PART II: This is the second of a three-part series on global climate science and its relationship to global warming. Part 3 will address atmospheric and oceanic circulation.
The geological record shows the sea level has risen by 120 meters over the past 20 kilo-years (KYs). This rise was non-linear (see table). Values ranged from 2 mm/year to 12 mm/year. A recent graph of estimated sea level rise (SLR) rates, on a 250-year time unit, showed a maximum rate of about 18 mm/year.
The range of uncertainty in sea level rise over the past century is significant.
In addition, there are undoubtedly major spikes in the SLR rate that occurred due to melt water pulses. An example of such an event was when ancient Lake Missoula in Idaho and Montana (the size of Lake Erie formed by an ice dam) gave way about 16 KYs ago. When such an event occurs, the SLR rates likely exceed a rate of 100 mm/year for a few weeks.
For the contemporary period, some reports have claimed rates as high as 8 mm/year, but such rates have proven to be erroneous. Still other reports have argued the rate has been no more than 1 mm/year over the last 200 years. The 1995 UN report (The Intergovernmental Panel on Climate Change report) provides a reasonable perspective, with a range of sea level rise rate observations of 1.0-2.5 mm/year over the past century.
Past century changes
A measurement of sea level is confounded by several factors. These include coastal subsidence, the Earth's crustal rebound, and oceanic-atmospheric interactions such as the El Niño. Crustal rebound is still underway from the compression applied by ice during the last Ice Age. This has to be isolated from measurements taken from tidal gauges. And El Niño can lead to water being pushed substantially higher in parts of the Pacific.
The range of uncertainty in SLR over the past century is very wide for both the observed and the estimated rise. For the observed rise, we see an uncertainty range of 15 cms. This is based on estimates for low, middle, and high values of 10, 18 and 25 cms.
The authors of the UN report ask, "Can this sea level rise over the past century be explained?" Below are estimates for each of the SLR components along with comments by the UN:
- Thermal expansion: Low, middle and high estimates were 2, 4 and 7 cms. They noted that observational data are still too sparse to make global scale estimates. With a UN-estimated average global temperature rise of 0.3-0.6°C, some thermal expansion would be likely.
- Changes in surface/ground water: Estimates are very uncertain and speculative (-5.0, 0.5, and +7.0 cms.
- Glaciers/small ice caps: While many of the world's glaciers have retreated, there are mixed signals in the data. Further, continuous long-term measurements of the mass balances are very limited. Based on data availability and simple models: 2.0, 3.5, and 5.0 cms.
- Greenland ice-sheet: There is insufficient evidence from models/data: -4.0, 0.0, and +4.0 cms.
- Antarctic ice-sheet: There is insufficient evidence from models/data: --14.0, 0.0, and +14.0 cms.
Note that the zero values above should be interpreted as a measure of the current poor state of knowledge. The total estimated rise thus has values of -19.0, 8.0, and +37.0 cms, a range of 56 cms.
Clearly there is still uncertainty, both in our ability to measure these recent sea level changes, and also in our ability to analyze the situation and estimate these changes. Indeed, the estimated uncertainty is nearly four times greater than the observed uncertainty.
Next 100 years
The huge uncertainty seen in the above SLR estimates does not bode well for our ability to forecast the future. Just as global warming forecasts for temperature increases have fallen over time, so have those for sea level rise. Forecasts have declined since the early 1980's, when a 7-8 meter rise was cited (for the year 2130). This level fell to less than 1 meter by 1990, and to less than a half a meter (49 cms) in 1995. Today, it is down to as low as 21 cms. Forecasts of future SLR suffer not only from a weak understanding of the natural processes involved, but also from a less than perfect picture of the likely future global warming.
A comparison can be made (see table) between the maximum rise (complete melting of the cryosphere) against the current UN forecast. The UN forecast's warming is set at 2 1/2°C, a level many skeptics would argue is too high. This level is the assumed temperature response due to a doubling of the greenhouse gas concentration. SLR contributions for three components were:
- Antarctic ice-sheet: A complete melting here would lead to a 76-meter rise. Yet the UN prediction for the next century is for an actual reduction of 1cm, based on higher water vapor levels occurring with the assumed global warming.
- Greenland ice-sheet: Here, the ultimate rise would be 7.6 meters, versus a forecast of only 6 cms.
- Glaciers/small ice caps: Here the ultimate sea level rise is small, only 0.5 meters, whereas the forecasted rise is now up to 16 cms.
Bringing these together, we see that the UN forecast for these three components for the next century is 21 cms, versus the ultimate potential of 84.1 meters. This is less than 0.25% of the ultimate, hardly an apocalyptic outlook. Adding in a thermal expansion contribution of 28 cms, we get an overall forecast of 49 cms.
Location map of the Larsen B Ice Shelf in Western Antarctica (EAIS - Eastern Antarctica Ice Shelf; WAIS - West Antarctica Ice Shelf).
In spite of this outlook of lowered SLR forecasts and of huge uncertainties in the estimates, this area is the center of hype and fear mongering. Perhaps the area where fear mongering is used the most is the Antarctic ice reservoirs, and the potential SLR that would result from a complete meltdown.
Two areas are singled out to illustrate this distortion. These are the Larsen Ice Shelf and the West Antarctic Ice Sheet (WAIS). Note the difference here between an ice shelf and an ice sheet. An ice sheet defines the ice reservoir over land that represents over 97% of the ice volume. In contrast, an ice shelf is ice floating at the edge of an ice sheet. Melting of an ice shelf would not contribute to sea level rise as shelves are already displacing an equal volume of water.
The Larsen Ice Shelf is situated next to the Antarctic Peninsula, the land that stretches north towards Chile and Argentina. As an ice-shelf, it is frozen to the ice sheet on the peninsula. This shelf is hardly representative of the conditions in Antarctica. First, it is substantially north of the South Pole, and situated above the Antarctic Circle. Secondly, an ice shelf is impacted severely by ocean currents. And finally, it is an ice shelf. Yet, there have been many articles about loss of a piece of this shelf.
One of the more distressing aspects of this published information is how little the public is made aware that an ongoing debate exists in the scientific community on this subject. Frequently, the headlines come down on the side of anthropogenic warming as the villain.
The real cause of any instability may turn out to be natural, due to the presence of geothermal heat at the base of the ice sheet. For example, there is a lake under the ice sheet, called Lake Vostok. This lake is 2 1/2 miles below the ice surface. It is about the size of Lake Ontario and is over 1,500 ft deep. The lake exists due to geothermal energy.
One can conclude that the odds are much higher that WAIS instability, if it exists, will turn out to be nature-based. The UN report notes that if collapse ever occurs, it will probably be due more to climate changes over the last 10,000 years, rather than any contemporary global warming.
Analysis of each of the above subjects is far from complete. The assessments to date do not represent any sort of apocalypse. There is a frequent imbalance between headlines and the facts. This subject of global warming is so complex that the media needs to be more cautious and precise, and to place this subject into context. And it needs to be on a constant vigil against those who would take advantage of it to further their own political agenda.
There is considerable agreement that the Earth has seen a modest amount of surface warming over the past century, due either to anthropogenic or natural causes. Although our knowledge of this subject is very incomplete, there is a reasonable probability that this trend will continue and we should not be surprised at a modest sea level rise.
One key question that remains to be answered is what fraction of any rise is due to natural warming and what is due to anthropogenic warming?
In the meantime, areas that may be vulnerable need to be prepared for a 1-2 foot rise over the next century. This may mean doing nothing in some areas to building or extending dikes and seawalls in other areas. It may mean discouraging seaside dwellings in some areas or relocation of some populations in other areas.