12 December 2013
Researchers from the University of Southampton and the Australian National University report that sea-level rise since the industrial revolution has been fast by natural standards and – at current rates – may reach 80cm above the modern level by 2100 and 2.5 metres by 2200.
The team used geological evidence of the past few million years to derive a background pattern of natural sea-level rise. This was compared with historical tide-gauge and satellite observations of sea-level change for the ‘global warming’ period, since the industrial revolution. The study, which was funded by the Natural Environment Research Council (iGlass consortium) and Australian Research Council (Laureate Fellowship), is published in the journal Scientific Reports.
Lead author Professor Eelco Rohling, from the Australian National University and formerly of the University of Southampton, says: “Our natural background pattern from geological evidence should not be confused with a model-based prediction. It instead uses data to illustrate how fast sea level might change if only normal, natural processes were at work. There is no speculation about any new mechanisms that might develop due to man-made global warming. Put simply, we consider purely what nature has done before, and therefore could do again.”
Co-author Dr Gavin Foster, a Reader in Ocean and Earth Science at the University of Southampton, who is based at the National Oceanography Centre, Southampton (NOCS), explains: “Geological data showed that sea level would likely rise by nine metres or more as the climate system adjusts to today’s greenhouse effect. But the timescale for this was unclear. So we studied past rates and timescales of sea-level rise, and used these to determine the natural background pattern.”
Co-author Dr Ivan Haigh, lecturer in coastal oceanography at the University of Southampton and also based at NOCS, adds: “Historical observations show a rising sea level from about 1800 as sea water warmed up and melt water from glaciers and ice fields flowed into the oceans. Around 2000, sea level was rising by about three mm per year. That may sound slow, but it produces a significant change over time.”
The natural background pattern allowed the team to see whether recent sea-level changes are exceptional or within the normal range, and whether they are faster, equal, or slower than natural changes.
Professor Rohling concludes: “For the first time, we can see that the modern sea-level rise is quite fast by natural standards. Based on our natural background pattern, only about half the observed sea-level rise would be expected.
“Although fast, the observed rise still is (just) within the ‘natural range’. While we are within this range, our current understanding of ice-mass loss is adequate. Continued monitoring of future sea-level rise will show if and when it goes outside the natural range. If that happens, then this means that our current understanding falls short, potentially with severe consequences.”
Tim Folger an American science and nature writer, contributing editor at the Discover Magazine and a science writer for other magazines, wrote an article about sea-levels rice in September 2013 issue of National Geographic. In this article “Rising Seas” he argues that
“A profoundly altered planet is what our fossil-fuel-driven civilization is creating, a planet where Sandy-scale flooding will become more common and more destructive for the world’s coastal cities. By releasing carbon dioxide and other heat-trapping gases into the atmosphere, we have warmed the Earth by more than a full degree Fahrenheit over the past century and raised sea level by about eight inches. Even if we stopped burning all fossil fuels tomorrow, the existing greenhouse gases would continue to warm the Earth for centuries. We have irreversibly committed future generations to a hotter world and rising seas.
In May the concentration of carbon dioxide in the atmosphere reached 400 parts per million, the highest since three million years ago. Sea levels then may have been as much as 65 feet above today’s; the Northern Hemisphere was largely ice free year-round. It would take centuries for the oceans to reach such catastrophic heights again, and much depends on whether we manage to limit future greenhouse gas emissions. In the short term scientists are still uncertain about how fast and how high seas will rise. Estimates have repeatedly been too conservative.”
“Six years ago the Intergovernmental Panel on Climate Change (IPCC) issued a report predicting a maximum of 23 inches of sea-level rise by the end of this century. ………….. As the IPCC prepares to issue a new report this fall, in which the sea-level forecast is expected to be slightly higher, gaps in ice-sheet science remain. But climate scientists now estimate that Greenland and Antarctica combined have lost on average about 50 cubic miles of ice each year since 1992—roughly 200 billion metric tons of ice annually. Many think sea level will be at least three feet (about 90cm) higher than today by 2100. Even that figure might be too low.”
