Direct impacts

Climate change is altering our world in all sorts of ways. In this data introduction, we’ll explore some of the direct impacts of climate change. Direct impacts include global warming, rise of sea levels, extreme weather, forest fires, drought, and flooding. Later in the course, you’ll learn about the societal impacts of those changes, which we call indirect impacts, but for now, let’s focus on one direct impact: sea-level rise.

As you have likely learned, greenhouse gas emissions are trapping heat in Earth’s atmosphere, causing a general rise in global average temperatures. As global temperatures increase, water that was once stored in polar ice sheets and glaciers melts and enters the oceans. As the water gets warmer, it also expands and so, when temperatures in the ocean go up, so do sea levels.

The long view

Sea levels have fluctuated throughout the history of the Earth. Natural processes such as plate tectonics have shifted continents and have both raised and lowered the sea floor. Ice ages and warming periods have alternated over hundreds of thousands of years, causing sea levels to rise and fall. Some people claim that the sea-level rise we’re seeing today is part of one of these long-term natural cycles. However, records from coral and ocean sediment cores tell us that for most of the last 2,000 years, as temperatures stabilized after the last ice age, sea levels have remained relatively consistent.

Around the turn of the 20th century, that changed: 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. The trend accelerated in the 30 years between 1993 and 2023 as sea levels rose another 4 inches. In Chart 1, you can see why the trend is so alarming. These changes are accelerating rapidly, and they coincide with a change in human activity: the burning of fossil fuels.

The trend is even more apparent when we overlay global temperature anomaly (below in Chart 2, in gray) alongside sea-level rise (in blue). Temperature anomaly is a way of measuring the increase or decrease in temperature from a baseline average.

See? Change

Let’s dig into the last 150 years of data and see how the rate of sea-level rise has changed. Chart 3 below shows a zoomed-in view of how sea levels have changed 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 measurements from all over the globe.

In the twentieth century, sea levels rose an average of 1.7 millimeters (0.07 inch) per year. By 1993, the rate had accelerated to 2 millimeters (0.08 inch), and in 2016, it reached 3.4 millimeters (0.13 inches) per year. By 2022, it had increased to 4.4 millimeters (0.17 inches) per year, more than double the long-term trend in the twentieth century. By 2050, researchers predict that the rate will increase to 6.6 millimeters (0.26 inches).² This rate of acceleration could result in the sea level along the U.S. coastline rising as much as 12 inches by 2050. That’s as much increase as was recorded in the 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 course of 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 per-year numbers are going to get bigger.
3. As you can tell in Chart 2, it takes a while for sea-level rise to catch up with the increase in temperatures.

We won’t know exactly what a 1.5- or 2-degree rise in global temperature will mean for sea levels until after we’ve reached that threshold. We don’t really know how high the seas will rise by the end of this century, but the National Oceanic and Atmospheric Administration (NOAA) estimates that the oceans could rise between 1 and 8 feet (0.4 to 2.4 meters) by the year 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, let’s 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 Earth’s population lives 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.

Sea-level rise is a huge problem for humanity because coastal regions are very densely populated, and coastal populations are increasing. By the end of the twenty-first century, more than 410 million people are expected to be at risk from rising seas. This will not only affect small island nations 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 drive massive migrations on every continent. Coastal communities will be even more vulnerable to extreme weather events like flooding, hurricanes, and typhoons. Coastal regions are critical to international economics and infrastructure, including fisheries, shipping, and tourism — which is why so many people live along the coast.

To make matters worse, the charts above deal with the average global sea-level rise. In reality, sea-level changes are different from place to place and are based on factors such as geography and gravitational pull. Coastal regions will experience sea-level rise unequally, depending on available infrastructure, such as levees and dams. In some regions, sea-level rise has been and will be far worse than the global average would indicate, thanks to issues like erosion and drilling. 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 the consequences of this direct impact of climate change.

If you live inland, maybe in Colorado or Iowa, you might think you’re safe from this direct impact. But even landlocked regions will be affected by the indirect effects 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 40 percent of humanity who live there. But sea-level rise will affect all of us, not just those who live on the coast. Sea-level changes, like climate change in general, will affect communities unequally. As you review the data and tools in this article, think about what people and regions of the world will be most at risk from the direct impacts of climate change. If you want to explore more about what sea levels will mean for your community or your favorite coastal cities around the world, you can check out these tools, maintained by NOAA and NASA:

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