During the Pliocene geological epoch, three million years ago, Earth faced a rise in temperature. The latest research funded by the Marsden Fund involving researchers from the Antarctic Research Centre, Waikato University and other nations has found that one-third of the ice sheet melted in Antarctica during this period causes a rise in current sea level by 20 meters in the centuries to come.
Past changes in sea level were measured by drilling of cores at the Whanganui Basin in New Zealand. It contains marine sediments of the highest resolution in the entire planet. Then a record of worldwide change in the sea-level was constructed with much more precision than previously estimated.
During the Pliocene period, the concentrations of carbon dioxide in the atmosphere were more than 400 ppm and the temperature of Earth was two Celsius higher than pre-industrial times. Warming exceeding two celsius might lead to widespread melting in Antarctica dragging the future of the planet back to three million years before.
Worldwide protests under the hashtag #FridaysForFuture were organized under the leadership of Greta Thunberg as people have realized the urgency to keep the levels of global warming below that of the target of two degrees Celsius set by the Paris Agreement. She criticized the United Nations for not acting on the evidence provided by the scientific community.
The current rate of worldwide emissions might take us back to the Pliocene period within 2030 thereby passing the target of the Paris Agreement. A pressing question is how fast would the sea levels rise. As per a special report on oceans and cryosphere by IPCC (Intergovernmental Panel on Climate Change) polar ice caps and glaciers are losing mass at an alarming rate, making it difficult to constrain the contribution of Antarctic ice sheets to a future rise in sea level. Following the current trend of emissions, the global sea level is likely to rise by 1.2 meters by the end of the century with an upper limit of two meters.
In 2015, sediment cores deposited during the Pliocene, preserved under Whanganui Basin were drilled. Timothy Naish, working for 30 years in this area detected more than 50 fluctuations in the worldwide sea level in the last 3.5 million years in the history of the planet. Sea levels changed in response to the climate cycles also known as Milankovitch cycles. These are caused due to long-term changes in the orbit of the Earth every 20,000, 40,000 and 100,000 years.
Sea levels are estimated to have been varied by several meters but the exact number has been difficult to obtain due to the deformation processes of Earth and the cycles’ incomplete nature. In the research, a theoretical relationship between particle size carried by waves on the continental shelf and the depth till seabed were used. This was then applied to 800 meters of drill core and outcrop that represented sediment sequences spanning a period of 2.5 to 3.3 million years before.
Fluctuations in the global sea level in the Pliocene were between five to twenty-five meters. This figure is adjusted to the local tectonic land movements and regional changes in the sea level due to gravitational and crustal changes also termed as PlioSeaNZ sea-level record.
During the Pliocene period, the geography of the continents, oceans, and size of polar ice sheets was similar to the present times, with an ice sheet on Greenland in the warmest times. Melting of this ice sheet would lead from five to 25 meters of the rise in sea level at the Whanganui Basin. 90 percent of heat from global warming has gone to the ocean, mainly to the Southern Ocean on the margins of the ice sheet at Antarctica.
Deepwater upwelling in addition to entering ice shelf cavities is observed around Antarctica now. The ice sheet is thinning the fastest around the Amundsen Sea of West Antarctica, where maximum ocean heating is observed. One-third of the ice sheet of Antarctica equalling nearly 20 meters of the rise in sea level is situated below the sea level which can collapse from ocean heating.
Thus if global temperatures are allowed to rise more than two degrees celsius, huge portions of the ice sheet could get melted in the coming times, changing the entire shoreline of the world.