Folger discusses with engineers, scientists, architects and citizens from New York, Miami, New Orleans and Rotterdam, Netherlands about the factors contributing to sea-level rise and the current projections. One of the scientists being interviewed for the article was Gavin Foster, a geochemist at the National Oceanography Centre, University of Southampton and Principal Investigator of the Descent into the Icehouse Project. Gavin says that
“With business as usual, the concentration of carbon dioxide in the atmosphere will reach around a thousand parts per million by the end of the century. …..Such concentrations haven’t been seen on Earth since the early Eocene epoch, 50 million years ago, when the planet was completely ice free. According to the U.S. Geological Survey, sea level on an iceless Earth would be as much as 216 feet higher than it is today. It might take thousands of years and more than a thousand parts per million to create such a world—but if we burn all the fossil fuels, we will get there.
No matter how much we reduce our greenhouse gas emissions, we’re already locked in to at least several feet of sea-level rise, and perhaps several dozens of feet, as the planet slowly adjusts to the amount of carbon that’s in the atmosphere already. A recent Dutch study predicted that the Netherlands could engineer solutions at a manageable cost to a rise of as much as five meters, or 16 feet. Poorer countries will struggle to adapt to much less. At different times in different places, engineering solutions will no longer suffice. Then the retreat from the coast will begin. In some places there will be no higher ground to retreat to.”
Read the article here
Carbon Dioxide the Dominant Control on Global Temperature and Sea Level Over the Last 40 Million Years
The study “Relationship between sea level and climate forcing by CO2 on geological timescales” by Dr Gavin Foster and Professor Eelco Rohling, published in the Proceedings of the National Academy of Sciences last month had quite an impact on the science websites and blogs.
In his Skeptical Science blogpost, Rob Painting discusses the key points of the study which has engaged scientists in a constructive debate in the comments section.
- Because the water contained in land-based ice sheets is ultimately derived from the ocean, over long (geological) timescales global sea level is largely determined by global temperature and, consequently, the temperature-dependent volume of ice stored on land.
- Since the concentration of carbon dioxide in the atmosphere (The Greenhouse Effect) exerts such a powerful influence on global and polar temperature, it therefore follows that it should also exhibit a strong relationship with global sea level over geologic intervals of time.
- Foster & Rohling (2013) examined time slices of paleo data covering the last 40 million years to uncover the details of this carbon dioxide-sea level relationship. Surprisingly, they found a consistent and robust relationship between carbon dioxide and sea level irrespective of other contributing factors.
- Based on the concentration of carbon dioxide in the atmosphere as of 2011, the authors estimated that future sea level is committed to rise 24 metres (+7/-15 m) above present-day once the land-based ice sheets have fully responded to the warming and the Earth is once more in equilibrium.
- The authors estimated that this sea level rise will likely take place over many centuries, if not several thousand years, but it nevertheless represents the long-term consequences of human industrial activity, and is further evidence that CO2 is the Earth’s “main control knob” for global temperature.
Continue reading here
The study “Relationship between sea level and climate forcing by CO2 on geological timescales” by Dr Gavin Foster and Professor Eelco Rohling which was this week published in the Proceedings of the National Academy of Sciences had quite an impact on the science websites and blogs.
In an interview in Katy Edgington (ScienceOmega.com), Gavin “expounded on the link between CO2 and the seas, and how the correlation exhibited in this study could influence our forecasts for the future.”
While sea level change is arguably one of the most long-lasting and significant impacts of anthropogenic climate change, long-range predictions of the change that can be expected as the oceans warm and the continental ice sheets melt are quite uncertain.
“This is largely because the complex processes involved in the melting of the continental ice sheets are difficult to incorporate into climate models,” Dr Foster pointed out. “We currently rely on semi-empirical methods to describe their behaviour; methods which remain as yet untested.”
Dr Foster and his colleagues approached the problem in a slightly different way, by delving into the rich archive of the geological past to find examples of warmer worlds that could provide an insight into how the Earth may behave in a warmer future. Focusing on the relatively recent past – the last 40 million years – they minimised the impact of changes in continental configuration, for example.
“The main advantage of looking at the geological past, and what makes it worthwhile, is that it represents a reality – a state which we can be sure the Earth system once occupied,” explained Dr Foster. “It inherently includes all feedbacks involved in the system whether we currently know about them or not; this is not the case with modelling, of course, which draws directly on the state of our current knowledge.”
