In recognition of Earth Day, President Obama walks the Anhinga Trail at Everglades National Park in Florida on April 22, 2015. (Susan Walsh/Associated Press)

Peter Engelke, a senior fellow at the Atlantic Council, and J.R. McNeill, professor of history at Georgetown University, are co-authors of the new book “The Great Acceleration.”

The first Earth Day, held April 22, 1970, was designed to draw popular attention to environmental causes and the need to protect nature. It succeeded. At age 46, Earth Day continues to focus our minds on preserving the natural world, if only for a brief moment each year.

But what if a basic assumption about our planet, one that we all make on Earth Day and every other day, is wrong? What if, in 2016, we no longer inhabit the Earth we once did? What if the nature we seek to protect has already been profoundly altered — by us? Would that undercut the logic of Earth Day?

Many scientists and scholars wonder if the Earth has entered a new epoch in its 5 billion-year history. They are debating whether we should officially declare the end of the Holocene, the geological epoch that began 11,700 years ago, and the start of what they are calling the Anthropocene. The Holocene was great for our species. The climate was remarkably stable, helping us prosper as never before in humankind’s 200,000-year history. The Anthropocene concept, as the root of the term suggests, rests on the notion that human activity has created a new version of an old planet.

That new version of Earth began with the environmental turbulence called the Great Acceleration, a term now used to describe the initial decades of the Anthropocene. Since about 1945, human interference in the Earth’s ecology has reached a new intensity, the result of more humans, more energy use and more transformative technologies, among other things.

According to this new way of thinking, the Great Acceleration (of human impact on the global environment) helped launch the Anthropocene (a new interval in Earth’s history). Both began around the middle of the 20th century. The Great Acceleration will eventually fade. The Anthropocene will last for many thousands of years, and perhaps millions.

By 1970, the Great Acceleration had already been underway for a quarter century. Earth Day and the mass environmental movement of the 1970s led to important reforms in the United States and elsewhere, especially in pollution control. Enforced air- and water-quality standards prevented millions of premature deaths in the United States and tens of millions worldwide.

But environmentalism did not slow the Great Acceleration much. It remains underway, in particular as the fruits of the American-led postwar economic system have spread around the world. (China and India are the two most consequential beneficiaries.) The Great Acceleration’s ecological results are all around us: biodiversity loss, soil erosion, ocean acidification, overfishing, aquifer depletion, rapid carbon loading of the atmosphere and oceans, deforestation, altered global geochemistry (the carbon, nitrogen, sulfur and phosphorous cycles, among others), and a thousand other markers of change. The latest studies of climate change and sea-level rise suggest that neither is slowing down.

While the future is known to no one, a grasp of current trends in historical perspective helps us assess the possibilities for the Anthropocene.

Many changes that the Great Acceleration have brought are here to stay. The extinctions of species, now happening unusually fast thanks to human actions, change the planet’s biology forever. Some radionuclides from atomic testing will be around for 100,000 years. Some of the roughly 500 billion tons of concrete poured since 1950 will remain visible in the geological record for eons. Hence the notion of a world enduringly changed by human beings.

Some other environmental changes of the Great Acceleration, however, will prove fleeting. Compelling science and prudent international action checked the loss of stratospheric ozone over the past 25 years, and in the decades ahead the ozone layer should rebuild itself. Political will and technical fixes have also reduced acid rain in North America and Europe, and parallel efforts in China lately have begun to bear fruit.

Most important, many changes accompanying the Great Acceleration remain open-ended. That is another way of saying that the Anthropocene, while here for a very long time still to come, is not yet fully shaped. Far from it.

Earth Day reminds us to think hard about the future we want for our planet, ourselves and all those who will come after us. The Great Acceleration already has nudged the Earth into a new state. It is up to us to determine its contours. Can we slow the pace of environmental change, perhaps by shifting away from reliance on fossil fuels, enough to make the Anthropocene a comfortable epoch for ourselves? We’ve done a lot since the first Earth Day to curtail the ecological disruptions we cause. While the Anthropocene remains young, and our options remain open, we must do much more.