Why did glacial cycles intensify a million years ago?

glacial cycles intensify

A major event seems to have impacted our Planet about one million years ago, during an episode called “mid-Pleistocene transition”: before this period, the cycles between glacial (colder) and interglacial (warmer) periods followed each other every 41,000 years, but they then intensified, sometimes giving rise to glacial periods lasting over 100,000 years. Paleoclimatologists have long debated the precise origin of this abrupt climate change.

Extending from 2.58 million years to 11,700 years before the present, the Pleistocene is a geological period covering the majority of recent known glaciations. The climate is characterized by intense cycles of glaciation, allowing the ice to cover more than 30% of the Earth’s surface during the maximum of glaciation: we find imposing glaciers in New Zealand, in the Ethiopian mountains or in the Atlas Mountains, in North Africa.

But these glacial cycles, characterized by a period of 41,000 years more than a million years ago, seem to have been largely impacted by a still unknown phenomenon to give the longer cycles we observe today.

Milankovitch cycles at the origin of these variations?

Scientists have been hesitating for a long time about the origin of this phenomenon. The most likely reason currently accepted is the Milankovitch cycles, cyclical variations in the Earth’s orbit that affect the amount of energy absorbed by the Earth.

On a geological scale, this is indeed the main natural driver for the alternation of warm and cold periods over several million years. However, some researchers argue that if the Milankovitch cycles did not undergo major changes a million years ago, another internal phenomenon was probably at work.

The role of the North Atlantic

Researchers at Columbia University’s Earth Institute analyzed sediment cores from the North and South Atlantic, looking specifically at the concentrations of the isotope Neodymium, a chemical element used as a tool for studying past ocean currents.

They found that a major Atlantic current system, the AMOC (Atlantic Meridional Circulation), underwent a severe weakening at a time coinciding with the “Mid-Pleistocene Transition” in the northern ocean. The researchers in the study link this change in ocean current to the greater presence of glaciers in the northern hemisphere, creating a more intense global cooling.

This discovery sheds light on the origin of a recent climate transition that is still unknown, and highlights the importance of the North Atlantic and ocean currents in past and future climate variations.

Oceans: the effects of plastics on animals and the environment

The first effect of plastic waste is a visual impact: debris washed up on the banks of rivers, beaches, piles of floating objects at sea. But some less directly apparent effects are also more severe.

Every second, hundreds of tons of waste (out of the 4 billion generated annually) ends up at sea, much of it plastic. Floating objects or microparticles, this plastic waste is deposited on beaches, dispersed at sea and found on the seabed. What effects do they have on man and his environment?

Highly resistant, abandoned or accidentally lost nets constitute a significant cause of mortality in animals; associations estimate that 100,000 marine mammals and one million birds die from strangulation or choking in these traps around the world each year. 

Transport of invasive species via plastic

Not many studies have been conducted, but the risk is there, with its environmental, health and economic consequences (invasive algae, bacteria in oyster or fish farming areas).
The recent discovery of the insect Halobates sericeus living on floating plastics in the Pacific is a perfect illustration. A 2007 study showed that the spread of species in subtropical waters has almost doubled due to debris. It tripled in temperate waters.

The real danger is the alteration of ecosystem balance caused by the transport of invasive species over long distances. Some wastes are useful floating supports for bacteria (specific pathogens for marine organisms such as humans, vibrios for example), unicellular or invertebrates, worms, insects, capable of acclimatizing in an area other than their original biotope, especially concerning climate change 

Ingestion of plastic waste by animals

The ingestion of plastic waste is another cause of mortality, affecting about 660 species. Seabirds sting floating pieces of plastic, and turtles confuse them with jellyfish.

Due to their components (plasticizers, additives) and the possible release of adsorbed contaminants, plastics are also suspected to be a source of water contamination. But in reality, the measured concentrations are too low for the substances released during their degradation to constitute a significant toxicity risk. The level of chemical contaminants remains limited but more dangerous for the filtering of marine organisms that accumulate them.

Large whales filter seawater, ingesting large quantities of microplastics. Molluscs, such as mussels, filter m3 of water containing microparticles. Plankton, invertebrates or small fish can also ingest Microwaste. However, the rate of ingestion remains very anecdotal for the species consumed (less than 0.01% of commercial fish), and there is no trace of it on our plates. Although micro waste can block the digestive and respiratory systems of some individuals, it is not digested due to the lack of suitable enzymatic equipment.