Global sea level rise

 Review of Sea Level rise with climate change    

Climate change is a serious issue, but the evidence suggests, world isn’t going to, and maybe really can't, reduce fossil fuel consumption in the near future.  With this we need to focus on adapting, one thing we’ll have to deal with is rising sea levels. Global mean sea level has risen about 8–9 inches (21–24 centimeters) since 1880. The rising water level is mostly due to a combination of melt water from glaciers and ice sheets and thermal expansion of seawater as it warms. In 2023, global mean sea level was 101.4 mm's (3.99 inches) above 1993 levels, making it the highest annual average in the satellite record. In some ocean basins, sea level has risen as much as 6-8 inches (15-20 cm's) since the start of the satellite record. Regional differences exist because of natural variability in the strength of winds and ocean currents, which influence how much and where the deeper layers of the ocean store heat. The global mean water level in the ocean rose by 0.14 inches (3.6 mm's)/ year from 2006–2015, which was 2.5 times the average rate of 0.06 inches (1.4 mm's)/ year throughout most of the twentieth century. By the end of the century, global mean sea level is likely to rise at least one foot (0.3 meters) above 2000 levels, even if greenhouse gas emissions follow a relatively low pathway in coming decades.

Energy generation from renewable sources is far less than from fossil fuels. The rate of global sea level rise is accelerating. In many locations along the US coastline, the rate of local sea level rise is greater than the global average due to land processes like erosion, oil and groundwater pumping and subsidence. From the 1970s up through the last decade or so, melting and heat expansion were contributing roughly equally to observed sea level rise. But the melting of mountain glaciers and ice sheets has accelerated. Between 1993 and 2023 mean sea level has risen across most of the world ocean. Rates of local sea level on the coast can be larger than the global average due to geological processes like ground settling or smaller than the global average due to processes like the centuries-long rebound of land masses from the loss of ice-age glaciers. The decadal average loss from glaciers in the World Glacier Monitoring Service’s reference network quintupled over the past few decades, from the equivalent of 6.7 inches (171 mm's) of liquid water in the 1980s, to 18 inches (460 mm's) in the 1990s, to 20 inches (-500 mm's) in the 2000s, to 33 inches (850 mm's) for 2010-2018.

Past and future sea level rise at specific locations on land may be more or less than the global average due to local factors: ground settling, upstream flood control, erosion, regional ocean currents, and whether the land is still rebounding or resettling from the compressive weight of vanished Ice Age glaciers. In the United States, the fastest rates of sea level rise are occurring in the Gulf of America (formerly Gulf of Mexico) from the mouth of the Mississippi westward, followed by the mid-Atlantic. Only in Alaska and a few places in the Pacific Northwest are sea levels falling today, although that trend will reverse in the future if the world follows a pathway with high greenhouse gas emissions. In the US, almost 30 % of the population lives in coastal areas, where sea level rise plays a role in flooding, shoreline erosion and hazards from storms. Globally, 8 of the world’s 10 largest cities are near a coast, according to the UN Atlas of the Oceans. In urban settings along coastlines around the world, rising seas threaten infrastructure necessary for local jobs and regional industries. Roads, bridges, subways, water supplies, oil and gas wells, power plants, sewage treatment plants, landfills, the list is practically endless, are all at risk from sea level rise. Higher background water levels mean that deadly and destructive storm surges, such as those associated with Hurricane Katrina, “Superstorm” Sandy, and Hurricane Michael, push farther inland than they once did. Higher sea level also means more frequent high-tide flooding, sometimes called “nuisance flooding” because it isn't generally deadly or dangerous, but it can be disruptive and expensive. Around the US, nuisance flooding has increased dramatically in the past 50 years. 

Ice loss from the Greenland Ice Sheet increased seven-fold from 34 billion tons/year between 1992-2001 to 247 billion tons/year between 2012 and 2016. Antarctic ice loss nearly quadrupled from 51 billion tons/year between 1992 and 2001 to 199 billion tons/year from 2012-2016. As a result, the amount of sea level rise due to melting (with a small addition from groundwater transfer and other water storage shifts) from 2005–2013 was nearly twice the amount of sea level rise due to thermal expansion. The global average ocean rise is less than the relative rise of sea level for most of the eastern seaboard of the US Glacial rebound is a fairly well-known phenomenon. The earth’s surface was pushed down by the weight of 10,000’ of ice. After the weight of the glaciers was removed, that depression is now returning to its original state, i.e., glacial rebound. Less often considered is glacial subsidence. Imagine where the earth’s inner material went as the glacier depressed the crustal material and where now the material comes from that allows the crust to rebound. Much of the returning mantle which buoys up the crust and creates glacial rebound comes from the area to the south of the greatest weight of the glacier. This would be the Atlantic coast of the US, leading to its subsidence and adds to the relative ocean level rise for our eastern seaboard

