Global sea levels have risen at least 6 meters with just 1-2 degrees of warming

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Scientists have long established that increasing global temperatures causes substantial melting of ice from the polar regions and this melting then causes sea levels to rise thereby affecting coastal areas of the world. Now a new review of almost 30 years of research on historic effects of melting polar ice sheets has revealed that global found that global sea levels have risen at least six meters, or about 20 feet, during multiple occasions in the past three million years and has happened with increase of just 1-2 degrees (Celsius) in global mean temperatures.

Anders Carlson, an Oregon State University glacial geologist and paleoclimatologist, and co-author on the study explains that ice sheets in Greenland and Antarctic have been major contributors to the sea level rise above modern levels. Carlson pointed out that current carbon dioxide levels are equivalent to those about three million years ago – a time when sea level was at least six meters higher because the ice sheets were greatly reduced.

Carlson added that though it takes time for global warming to start affecting the ice sheets and to cause subsequent melting, it doesn’t take forever and that there is evidence that we are likely seeing that transformation begin to take place now.

Co-author Peter Clark, an OSU paleoclimatologist, said that because current carbon dioxide, or CO2, levels are as high as they were 3 million years ago, “we are already committed to a certain amount of sea level rise.”

“The ominous aspect to this is that CO2 levels are continuing to rise, so we are entering uncharted territory,” Clark said. “What is not as certain is the time frame, which is less well-constrained. We could be talking many centuries to a few millennia to see the full impact of melting ice sheets.”

During those times, atmospheric carbon dioxide levels peaked around 280 parts per million, but today’s levels are around 400 ppm and rising. The team of researchers looked at the last time period when carbon dioxide was this high – about 3 million years ago – but couldn’t get a confident estimate on sea-level rise, in part due to land motion that has distorted the position of past shorelines.

The review, which was led by Andrea Dutton of the University of Florida, summarized more than 30 years of research on past changes in ice sheets and sea level. The review showed that changes in Earth’s climate and sea level are closely linked, with only small amounts of warming needed to have a significant effect on seal levels. Those impacts can be significant.

Dutton and the team assessed evidence of higher sea levels during several periods to understand how polar ice sheets respond to warming. Combining computer models and observations from the geologic record, they found that during past periods with average temperatures 1 to 3 °C (1.8 to 5.4 °F) warmer than preindustrial levels, sea level peaked at least 20 feet higher than today.

“As the planet warms, the poles warm even faster, raising important questions about how ice sheets in Greenland and Antarctica will respond,” she said. “While this amount of sea-level rise will not happen overnight, it is sobering to realize how sensitive the polar ice sheets are to temperatures that we are on path to reach within decades.”

Though six meters (or about 20 feet) of sea level rise doesn’t sound significant, it is a lot for cities and populations dwelling in coastal areas across the globe. Sea level rise of anywhere between 10 to 20 feet could be catastrophic to the hundreds of millions of people living in these coastal zones, the researchers added.

Researchers pointed out several cities around the world which are having comparatively lower elevation. Florida with elevation of 50 feet or less, Miami with an average elevation of six feet are a couple of places that could be affected by the sea level rise.

Dhaka in Bangladesh, which is one of the world’s most populous cities with 14.4 million inhabitants, is also located in low-lying areas that could be greatly impacted due to sea level rise.

Tokyo and Singapore also have been singled out as extremely vulnerable to sea level rise.

“The influence of rising oceans is even greater than the overall amount of sea level rise because of storm surge, erosion and inundation,” said Carlson, who studies the interaction of ice sheets, oceans and the climate system on centennial time scales. “The impact could be enormous.”

The Science review is part of the larger Past Global Changes, or PAGES, international science team. A working group known as PALSEA2 (Paleo constraints on sea level rise) used past records of local change in sea level and converted them to a global mean sea level by predicting how the surface of the Earth deforms due to changes in ice-ocean loading of the crust, along with changes in gravitational attraction on the ocean surface.

Independently, Greenland and Antarctic ice sheet volumes were estimated by observations from adjacent ocean sediment records and by ice sheet models.

“The two approaches are independent of one another, giving us high confidence in the estimates of past changes in sea level,” Carlson said. The past climates that forced these changes in ice volume and sea level were reconstructed mainly from temperature-sensitive measurements in ocean cores from around the globe, and from ice cores.

“Improving our understanding of individual polar ice sheet contributions to global mean sea level is a key challenge,” the authors point out. “An important uncertainty for future projections of the Greenland ice sheet is the threshold temperature beyond which it undergoes irreversible retreat, with current estimates ranging from 1 to 4 degrees C. above pre-industrial temperatures.”

The findings have been published in journal Science.