Direct impacts

Climate change is the long-term heating of the Earth’s climate due to human activities, particularly the emission of greenhouse gases. This data introduction will explore some of the direct impacts of climate change. These include rising temperatures and sea levels, extreme weather, forest fires, drought, and flooding. Later in the course, you’ll learn about indirect impacts and how those changes affect all of society, but for now, let’s focus on one direct impact: sea-level rise.

As you might know, greenhouse gas emissions trap heat in Earth’s atmosphere, causing global average temperatures to rise. Greenhouse gases are caused by burning fossil fuels. As global temperatures increase, water once stored in polar ice sheets and glaciers melts and enters 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 risen and fallen many times throughout history. Our planet has gone through ice ages and warming periods over hundreds of thousands of years. Some people believe the sea-level rise today is part of this natural cycle. However, coral records and ocean sediment cores tell us sea levels have remained fairly steady for about 2,000 years. Sediment cores are collected from beneath the Earth, and samples show layers of sediment over time.

Beginning in the early 20th century, sea levels suddenly started to climb, and they haven’t stopped. From 1900 to 1993, historical tide records tell us that sea levels rose 4 to 5 inches. In the 30 years between 1993 and 2023, the trend continued as sea levels rose another 4 inches. In Chart 1, you can see why the trend is so alarming. These changes are speeding up. They correspond to an increase in the burning of fossil fuels by humans, such as natural fuels like gas or coal.

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

See? Change

Let’s look at the last 150 years of data and see how the rate of sea-level rise has changed. Below, Chart 3 shows a closer view of sea levels since 1880. How do we know about these changes? From 1880 to 1993, researchers relied on historical sea-level recordings collected from different ports worldwide.¹ In 1993, NASA started measuring sea levels with satellites, which provided more exact global measurements.

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) per year. In 2016, it again rose to an average of 3.4 millimeters (0.13 inches). By 2022, it had increased to 4.4 millimeters (0.17 inches) per year. This was more than double the long-term trend in the twentieth century. By 2050, researchers predict the rate will increase to 6.6 millimeters (0.26 inches).² This rate of acceleration could result in the sea level along the United States coastline rising as much as 12 inches by 2050. It’s the same increase that was recorded over 100 years between 1920 and 2020.³

We’re in over our heads

These might sound 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 the 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 accelerating, so those increases every year will get bigger.
3. As you can tell in Chart 2, it takes a while for sea-level rise to catch up with the temperature increases.

We won’t know what a 1.5- or 2-degree rise in global temperatures will mean for sea levels until 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 research focuses on understanding and preserving 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 are almost certain that sea levels will continue to rise until 2100. How much they rise, however, will depend on how quickly humanity moves to reduce our greenhouse gas emissions.

Humanity and the sea

What does the variation in the level of sea rise mean for the world? What does it mean for you and your community? To begin answering those questions, take a look at Map 1, below.

Did you notice that you can recognize the shape of most of the continents even though there are no borders, no coastlines, no oceans—just population density spikes? That’s because a lot of the population on Earth lives 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. In fact, eight of the 10 largest cities in the world are on a coastline.

Sea-level rise is a huge problem because coastal areas are heavily populated. On top of that, coastal populations are growing. By the end of the twenty-first century, more than 410 million people will be at risk from rising sea levels. This will affect small islands in the Pacific Ocean, such as Kiribati and Tuvalu, whose very existence is threatened. If sea levels continue to rise at the current rates, flooding and weather events will lead to massive migrations on every continent. Coastal communities will be open to extreme weather events like flooding, hurricanes, and typhoons. Coastal areas are essential to many international businesses and infrastructure, including fishing industries and tourism. This is why so many people choose to live there.

The charts above deal with the average global sea-level rise. In reality, sea-level changes vary from place to place. Coastal regions will experience sea-level rise unequally, depending on geography and protective structures such as levees and dams. In some areas, the rising sea level is worse than the global average. These problems are worsened by drilling or erosion, which is the wearing away of the Earth’s surface over time. 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’re safe from this direct impact. But even inland regions will be affected by the indirect impacts of coastal flooding: economic damage, supply-chain disruption, migration, food shortages, and other unforeseen 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 all over the world and the people who live there. But 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 on the planet will be most at risk. If you want to explore more about what sea levels will mean for your community or coastal cities around the world, you can check out these tools:

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