Tag Archives: sea level rise

Too late to turn back Rising oceans?

Rising oceans: Too late to turn the tide?.

If sea levels rose to where they were during the Last Interglacial Period, large parts of the Gulf of Mexico region would be under water (red areas), including half of Florida and several Caribbean islands. (Credit: Illustration by Jeremy Weiss)

ScienceDaily (July 18, 2011) — Thermal expansion of seawater contributed only slightly to rising sea levels compared to melting ice sheets during the Last Interglacial Period, a University of Arizona-led team of researchers has found.

The study combined paleoclimate records with computer simulations of atmosphere-ocean interactions and the team’s co-authored paper is accepted for publication in Geophysical Research Letters.

As the world’s climate becomes warmer due to increased greenhouse gases in the atmosphere, sea levels are expected to rise by up to three feet by the end of this century.

But the question remains: How much of that will be due to ice sheets melting as opposed to the oceans’ 332 billion cubic miles of water increasing in volume as they warm up?

For the study, UA team members analyzed paleoceanic records of global distribution of sea surface temperatures of the warmest 5,000-year period during the Last Interglacial, a warm period that lasted from 130,000 to 120,000 years ago.

The researchers then compared the data to results of computer-based climate models simulating ocean temperatures during a 200-year snapshot as if taken 125,000 years ago and calculating the contributions from thermal expansion of sea water.

The team found that thermal expansion could have contributed no more than 40 centimeters — less than 1.5 feet — to the rising sea levels during that time, which exceeded today’s level up to eight meters or 26 feet.

At the same time, the paleoclimate data revealed average ocean temperatures that were only about 0.7 degrees Celsius, or 1.3 degrees Fahrenheit, above those of today.

“This means that even small amounts of warming may have committed us to more ice sheet melting than we previously thought. The temperature during that time of high sea levels wasn’t that much warmer than it is today,” said Nicholas McKay, a doctoral student at the UA’s department of geosciences and the paper’s lead author.

McKay pointed out that even if ocean levels rose to similar heights as during the Last Interglacial, they would do so at a rate of up to three feet per century.

“Even though the oceans are absorbing a good deal of the total global warming, the atmosphere is warming faster than the oceans,” McKay added. “Moreover, ocean warming is lagging behind the warming of the atmosphere. The melting of large polar ice sheets lags even farther behind.”

“As a result, even if we stopped greenhouse gas emissions right now, the Earth would keep warming, the oceans would keep warming, the ice sheets would keep shrinking, and sea levels would keep rising for a long time,” he explained.

They are absorbing most of that heat, but they lag behind. Especially the large ice sheets are not in equilibrium with global climate,” McKay added. ”

Jonathan Overpeck, co-director of the UA’s Institute of the Environment and a professor with joint appointments in the department of geosciences and atmospheric sciences, said: “This study marks the strongest case yet made that humans — by warming the atmosphere and oceans — are pushing the Earth’s climate toward the threshold where we will likely be committed to four to six or even more meters of sea level rise in coming centuries.”

Overpeck, who is McKay’s doctoral advisor and a co-author of the study, added: “Unless we dramatically curb global warming, we are in for centuries of sea level rise at a rate of up to three feet per century, with the bulk of the water coming from the melting of the great polar ice sheets — both the Greenland and Antarctic Ice Sheets.”

According to the authors, the new results imply that 4.1 to 5.8 meters, or 13.5 to 19 feet, of sea level rise during the Last Interglacial period was derived from the Antarctic Ice Sheet, “reemphasizing the concern that both the Antarctic and Greenland Ice Sheets may be more sensitive to warming temperatures than widely thought.”

“The central question we asked was, ‘What are the warmest 5,000 years we can find for all these records, and what was the corresponding sea level rise during that time?'” McKay said.

Evidence for elevated sea levels is scattered all around the globe, he added. On Barbados and the Bahamas, for example, notches cut by waves into the rock six or more meters above the present shoreline have been dated to being 125,000 years old.

“Based on previous studies, we know that the sea level during the Last Interglacial was up to 8.5 meters higher than today,” McKay explained.

“We already knew that the vast majority came from the melting of the large ice sheets in Greenland and Antarctica, but how much could the expansion of seawater have added to that?”

Given that sea surface temperatures were about 0.7 degrees warmer than today, the team calculated that even if the warmer temperatures reached all the way down to 2,000 meters depth — more than 6,500 feet, which is highly unlikely — expansion would have accounted for no more than 40 centimeters, less than a foot and a half.

“That means almost all of the substantial sea level rise in the Last Interglacial must have come from the large ice sheets, with only a small contribution from melted mountain glaciers and small ice caps,” McKay said.

