The creeping menace

Remember the song… "New York, New York, is a helluva town / The Bronx is up and the Battery's down..."

Should we add the words …"A shame the city is going to drown…" ?

The problem is that much of New York is less than 5m above sea level. In fact many parts of the city are less than 2m above sea level and that includes large areas of Coney Island, Queens, Brooklyn, the Bronx and Staten Island, as well as neighboring cities in New Jersey. This places dwellings, port facilities, roads, bridges, subways, airports, industrial and other structures at risk of inundation when sea levels rise above 1m, less if accompanied by a winter storm surge or hurricane.

New York is not the only city threatened by flooding due to sea level rise. It will eventually affect Baltimore, Boston and Halifax, even Washington, threatening hundreds of thousands of houses, displacing millions of people and causing massive social and economic dislocation. US Federal, State and City governments are well aware of this threat but rightly feel it poses no short-term danger, while admitting it will occur.

Just as serious is the effect of sea level rise on the coastline. Bruuns Rule states that on average each 1cm of sea level rise results in about 1m of coastal recession. In other words, for each metre of sea level rise, the coastline is eroded by 100m threatening property, infrastructure and land located near and within 100m. of the existing coastline.

Risk of flooding is not going to suddenly become a looming danger. Sea level rise occurs very slowly and there should be plenty of warning of any impending threat. So why worry? New York and other great cities along America’s north-east coast are safe from flooding by rising sea level for at least a century, aren’t they? Well, probably not. If global warming continues unabated, flooding is likely to occur much sooner than previously thought.

Prior to the industrial revolution, sea level is estimated to have risen at about 0.2mm/year. In the 19th and 20th century, it reached an average rate of about 1.7 mm/year. From 1961 to 2003 the rate was 1.8 mm/year and increased to 3.1 mm/year from 1993 to 2003. This rate will continue to increase as ocean surface and air temperatures rise further, primarily due to the greenhouse and ice albedo feedback effects. These are bringing about acceleration in the four principal causes of sea level rise, which are:

· Melting of mountain based snow and ice, excluding polar ice caps, may elevate see level by 0.5-1m. and is expected to have fully melted by 2200.

Snow and ice permanently cover mountain ranges at higher altitudes and lower land at higher latitudes. Some of this cover melts seasonally or flows into glaciers which move it to the sea or a warmer altitude where it melts, producing water flows into lakes, riversand aquifers. These losses are normally compensated for by new snowfall and ice formation.

Global warming is causing this snow and ice to melt more quickly than it is being replaced and in many locations precipitation is falling as rain rather than snow. The result is that with few exceptions, the vast majority of the earths 150,000-160,000 mountain glaciers are in retreat and the area of land covered by snow and ice is shrinking.

Both events are occurring at an increasing rate for two reasons. First: solar energy hitherto reflected back into space is trapped by the greenhouse effect increasing air and surface temperatures. Second: land and sea which has lost its cover of snow or ice is now absorbing solar radiation, increasing their temperature.

· Melting of Ice Caps:

The Greenland Ice Cap has the potential to raise sea levels by about 7m. and is presently loosing over 250 gigatonnesof ice per annum. 1 gigatonne = 1 cubic kilometre of water.

The cap is melting at all its margins because global warming has caused melting of sea and land based snow and ice, with the result that solar energy is being absorbed, heating land on which the ice cap rests. In addition, increasing atmospheric CO2 is radiating solar energy reflected by the ice cap back on to it.

The result is that snow falling on the ice cap is melting rather than compacting into new ice. In the south precipitation previously falling as snow, is more frequently falling as rain. Combined, these cause the formation of lakes on the surface of the ice cap. This water eventually erodes the lake bed, seeping through it until it reaches bed-rock on which the ice cap rests, providing lubricant enabling the ice to move, become stressed, crack, break-up and discharge into glaciers draining into the sea.

The effect is that a growing number of Greenland glaciers have less impeded access to the sea enabling them to move more rapidly, draining greater volumes of ice from the interior of the ice cap. In recent years an increase of over 7 percent per annum in the volume of ice discharged into the sea has been recorded by the GRACE satellites.

The West Antarctic Ice Cap has the potential to raise sea level by 6-9m. and is currently loosing over 130 gigatonnes of ice per annum and accelerating.

