A study published in March 2012 in Environmental Research Letters discussed the likely results of projected sea level rise (SLR) on coastal areas of the United States in the near future. In the write-up, Claudia Tebaldi, senior scientist at Climate Central, maintains that the potential multi-meter sea level rise (SLR) in coming centuries “threatens permanent submersion or displacement of extensive coastal land, infrastructure and ecosystems.”
But even the shorter-term SLR expected in coming decades, Tebaldi finds, “will still impact low-lying coastal areas by augmenting the water levels reached by storm surges.”
The study predicts that:
The frequency of surges currently reaching a given height will thus increase… Equivalently, the water level associated with any given frequency will grow, and communities should expect to see waters reach progressively new heights. These trends will very likely force changes in risk assessments related to extreme events, such as the delineation of 100 yr floodplains, that influence coastal policy and development…
However, that same month, climate-change critic Christopher Monckton gave a presentation to the California State Assembly alleging that dire predictions about sea level rise are overblown. His presentation included a slide (no. 39) stating that the “sea level is rising at just 1.3 inches per century.”
Monckton is talking specifically about short-term sea level trends. In a blog comment, he expands:
The IPCC predicts a [one-to-two-foot] sea-level rise by 2100, with a best estimate [of 1 feet 5 inches]. Yet for the past eight years sea level has been rising at a rate equivalent to just 1.3 inches per century, and sea level last year was lower than in any of the previous seven years.
Why such a discrepancy? Why do some knowledgeable people predict calamitous sea level rise, whereas others claim it’s no big deal?
Long-Term Picture Is Most Important
Monckton, clearly, is talking about sea level change only in the past eight years. And, as I pointed out in another article, climate change takes place on various time scales (see “The Climate Change Controversy — Are Global Temperatures Really Increasing.”) So when you examine trends around things like global temperatures and sea levels, you need to pay attention to the length of time.
Roy Spencer, a climatologist and well-known dissenter of human-caused climate change, has written that it’s hard to tell through short time scales whether global temperatures are really rising:
There is no way to know, because natural year-to-year variability in global temperature is so large, with warming and cooling occurring all the time. What we can say is that surface and lower atmospheric temperature have risen in the last 30 to 50 years, with most of that warming in the Northern Hemisphere… But there is no way to know if temperatures are continuing to rise now — we only see warming (or cooling) in the rear-view mirror, when we look back in time.
The Intergovernmental Panel on Climate Change (IPCC) 2007 report included the sea level chart shown here, going back to the 1870s. After looking at results from multiple studies, IPCC assessed the rate of sea level rise for the 20th century at 1.8 millimeters per year, plus or minus 0.5 mm.
However, the rate of sea level rise appears to be accelerating.
The European Space Agency’s (ESA) environmental satellites generated the sea level chart show below, representing about the last 20 years. According to the agency, the rate of mean sea level rise since 1993 has been about 3.18 mm per year. Extrapolating that rate would result in 318 millimeters — or 12.5 inches — per century. That figure is a mean; ESA points out that sea levels exhibit “marked regional differences.” For example, easterly trade winds cause sea levels to mound up higher in the western Pacific during normal conditions. The west-to-east difference in sea level decreases during El Niño years and increases during La Niña years.
In fact, this ESA satellite data, showing a greater-than-3-mm/yr sea level rise, is the same data to which Monckton referred in his California State Assembly presentation, where he asserted that “sea level is rising at just 1.3 inches per century.” The long-term trend tells a very different story; sea levels are rising almost 10 times as fast as reported by Monckton.
Below is another sea level chart, this one from the University of Colorado, based on satellite data. You can see how the discrepancy in rates of sea level rise can vary depending on the time frame with which you analyze. If you only consider the past six or eight years, the mean sea level rise will appear lower, because your mean will be affected by the downward spikes in 2008 and 2011. Look at a longer-term trend, back to 1993, and you see a steady SLR of about 3.1 mm/yr, similar to the results from the ESA data.
The problem with focusing on short-term trends is that sea level can vary a lot from year to year, as water gets swapped out between the oceans and the continents. An August 2011 report from NASA shows how the switch from El Niño to La Niña conditions from 2010 to 2011, respectively, actually caused global sea levels to drop 5 mm in 2010.
Because of the strong La Niña, rain piled up on the continents, for example in Australia, the Amazon, and the southern United States. All that rain came from ocean evaporation. In the NASA report, Carmen Boening, an oceanographer from Jet Propulsion Laboratory (JPL), was quoted, “This year, the continents got an extra dose of rain, so much so that global sea levels actually fell over most of the last year.”
Also in the report, JPL climate scientist Josh Willis cautioned those who think that global sea levels are declining that it is a short-term phenomenon. He noted that sea level drops such as the one in 2010 cannot last and that the trend remains solidly upward over the long run. Water flows downhill, so the 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,” said Willis. “But El Niño and La Niña always take us on a rainfall roller coaster, and in years like this they give us sea-level whiplash.”
Troubled Waters Ahead?
Lee R. Kump and colleagues, in The Earth System (Prentice Hall, 2010), wrote that “the increase in global sea level during the 20th century parallels the rise in global mean surface temperature” and that, because water normally expands as it warms, “approximately half of the rise in sea level can be attributed to simple thermal expansion of surface-ocean water.” Climate scientists believe that the remaining sea level rise is due to the melting of ice sheets and glaciers.
How much sea levels will rise in the future is hard to predict. The effects of both thermal expansion and melting of ice will vary depending on how much temperatures actually rise. Also, it’s hard to predict in advance what will happen with the major ice sheets on Greenland and Antarctica. But the potential is evident from what Kump wrote:
The Greenland ice sheet contains enough water to raise sea level by approximately 7 m, were it to melt entirely. The Antarctic ice sheet contains much more water — some 60 to 70 m of equivalent sea-level rise.
IPCC views such a scenario as unlikely, though, as we see from its 2007 report:
Abrupt climate changes, such as the collapse of the West Antarctic Ice Sheet, the rapid loss of the Greenland Ice Sheet or large-scale changes of ocean circulation systems, are not considered likely to occur in the 21st century, based on currently available model results. However, the occurrence of such changes becomes increasingly more likely as the perturbation of the climate system progresses.
The ESA says, in its background materials on sea level rise:
The global mean level of the oceans is one of the most important indicators of climate change. It incorporates the reactions from several different components of the climate system. Precise monitoring of changes in the mean level of the oceans, particularly through the use of altimetry satellites, is vitally important, for understanding not just the climate but also the socioeconomic consequences of any rise in sea level.
[Editor's Note: This article is the second in a series examining key arguments about climate change. The two other articles examine whether global temperatures are really increasing and whether Arctic sea ice is really shrinking.]