Earlier this summer, Pope Francis riveted the world with the release of "Laudato Si," the papal encyclical on the environment. Outlining our unique place in history, this simple, powerful, and very human message called on people of all faiths and nations to recognize changes that need to be made to ensure a future for both nature and human society.

At once a studied argument and a call to action, the encyclical in spirit can be summarized in one sentence: "Never have we so hurt and mistreated our common home as we have in the last 200 years."

As Philadelphians, we anticipate the pope's visit as a chance for the people of our region to bring that message home - to understand accepted climate science and to take action.

Our own work at the Academy of Natural Sciences of Drexel University builds on more than 70 years of efforts to understand and manage the ways people affect our planet's life-support system. We're especially interested in streams and rivers (supplying 60 percent of the nation's drinking water), in the variety of living things (which in turn supply, well, almost everything), and the threats to their survival. But, as the encyclical made clear, global climate change has the potential to overshadow all our interactions with the environment. As long ago as the mid-19th century, scientists understood that emissions of greenhouse gases could drastically alter the state of the Earth.

Now these early predictions have been borne out, and we are moving toward conditions not seen in recorded history. To say that climate change poses a threat to the ecosystem is like saying a burning building poses a threat to the furniture. Scientists of earlier eras could treat the greenhouse effect as a hypothetical curiosity; we don't have that luxury.

The leading economist Jeffrey Sachs, a key Vatican adviser who helped organize the encyclical, echoes this in a recent article: "The pseudo-debate about climate science has always been about politics, not science." To mark the pope's visit to Philadelphia, Sachs expanded on these thoughts as the first of several distinguished panelists at a free public forum, "Climate Change: A New Dialogue," held Monday at the academy.

Last year's National Climate Assessment did not equivocate: "Climate change, once considered an issue for a distant future, has moved firmly into the present. ... We know with increasing certainty that climate change is happening now." In scientific circles, the reality of this hasn't been a subject of serious debate for years. By several estimates, 97 percent of climate scientists agree that the world is warming due to human actions.

We're not climate scientists. We study Earth's living systems. As such, we wouldn't presume to make expert statements on highly specialized fields like atmospheric chemistry and climatology. But we are part of the world's scientific enterprise, and we understand the level of scrutiny and testing faced by any scientific conclusion. We know well the gauntlet of data collection, peer review, experimental replication, conference debates, and ongoing reevaluation that any new idea must go through to become part of the scientific consensus. To have 97 percent of scientists in any field agree on the broad outlines of a principle is about as close to certainty as science ever gets.

Again, we're not climate scientists, but we are the ones who try to figure out what climate change will do to the natural systems on which we all rely. And we already see hints of what is happening.

Academy scientists are watching sea-level rise unfold in the coastal wetlands of our region. Migration patterns, from birds to bugs, are shifting, and patterns of plant growth are moving steadily north.

In the rawer conditions of Mongolia, where academy researchers have worked for decades, we are indeed watching definitive changes in climate, measured by science and confirmed by the deep local knowledge of the nomadic herders who aid our work.

While we waste time arguing settled science - that climate change is occurring - we lose sight of the myriad human costs this change is initiating. Sea-level rise, superstorms, temperature extremes, and catastrophic flooding are all just words until they happen. When they happen, lives and livelihoods are destroyed, often irrevocably.

The papal encyclical went to the heart of the matter, highlighting the human costs and the moral imperative of recognizing and addressing the problem. President Obama echoed this in announcing new regulations to limit greenhouse gases: "We're the first generation to feel the impact of climate change and the last generation that can do something about it."

We commend the president for stating the issue so plainly, and we especially commend Pope Francis and the Pontifical Academy of Sciences for making such a compelling case for action. Speaking as both scientists and citizens, we say the debate is over. It's time to get to work.

Roland Wall is senior director of environmental initiatives at the Academy of Natural Sciences of Drexel University. rwall@ansp.org

David Velinsky is vice president for environmental research at the Academy of Natural Sciences of Drexel University and head of the department of biodiversity, earth, and environmental science at Drexel University. velinsky@ansp.org

Ted Daeschler is vice president for collections and associate curator of vertebrate biology at the Academy of Natural Sciences of Drexel University and an associate professor in the Drexel University College of Arts and Sciences. daeschler@ansp.org