Over 400 glaciers on the Antarctic Peninsula are in retreat and in summer, snow covering large areas of adjacent land melt permitting absorption of solar radiation. Collapse of the Larsen B. ice shelf in 2002 has enabled many glaciers draining the Peninsula ice cap to significantly increase in speed and discharge much greater quantities of ice into the Southern Ocean.

The West Antarctic ice cap is a marine ice sheet largely grounded on the sea bed and is highly susceptible to rapid melting both on its surface and underside. The latter will result in erosion and weakening of the ice anchoring the cap to the sea floor. When this occurs it will cause large portions of the cap to break-away at the margins and float, precipitating rapid sea level rise.

Glaciers draining the largest portion of the West Antarctic Ice Cap are blocked from direct access to the sea by the massive Rönne and Ross ice shelves, slowing the amount of ice they discharge into the sea. The seaward edges of both ice shelves show significant susceptibility to warming temperatures, causing the calving of massive icebergs.

These ice shelves will eventually collapse permitting more rapid discharge of glaciers and melting of the ice cap. Evidence for this is seen in the massive increase in the flow of glacier discharge into the Amundsen Sea, particularly from the Pine Island Glacier. Stability of the ice cap over the next 20 years is far from certain and gives rise to speculation that the entire ice cap could rapidly melt causing it to disintegrate within a few hundred years, rather than millennia, contributing to relatively rapid rise in sea level.

The East Antarctic Ice Cap, has the potential to raise sea levels by 55-70m. It is currently losing 50-70 gigatonnes per annum. That rate is increasing.

The East Antarctic Ice Cap is entirely land based. Until a few months ago, it was widely accepted that temperatures were far too low to permit any part of the ice cap to melt. At the very worst, the ice cap was unaffected by global warming and at best it was actually growing.

Given that precipitation over Antarctica is very low producing very little new ice and that much of its coastline is exposed to warming sea and air temperatures, those conclusions were always optimistic and have now been proven wrong by GRACE satellite measurements. Global warming is already melting the ice cap at its fringes and traces of lakes having formed on the surface of the ice cap and draining away through the ice have been found, indicating similar dynamics to those evident in Greenland.

Thermal expansion of seawater occurs as a result of rising ambient temperatures, particularly those affecting the top 500m. It is regarded as having contributed up to 50 percent of the rise in sea level that has occurred over the last century.

Scientific consensus is that on current levels of green-house gas emissions temperatures could rise by 5-6C and that to limit warming to 2C, anthropomorphic greenhouse gas emissions must peak by 2015 and reduce to near zero by 2050. Refusal of the largest emitters, the USA, China, India and Japan to commit to those reductions makes a global warming limit of 2C by 2100 impossible.

Given these considerations, rise in sea level by 2100 due solely to thermal expansion is estimated to be between, at best 75mm and 255mm at worst.

Extreme climate events which have a significant effect on local sea level include storm surges, king tides, cyclones/hurricanes and temperature increase (and decrease), particularly in polar regions where temperature rise tends to be double the global average. Increased sea temperature is likely to increase the incidence of hurricanes and their severity.

Expert evidence given to the Australian House of Representatives Committee on Climate Change, paragraph 2.60-64, warn that as sea level rises, so does the incidence of dangerous climate events increasing the effects of sea level. An increase in sea level of 0.2m is likely to result in a 10 fold increase in the number of severe climate events. An increase of 0.5m could increase the number of such events to one a day, vastly exacerbating the dangers arising from elevated sea level, causing it to rise further - a feedback situation.

Taking into account 2009 National Centre for Atmospheric Research estimates, it may well be at least 25-50cm higher along the NE coast of America. In summary, by 2050 many of the great cities of the New England States and all of their coastline could be exposed to a rise of 1.0-1.4m in sea level.

Expert evidence given to the Committee concluded that global sea level could rise by 1.4m by 2100 but that it could be substantially higher (para 24.9). This paper concludes that on the basis of currently available data global sea level is likely to rise by at least 0.9m by 2050 and probably by over 2.0m by 2100.

It would be imprudent for governments and other organisations to assume that planning and implementation of mitigating action is not urgent. It can not wait until the latter half of the century because the scientific consensus on sea levels rising by 1.1m by 2100 is likely to become a reality 50 years sooner.

New York, New York is a helluva town and it would be catastrophic if it were to drown. But it will, within 40-50 years, unless global action is taken to ensure greenhouse gas emissions reach a peak no later than 2015 and increase in pre-industrial temperature is kept below 2C. Those outcomes seem unlikely.