In “Sand County Almanac,” Aldo Leopold launched one of the most remarkable ideas in the history of conservation when he proposed we have ethical duties to the land and to the plants and animals that live on it.

Ever since humans lived in tribes, there were rules of conduct for the tribe members. The rules limited the action of individuals, but they served to benefit the community. What Leopold called “the land ethic” simply enlarged the boundary of that community to include soils, waters, plants and animals.

Leopold’s land ethic essentially is an ethic of conservation. It is designed to preserve the biotic community in the same way rules of conduct preserve the integrity of a human community.

Much has changed in the seven decades since “Sand County Almanac” was published. Human enterprise has become an unprecedented threat to the environment, and we are in need of a “land ethic” now more than ever.

The population has grown from 2.5 billion to more than 7 billion. Clean water is becoming scarce and resources are dwindling. Pollution is spreading across land and sea. The Pacific Ocean has sprouted an island of floating garbage bigger than Texas.

Species are becoming extinct between 100 and 1,000 times faster than normal, mostly because of deforestation and loss of habitat. It is not just individual species but whole ecosystems that are now being threatened.

Anthropogenic climate change is ramping up. Atmospheric CO2, the chief driver of global warming, has passed 400 parts per million, higher than it’s been in 400,000 years.

Glaciers are melting and the polar ice caps are shrinking. The arctic ice is melting even faster than predicted, and all that melt water is pouring into the ocean.

The sea level at Miami Beach already has risen 6 inches and some lawns there are regularly being flooded at high tide. Greenland alone contains enough ice to raise the sea level another 20 feet, and that would wipe out a good part of many coastal cities, including Washington, D.C.

Critics of climate change say that, even if it’s true that human activity is to blame, the problem would just cost too much to fix. But it will cost the economy even more in the future if we do nothing, because climate change is not going away.

If the sea level were to rise another 20 feet or more, what would it cost to move a city like Miami? And where would they put it?

No one can guess the ecological and human cost of species extinction. There are about 2 million species that we know about, and as many as 10 million that remain undiscovered. Of the known species, only a few have as yet been examined by pharmaceutical companies.

So far, a third of all prescription drugs have been derived from compounds originally found in plants and animals, including life-saving antibiotics, blood thinners, inflammation controls, sedatives, cardiac stimulants and anti-cancer drugs. It’s hard to put a price on that.

We are caught in an ethical conflict between short-term and long-term values. Our brains apparently have evolved to think in terms of short-term benefits. Clinging to small groups and focusing on short-term gains has led to longer life and more offspring.

And so, we are hard-wired to value the present and to discount, or undervalue, the future. But conservation is all about the future. The chant of the economist is “grow the economy,” and the response of the conservationist is “save the earth.”

Combining the two visions will be very difficult. “But combine them we must,” says Wilson, “because a universal environmental ethic is the only guide by which humanity and the rest of life can be safely conducted through the bottleneck into which our species has foolishly blundered.”

The bottleneck is a constriction of growing population, increasing consumption, climate change, shrinking biodiversity and decreasing renewable resources. Getting through that bottleneck successfully will require a sea change in our culture and thinking.

Black Elk said, “We don’t inherit the earth from our ancestors, we borrow it from our children.” But we are not so much borrowing as stealing it. Borrowing implies an intention to return the object pretty much as it was, and we have no intention to handing the earth off to our children in the same shape that we found it.

Leopold spoke more pointedly: “When the logic of history hungers for bread and we hand out a stone, we are at pains to explain how much the stone resembles bread.”

We certainly will be hard-pressed to explain our ethical blunder in handing out those stones.

The earth is our home, a beautiful planet teeming with an amazing complexity of life, a planet whose soil has given birth to the great human adventure. We have a duty to ourselves and to our children to make sure it remains intact.

Richard Weber is a nature enthusiast living in Ellis County.