A new study in Nature uses stalagmites from Peruvian caves to track monsoon patterns over 450,000 years. Researchers found that Asian and South American monsoons usually act like a seesaw, but exceptions exist. These findings help experts understand how climate changes affect water supplies for billions of people across the globe.
In places like South Asia, Africa and parts of the Americas, monsoon rains supply water for drinking, crops and hydropower for billions of people. Understanding the patterns that monsoons tend to follow can help people plan for drought risks affecting agriculture, reservoirs and hydropower across distant regions.
While past research has established patterns between Asian and South American monsoons, in particular, a better record of past monsoons can help researchers understand when these patterns have broken down in the past and which climate changes might explain those exceptions. A new study, published in Nature, lends insight into these dynamics with the help of stalagmites from a cave in Peru.
A monsoon seesaw
Researchers know that Asian and South American monsoons generally behave like a seesaw: one region experiences strong monsoons while monsoons in the other weaken, bringing drier conditions. This occurs seasonally, over the course of a year, but also over much longer timescales of thousands of years. Scientists have linked this to changes in Earth's orbit and axial tilt over time, which affect how much sunlight reaches the surface in different areas.
The authors of the new study write, "The global monsoon is controlled by the location of the Intertropical Convergence Zone (ITCZ) and the strength of incoming solar radiation (insolation) in each hemisphere. For this reason, variations in monsoon precipitation are typically antiphased between the two hemispheres and reflect the mean position of the ITCZ as a direct result of orbital (Milankovitch) forcing."
Past research utilized cave records inferred from stalagmite analysis to trace Asian monsoon changes over roughly 690,000 years and show strong links to orbital cycles. Earlier Southern Hemisphere cave records have been more sparse, generally covering only the last two ice-age cycles. While these records pointed to the seesaw effect, a longer record of rainfall activity in South America would help to support the idea and provide further insight.
Insights from Peruvian caves
In an effort to map out monsoon activity over a longer timescale in South America, the team combined oxygen-isotope measurements from 30 stalagmites in three Peruvian caves near the Andes Mountains, using radioactive dating to establish their ages. The resulting data spanned roughly 450,000 years of summer monsoon variability with a few gaps. The researchers checked the rainfall interpretation against modern cave monitoring, nearby lake sediments and other South American cave records. They then compared the reconstruction with Asian monsoon records, other ancient climate evidence and existing ocean-circulation simulations.
Comparison with Asian records confirmed that the two monsoons usually changed in opposite directions, broadly following Earth's roughly 23,000-year orbital wobble. However, the team identified 10 episodes when the records showed simultaneous weakening or strengthening instead. The shared weakening occurred during several transitions out of ice ages and during some warmer intervals, challenging the simple north–south rainfall seesaw.
The two episodes of shared strengthening occurred during the penultimate ice age, which lasted from roughly 194,000 to 135,000 years ago. The researchers say that the synchronous events lasted for several thousand years before the two records reverted to their previous behavior.
Causes of shared highs and lows
The researchers think that these anomalous synchronized periods might indicate that ocean temperatures, greenhouse gases and temperature contrasts between hemispheres can modify the usual orbital influence. It seems that the global monsoon reorganizes itself in response to these changes, especially during intervals of rapid warming.
The study authors write, "Among the 10 synchronous monsoon events of the past 450 ka, interhemispheric weak monsoon intervals (WMIs) during terminations occur regularly, with a periodicity of about 100 kyr. These interhemispheric WMIs occur within intervals of major increases in atmospheric CO₂ and global temperature, which were intervals of reduced meridional temperature gradient.
"As monsoons are driven by temperature gradients (that is, the temperature-driven pressure differences between land and sea), the global reduction in meridional temperature gradients during terminations may be partly responsible for interhemispheric WMIs."