According to co-author Bette Otto-Bliesner, senior scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., getting the same estimate of the role ocean expansion played on sea level rise increases confidence in the data and the climate models.

“The models allow us to attribute changes we observe in the paleoclimate record to the physical mechanisms that caused those changes,” Otto-Bliesner said. “This helps tremendously in being able to distinguish mere correlations from cause-and-effect relationships.”

The authors cautioned that past evidence is not a prediction of the future, mostly because global temperatures during the Last Interglacial were driven by changes in Earth’s orbit around the sun. However, current global warming is driven by increasing greenhouse gas concentrations.

The seasonal differences between the northern and the southern hemispheres were more pronounced during the Last Interglacial than they will be in the future.

“We expect something quite different for the future because we’re not changing things seasonally, we’re warming the globe in all seasons,” McKay said.

“The question is, when we think about warming on a global scale and contemplate letting the climate system change to a new warmer state, what would we expect for the ice sheets and sea levels based on the paleoclimate record? The Last Interglacial is the most recent time when sea levels were much higher and it’s a time for which we have lots of data,” McKay added.

“The message is that the last time glaciers and ice sheets melted, sea levels rose by more than eight meters. Much of the world’s population lives relatively close to sea level. This is going to have huge impacts, especially on poor countries,” he added.

“If you live a meter above sea level, it’s irrelevant what causes the rise. Whether sea levels are rising for natural reasons or for anthropogenic reasons, you’re still going to be under water sooner or later.”

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Effects of climate change in Arctic more extensive than expected; a clever proposal for climate change legislation

Effects of climate change in Arctic more extensive than expected, report finds.

ScienceDaily (May 4, 2011) — A much reduced covering of snow, shorter winter season and thawing tundra: The effects of climate change in the Arctic are already here. And the changes are taking place significantly faster than previously thought. This is what emerges from a new research report on the Arctic, presented in Copenhagen this week. Margareta Johansson, from Lund University, is one of the researchers behind the report.

Together with Terry Callaghan, a researcher at the Royal Swedish Academy of Sciences, Margareta is the editor of the two chapters on snow and permafrost.

“The changes we see are dramatic. And they are not coincidental. The trends are unequivocal and deviate from the norm when compared with a longer term perspective,” she says.

The Arctic is one of the parts of the globe that is warming up fastest today. Measurements of air temperature show that the most recent five-year period has been the warmest since 1880, when monitoring began. Other data, from tree rings among other things, show that the summer temperatures over the last decades have been the highest in 2000 years. As a consequence, the snow cover in May and June has decreased by close to 20 per cent. The winter season has also become almost two weeks shorter — in just a few decades. In addition, the temperature in the permafrost has increased by between half a degree and two degrees.

“There is no indication that the permafrost will not continue to thaw,” says Margareta Johansson.

Large quantities of carbon are stored in the permafrost.

“Our data shows that there is significantly more than previously thought. There is approximately double the amount of carbon in the permafrost as there is in the atmosphere today,” says Margareta Johansson.

The carbon comes from organic material which was “deep frozen” in the ground during the last ice age. As long as the ground is frozen, the carbon remains stable. But as the permafrost thaws there is a risk that carbon dioxide and methane, a greenhouse gas more than 20 times more powerful than carbon dioxide, will be released, which could increase global warming.

“But it is also possible that the vegetation which will be able to grow when the ground thaws will absorb the carbon dioxide. We still know very little about this. With the knowledge we have today we cannot say for sure whether the thawing tundra will absorb or produce more greenhouse gases in the future,” says Margareta Johansson.

Effects of this type, so-called feedback effects, are of major significance for how extensive global warming will be in the future. Margareta Johansson and her colleagues present nine different feedback effects in their report. One of the most important right now is the reduction of the Arctic’s albedo. The decrease in the snow- and ice-covered surfaces means that less solar radiation is reflected back out into the atmosphere. It is absorbed instead, with temperatures rising as a result. Thus the Arctic has entered a stage where it is itself reinforcing climate change.

The future does not look brighter. Climate models show that temperatures will rise by a further 3 to 7 degrees. In Canada, the uppermost metres of permafrost will thaw on approximately one fifth of the surface currently covered by permafrost. The equivalent figure for Alaska is 57 per cent. The length of the winter season and the snow coverage in the Arctic will continue to decrease and the glaciers in the area will probably lose between 10 and 30 per cent of their total mass. All this within this century and with grave consequences for the ecosystems, existing infrastructure and human living conditions.

