from ground-level ozone exposure.”