Direct impacts

Climate change is the long-term heating of the Earth’s climate due to human activities. We will explore some of the direct impacts of climate change. These direct impacts include rising temperatures and sea levels, extreme weather, forest fires, drought, and flooding. Later, you’ll learn how those changes affect society as a whole, which we call indirect impacts. But for now, let’s focus on one direct impact: sea-level rise.

The burning of fossil fuels like coal creates greenhouse gases. These gases trap heat in the Earth’s atmosphere, causing global average temperatures to rise. As temperatures increase, polar ice sheets and glaciers melt, flooding water into the oceans. As the water warms, it also expands. So, when temperatures in the ocean go up, sea levels do, too.

The long view

Sea levels have gone up and down throughout history. The planet has seen ice ages and warming periods over hundreds of thousands of years. Some people believe sea-level rise today is part of this natural cycle. However, scientists have studied coral records and collected layers of sediment from the oceans to learn about the past. From these, we know sea levels have remained steady for the last 2,000 years.

Early in the twentieth century, sea levels suddenly started to climb. From 1900 to 1993, records tell us that sea levels rose 4 to 5 inches. In the 30 years between 1993 and 2023, sea levels rose another 4 inches. In Chart 1, you can see why the trend is so alarming. These changes are speeding up, and they match an increase in the burning of fossil fuels by humans.

The trend is even clearer when we put global temperature anomaly alongside sea-level rise. An anomaly is a change from something considered normal. Temperature anomaly shows us the increase or decrease in temperature from the average. It is shown below in Chart 2 (in gray) alongside sea-level rise (in blue).

See? Change

Let’s look at the rate of sea-level rise over the last 150 years. Chart 3 shows sea levels since 1880. How do we know about these changes? From 1880 to 1993, researchers used sea-level recordings collected around the world.¹ In 1993, NASA started measuring sea levels with satellites, which provided more exact global measurements. NASA is the U.S. space agency.

In the twentieth century, sea levels rose an average of 1.7 millimeters (0.07 inch) per year. By 1993, the rate had increased to 2 millimeters (0.08 inch). In 2016, it rose to 3.4 millimeters (0.13 inches). By 2022, the average increase grew to 4.4 millimeters (0.17 inches) per year. Researchers are already looking ahead to 2050. By then, they predict the rate will increase to 6.6 millimeters (0.26 inches).² This could result in the sea level along the United States coastline rising as much as 12 inches by 2050. That’s the same increase recorded in the 100 years between 1920 and 2020.³

We’re in over our heads

These might seem like small numbers. After all, 3 millimeters is about the length of a sesame seed. So, what’s the big deal? Well, there are three issues:

1. Over decades, those small numbers really add up. If you stack enough sesame seeds, things get messy quickly.
2. The rate of sea-level rise is quickly growing, so those yearly increases will get bigger.
3. As you can tell in Chart 2, it takes a while for sea-level rise to catch up with higher temperatures.

We won’t know what a 1.5- or 2-degree rise in global temperature will mean for sea levels until after it happens. We also don’t know how high the seas will rise in the next 100 years. The National Oceanic and Atmospheric Administration (NOAA) is a United States government agency. Its goal is to research and protect ocean resources. NOAA estimates that the oceans could rise between 1 and 8 feet (0.4 to 2.4 meters) by 2100 (see Chart 4). That’s a big difference. Scientists think that sea levels will continue to rise until 2100. How much they rise, however, will depend on how quickly we reduce our greenhouse gas emissions.

Humanity and the sea

What do the differences in the level of sea rise mean for the planet? What does it mean for you and your community? To start answering those questions, let’s take a look at Map 1, below.

Did you notice that you can recognize the shape of most of the continents from the population density spikes? This is because many people on Earth live very close to the ocean. Take a look at Map 2 below. About 40 percent of Earth’s population lives within 100 kilometers (about 60 miles) of a coast. Eight of the top 10 largest cities in the world are on a coastline.

Coastal areas are heavily populated and growing. Coastal areas are essential to many international businesses and development. They contribute to global industries like fishing and tourism. That’s why so many people choose to live near the coast. By the end of the twenty-first century, more than 410 million people will be at risk from rising sea levels. It will affect small islands in the Pacific Ocean, such as Kiribati and Tuvalu. These islands are at risk of being severely damaged or destroyed. If sea levels continue rising quickly, flooding and weather events will cause population shifts on every continent. People will migrate in and out of new areas. Coastal communities might experience extreme weather events like flooding, hurricanes, and typhoons.

The charts above deal with the average global sea-level rise. However, sea-level changes are different from place to place. Coastal regions will experience sea-level rise in unequal ways. How they are affected depends on geography and man-made protections, like dams. In some areas, the rising sea level is worse than the global average. These problems are worsened by issues such as drilling for resources. Despite less than a foot of global sea-level rise, high-tide flooding is 300 percent to 900 percent more frequent than 50 years ago.⁴ Coastal communities are already experiencing what climate change can do.

If you live inland, you might think you are safe from rising sea levels. But even regions farther from the coast will be affected by the indirect impacts of coastal flooding: economic damage, supply-chain disruption, migration, food shortages, and other unknown changes. You’ll learn more about these soon.

Conclusion

Sea-level rise is caused by climate change, which is caused by human activity. It threatens coastlines around the world and the people who live there. However, sea-level rise will affect all of us, not just those who live on the coast. And sea-level changes, like climate change in general, will affect communities unequally. As you review this article, think about what people and regions will be most at risk. You might want to explore more about what sea levels will mean for your community or coastal cities around the world. Check out these tools for more:

NOAA Sea Level Rise Viewer: This tool shows how the U.S. coastline will change if sea levels rise between 1 and 10 feet. For some locations — like Fulton Street in Manhattan — you can see a visual estimate of how a place will change based on sea- level increase.

NASA’s IPCC AR6 Sea Level Projection Tool: This tool allows you to see predicted sea levels around the world in different years based on several different warming scenarios.

1 Church, J.A., White, N.J. “Sea-Level Rise from the Late 19th to the Early 21st Century.” Surv Geophys 32, 585–602 (2011). https://link.springer.com/article/10.1007/s10712-011-9119-1
2 Note that these estimates vary depending on the source. However, the general trend of an increasing rate of increase is consistent across sources.
3 EPA. “Climate Change Indicators: Sea Level.” https://www.epa.gov/climate-indicators/climate-change-indicators-sea-level#ref1
4 Lindsey, R. “Climate Change: Global Sea Level.” Climate.gov. https://www.climate.gov/news-features/understanding-climate/climate-change-global-sea-level