New estimates also show that by 2100, the sea level will have risen by between 0.9 and 1.6 metres, which is approximately twice the increase predicted by the UN’s panel on climate change, IPCC, in its 2007 report. This is largely due to the rapid melting of the Arctic icecap. Between 2003 and 2008, the melting of the Arctic icecap accounted for 40 per cent of the global rise in sea level.

“It is clear that great changes are at hand. It is all happening in the Arctic right now. And what is happening there affects us all,” says Margareta Johansson.

The report “Impacts of climate change on snow, water, ice and permafrost in the Arctic” has been compiled by close to 200 polar researchers. It is the most comprehensive synthesis of knowledge about the Arctic that has been presented in the last six years. The work was organised by the Arctic Council’s working group for environmental monitoring (the Arctic Monitoring and Assessment Programme) and will serve as the basis for the IPCC’s fifth report, which is expected to be ready by 2014.

Besides Margareta Johansson, Torben Christensen from Lund University also took part in the work.

More information on the report and The Artic as a messenger for global processes – climate change and pollution conference in Copenhagen can be found at:

http://amap.no/Conferences/Conf2011/

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2 °C or not 2 °C? That is the climate question

Targets to limit the global temperature rise won’t prevent climate disruption. Tim Lenton says that policy-makers should focus on regional impacts.

As a scientist who works on climate change, I am not comfortable with recommending policy. Colleagues frown on it, and peer review of scientific papers slams anything that could be construed as policy prescription. Yet climate science is under scrutiny in multiple arenas, and climate scientists have been encouraged to engage more openly in societal debate.

I don’t want to write policies, but I do want to ensure that global efforts to tackle the climate problem are consistent with the latest science, and that all useful policy avenues remain open. Ongoing negotiations for a new climate treaty aim to establish a target to limit the global temperature rise to 2 °C above the average temperature before the industrial revolution. But that is not enough.

The target is linked to the United Nations Framework Convention on Climate Change (UNFCCC), which aims to “prevent dangerous anthropogenic interference with the climate system”. But that noble objective is nearly 20 years old and is framed too narrowly, in terms of the “stabilization of greenhouse gas concentrations in the atmosphere”. Long-term goals to limit temperature or concentrations have so far failed to produce effective short-term action, because they do not have the urgency to compel governments to put aside their own short-term interests.

“Global average warming is not the only kind of climate change that is dangerous.”

Global average warming is not the only kind of climate change that is dangerous, and long-lived greenhouse gases are not the only cause of dangerous climate change. Target setters need to take into account all the factors that threaten to tip elements of Earth’s climate system into a different state, causing events such as irreversible loss of major ice sheets, reorganizations of oceanic or atmospheric circulation patterns and abrupt shifts in critical ecosystems.

Such ‘large-scale discontinuities’ are arguably the biggest cause for climate concern. And studies show that some could occur before global warming reaches 2 °C, whereas others cannot be meaningfully linked to global temperature.

Disruption of the south- or east-Asian monsoons would constitute dangerous climate change, as would a repeat of historic droughts in the Sahel region of Africa or a widespread dieback of the Amazon rainforest. These phenomena are not directly dependent on global average temperature, but on localized warming that alters temperature gradients between regions. In turn, these gradients are influenced by uneven distribution of anthropogenic aerosols in the atmosphere.

Equally, an abrupt shift in the regions in which dense masses of water form in the North Atlantic could dangerously amplify sea-level rises along the northeastern seaboard of the United States. But the point at which that will occur depends on the speed of climate change more than its magnitude.

Even when a threshold can be directly related to temperature, as with the melting of ice sheets, it is actually the net energy input that is important. The rapid warming of the Arctic in recent years is attributable less to increasing carbon dioxide levels than to reductions in emissions of sulphate aerosols (which have a cooling effect), and to increases in levels of warming agents, including black-carbon aerosols and the shorter-lived greenhouse gases methane and tropospheric ozone.

Ultimately, crucial climate events are driven by changes in energy fluxes. However, the one metric that unites them, radiative forcing, is missing from most discussions of dangerous climate change. Radiative forcing measures the change in the net imbalance of energy that enters and leaves the lower atmosphere; it is a better guide to danger than greenhouse-gas concentrations or global warming. It takes into account almost all anthropogenic activities that affect our climate, including emissions of methane, ozone-producing gases and hydrofluorocarbons, and changes in land use and aerosol levels.

I suggest that the UNFCCC be extended. The climate problem, and the political targets presented as a solution, should be aimed at restricting anthropogenic radiative forcing to limit the rate and gradients of climate change, before limiting its eventual magnitude.

