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Stefan Lovgren for National Geographic News Updated April 26, 2004 ||
 * =**Warming to Cause Catastrophic Rise in Sea Level?**=

// From the melting of the icecap ////on Mount Kilimanjaro, Africa's tallest peak, to the loss of coral reefs as oceans become warmer, the effects of global warming are often clear. // //However, the biggest danger, many experts warn, is that global warming will cause sea levels to rise dramatically. Thermal expansion has already// **__//raised the oceans 4 to 8 inches//__** //(10 to 20 centimeters). But that's nothing compared to what would happen if, for example, Greenland's massive ice sheet were to melt.// //The current rate of warning is unprecedented, however. It is apparently the fastest warming rate in millions of years, suggesting it probably is not a natural occurrence. And most scientists believe the rise in temperatures will in fact accelerate. The United Nations-sponsored Intergovernmental Panel on Climate Change reported in 2001 that the// **__//average temperature is likely to increase by between 1.4 and 5.8 degrees Celsius (2.5 and 10.4 degrees Fahrenheit) by the year 2100//__**//. So far, the rise in sea level is because warmer water takes up more room than colder water, which makes sea levels go up, a process known as thermal expansion.// //A recent Nature study suggested that Greenland's ice sheet will begin to melt if the temperature there rises by 3 degrees Celsius (5.4 degrees Fahrenheit). That is something many scientists think is likely to happen in another// **//hundred years.//** //The complete melting of Greenland would raise sea levels by// __**//7 meters (23 feet).//**__ //But even a partial melting would cause a one-meter (three-foot) rise. Such a rise would have a devastating impact on low-lying island countries, such as the Indian Ocean's Maldives, which would be entirely submerged.// == Source:  National Geographic - Warming to Cause Catastrophic Rise in Sea Level? http://news.nationalgeographic.com/news/2004/04/0420_040420_earthday.html **RISING OCEAN LEVELS - SEVERE** == = =

//The permanent Greenland ice sheet covers almost 700,000 square miles - 1,500 miles north to south and 700 miles east to west at the widest point. The ice sheet is typically over a mile thick. That's __**750,000,000,000,000 gallons of water, or 2.85m cubic km.**__ (Estimates vary a bit.) If that much water were dumped into the ocean, __**sea levels would rise 7.2m (23.6 feet)**__.// //If we continue with current global warming trends, global temperatures will rise by around 4°C by the end of this century, and sea levels will __**rise 10 to 23 inches**__, due to expansion of sea water as the oceans warm, increased ice melt runoff from all over the world and other causes, but not counting major ice sheet disintegration. For this scenario we take the mid-range rise for all other causes (the same as our Moderate case) and add the Greenland ice melt estimate giving a __**total of about 7.6m (25 feet).**__// //Scientists estimate that at these global warming levels, the Greenland ice sheet will eventually disappear. How long is complicated and uncertain to calculate. If the ice melts slowly and steadily, it will take thousands of years. But more rapid events can happen. For example, glaciers that were locked to the ground can suddenly start surging forward when sea water at their outlets rises or warms, or lubricating water percolates down to the glacier base. Researchers warn that once temperatures reach a certain level, unpredictable events involving rapid feedback might happen on a time scale of decades. We don't know, but if we keep warming the planet, we should study it further. A quick collapse (in a century or so) would be a catastrophe.// =**RISING OCEAN LEVELS - EXTREMELY SEVERE**=

//As we described in the Severe case (Greenland Ice Sheet Disintegrates), if current warming trends continue, global temperatures will rise by around °C by the end of this century, leading to sea level rises and the possibility of major ice sheet disintegration. For this scenario we assume not only that Greenland ice disappears but also that the West Antarctic Ice Sheet (WAIS) disintegrates. If WAIS collapses the best estimate is that global __**sea levels would increase by another 3.3m.**__ The total sea level rise for this combination would be __**approximately 11m (36 feet.)**__//

