Klimaforskning - hjemmeside
Startet av Jostemikk, mars 08, 2012, 15:42:17 PM
SitatLike mercury in a thermometer, ocean waters expand as they warm. This, along with melting glaciers and ice sheets in Greenland and Antarctica, drives sea levels higher over the long term. For the past 18 years, the U.S./French Jason-1, Jason-2 and Topex/Poseidon spacecraft have been monitoring the gradual rise of the world's ocean in response to global warming. While the rise of the global ocean has been remarkably steady for most of this time, every once in a while, sea level rise hits a speed bump. This past year, it's been more like a pothole: between last summer and this one, global sea level actually fell by about a quarter of an inch, or half a centimeter. So what's up with the down seas, and what does it mean? Climate scientist Josh Willis of NASA's Jet Propulsion Laboratory, Pasadena, Calif., says you can blame it on the cycle of El Niño and La Niña in the Pacific.
SitatWillis said that while 2010 began with a sizable El Niño, by year's end, it was replaced by one of the strongest La Niñas in recent memory.
SitatBut for those who might argue that these data show us entering a long-term period of decline in global sea level, Willis cautions that sea level drops such as this one cannot last, and over the long-run, the trend remains solidly up. Water flows downhill, and the extra rain will eventually find its way back to the sea. When it does, global sea level will rise again. "We're heating up the planet, and in the end that means more sea level rise," says Willis.
SitatNine long and nearly continuous sea level records were chosen from around the world to explore rates of change in sea level for 1904–2003. These records were found to capture the variability found in a larger number of stations over the last half century studied previously. Extending the sea level record back over the entire century suggests that the high variability in the rates of sea level change observed over the past 20 years were not particularly unusual. The rate of sea level change was found to be larger in the early part of last century (2.03 ± 0.35 mm/yr 1904–1953), in comparison with the latter part (1.45 ± 0.34 mm/yr 1954–2003). The highest decadal rate of rise occurred in the decade centred on 1980 (5.31 mm/yr) with the lowest rate of rise occurring in the decade centred on 1964 (−1.49 mm/yr). Over the entire century the mean rate of change was 1.74 ± 0.16 mm/yr.
SitatABSTRACTEstimates of regional patterns of global sea level change are obtained from a 1° horizontal resolutiongeneral circulation model constrained by least squares to about 100 million ocean observations and manymore meteorological estimates during the period 1993–2004. The data include not only altimetric variability,but most of the modern hydrography, Argo float profiles, sea surface temperature, and other observations.Spatial-mean trends in altimetric data are explicitly suppressed to isolate global average long-term changesrequired by the in situ data alone. On large scales, some regions display strong signals although fewindividual points have statistically significant trends. In the regional patterns, thermal, salinity, and massredistribution contributions are all important, showing that regional sea level change is tied directly to thegeneral circulation. Contributions below about 900 m are significant, but not dominant, and are expectedto grow with time as the abyssal ocean shifts. Estimates made here produce a global mean of about 1.6 mmyr, or about 60% of the pure altimetric estimate, of which about 70% is from the addition of freshwater.Interannual global variations may be dominated by the freshwater changes rather than by heating changes.The widely quoted altimetric global average values may well be correct, but the accuracies being inferredin the literature are not testable by existing in situ observations. Useful estimation of the global averagesis extremely difficult given the realities of space–time sampling and model approximations. Systematicerrors are likely to dominate most estimates of global average change: published values and error barsshould be used very cautiously.
SitatAlthough intense interest exists in the global averagevalue of sea level change, and the plausible inference ofan average rise, actually obtaining a useful result provesextremely difficult. If errors in the altimetric data arefully understood (not clear), estimates of an averagerise near 3 mm yr (e.g., CN2004) are sensible, butcurrently untestable against in situ datasets. Severalproblems exist:
SitatThe TOPEX/Poseidon mission is nearing the completion of its twelveth year. The remarkable length of the record implies that the global rate of sea level change can be estimated from this single altimeter with striking reliability. The currently accepted value is 2.5±0.5 mm/year.However, every few years we learn about mishaps or drifts in the altimeter instruments,errors in the data processing or instabilities in the ancillary data that result in rates of change that easily exceed the formal error estimate, if not the rate estimate itself. In all these cases the intercomparision with external sources, mainly contemporary altimeter satellites, like ERS-1 and ERS-2, were pivotal to the uncovering and correction of the problems.With the missions of Jason-1 and Envisat now on the way for a few years, more differences between the missions pop-up. Neither of these missions currently ﬁt the established rates. It seems that the more missions are added to the melting pot, themore uncertain the altimetric sea level change results become.This presentation highlights a number of issues in altimeter data and their processing that have an impact on the global rates of sea level change, such as: geographical sampling, temporal sampling, various forms of altimeter drift, wet tropospheric correction, and other models. Ultimately, this exercise will tell us more about what we know about altimetric sea level change, and what not.
SitatAbstractWithout sea-level acceleration, the 20th-century sea-level trend of 1.7 mm/y would produce a rise of only approximately 0.15 m from 2010 to 2100; therefore, sea-level acceleration is a critical component of projected sea-level rise.[/b] To determine this acceleration, we analyze monthly-averaged records for 57 U.S. tide gauges in the Permanent Service for Mean Sea Level (PSMSL) data base that have lengths of 60–156 years. Least-squares quadratic analysis of each of the 57 records are performed to quantify accelerations, and 25 gauge records having data spanning from 1930 to 2010 are analyzed. In both cases we obtain small average sea-level decelerations. To compare these results with worldwide data, we extend the analysis of Douglas (1992) by an additional 25 years and analyze revised data of Church and White (2006) from 1930 to 2007 and also obtain small sea-level decelerations similar to those we obtain from U.S. gauge records.
Sitat fra: Jostemikk på mars 11, 2012, 16:15:33 PMHvorfor forsøker hele tiden mainstreams klimaautoriteter å fortelle verden det motsatte av virkeligheten?
Sitat fra: Jostemikk på mars 11, 2012, 16:15:33 PMSe ekstra nøye på 1998. Det var "Super-El Ninoens år". Denne El Ninoen startet allerede i 1997, mens slutten på 1998 var et skifte til La Nina.Havet sank mens havet tømte seg for varme. Så skjer nøyaktig motsatt i 1999 og framover. Da opplever vi en La Nina, og havnivået stiger igjen fordi La Nina fører til at havet atter en gang varmes opp.
Sitat fra: Okular på april 23, 2012, 16:17:34 PMSitat fra: Jostemikk på mars 11, 2012, 16:15:33 PMSe ekstra nøye på 1998. Det var "Super-El Ninoens år". Denne El Ninoen startet allerede i 1997, mens slutten på 1998 var et skifte til La Nina.Havet sank mens havet tømte seg for varme. Så skjer nøyaktig motsatt i 1999 og framover. Da opplever vi en La Nina, og havnivået stiger igjen fordi La Nina fører til at havet atter en gang varmes opp.Veit dette blir et svar på et litt gammelt innlegg, men ...Grunnen til at havnivået synker under El Niño og stiger under La Niña i de vestlige delene av Stillehavet (hvor alle lokalitetene på figuren din hører hjemme (vel, et par ligger vel nærmere midten, så som Kiribati)) er ikke at havet lagrer varme under en La Niña og frigir det under en El Niño (selv om det jo stemmer at det er det som skjer - i Stillehavet, vel og merke, ikke i verdenshavet for øvrig; der er det motsatte tilfelle). Det er fordi vindene under en La Niña presser overflatevannet i det tropiske Stillehavet mot vest (Pacific Warm Pool) samt snur og drar det mot øst under en El Niño.