Rising waters

Even if humanity were to stop throwing carbon dioxide and methane into the atmosphere today, a catastrophic rise in sea levels of six meters may be inevitable. Two previous prehistoric interglacial periods, in which the carbon dioxide content of the atmosphere was believed to be about what it is today, resulted in dramatic rising of the oceans.
High-latitude ice sheets are melting, and given that global warming is most pronounced in the Arctic, it may already be too late to stop a rise in sea levels that would flood out hundreds of millions around the world. Two new papers, the latest in a series of scientific studies, paint a picture considerably less rosy than conventional ideas that major damage can still be avoided.
One of these papers, a nine-scientist report led by geologist Andrea Dutton at the University of Florida published in the journal Science, found that modest rises in global temperatures in the past led to sea levels rising at least six meters. She summarised the findings this way to Climate Central:
“Even if we meet that 2 degrees C target, in the past with those types of temperatures, we may be committing ourselves to this level of sea level rise in the long term. The decisions we make now about where we want to be in 2100 commit us on a pathway where we can’t go back. Once these ice sheets start to melt, the changes become irreversible.”
Professor Dutton was referencing the widely held belief that catastrophic damage can be avoided if global warming is held to no more than 2 degrees C. from pre-industrial levels. The ‘permissible’ level may be less than that, however. More sophisticated ‘sea-level reconstructions’ through interdisciplinary studies of geological evidence and better understanding of the behaviour of ice sheets enabled the paper’s authors to infer that temperatures only slightly higher than what we are experiencing today upset the climatic balance.
More alarming, the level of carbon dioxide in the atmosphere then was lower than it is today. Although geological forces pushing and pulling Earth’s surface can’t be precisely calculated, and thus introduce uncertainty in the actual level of the oceans in the geologic record, the greater uncertainty lies at the higher level of estimates. The paper’s summary said: “Noticeably, during these two periods, carbon dioxide in the atmosphere remained around 280 parts per million (ppm). The scientists also looked at sea level during the Pliocene, three million years ago, when carbon dioxide levels reached around 400 ppm – similar to today’s levels. They hypothesized that sea level was at least 6 metres higher than today and potentially substantially higher. … While the global average temperature rises of 1 to 3°C seem small, they were, like today, linked with magnified temperature increases in the polar regions which sustained over many thousands of years.”
A second paper, State of the Climate in 2014, reports that Arctic sea-surface temperatures are rising faster than overall global temperatures, ice caps across the Northern Hemisphere continue to shrink, record high permafrost temperatures are being recorded in northern Alaska and melting of the Greenland ice cap is accelerating. The paper, a collaboration of 413 scientists from 58 countries, reports that, even if greenhouse gases were frozen at current levels, the oceans would continue to warm for centuries and thus lead to rising sea levels.
Carbon dioxide thrown into the air stays in the atmosphere for a long time and warming oceans will retain added heat and transfer that back to the atmosphere. This is already leading to warming oceans, State of the Climate reports: “Increasing concentrations of greenhouse gases are preventing heat radiated from Earth’s surface from escaping into space as freely as it used to; most of the excess heat is being stored in the upper ocean. As a result, upper ocean heat content has increased significantly over the past two decades.”
The Science and State of the Climate papers back previous studies that conclude “there is no going back” – the excess heat stored in oceans will be released back into the atmosphere for centuries to come – and that Earth is crossing multiple points of no return.
Two worrisome trends are that the eight lowest Arctic Ocean sea-ice extents have all occurred in the past eight years, and that the extent of the melting of the Greenland ice sheet during summer 2014 was faster than the 1981-to-2000 average 90 percent of the time. Antarctic ice is not yet showing accelerated melting, State of the Climate reports, but the paper does note that short-term extremes in temperatures have become more frequent on the continent.
Nor does that mean that all is well in Antarctica. Two scientific papers published in 2014 suggest the West Antarctic ice sheet has become dangerously weakened. One finds that a “large sector of the West Antarctic ice sheet … has passed the point of no return” and the other finds that the ice sheet has become sufficiently unstable to possibly collapse in as few as 200 years. That is a long time by ordinary human standards, but very brief in geological terms, and would add greatly to rising sea levels.
So what would a six-meter increase in ocean levels mean? More than 440,000 square miles (1.14 million square kilometres), where 375 million people, would go under water, according to Climate Central.
The current path humanity is walking is to throw more greenhouse gases into the atmosphere. Current plans by political leaders to reduce greenhouse-gas emissions by 2050 and completely by 2100 are woefully inadequate, but even those goals will be difficult to achieve.
This article has been excerpted from: ‘We may have already committed ourselves to 6-meter sea-level rise’. Courtesy: Counterpunch.org