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=IPCC (Intergovernmental Panel on Climate Change) -=  __**Global sea level rose by about 120 m**__ during the several millennia that followed the end of the last ice age (approximately 21,000 years ago), and stabilised between 3,000 and 2,000 years ago. Sea level indicators suggest that global sea level did not change significantly from then until the late 19th century. The instrumental record of modern sea level change shows evidence for onset of sea level rise during the 19th century. Estimates for the 20th century show that global __**average sea level rose at a rate of about 1.7 mm yr**__–1. Satellite observations available since the early 1990s provide more accurate sea level data with nearly global coverage. This decade-long satellite altimetry data set shows that since 1993, __**sea level has been rising at a rate of around 3 mm y**__r–1, significantly higher than the average during the previous half century. Coastal tide gauge measurements confirm this observation, and indicate that similar rates have occurred in some earlier decades. In agreement with climate models, satellite data and hydrographic observations show that sea level is not rising uniformly around the world. In some regions, rates are up to several times the global mean rise, while in other regions sea level is falling. Substantial spatial variation in rates of sea level change is also inferred from hydrographic observations. Spatial variability of the rates of sea level rise is mostly due to non-uniform changes in temperature and salinity and related to changes in the ocean circulation. Near-global ocean temperature data sets made available in recent years allow a direct calculation of thermal expansion. It is believed that on average, over the period from 1961 to 2003, thermal expansion contributed about one-quarter of the observed sea level rise, while melting of land ice accounted for less than half. Thus, the full magnitude of the observed sea level rise during that period was not satisfactorily explained by those data sets, as reported in the IPCC Third Assessment Report. During recent years (1993–2003), for which the observing system is much better, thermal expansion and melting of land ice each account for about half of the observed sea level rise, although there is some uncertainty in the estimates. The reasonable agreement in recent years between the observed rate of sea level rise and the sum of thermal expansion and loss of land ice suggests an upper limit for the magnitude of change in land-based water storage, which is relatively poorly known. Model results suggest no net trend in the storage of water over land due to climate-driven changes but there are large interannual and decadal fluctuations. However, for the recent period 1993 to 2003, the small discrepancy between observed sea level rise and the sum of known contributions might be due to unquantified human- induced processes (e.g., groundwater extraction, impoundment in reservoirs, wetland drainage and deforestation). Global sea level is projected to rise during the 21st century at a greater rate than during 1961 to 2003. Under the IPCC Special Report on Emission Scenarios (SRES) A1B scenario by the mid-2090s, for instance, __**global sea level reaches 0.22 to 0.44 m above 1990 levels, and is rising at about 4 mm yr**__–1. As in the past, sea level change in the future will not be geographically uniform, with regional sea level change varying within about ±0.15 m of the mean in a typical model projection. Thermal expansion is projected to contribute more than half of the average rise, but land ice will lose mass increasingly rapidly as the century progresses. An important uncertainty relates to whether discharge of ice from the ice sheets will continue to increase as a consequence of accelerated ice flow, as has been observed in recent years. This would add to the amount of sea level rise, but quantitative projections of how much it would add cannot be made with confidence, owing to limited understanding of the relevant processes. Figure 1 shows the evolution of global mean sea level in the past and as projected for the 21st century for the SRES A1B scenario //FAQ 5.1, Figure 1. Time series of global mean sea level (deviation from the 1980-1999 mean) in the past and as projected for the future. For the period before 1870, global measurements of sea level are not available. The grey shading shows the uncertainty in the estimated long-term rate of sea level change (__[|Section 6.4.3]__). The red line is a reconstruction of global mean sea level from tide gauges (__[|Section 5.5.2.1]__), and the red shading denotes the range of variations from a smooth curve. The green line shows global mean sea level observed from satellite altimetry. The blue shading represents the range of model projections for the SRES A1B scenario for the 21st century, relative to the 1980 to 1999 mean, and has been calculated independently from the observations. Beyond 2100, the projections are increasingly dependent on the emissions scenario (see __[|Chapter 10]__ for a discussion of sea level rise projections for other scenarios considered in this report). Over many centuries or millennia, sea level could rise by several metres (__[|Section 10.7.4]__).//
 * Frequently Asked Question 5.1**
 * Is Sea Level Rising?**