Global Warming: A Sea Change

“The coastlines of the United States and the world are major centers of economic, social, and cultural development, and coastal areas are home to critical ecological and environmental resources. Climate change poses a number of risks to coastal environments. Foremost among these is sea level rise, which threatens people, ecosystems, and infrastructure directly and also magnifies the impacts of coastal storms.” — National Research Council (2010)

Sea Level Rise:

Currently, about 160 million people live in locations that are 1 meter above sea level or lower, therefore, even small changes in sea level can greatly impact millions of people around the world. Sea level gradually rose in the 19th and 20th centuries and is currently rising at an increased rate, after a period of little change for the 6,000 years before the year 1800 (NRC, 2010, IPCC, 2007). When climate warms, ice on land melts and flows back into the oceans raising sea levels. Also, when the oceans warm, the water expands (thermal expansion) which raises sea levels.

According to The Copenhagen Diagnosis (2009):

“Satellites show great global average sea-level rise (3.4 mm/yr over the past 15 years) to be 80% above past IPCC predictions. This acceleration in sea-level rise is consistent with a doubling in contribution from melting of glaciers, ice caps and the Greenland and West-Antarctic ice-sheets. By 2100, global sea-level is likely to rise at least twice as much as projected by Working Group 1 of the IPCC AR4, for unmitigated emissions it may well exceed 1 meter. The upper limit has been estimated as 2 meters sea-level rise by 2100. Sea-level will continue to rise for centuries after global temperature have been stabilized and several meters of sea level rise must be expected over the next few centuries.”

Since 1870, global sea level has risen by about 20 centimeters (IPCC, 2007). Since 1993, sea level has been accurately measured globally from satellites. Before that time, the data come from tide gauges at coastal stations around the world. Satellite and tide-gauge measurements show that the rate of sea level rise has accelerated. Statistical analysis reveals that the rate of rise is closely correlated with temperature: the warmer it gets, the faster sea level rises (Rahmstorf 2007). For the period 1961-2003, thermal expansion contributed ~40% to the observed sea level rise, while shrinking mountain glaciers and ice sheets have contributed ~60% (Domingues et al., 2008). Fig. 1 (Copenhagen Diagnosis, 2009) shows the trend since 1970.

Fig. 2 (Colorado Center for Astrodynamics Research) shows the current sea level change data using seasonally adjusted values from TOPEX and Jason.

Figure 2: Current satellite-measured sea level change

Based on a number of studies since the IPCC 2007 reports, the synthesis document of the 2009 Copenhagen Climate Congress (Richardson et al., 2009) concluded that “updated estimates of the future global mean sea level rise are about double the IPCC projections from 2007.” Sea level will continue to rise for many centuries after global temperature is stabilized, since it takes that much time for the oceans and ice sheets to fully respond to a warmer climate. Some recent estimates of future rise are shown in Fig. 3 (Copenhagen Diagnosis, 2009). A business as usual approach to emissions results in global warming that is likely to raise sea level by several meters in coming centuries, leading to the loss of many major coastal cities and entire island nations.

Figure 3: Various sea level rise projections

A review of the literature by the U.S. National Research Council (2010) shows that by 2100 sea levels are projected to rise between 2.5 ft. and 6.5 ft. Although there is disagreement in the exact value, there is strong agreement that sea levels will rise substantially in the coming centuries and will cause significant economic and social devastation.

Lemonick (2010) writes in the article The Secret of Sea Level Rise: It Will Vary Greatly by Region:

As the world warms, sea levels could easily rise three to six feet this century. But increases will vary widely by region, with prevailing winds, powerful ocean currents, and even the gravitational pull of the polar ice sheets determining whether some coastal areas will be inundated while others stay dry.

As shown in Fig. 4 (NOAA, 2009) sea level change since 1958 in the U.S. has varied dramatically by region.

Figure. 4: Regional sea level changes since 1958 in the US vary by region

Impact on the Coastal Environment:

Figure 5: Coastal Impacts of Climate Change (IPCC, 2007)

Fig. 5 shows a simple diagram of the influences of climate change and sea level rise on the coastal environment. The coastal environment can be divided into two subsets: natural and societal. Terrestrial-sourced hazards include river floods and inputs of sediment or pollutants; marine-sourced hazards include storm surges, energetic swell and tsunamis. Some coastal countries and communities are more able to adapt to these changes thus minimizing the impacts of climate change, others have fewer options and hence are much more vulnerable to climate change. Additionally, human population growth in many coastal regions is both increasing socio-economic vulnerability and decreasing the resilience of coastal systems.

Fig. 6 (IPCC, 2007) summarizes the main climate drivers for coastal systems, their trends due to climate change, and their main physical and ecosystem effects. (Trend: ↑ increase; ? uncertain; R regional variability).

