New York and Berkeley, California — President Barack Obama opened a new chapter in America’s role in solving global problems in his speech to the U.N. General Assembly. By calling for the U.S. to re-engage in the global community, he has set us on a new course to preserve American liberty. The preservation of liberty has been the most powerful unifying political commitment for generations of Americans. With global warming, the threats to our liberty are now tied more than ever before to the actions of all nations. The climate change negotiations in Copenhagen this December and in D.C. provide a critical opportunity for the U.S. to start down this new path.

Many of us in the United States have not personally experienced the absence of liberty nor the fear of political insecurity. Yet, if we do not act quickly to address global warming, our children and grandchildren will likely confront these challenges with increasing constraints on their choices and their well-being. Decisions underlying our children’s liberty can no longer be governed simply by election cycles. These decisions are upon us today and they cannot wait.

We write this as father and daughter, with different windows on global issues: one with a 50-year career in diplomacy, the other with a 20-year career in environmental science. Despite our diverse backgrounds, we both see climate change as the biggest and most challenging threat on the horizon to American values and liberty.

The struggles for liberty and global security in the 20th century were rooted largely in the oppression within and the conflicts with the closed societies of ideologically driven despotisms — of Fascism, Nazism and Communism.

In the 21st century, the battlefields will be staggeringly more complex. Rising temperatures and changing precipitation patterns are already decreasing water availability, reducing air quality, and increasing frequency of floods, droughts and wildfires. If climate change continues unabated, these changes will greatly affect our children’s choices on where they live, what they eat and, most likely, how they are governed and the wars they fight.

The ingenuity of the U.S. and the global community has led to advancements in science and technology that have enabled us to tackle tremendous challenges in the past. We must harness this ingenuity; however, we cannot wait for the scientific or technological breakthrough that could solve climate change. The changes that humans are inflicting on our Earth are destabilizing our life support system. Once this is broken, we may never be able to fix it.

To address the increasingly complex needs of security and liberty, we need international cooperation at a level never imagined in the past. As climate change takes its toll over the coming decades there is no individual future of liberty and no individual national future of security. Security and liberty will be determined by the global community.

After the fall of the Berlin wall, the United States could have chosen to join other nations in building new collective strategies to respond to global challenges. At that moment, the U.S. had the opportunity to deploy its commitment to help the world prepare for these new challenges and preserve liberty in the 21st century. Instead the U.S. chose to continue to deploy liberty as the warrior — as the liberator — rather than collaborator.

If our children are to overcome the hurdles of liberty in this century we will need to cultivate a role for the U.S. as a collaborator and not as the liberator. However, Americans face a particular challenge in taking on this role. Some argue that climate policies represent yet another subterfuge of the state to curb the liberty of individual citizens.

This very American obsession with individual liberty has been an obstacle for the U.S. in playing a leadership role on global issues. Following WWII, the U.S. was the evangelist of international law, agreements and organizations; however, in recent years, we have increasingly operated as a "lone ranger."

The unique American experience with liberty has magnified our obsession with the sanctity of our national sovereignty. The United States has chosen to avoid agreements that would limit individual freedoms. The U.S. Congress has not ratified a treaty for decades and presidents regularly opt out of international negotiations.

Yet, at the U.N. General Assembly, President Obama pledged a reversal of the U.S. spiral into selfish survival. President Obama and the U.S. Congress together must seize the moment at Copenhagen and in Washington to make good on this pledge and establish the U.S. as the credible leader for the preservation of liberty for future generations.

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    William Luers was the American Ambassador to Czechoslovakia and Venezuela and Dr. Amy Luers is a climate scientist and Environmental Program Manager at Google.org. All the views stated here are her own.