Jonah Patel
Sea levels have been rising due to human-caused global warming, with rates of increase being unprecedented over the past 2,500 years. The rising water level is mainly due to the melting of ice sheets and glaciers and the thermal expansion of seawater as it warms. The degree of sea level rise varies based on different emission scenarios. The lowest scenario, RCP2.6, would see greenhouse gas emissions low enough to limit warming to 2°C by 2100, with a sea level rise of about 17 inches. The moderate scenario, RCP4.5, would see emissions peak in the next two decades, with a sea level rise of 14-28 inches. The highest scenario, RCP8.5, would see emissions continue to rise, resulting in a sea level rise of 20-38 inches. Extreme sea levels are triggered by storm surges, tides, and waves, causing severe hazards to human settlements and coastal ecosystems.
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What You'll Learn
Global sea levels have risen 8-9 inches since 1880
Global sea levels have risen by about 8-9 inches (21-24 centimeters) since 1880. This rise in sea level is mainly due to the melting of glaciers and ice sheets, as well as the thermal expansion of seawater as ocean temperatures increase. The rate of sea level rise has accelerated in recent decades, more than doubling from 0.06 inches (1.4 millimeters) per year during most of the 20th century to 0.14 inches (3.6 millimeters) per year between 2006 and 2015.
This acceleration is largely driven by increased ice melting in Greenland and Antarctica, and it has the potential to significantly impact coastal regions around the world. For example, the rate of local sea level rise along the US coastline is often greater than the global average due to factors such as erosion, oil and groundwater pumping, and subsidence. High-tide flooding has become significantly more frequent, with a 300-900% increase compared to 50 years ago.
The extent of future sea level rise will depend on our ability to reduce greenhouse gas emissions and slow the melting of major ice sheets. According to projections, if significant emissions reductions are achieved, the global sea level in 2100 is expected to be around 0.6 meters (2 feet) higher than in 2000. However, if high emissions and rapid ice sheet collapse occur, models predict that the sea level rise could reach upwards of 2 meters (6.6 feet) by 2100 and even exceed 3 meters (10 feet) by 2150.
The impacts of sea level rise are already being felt around the world, with coastal flooding and erosion causing economic losses and threatening communities. For instance, by 2030, flooding along the US Gulf Coast could result in economic losses of up to $176 billion. Additionally, the Mi'kmaq community is considering relocation due to widespread coastal erosion.
Understanding the levels that constitute moderate, severe, and extreme sea-level rise is crucial for preparedness and mitigation. According to the RCP (Representative Concentration Pathway) scenarios, the "moderate" scenario, RCP4.5, projects a sea level rise of 36-71 cm (14-28 inches) by 2100. The "severe" scenario, RCP8.5, estimates a rise of 52-98 cm (20.5-38.5 inches) by 2100. The extreme scenario, represented by SSP-8.5 with ice cliff instability, projects a sea level rise of 9.5-16.2 meters (31-53 feet) by the year 2300.
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The rate of sea level rise is accelerating
The extent of sea level rise varies across different regions. In many locations along the U.S. 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. By 2050, coastal flooding in the US is likely to rise tenfold, with four "moderate" flooding events per year. The western Gulf of America (formerly the Gulf of Mexico) is particularly vulnerable, with sea levels expected to rise by 16-18 inches above 2020 levels by 2050.
The impact of sea level rise is not limited to the United States. By 2030, flooding along the US Gulf Coast could cause economic losses of up to US$176 billion. In Mexico, damage to tourism hotspots like Cancun and Cozumel due to sea level rise and storm surges could amount to US$1.4–2.3 billion. In Australia, erosion and flooding of Queensland's Sunshine Coast beaches are likely to intensify by 60% by 2030, impacting tourism significantly. By 2050, 12 major African cities could sustain cumulative damages of US$65 billion to US$137.5 billion in a "moderate" climate change scenario.