Figure 6: Main climate drivers for coastal systems, their trends due to climate change, and their main physical and ecosystem effects

Fig. 7 (Ibid) summarizes the climate-related impacts on socio-economic sectors in coastal zones.

Figure 7: Summary of the climate-related impacts on socio-economic sectors in coastal zones

Beaches, Rocky Shorelines and Cliffed coasts:

Most of the world’s sandy shorelines have retreated during the past century and any rise in sea level will further exacerbate global beach erosion. One half or more of the Mississippi and Texas shorelines have eroded at average rates of 3.1 to 2.6 m/yr since the 1970s, while 90% of the Louisiana shoreline eroded at a rate of 12.0 m/yr. In Nigeria, retreat rates up to 30 m/yr have been reported . Along the eastern coast of the United Kingdom 67% of the coastline has experienced a shoreline retreat from the low-water mark over the past century (Ibid).

The combined effects of beach erosion and storms can lead to the erosion or inundation of other coastal systems. For example, an increase in wave heights in coastal bays is a secondary effect of sandy barrier island erosion in Louisiana, and increased wave heights have enhanced erosion rates of bay shorelines, tidal creeks and adjacent wetlands (Ibid).

Hard rock cliffs have a relatively high resistance to erosion, but cliffs formed in softer conditions are likely to retreat more rapidly in the future due to increased erosion resulting from sea-level rise. Changes in temperature, precipitation, sea level, and wave climate affect the stability of soft rock cliffs (Ibid).

Deltas:

Deltas are highly sensitive to sea-level rise. Rates of relative sea-level rise can greatly exceed the global average in many heavily populated deltaic areas due to subsidence (sinking of the land) caused by human activities Examples include the Chao Phraya delta, Mississippi River delta, and the Changjiang River delta. Sea-level rise increases the potential for flooding, especially for the most populated cities on these deltas – Bangkok, New Orleans and Shanghai (Ibid).

Delta plains, particularly those in Asia, are densely populated and large numbers of people are often impacted as a result of external terrestrial influences (river floods, sediment starvation) and/or external marine influences (storm surges, erosion). Fig. 8 (Ericson et al., 2006) shows the relative vulnerability of coastal deltas as shown by the population potentially displaced by current sea-level trends in the year 2050. (Note that sea level trends are actually greater than projected by the IPCC so this figure is too conservative in its timing. Much of the impact will likely occur well before 2050.)

Figure 8: Relative vulnerability of coastal deltas as shown by the population potentially displaced by current sea-level trends by the year 2050

Ericson et al. (2006) estimated that nearly 300 million people inhabit a sample of 40 deltas globally, including all the large megadeltas. Average population density is 500 people/km² with the largest population in the Ganges-Brahmaputra delta, and the highest density in the Nile delta. The authors estimate that more than 1 million people will be directly affected by 2050 in three megadeltas: the Ganges-Brahmaputra delta in Bangladesh, the Mekong delta in Vietnam and the Nile delta in Egypt. More than 50,000 people are likely to be directly impacted in each of a further 9 deltas, and more than 5,000 in each of a further 12 deltas.

Estuaries & Lagoons:

Global mean sea-level rise will force existing coastal plant and animal communities to move inland. One of the greatest potential impacts of climate change on estuaries may result from changes caused by increased freshwater runoff. Freshwater inflows into estuaries influences water residence time, nutrient delivery, vertical stratification, salinity and control of phytoplankton growth rates. Changes in the timing of freshwater delivery to estuaries could lead to a disruption of the early growing phases of many estuarine and marine fishery species from the available habitat. Increased water temperature may lead to increased algae blooms which will reduce the availability of light, oxygen and carbon for other estuarine species (IPCC, 2007).

Mangroves, Saltmarshes & Sea Grasses:

According to the IPCC (2007), “Coastal vegetated wetlands are sensitive to climate change and long-term sea-level change as their location is intimately linked to sea level. Modeling of all coastal wetlands (but excluding sea grasses) by McFadden et al. (2007a) suggests global losses from 2000 to 2080 of 33% and 44% given a 36 cm and 72 cm rise in sea level, respectively. Regionally, losses would be most severe on the Atlantic and Gulf of Mexico coasts of North and Central America, the Caribbean, the Mediterranean, the Baltic and most small island regions due to their low tidal range.”

“Mangrove forests dominate intertidal subtropical and tropical coastlines between 25^oN and 25^oS latitude. Mangrove communities are likely to show a blend of positive responses to climate change, such as enhanced growth resulting from higher levels of CO₂ and temperature, as well as negative impacts, such as increased saline intrusion and erosion, largely depending on site-specific factors.”

Sea grasses appear to be declining around many coasts due to human impacts, and this is expected to accelerate if climate change alters environmental conditions in coastal waters. However, for some species of sea grass, increased dissolved CO₂ gas may act as a “fertilizer” to increase growth (Ibid).