The inclusion of data from points at which the global temperature was increasing and decreasing allowed Dr Foster and co-author Professor Eelco Rohling – also from Ocean and Earth Science at the University of Southampton – the best chance of spotting trends and relationships.
“As many people are aware, CO2 is a potent greenhouse gas responsible for a significant portion of the greenhouse effect,” stated Dr Foster. “On geological timescales, the concentration of CO2 in the atmosphere is determined by subtle imbalances in the amount of CO2 coming out of volcanoes and the amount being removed by silicate weathering.”
Over the last 40 million years CO2 concentration has changed quite dramatically, from 1200 parts per million (ppm) 40 million years ago to 180 ppm during the last glacial maximum.
“We show here that ice volume and sea level, and hence global temperature, have changed in concert,” stated Dr Foster. “Correlation does not prove causation of course, but we have known from fundamental physics for over 100 years that if you change atmospheric CO2 concentration you change global temperature by ~1ºC per doubling. This basic response is amplified by other processes that operate in the atmosphere and the result is something like a 2–5ºC global temperature change for a doubling of CO2.”
By comparing reconstructions of atmospheric CO2 concentrations and sea level over the past 40 million years, researchers based at the National Oceanography Centre, Southampton have found that greenhouse gas concentrations similar to the present (almost 400 parts per million) were systematically associated with sea levels at least nine metres above current levels.
The study determined the ‘natural equilibrium’ sea level for CO2 concentrations ranging between ice-age values of 180 parts per million and ice-free values of more than 1,000 parts per million.
It takes many centuries for such an equilibrium to be reached, therefore whilst the study does not predict any sea level value for the coming century, it does illustrate what sea level might be expected if climate were stabilized at a certain CO2 level for several centuries.
Lead author Dr Gavin Foster, from Ocean and Earth Science at the University of Southampton which is based at the centre, said, “A specific case of interest is one in which CO2 levels are kept at 400 to 450 parts per million, because that is the requirement for the often mentioned target of a maximum of two degrees global warming.”
The researchers compiled more than two thousand pairs of CO2 and sea level data points, spanning critical periods within the last 40 million years. Some of these had climates warmer than present, some similar, and some colder. They also included periods during which global temperatures were increasing, as well as periods during which temperatures were decreasing.
“This way, we cover a wide variety of climate states, which puts us in the best position to detect systematic relationships and to have the potential for looking at future climate developments,” said co-author Professor Eelco Rohling, also from Ocean and Earth Science at the University of Southampton.
The researchers found that the natural relationship displays a strong rise in sea level for CO2 increase from 180 to 400 parts per million, peaking at CO2 levels close to present-day values, with sea level at 24 +7/-15 metres above the present, at 68 per cent confidence limits.
“This strong relationship reflects the climatic sensitivity of the great ice sheets of the ice ages,” said Dr Foster. “It continues above the present level because of the apparently similar sensitivity of the Greenland and West Antarctic ice sheets, plus possibly some coastal parts of East Antarctica.”
According to the study, sea level stays more or less constant for CO2 changes between 400 and 650 parts per million and it is only for CO2 levels above 650 parts per million that the researchers again saw a strong sea level response for a given CO2 change.
“This trend reflects the behaviour of the large East Antarctic ice sheet in response to climate changes at these very high CO2 levels. An ice-free planet, with sea level 65 metres above the present, occurred in the past when CO2 levels were around 1200 parts per million.”
Professor Rohling said, “Sea level rises to these high values will take many centuries, or even millennia, but the implications from the geological record are clear – for a future climate with maximum warming of about two degrees Centigrade, that is with CO2 stabilized at 400 to 450 parts per million, sea level is set to steadily rise for many centuries, towards its natural equilibrium position at around 24 +7/-15 metres, at 68 per cent confidence. In Intergovernmental Panel on Climate Change terms, this is a likely rise of at least nine metres above the present. Previous research indicates that such rises above present sea level may occur at rates of roughly one metre per century.”
Based on these results, which document how the Earth system has operated in the past, future stabilization of CO2 at 400-450 parts per million is unlikely to be sufficient to avoid a significant steady long-term sea level rise.
The study “Relationship between sea level and climate forcing by CO2 on geological timescales” is published this week online ahead of print in Proceedings of the National Academy of Sciences of the United States of America
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