Editors Vox is a blog of the AGU publications service.
About 75,000 to 10,000 years ago, there was a series of sudden and dramatic changes in precipitation patterns in the tropics, possibly triggered by changes in the circulation of ocean waters. A recent article in Geophysical reviews examines the evidence for these “tropical hydroclimatic events” and explores the potential causes. Here, one of the authors explains these abrupt climate change events of the past in more detail and suggests how this can inform our understanding of potential future climate change.
What are “Tropical Hydroclimatic Events” (THE)? When and where did they happen?
When we look back at past climatic variations in the continental regions of the tropics and sub-tropics (~ 30ºN-30ºS), it is quite clear that there have been times when the climate has changed abruptly, causing areas that were once wet to become quite dry, and areas that were once dry to become much wetter, disrupting plant and animal communities, as well as people living in the area. We call these changes “Tropical Hydroclimatic Events (TEA)”. The changes affected large areas (and lasted for centuries in some cases, before the climate returned to previous conditions. There were at least half a dozen of these THE around 10,000 to 75,000 years ago.
Where do we find the scientific evidence for these events?
Changes in precipitation regimes (regional hydroclimatic conditions) can be observed by studying different indicators of past climate. The evidence is recorded in numerous “natural records” – lake sediments, stalagmites in caves, as well as in marine sediments where rivers have carried a record of conditions on adjacent continents offshore.
In these archives, evidence of hydrological changes is recorded by many different climatic indicators, such as pollen, diatoms, organic biomarkers, and the physical and geochemical properties of sediments, or by the isotopes of oxygen in carbonate. calcium which forms the stalagmites.
How are changes in the Atlantic Reversal Meridional Circulation (AMOC) related to THE?
There is strong evidence that THE occurred around the same time that major changes in the North Atlantic overturning circulation (AMOC) occurred. When AMOC slowed down, the reduction in energy transported to the northern hemisphere via the ocean was reduced. This caused a change in atmospheric circulation to compensate. The result was a southward shift of the equatorial rainfall belt, bringing much more precipitation in some areas, while leaving other areas quite dry.
What have been the ripple effects of TEA on different types of climates around the world?
Large areas of the world have undergone abrupt hydroclimatic changes. Although the oceanic changes that appear to have triggered the THE began in the North Atlantic, the atmospheric response occurred across the tropics, affecting all parts of this region, from Peru and Brazil north to the tropics. ‘Australia and New Guinea in the southern hemisphere, and from central America to tropical Africa, India and southern China in the northern hemisphere.
What physical mechanisms may have played a role in these climate changes?
The main change caused by a slowdown in AMOC was a southward shift of the Intertropical Convergence Zone (ITCZ) and associated areas of heavy convective precipitation. This area migrates through the equatorial region seasonally, but during periods of TH its mean position has shifted further south, resulting in a redistribution of seasonal precipitation across the tropics.
Do these past climate events help us understand potential future climate changes?
Today there is a great deal of interest in “tipping points” and “thresholds” at which the climate can suddenly change. TEA are dramatic examples of sudden changes in climate that we know have occurred in the past, affecting almost half of the Earth’s surface. In the case of THE, the tipping point appears to have been a change in the Atlantic Reversal Meridional Circulation (AMOC) which has been significantly reduced in strength. These changes were linked to the sudden inflow of fresh water into the North Atlantic.
Today we see that the Greenland ice sheet is rapidly losing mass, with much of this fresh water entering the North Atlantic. Although there has been much more ice entering the North Atlantic in the past (from continental ice caps and arctic sea ice), current and future warming could cause the freshwater to increase rapidly. Greenland; if this affects the strength of AMOC, we should expect to see a ripple effect in terms of extensive hydroclimatic changes across the tropics. With billions of people living in the region today, this prospect requires careful consideration.
What are some of the unresolved questions that require further research, data collection or modeling?
Our understanding of the link between AMOC and THEs changes is still quite simplistic and requires further study, involving oceanographic measurements, climate modeling, and research on climate dynamics. Improvements in the dating of past changes, both in the oceans and on land, are particularly needed, in order to trace the advances and lags in the ocean-atmosphere system. It is also important to recognize that the rapid changes in continental hydroclimate in some cases have occurred within decades. This would likely have had significant impacts on flora and fauna in many parts of the tropics, leading to species migration and possibly local extinctions. People living in the tropics at this time were also said to have been severely affected. This subject certainly deserves further study.
—Raymond S. Bradley ([email protected],