Coral Reefs:

Reefs have deteriorated as a result of a combination of human impacts such as over fishing and pollution from adjacent land masses, together with an increased frequency and severity of bleaching and ocean acidification associated with climate change. (See my blog post about ocean acidification titled: The 800 lb. Gorilla in the Ocean)

Coral bleaching occurs with the loss of symbiotic algae and/or their pigments and has been observed on many reefs since the early 1980s. It may have previously occurred, but gone unrecorded. Slight paling occurs naturally in response to normal seasonal increases in sea surface temperature (SST) and solar radiation. Corals bleach white in response to unusually high SST (~1^oC above average seasonal maxima, often combined with high solar radiation). Some corals recover their natural color when environmental conditions improve but their growth rate and reproductive ability may be significantly reduced for a substantial period. If bleaching is prolonged, or if SST exceeds 2^oC above average seasonal maxima, corals die. Branching species appear more susceptible than massive corals (IPCC, 2007).

Major bleaching events were observed in 1982-83, 1987-88 and 1994-95 and some more recent bleaching outbreaks are shown in Fig. 9 (Ibid) below:

Figure 9: Maximum monthly mean sea surface temperature for 1998, 2002 and 2005, and locations of reported coral bleaching

According to the IPCC (2007) global climate model results imply that thermal thresholds will be exceeded more frequently with the consequence that bleaching will recur more often than reefs can sustain, perhaps almost annually on some reefs in the next few decades. If the threshold remains unchanged, more frequent bleaching and mortality seems inevitable. Bleaching events reported in recent years have already impacted many reefs, and their more frequent recurrence is very likely to further reduce both coral cover and diversity on reefs over the next few decades.

Consequences for Human Society:

Fig. 7 above summarizes the climate-related impacts on socio-economic sectors in coastal zones. Vulnerability to the impacts of climate change, including the higher socio-economic burden imposed by present climate-related hazards and disasters, is very likely to be greater on coastal communities of developing countries than in developed countries due to inequalities in adaptive capacity. For example, one quarter of Africa’s population is located in resource-rich coastal zones and a high proportion of GDP is exposed to climate-influenced coastal risks. In Guyana, 90% of its population and important economic activities are located within the coastal zone and are threatened by sea-level rise and climate change. Low-lying densely populated areas in India, China and Bangladesh and other deltaic areas are highly exposed, as are the economies of small islands (Ibid).

Fig. 8 (Ibid) shows the key hotspots of societal vulnerability in coastal zones along with examples described in Chapter 6 of the IPCC WG II report. Fig. 9 (Anthoff et al., 2006) shows large numbers of people and significant economic activity are exposed to sea-level rise, particularly in Asia. Fig. 10 (Overpeck & Weiss, 2009) shows the extent of a 1m and 6m future sea-level rise along the East and Gulf coasts of the United States and for selected major coastal cities.

Figure 8: Key hotspots of societal vulnerability in coastal zones with examples from WG II Ch. 6

Figure 9: World population, area and economy affected by a 1m sea level rise.

Figure 10: Extent of a 1m and 6m future sea-level rise along the East and Gulf coasts of the U.S.

A recent paper published by some of the top hurricane researchers in the field (Knutson, et al. 2010) concludes:

…future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100.

When combined with rising sea levels, storm surge inundation (which causes more than 90% of the casualties in hurricanes, typhoons, and cyclones) will be more common and more extensive. As the population grows and building accelerates near coastal locations, the economic damage from storms will increase. Hurricane Katrina, which struck the Gulf Coast on August 29, 2005, was the single largest natural disaster loss in the history of the U.S. insurance industry. Insurance companies paid $41 billion arising from 1.7 million claims for damage to homes, businesses and vehicles to policyholders in six states. “While 2005 was by far the worst year ever for insured catastrophe losses in the U.S., future storms could prove even costlier, reaching upwards of $100 billion,” said Dr. Robert Hartwig, an economist and president of the Insurance Information Institute. In the United States alone, the value of coastal property exposed to hurricanes increased by 24 percent, or $1.7 trillion, from $7.2 trillion in 2004 to $8.9 trillion by year-end 2007, according to AIR Worldwide (Insurance Information Institute, 2008).

Figure 11: Value of insured coastal property vulnerable to hurricanes in the U.S.

Rising sea levels will flood the word’s largest ports. According to The Tipping Points Report (2009) commissioned jointly by Allianz, a leading global financial service provider, and WWF, a leading global environmental NGO, a rise in sea level by 0.5 meters by 2050 could put at risk more than $28 trillion worth of assets in the world’s largest coastal cities. The value of infrastructure exposed in port mega-cities, is just $3 trillion at present. A hurricane in New York, which could cost $1 trillion now, would mean a $5 trillion insurance bill by the middle of the century. Insurance rates will rise and taxes will increase to pay for the recovery and to move ports inland.

Next post: Freshwater Resources

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