I see two urgent, existential threats not just to our country, but to our planet — nuclear proliferation and climate change. There are enough nuclear weapons in the world to destroy humankind many times over; we should be doing all we can to ensure that nuclear weapons do not end up in the hands of terrorists or states who seek to harm America. Climate change, gone unchecked, promises to destroy food systems, spark famines, violent conflicts, and mass migrations, destabilize governments, and cause deadly extreme weather events — and in fact, this is already happening all around the world.There is no silver bullet to reducing the risk of nuclear warfare with other nation-states. Doing so will take expert, sustained, and multilateral efforts led by American diplomats and other government officials to induce states to abandon their attempts to develop nuclear weapons. I support the Iran deal and agree with most experts who view it as effective. The cavalier attitude of the Trump Administration to the possibility of nuclear warfare with North Korea should keep us all up at night, and I support changing American law so the President cannot unilaterally decide to launch a nuclear strike, along with reopening a multilateral dialogue with the North Korean regime.Preventing nuclear weapons from being acquired by terrorists first and foremost means reducing and securing the amount of material that could be used to build a nuclear weapon or “dirty bomb.” President Obama’s Nuclear Security Summit initiative induced 14 countries to eliminate their weapons-grade plutonium and uranium; it is unfortunate that our country’s current leadership lacks the credibility and skill to continue these critically important efforts. Russia is a dangerous source of unsecured nuclear material and so it is especially incumbent on President Trump, who boasts of Putin’s admiration for him, to do far more to ensure that it does not fall into the hands of terrorist groups. American leadership and sustained, dogged diplomacy are the most important ingredients for preventing loss of life due to a nuclear event.The threat of climate change requires that we accelerate the pace of our transition to clean energy. I support tools including carbon taxes, renewable fuel standards, renewable electricity standards, and incentives for clean energy development to drive that transition forward. We know that these tools work, while also creating safe, good jobs, reducing energy costs, and making our communities cleaner and healthier. And if we want to leave a stable, livable world to our children and grandchildren, it is past time to implement them.