Looking further ahead, projections for sea level rise by the year 2100 vary depending on emission scenarios. In a moderate scenario, sea levels are projected to rise by 36–71 cm (14–28 in) by 2100. On a pathway with very high rates of emissions and rapid ice sheet collapse, sea levels could rise by 2 meters (6.6 feet) or more by 2100. Even with the lowest possible greenhouse gas emissions and warming (1.5 degrees C), global mean sea level is projected to rise by at least 0.3 meters (1 foot) above 2000 levels by 2100. By 2300, in a worst-case scenario with ice cliff instability, the projected range for total sea level rise is estimated to be 9.5–16.2 meters (31–53 feet).
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Coastal flooding will increase tenfold by 2050
The global average sea level has risen by 8–9 inches (21–24 cm) since 1880, with the rate of increase more than doubling from 0.06 inches (1.4 mm) per year in the 20th century to 0.14 inches (3.6 mm) per year from 2006 to 2015. This rate of increase is expected to continue to accelerate, with the global mean sea level projected to rise by at least 0.3 meters (1 foot) above 2000 levels by 2100, even in the best-case scenario of the lowest possible greenhouse gas emissions and warming (1.5 degrees C). In a worst-case scenario with very high emissions and rapid ice sheet collapse, the sea level could rise by as much as 2 meters (6.6 feet) or even 3.7 meters (12 feet) by 2100.
The impact of sea-level rise on coastal flooding is already being felt, with high-tide flooding 300-900% more frequent than it was 50 years ago. By 2050, it is projected that coastal flooding in the US will increase tenfold, with four "moderate" flooding events per year. This will have significant economic impacts, with flooding along the US Gulf Coast expected to cause up to US$176 billion in economic losses by 2030.
The increase in coastal flooding is not limited to the US. By 2050, 12 major African cities are projected to sustain cumulative damages of US$65 billion in a "moderate" climate change scenario, with damages potentially tripling in the worst case. Coastal cities are increasingly experiencing flooding on days with less extreme tides or little wind, and this type of tidal flooding is expected to increase in depth, frequency, and extent.
The rising sea levels also have other impacts, such as inundating low-lying wetlands and dry land, eroding shorelines, increasing the flow of saltwater into estuaries and groundwater aquifers, and making coastal infrastructure more vulnerable to storm damage. Accurate estimates of global mean sea-level rise are crucial for understanding and mitigating these impacts.
To summarize, coastal flooding is expected to increase significantly by 2050, with projections of a tenfold increase in the US and impacts on multiple regions globally. The rising sea levels will have economic, environmental, and social consequences, highlighting the urgency of addressing climate change and adapting to its impacts.
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Ice sheet collapse will cause rapid sea level rise
The collapse of ice sheets can cause rapid sea level rise, leading to devastating consequences for coastal communities and ecosystems. Ice sheets are massive bodies of land ice that cover more than 50,000 square kilometers (20,000 square miles). The Antarctic Ice Sheet and the Greenland Ice Sheet are the two largest ice sheets on Earth, containing about 90% of the planet's freshwater.
The partial collapse of the Eurasian Ice Sheet nearly 15,000 years ago caused global sea levels to rise by up to 45 feet in less than 400 years. This event, known as the Last Glacial Maximum, was marked by low global temperatures and the presence of massive ice sheets. Similarly, the West Antarctic Ice Sheet is currently undergoing dramatic ice loss, with glaciers accelerating and suggesting underlying instability. The melting of the Thwaites Glacier in West Antarctica, for instance, could contribute several centimeters to sea level rise by the end of the century.
The Greenland and Antarctic Ice Sheets hold enough water to raise sea levels by roughly 65 meters (over 210 feet) if they were to melt entirely. While this scenario is unlikely in the foreseeable future, even a small fraction of ice sheet collapse can have significant impacts. For example, a rise of 0.5 meters in sea level would cause what is currently a 100-year flood event in Australia to occur several times a year. Additionally, the melting of ice sheets can also lead to the contamination of freshwater sources, the inundation of farmland with saltwater, and the displacement of millions of people living in low-lying coastal areas.