In the natural world, rising sea level creates stress on coastal ecosystems which provide recreation, protection from storms, and habitat for fish and wildlife, including commercially valuable fisheries. As seas rise, saltwater is also contaminating freshwater aquifers, many of which sustain municipal and agricultural water supplies and natural ecosystems. Sea level is measured by two main methods: tide gauges and satellite altimeters. Tide gauge stations from around the world have measured the daily high and low tides for more than a century, using a variety of manual and automatic sensors. Using data from scores of stations around the world, scientists can calculate a global average and adjust it for seasonal differences. Since the early 1990s, sea level has been measured from space using radar altimeters, which determine the height of the sea surface by measuring the return speed and intensity of a radar pulse directed at the ocean. The higher the sea level, the faster and stronger the return signal is. The environmental effects of rising greenhouse gases are many: Increased ocean temperature fuels hurricane intensity, and increased evaporation from warmer oceans feeds high cloud moisture, leading to more severe rainstorms where warmer air meets cooler air. However, by increasing the capacity of clouds to carry water, this phenomenon decreases the amount of rain in hotter areas. From the 1970s up through the last decade or so, melting and heat expansion were contributing roughly equally to observed sea level rise. But the melting of mountain glaciers and ice sheets has accelerated.

The amount of sea level rise due to melting (with a small addition from groundwater transfer and other water storage shifts) from 2005–2013 was nearly twice the amount of sea level rise due to thermal expansion. So, should we care if renewable energy generation doesn’t catch up with burning fossil fuels? Note that different scenarios are derived from models based on different rates of greenhouse gas emission. Imagine a 3-foot rise in ocean levels around New York City by the end of the century if the current rate of change in greenhouse gas emissions remains constant. Or, if we don’t control global greenhouse gas emissions, imagine a 5’ rise. The proportion of our energy to be derived from renewable sources is not inconsequential. To estimate how much of the observed sea level rise is due to thermal expansion, scientists measure sea surface temperature using moored and drifting buoys, satellites and water samples collected by ships. Temperatures in the upper half of the ocean are measured by a global fleet of aquatic robots. Deeper temperatures are measured by instruments lowered from oceanographic research ships. To estimate how much of the increase in sea level is due to actual mass transfer, the movement of water from land to ocean, scientists rely on a combination of direct measurements of melt rate and glacier elevation made during field surveys, and satellite-based measurements of tiny shifts in Earth’s gravity field. When water shifts from land to ocean, the increase in mass increases the strength of gravity over oceans by a small amount. From these gravity shifts, scientists estimate the amount of added water.

In many locations along the US coastline, the rate of local sea level rise is greater than the global average due to land processes like erosion, oil and groundwater pumping, and subsidence. High-tide flooding is now 300% to more than 900% more frequent than it was 50 years ago. If we are able to significantly reduce greenhouse gas emissions, U.S. sea level in 2100 is projected to be around 0.6 meters (2 feet) higher on average than it was in 2000. On a pathway with high greenhouse gas emissions and rapid ice sheet collapse, models project that average sea level rise for the contiguous US could be 2.2 meters (7.2 feet) by 2100 and 3.9 meters (13 feet) by 2150. Global warming is causing global mean sea level to rise in two ways. First, glaciers and ice sheets worldwide are melting and adding water to the ocean. Second, the volume of the ocean is expanding as the water warms. A third, much smaller contributor to sea level rise is a decline in the amount of liquid water on land, aquifers, lakes and reservoirs, rivers, soil moisture. This shift of liquid water from land to ocean is largely due to people depleting ground water. As global temperatures continue to warm, additional sea level rise is inevitable. How much and by when depends mostly on the future rate of greenhouse gas emissions. But another source of uncertainty is whether big ice sheets in Antarctica and Greenland will melt in a steady, predictable way as the Earth gets warmer, or whether they will reach a tipping point and rapidly collapse.

Every four or five years, a task force reviews the latest sea level rise and issues a report on likely, and ‘unlikely but plausible’, amounts future sea level rise for different greenhouse gas and global warming. In the 2022 report, the task force concluded that even with the lowest possible greenhouse gas emissions and warming (1.5 degrees C), global mean sea level would rise at least 0.3 meters (1 foot) above 2000 levels by 2100. But with very high rates of emissions that trigger rapid ice sheet collapse, sea level could be as much as 2 meters (6.6 feet) higher in 2100 than it was in 2000. Many parts of the US can expect their local rate and overall amount of sea level rise to exceed the global average. Extrapolating from observed rates, sea levels on average along the contiguous US are expected to rise as much over the next 30 years (10-12 inches over 2020-2050) as they have over the last 100 years (1920-2020). In some regions, the increases will be even larger. In the western Gulf of America (formerly Gulf of Mexico), for example, sea level rise is likely to be about 16-18 inches higher than 2020 levels by 2050, almost a ½ foot higher than the national average. If we are able to significantly reduce greenhouse gas emissions, US sea level in 2100 is projected to be around 0.6 meters (2 feet) higher on average than it was in 2000. But with high greenhouse gas emissions and rapid ice sheet collapse, average sea level rise for the contiguous US could be 2.2 meters (7.2 feet) by 2100 and 3.9 meters (13 feet) by 2150.