Wednesday 9 October 2011
Director of carbon solutions company Cleaner Climate Kerry Wright responds to the question of how climate change contributes to increased storm activity over the past few years.
The facts about sea level rise
Scientists unanimously accepted that climate change and sea level rise are causing an increase in the frequency and ferocity of coastal storms such as the March 2007 storm that ravaged KwaZulu-Natal, and the huge storm that hit the entire Cape coast at the end of August 2008. To save our waves, we have to save our planet.
Global sea level has fluctuated widely in the geologic past. It stood 4-6 meters above the present level during the last interglacial period, 125 000 years ago, but was 120 m lower at the peak of the last ice age, around 20,000 years ago.
Before the mid-19th Century, sea level had been almost constant for 3000 years, the rate of rise not exceeding a few tenths of a millimetre per year. Twentieth century sea level trends, however, are substantially higher than those of the last few thousand years. There was a marked acceleration with the onset of the industrial age and warming of the earth’s atmosphere. The sea level rose about 17cm during the past century as determined from tide gauges in coastal harbours. Since 1993, an even higher sea level trend of 2.8 mm/yr has been measured from the TOPEX/POSEIDON satellite altimeter.
Higher temperatures are expected to further raise sea level through melting mountain glaciers and small ice caps, causing portions of Greenland and the Antarctic ice sheets to melt. Temperature rise also causes the oceans to expand, so not only is sea level rise attributable to the increased melt water, but what is already there is expanding, and thus rising. The IPCC estimates that the global average sea level will rise between 0.6 and 2 feet (0.18 to 0.59 meters) in the next century (IPCC, 2007).
What happens when the ice caps melt?
In September 2008, for the first time in human history, open water stretched all the way round the Arctic, making it possible to circumnavigate the North Pole. Melting ice opened up both the fabled northwest and northeast passages, in the most important geographical sign of the unexpectedly rapid progress of global warming. Both passages had been blocked by ice since the beginning of the last Ice Age, 125,000 years ago.
Professor Mark Serreze, a sea ice specialist at the official US National Snow and Ice Data Center (NSIDC), said that it provided further evidence that the Arctic ice cap may now have entered a "death spiral". Some scientists predict that it could vanish altogether in summer within five years. Scientists say that if the ice continues to melt at present rates, it will be possible to sail across the North Pole. They have long regarded the disappearance of the ice cap as inevitable as global warming takes hold, though until recently it was not expected until around 2070. Many scientists now predict that the Arctic Ocean will be ice-free in summer by 2030 – and a landmark study concluded that there could be no ice as early as 2013. The tipping point, experts believe, was the record loss of ice last year, reaching a level not expected to occur until 2050.
Recent observations of Greenland and the West Antarctic Ice Sheet raise concerns for the future. Satellites detect a thinning of parts of the Greenland Ice Sheet at lower elevations, and glaciers are disgorging ice into the ocean more rapidly, adding 0.23 to 0.57 mm/yr to the sea within the last decade. The West Antarctic Ice Sheet is also showing some signs of thinning. Either ice sheet, if melted completely, contains enough ice to raise sea level by 5-7 m. A global temperature rise of 2-5°C might destabilise Greenland irreversibly. Such a temperature rise lies within the range of several future climate projections for the 21st century.
The data now available raise concerns that the climate system, in particular sea level, may be responding more quickly than climate models indicate. Overall, these observational data underscore the concerns about global climate change. Previous projections, as summarised by the Intergovernmental Panel on Climate Change (IPCC), have not exaggerated but may in some respects even have underestimated the change, in particular for sea level.
The positive feedback loop
Melting ice isn’t just about causing a sea level rise, there are serious climatic implications. Arctic sea ice reflects sunlight, keeping the polar regions cool and moderating global climate. The rate of climate warming over the land masses of northern Alaska, Canada, and Russia could more than triple during periods of rapid sea ice loss, according to a study by scientists at the National Center for Atmospheric Research and the National Snow and Ice Data Center. The findings, released in June, raise concerns about the thawing of permafrost, or permanently frozen soil. The study suggests that if sea-ice continues to contract rapidly over the next few years, Arctic land warming and permafrost thaw will accelerate.
Melting permafrost is a big deal. Arctic soils are believed to hold 30 percent or more of all the carbon stored in soils worldwide, and thawing permafrost would release additional greenhouse gases that would further accelerate global warming. Last year, air temperatures over land in the western Arctic were unusually warm, reaching more than 2°C above the 1978-2006 average. This warming could be related to loss of sea ice. Using sophisticated climate change simulations, the study found that when there was rapid sea ice loss, the rate of Arctic land warming is 3.5 times greater than the average 21st century warming rates predicted in global climate models. While this warming is greatest over the ocean, the simulations suggest that it can penetrate up to 900 miles inland, especially in autumn. The study concluded that a decade of rapid sea ice loss could see autumn temperatures warm by as much as 5°C along the Arctic coasts of Russia, Alaska, and Canada.
You certainly might see a few more waves being ridden up there.
The role of the oceans in global warming is complex. The oceans are a sink for carbon dioxide that would remain in the atmosphere, but increased levels of CO² have led to ocean acidification. In addition, as ocean temperature climbs, the sea becomes less able to absorb excess CO². More CO² left in the atmosphere. Besides rising sea levels due to thermal expansion and melting of glaciers and ice sheets, and warming of the ocean surface, increasing sea temperature will lead to increased temperature stratification that could result in large-scale changes in ocean circulation. Vital circulations such as the Gulf Stream could just abruptly stop, with huge repercussions.
Implications of a rising sea level, and how it exacerbates coastal storm activity
There is consensus that climate change has, and will continue, to contribute to increased frequency and ferocity of storms globally. The main driving mechanism for all storms is the condensation of water vapor releasing energy/heat. Temperature differences cause instabilities and drive winds, and unstable disturbances grow into powerful storms. Worldwide there has been an exponential increase in the frequency and violence of storm activity, from hurricanes in the US to typhoons in the Philippines and Japan, to right here in South Africa.
The potential of sea level rise enhancing the impacts of coastal storms is real. This is especially so when storms events occur during high tidal periods.
Let’s look at the storm that battered Kwa-Zulu Natal in March 2007: a combination of the Highest Astronomical Tide (HAT – currently occurring every 19 years), and an intense storm in the southwest Indian Ocean resulted in wave conditions equivalent to a 1 in 500 year storm.
At the most conservative estimate, rising sea levels as a result of the global warming are likely to add 0.4m to mean sea level by 2100. This will bring similar sea level conditions experienced during the HAT once every 19 years to once every two weeks (the normal lunar cycle), making the joint probability of high tide levels and an intense storm much more likely. Combined with the intensifying development of the South African coastline, the cost of damages is likely to rise substantially.
So there’s the witches brew….a higher frequency of bigger storms, coupled with an increased probability that they will coincide with HAT equivalent conditions as sea levels continue to rise.
References
Department of Environmental and Tourism Affairs, Long Term Mitigation Study http://www.deat.gov.za
Intergovernmental Panel on Climate Change (IPCC) http://www.ippc.ch/
The Pew Centre on Global Climate Change http://www.pewclimate.org
NASA Earth Sciences division http://pita.gsfc.nasa.gov/metadot/index.pl?iid=1801
National Centre for Atmospheric Research http://www.ncar.ucar.edu/
National Snow and Ice Data Centre http://www.nsidc.org/
August 2005 Storm
March 2007 Storm
August 2008 Storm
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