Floods and flooding
Floods and flooding are significant natural events characterized by unusually high water levels in rivers, lakes, or coastal areas, often resulting in the inundation of land. These occurrences can be exacerbated by climate change, which leads to increased precipitation and higher sea levels. River floods typically arise from heavy rainfall or melting snow that overwhelms the river's base flow, particularly in urban areas where impervious surfaces like pavement accelerate runoff. Coastal flooding, on the other hand, often results from storm surges associated with severe weather events, including hurricanes, especially as rising sea levels make coastlines more vulnerable.
Global warming is believed to intensify these phenomena, contributing to more frequent and severe weather patterns, including record-breaking rainfall events. Areas with significant glacial melting or low-lying coastal regions face heightened risks as sea levels continue to rise, potentially leading to catastrophic flooding. Additionally, human activities such as urban development and land use changes complicate natural flood dynamics by increasing runoff and reducing the land's capacity to absorb water. As these climate-related changes unfold, understanding the implications of floods and their increasing likelihood is essential for communities worldwide.
Floods and flooding
Floods along rivers and storm surges along ocean shorelines may become more severe on average as a result of higher sea levels and greater precipitation associated with global warming.
Background
Floods along rivers occur when large amounts of precipitation, often combined with melting snow, cause the discharge of a river to increase far beyond its base flow. Floods may at times be even higher in cities because of rapid runoff of water from impervious surfaces such as pavement and into storm sewers. Global warming may increase the intensity and variability of precipitation in many regions, causing more intense flooding. Floods along maritime coastlines can result from storm surges, including those associated with hurricanes and other severe weather. Higher sea levels due to glacial melting during global warming and more frequent and intense hurricanes may result in more coastal flooding.
Warming of the Earth’s Climate
The average temperature of the atmosphere of Earth near its surface is variable, and it has been estimated in a number of ways. Nevertheless, the average temperature before 1920 is estimated to have been about 0.2° to 0.4° Celsius lower than it was from about 1940 to 1980. The average temperature from about 1980 to the early twenty-first century was about 0.4° Celsius higher than that from 1940 to 1980. Since 1980, the temperature has risen about 0.2° Celsius per decade. In 2023, according to NOAA's Climate.gov, the global temperature in July, August, and September was 1.0° Celsius above the long-term average, making these months the warmest in more than one hundred years. Scientists believe that a major reason for this increase in average temperature is the drastic increase in the atmospheric concentration of the greenhouse gas (GHG) carbon dioxide (CO2) through the twentieth century and into the early twenty-first. Much of this increase in CO2 was due to the burning of fossil fuels such as coal, oil, and natural gas.
Predictions of warming of the atmosphere in the future are much more difficult to make. For instance, a doubling of the CO2 content of the atmosphere from that of the first decades of the twenty-first century has been predicted to cause an increase in temperature of about 2° to 3° Celsius. Warming of the atmosphere to the year 2100 has been predicted, depending on the model, to increase the temperature by as little as 2° Celsius to as much as 8° Celsius.
Predicted Influence of Climate Change on Precipitation and Floods
The gradual increase in temperature is likely to lead to more frequent warm periods, such as the extreme heat wave that occurred in Europe in the summer of 2003. Also, snow should, on average, melt sooner in the spring and melt more rapidly and further to the northern and southern latitudes than it does now.
The increase in temperature of the atmosphere and oceans during global warming will also likely increase the amount of water that evaporates into the atmosphere. Thus, more intense precipitation should occur in some regions. In 2016, for example, Louisiana experienced record-breaking rainfall, which caused devastating flooding. On a regional scale, this effect is most likely to transport more water from the subtropics to higher latitudes. Thus, higher local rainfall may occur in some areas, causing more discharge into rivers and producing more flooding. This precipitation may be quite variable in different areas. For instance, some drainage basins in the northwestern United States have been predicted to have increased flood risk as a result of global warming. By contrast, drainage basins that receive much of their precipitation as snow have been predicted to have less flood risk in the spring as a result of there being less snow to melt. In addition, a 2021 study by Nature showed an increase in the frequency and intensity of extreme flood events.
