The changes in the environment since the first Earth Day 51 years ago are striking, but many challenges still persist.

On April 22, 1970, millions across the U.S. took part in protests, cleanups, and various activities for the first Earth Day. The event was the idea of Democratic Senator Gaylord Nelson of Wisconsin and became a defining moment for the rising environmental movement in America.

As Americans began to realize that industrial progress was affecting the environment and their health, CBS anchorman Walter Cronkite highlighted the issue in a special broadcast, saying Earth Day participants shared a "common cause of saving life from the deadly by-products of that bounty: the fouled skies, the filthy waters, the littered Earth."

The creation of the Environmental Protection Agency and the first major environmental laws happened the same year. Over time, efforts to address environmental issues have fluctuated. Although significant progress has been made in reducing air pollution, the threat of climate change has grown substantially. Let's examine a few environmental indicators to understand the progress — or lack thereof — since the first Earth Day 50 years ago.

Climate

The steep, unmistakable rises in the graphs below lie at the heart of the global warming issue. As more cars fill the roads and more coal and natural gas are burned for energy, the amount of carbon dioxide in the atmosphere continues to rise.

The current concentration of CO2 in the atmosphere is above 410 parts per million (ppm), compared to about 325 ppm in 1970 (and 280 ppm before the industrial revolution). The additional heat trapped by this CO2 has already caused global temperatures to increase by around 1 degree Celsius since preindustrial times. Under the 2015 Paris climate agreement, countries have committed to keeping global warming well below 2 degrees Celsius (3.6 degrees Fahrenheit) above preindustrial levels, aiming for a more ideal target of below 1.5 degrees Celsius (2.7 degrees Fahrenheit). So far, efforts to reduce emissions have not been sufficient to meet these goals.

Air Pollution

Among the fundamental issues that drove Nelson and his colleagues to hold the first Earth Day was the rampant, deadly pollution clogging U.S. skies. One of the worst air pollution disasters in the nation's history came in the fall of 1948, when weather conditions caused a stew of smog to build up in the industrial town of Donora, Pennsylvania, sickening thousands and killing 20 people. Congress passed various laws to limit air pollution in the ensuing years, but the Clean Air Act of 1970 was the landmark legislation that truly ushered in the stringent and comprehensive regulation of emissions from power plants, factories and cars.

The graphs below show how the law and its subsequent revisions have led to notable nationwide drops in three major pollutants: nitrogen dioxide, sulfur dioxide and lead. Nitrogen dioxide and sulfur dioxide can be harmful to human health when breathed in, and both react with other chemicals in the atmosphere to create the particles that contribute to smog.

Lead is highly toxic and can cause neurological and cardiovascular problems. One of the key sources of airborne lead pollution in the mid-20th century was leaded gasoline; since it was phased out, beginning in the mid-1970s, lead levels have plummeted. Many environmentalists and scientists who study air pollution are concerned that the considerable progress over the past 50 years might be stalled — or even reversed — by actions the EPA has taken during the Trump administration to weaken air pollution rules and enforcement.

Water

One of the most significant events in the U.S. water crisis was the fire on the Cuyahoga River on June 22, 1969. Pollution from industrial activities along the river, which flows from Akron to Cleveland, had fueled over a dozen fires since the 1800s, devastating fish populations. This catastrophic event played a key role in the passage of the Clean Water Act in 1972. This landmark legislation sought to curb pollution in the nation's waterways, tackling contaminants from industries, sewage plants, and agriculture.

The graphs below depict data from two significant U.S. water bodies: Lake Michigan and Lake Erie. The first graph shows the change in phosphorus levels, a crucial nutrient that contributes to toxic algal blooms, entering Lake Erie via the Maumee River in Toledo. The primary source of phosphorus in the lake is runoff from agricultural fields, which provides drinking water to 11 million people. Researchers are working to find ways to adjust farming practices to reduce phosphorus contamination.

The Lake Michigan graph illustrates a different type of pollution: polychlorinated biphenyls (PCBs), chemicals linked to cancer and other health problems. Although PCB production was banned in 1979, the levels of these compounds in the air and fish have decreased, yet they remain high enough for some states to advise limiting fish consumption from the lakes.

Waste

Americans are generating significantly more waste than they did half a century ago — and it's not solely due to population growth. On average, each person in the U.S. produces 4.5 pounds (2 kilograms) of trash per day, compared to just 3.25 pounds (1.4 kilograms) in 1970. The composition of waste has also shifted, with plastic waste now making up a larger portion than ever before.

This change is a direct result of the explosion of plastic products over recent decades, from under 50 million tons (45 million metric tons) produced in 1970 to more than 320 million tons (290 million metric tons) today. Although recycling and composting efforts for some materials have increased, a significant portion of waste still ends up in landfills. In 2017, 139.6 million tons (129.6 million metric tons) of the 267.8 million tons (243.7 million metric tons) of trash generated were dumped in landfills. However, this was a slight improvement from the 145.3 million tons (131.8 million metric tons) discarded in 1990.

It's evident that both the U.S. and the world have made progress in understanding humanity's impact and the need to protect the environment. However, much work remains. As Nelson stated in 1984, "The ultimate test of man's conscience may be his willingness to sacrifice something today for future generations whose words of thanks will not be heard."

This article was originally published in Scientific American and is republished here as part of Covering Climate Now, a global collaboration in journalism focused on enhancing climate coverage.