From about a thousand years ago through to the mid 19th Century, global sea levels remained remarkably stable. Together with overall global temperatures, sea surface heights stayed at about the same levels until the late 1800s. At that time, an initiation of large-scale burning of oil, gas and coal dumped heavy volumes of greenhouse gasses into the atmosphere. The Earth System began to warm and seas began a slow upward climb.
(Global sea level rise since 1870. Image source: Dr. James Hansen.)
At first, the pace of sea level rise was minor — only hitting about 0.8 mm per year. But then, by around 1925, the rate of sea level rise more than doubled to 1.9 mm per year. The oceans, which at first only slowly accumulated heat, began a long term warming which eventually extended through almost every depth and region. This pace maintained until about 1992 when the oceans again hit a higher rate of rise at around 3.1 mm per year — a pace that then included a small but ominously growing portion of glacial melt.
Now, it appears that global warming is again pushing sea levels to rise even faster. As, over recent years, a number of ominous indicators pointed toward yet another surge in ocean surface levels.
In south Florida, the pace of sea level rise at local tidal gauges, by last year, had gone exponential. Along the U.S. East Coast, a sudden jump in sea level during recent years was blamed on a slowing down of the Gulf Stream due to freshwater melt pulses hitting the North Atlantic.
All over the world’s frozen regions, the great land glaciers — especially in Greenland and Antarctica — have been destabilizing. Melting, cracking, and clamoring as their gargantuan, mountain-like forms assembled in an ever-speeding march to the seas. This great rush of freshwater melt and ice is already causing an ocean-threatening slow-down of Atlantic circulation. And in the Southern Ocean surrounding Antarctica an ominous bulge of water near the southern polar zone became an indicator of an increasing rate of melt from some of the largest glaciers on Earth. A bulge that was 2 centimeters higher than the global average along melting and thawing Antarctic shores.
Global Sea Level Rise On Upward Curve?
Recently, the global sea level rise measure — AVISO — also took an unsettling leap. With satellite captures of the world ocean showing a strong surge in sea level rise throughout 2014 and into 2015. A spike that displays vividly as a hockey-stick like jog at the tail end of the measure below:
(Big spike in sea level rise plainly visible in the AVISO measure. Image source: AVISO.)
It’s an upward jump representing nearly a 1 centimeter spike in the rate of sea level rise over the past six months.
By itself, this jump in sea level would be something to worry over. But new findings paint an even starker picture. For a recent study, headed by Shuang Yi and published on April 30 in Geophysical Research Letters provides evidence that, since 2010, annual rates of global sea level rise have shown a strong uptick. The study, entitled An Increase in the Rate of Global Mean Sea Level Rise Since 2010, notes:
The global mean sea level (GMSL) was reported to have dropped 5 mm due to the 2010/11 La Niña and have recovered in one year. With longer observations, it is shown that the GMSL went further up to a total amount of 11.6 mm by the end of 2012, excluding the 3.0 mm/yr background trend. A reconciled sea level budget, based on observations by Argo project, altimeter and gravity satellites, reveals that the true GMSL rise has been masked by ENSO-related fluctuations and its rate has increased since 2010. After extracting the influence of land water storage, it is shown that the GMSL have been rising at a rate of 4.4 ± 0.5 mm/yr for more than three years, due to an increase in the rate of both land ice loss and steric change.
In short, the study finds an average rate of sea level rise of 4.4 mm per year, or 30% faster than the annual rate from 1992 to 2009, during the period of 2010 to 2013. For these, more rapidly rising, sea levels the study identifies clear causes. The first is an increasing rate of land ice loss. The second is what is termed as ‘steric change’ — a scientific phrase that both identifies ocean thermal expansion due to warming combined with changes in ocean salinity, which also impacts sea surface height.
The April 30 study did not include the more recent sea level rise spike now showing up in the AVISO measure. So, at least for now, sea levels do appear to be sliding up some rather dangerous curves.
Hitting the More Difficult Rates of Sea Level Increase
Such a jump has stark implications for sea level by end century. A 4.4 mm per year rate of rise would equal just less than half a meter of increased sea level within one Century. This compares to the previous rate of rise which would have resulted in a 1 foot global jump within a one hundred year span.
A jump of this kind was, however, predicted with sea level rise by end of this Century expected to hit between 0.5 and 1 meters of increase in the IPCC measure and between 5 and 6 feet in US Coast Guard studies (most studies find a range between 3-9 feet for this Century). The 4.4 mm per year increase is rather ominous in that it already puts annual rates of rise in the IPCC mid-range. An early ramp up with fully eight and a half decades left to go in a Century that will certainly see substantial further increases in global heat accumulation.
(South Florida 6 meters of sea level rise before [left frame] and after [right frame]. Note that second image is an artist’s rendering based on flood analysis showing what a 6 meter sea level rise would look like for South Florida, should it occur. Image source: Tropical Audobon Society.)
Many planners use the IPCC measure or even more conservative indicators to prepare for sea level rise at their city, county and state shores. And the fact is these indicators may fall well short of reality at the coastlines. A stark circumstance that will become more and more difficult to manage as time moves forward.
Overall, a 2010 ramping in the rate of sea level rise is a bit soon. Similar further jumps leading up to potential worst case 1-4 cm per year levels would initiate a combination of dangerous impacts including untenable rates of rise for coastal regions, severe shocks to ocean circulation systems and overall ocean health, and potentially very dangerous impacts to the world’s weather. To this point Hansen’s paper entitled ‘Greenland Ice Sheet Mass Loss, Exponential?‘ is well worth a (re)read.
Similar Climate Conditions Saw 20 Meter Surges in Sea Level Due to Glacial Melt
With current greenhouse gas levels now in the range of 400-405 parts per million coinciding with substantial jumps in glacial melt and sea level rise, it may be worth taking a look back at times in the geological past when atmospheric heating conditions were similar to those seen today. The last time heat trapping gasses were seen at such high concentrations was at the height of the Pliocene warming 3-5 million years ago. That time saw temperatures in the range of 2-3 degrees Celsius warmer than Holocene averages. It was also a geological period that saw Antarctic and Greenland melt events that pushed seas up to 20 meters higher.
We are exceeding maximum Pliocene atmospheric CO2 thresholds at this time (well exceeding if you count in a 485 CO2 equivalent forcing from all greenhouse gasses added by human beings). And we will almost certainly enter Pliocene warming levels this century. So the melt pressure we are putting on the world’s ice sheets is likely to at least be in the 20 meter range for the (hopefully) longer term.
Hat tip to Colorado Bob
Hat tip to Wili