Predicted Influence of Climate Change on Maritime Coasts
Some scientists have suggested that hurricanes have already become more numerous and destructive as a result of global warming. For instance, by the early 2000s, researchers had noted an increase in Type 4 and 5 hurricanes since 1970. Others suggest that while the overall number of hurricanes may not be increased by climate change, the number of particularly destructive storms will grow. This increase in extreme weather can be expected to result in more and greater storm surges along coastlines, causing flooding. Many experts note that more powerful hurricanes also have the capacity to cause significant and damaging floods farther inland, with heavy rainfall replacing storm surges as the key flood threat. Hurricanes Harvey and Maria in 2017 and Hurricane Florence in 2018 were all widely seen as evidence of this trend.
The gradual increase in temperature over time should result in more glacial melting and an increase in the volume of liquid water on the planet. Some predict that even modest increases in GHGs in the atmosphere could melt the ice sheet on Greenland and much of the Antarctic ice sheet. These effects would lead gradually to a rise in sea level. Estimates of this rise have varied from an increase of 0.1 meter to 0.9 meter by the year 2100. This increase in sea level should result in more flooding of flat-lying areas along oceanic coastlines and an increased influence of storm surges.
This problem could be made worse in areas in which the land surface is gradually sinking, such as in southeastern England or in Venice, Italy. Venice is located at sea level, and a series of water canals inundate much of the city. The sea level rose relative to the land surface by about 16 to 21 centimeters on average from about 1900 to 2016, mostly as a result of the land subsiding because of the removal of groundwater for industry. Thus, Venice risks being submerged into the ocean if the sea level rises. Many island communities in the Pacific Ocean are similarly threatened by rising sea level and the resulting increased risk of destruction from storm surges and coastal flooding.
Climate scientists expect sea levels to continue to rise throughout the coming decades, even if governments take significant measures to limit the effects of climate change. According to the US Global Change Research Program in 2022, sea levels were expected to rise 10 to 12 inches along US coastlines during the next thirty years. This places most coastal communities at a significant risk for flooding.
Context
Some of the changes expected to result from global warming are difficult to predict. For instance, changes to the Gulf Stream associated with global warming could potentially be very important, causing vast changes in the climate. The Gulf Stream is the flow of warm, salty water from the subtropical Atlantic Ocean northward toward polar regions. The Gulf Stream loses heat to the atmosphere in cooler areas, so it becomes denser and sinks. The cooler water then flows to the south, along the eastern coast of North America. Warming of the atmosphere and ocean as a result of climate change could potentially keep the Gulf Stream warmer farther to the north, making the water less dense. This lower density might keep the surface waters from sinking and partially or even completely shut down this circulation cycle in the Atlantic Ocean, drastically affecting the climate. Northern latitudes might become wetter, much of the mid-latitudes could become dry, and tropical rainfall could shift. Thus, flooding and drought areas may shift drastically if the global climate continues to change.
Scientists also note that complicating both natural flood patterns and the influence of human-induced climate change are patterns of land use and development. Land near water bodies has always been attractive to human settlement for purposes of transportation and agriculture, despite the risk of flooding. Modern construction makes extensive use of impermeable surfaces, from buildings to paved roads, which prevent precipitation from draining into the ground and thereby increase flood intensity. Ongoing urban growth has caused even previously undesirable, low-lying, flood-prone areas to be developed, including wetlands that naturally play a major role in absorbing water. Resource pressures and other long-term effects of climate change may further exacerbate such land-use patterns, creating a cycle of ever-worsening flood risk in certain areas.
Key Concepts
- base flow: the normal amount of discharge in a river
- discharge: the volume of water that passes a point in a river in a certain interval of time
- evaporation: the absorption of liquid water into the air as water vapor
- flood: unusually high levels in bodies of water, causing some water to leave those bodies and pour onto land
- latitude: the distance of a point on Earth from the equator, measured in degrees of the planet’s curvature
- precipitation: rainfall, hail fall, or snowfall
- storm surge: an abnormal rise of the ocean along a shoreline as a result of storm winds
Bibliography
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