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How would this help? A given level of radiative forcing is reached long before the resulting global temperature change is fully realized, which brings urgency to the policy process. The 2 °C target would translate into a radiative forcing of about 2.5 Watts per square metre (W m?2), but to protect major ice sheets, we might need a tougher global target of 1.5 W m?2. We will still need a binding target to limit long-term global warming. And because CO2 levels remain the most severe threat in the long term, a separate target could tackle cumulative carbon emissions. But while we wait for governments to reach an agreement on CO2, we can get to work on shorter-lived radiative-forcing agents.

The beauty of this approach is that it opens separate policy avenues for different radiative-forcing agents, and regional treaties to control those with regional effects. For example, hydrofluorocarbons emissions could be tackled under a modification of the 1987 Montreal Protocol, which aimed to halt ozone depletion. And emissions of black-carbon aerosols and ozone-producing gases could be regulated under national policies to limit air pollution. This would both break the political impasse on CO2 and help to protect vulnerable elements of the Earth system.

Tim Lenton is professor of Earth system science in the College of Life and Environmental Sciences, University of Exeter, UK. e-mail: t.m.lenton@exeter.ac.uk

 

Rising Seas Look Inevitable

Rising Seas Look Inevitable – ScienceNOW.

Rising Seas Look Inevitable

on 10 January 2011, 5:44 PM | | 4
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Too late? Even if carbon dioxide emissions cease completely in 2100, warming in and around Antarctica (depicted in yellow and orange tones), as well as ocean warming there at depths between 500 and 1500 meters, will continue until the end of the millennium.
Credit: Gillett et al., Nature Geoscience, Advance Online Publication (2011)

It may be too late to stop the seas from eventually rising and flooding Earth’s coastlines. Even if humans manage to eliminate carbon dioxide emissions completely by the year 2100, ocean warming set in motion by the end of this millennium could trigger the collapse of the West Antarctic Ice Sheet and flood New York City, Hong Kong, and other coastal cities, a new study suggests.

Sea level rises when meltwater from land-based masses of ice, such as glaciers, flows into the ocean. But sea level also increases when heat from the atmosphere gets mixed into the upper layers of the ocean, causing that water to expand. In recent decades, this thermal expansion has provided, on average, only about one-quarter of the 1.8 millimeters of sea level rise seen each year, but its contribution is increasing, studies suggest.

Now researchers point to an even bigger threat from warm ocean waters. The floating ice shelves that ring Antarctica could melt. So could the seaward end of land-based ice streams. That would lead to a long-term, catastrophic rise in sea level.

The new analysis, conducted by Nathan Gillett, a climate scientist at the University of Victoria in Canada and his colleagues, considers a rosy scenario. The team assumes that carbon dioxide emissions will rise at moderate rates from now until 2100, when people will switch to renewable energy sources and stop producing carbon dioxide. In this scenario, atmospheric concentrations of the greenhouse gas peak at about 770 parts per million (approximately twice today’s level of approximately 390 ppm), Gillett says. Even though no new humanmade carbon dioxide emissions are produced after 2100 and terrestrial and marine ecosystems continue soaking it up, carbon dioxide levels remain above 550 ppm for the next 9 centuries. Oceans will be slow to soak up the atmospheric carbon dioxide, and terrestrial ecosystems—many of which have been storing carbon gradually for centuries—will begin to release some of that carbon after the year 2200, the model suggests. As a result, ocean warming persists throughout the millennium, the researchers reported online yesterday in Nature Geoscience.

Much of that warmth is mixed directly into surface seas by wave action. But some is injected into deeper ocean layers by the thermohaline circulation, a pattern of ocean currents that carries warm, salty water from the North Atlantic southward to the Antarctic. Overall, the team’s model suggests that the temperature of waters surrounding the icy continent at depths between 500 and 1500 meters will rise approximately 3?C between the years 2105 and 2995. Add that to an Antarctic surface warming of as much as 9?C since the mid-1800s, and that’s a recipe for melting ice. At particular risk is the West Antarctic Ice Sheet, a 2.2-million-cubic-kilometer, potentially unstable ice mass that sits on the sea floor at depths where Antarctic waters are warming the most.

Ocean warming alone will result in 25 centimeters of sea-level rise by 2100 and 1 meter by 3000, the researchers estimate. But if warming waters melt the major ice shelves of western Antarctica, which act like dams to hold immense quantities of ice on shore, the entire western portion of the Antarctic ice sheet could melt away. Previous studies hint that such a collapse could boost sea level as much as 4 meters, swamping coasts worldwide.

The team’s analysis “looks like a solid study, and the most interesting new result is the tie to the West Antarctic Ice Sheet,” says Richard Alley, a climate scientist at Pennsylvania State University, University Park. Human-caused warming, he says, could influence Antarctica’s land ice many centuries after we stop burning fossil fuels.