Rising ocean temperatures also contribute to the instability of ice sheets. Warm waters melt the ice shelves that help stabilize the ice sheet and slow their flow. This melting around the grounding line, where the ice sheet becomes a floating ice shelf, accelerates the flow of ice off the continent. Furthermore, the shape of the ground under the ice, known as the bedrock, also influences the melt rate, with some areas being more vulnerable to rapid melting.
To predict and prepare for the potential impacts of ice sheet collapse, scientists employ models that incorporate factors such as changing wind patterns, ocean currents, and bedrock shape. These models help to estimate sea level rise under different greenhouse gas emissions scenarios. For instance, on a pathway with high emissions and rapid ice sheet collapse, the average sea level rise for the contiguous United States could reach 2.2 meters (7.2 feet) by 2100 and 3.9 meters (13 feet) by 2150. However, even with low emissions, global mean sea level is projected to rise by at least 0.3 meters (1 foot) by 2100 compared to 2000 levels.
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Global warming is causing sea levels to rise
The rising sea levels pose significant risks to coastal ecosystems, including erosion, contamination of freshwater aquifers, and impacts on wildlife and fisheries. It is also leading to an increase in storm surges, causing greater damage and affecting more people. By 2050, coastal flooding in the US is expected to rise tenfold, with "moderate" flooding events occurring four times per year.
The extent of sea-level rise depends on future greenhouse gas emissions. On a pathway with high emissions and rapid ice sheet collapse, models project that sea levels could rise by 2.2 meters (7.2 feet) by 2100 and 3.9 meters (13 feet) by 2150 for the contiguous United States. Even with the lowest possible emissions and warming, the global mean sea level is still projected to rise by at least 0.3 meters (1 foot) by 2100.
By the year 2100, Egypt, Mozambique, and Tanzania are expected to be among the most affected African countries by annual flooding. Coastal cities like Shanghai and Jakarta are also vulnerable to sea-level rise, with Jakarta sinking by up to 28 cm per year between 1982 and 2010, leading to the Indonesian government's decision to relocate the capital.
Accurate estimates of global mean sea-level rise are crucial for understanding the vulnerability of coastal regions to flooding and erosion. Employing advanced models, researchers have found that the number of people affected by rising sea levels may be significantly higher than previously estimated.
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Frequently asked questions
The definitions of moderate, severe, and extreme sea level rise are relative and depend on the context and location. However, here are some estimates and projections based on different scenarios:
- Moderate: According to the RCP4.5 scenario, a moderate sea level rise by 2100 could be around 36-71 cm (14-28 inches). By 2050, a sea level rise of 0.2-0.3 meters is expected, which would cause more frequent flooding in some regions.
- Severe: A severe sea level rise could be considered when it starts causing significant impacts and hazards. For example, a rise of 0.5 meters could cause a 100-year flood in Australia to occur several times a year. By 2100, extreme weather events and flooding are expected to become more frequent, affecting various regions and cultural sites.
- Extreme: An extreme sea level rise could refer to scenarios where the rise is much higher than expected. For example, under the RCP8.5 pathway, sea levels could rise by 52-98 cm by 2100, and even higher in far-tail scenarios. By 2300, projections indicate a potential rise of 9.5-16.2 meters, which would be considered an extreme event.
The levels of sea-level rise vary depending on factors such as greenhouse gas emissions, the rate of ice sheet collapse, and local land processes. Human-caused global warming is a significant contributor, with melting ice sheets and glaciers, and the expansion of seawater as it warms being primary factors.
The impacts of moderate, severe, and extreme sea level rises vary regionally. Low-lying coastal areas and small island nations are particularly vulnerable to sea level rise. For example, the northern Gulf of Mexico, Atlantic Canada, and the Pacific coast of Mexico are expected to experience greater sea level rise. By 2100, Egypt, Mozambique, and Tanzania are projected to be among the most affected African countries by annual flooding.
