That the climate is changing is undeniable. Scientists have documented a marked increase in the temperature of the Earth and the oceans since the 19th century. Most of the warming has been in the past 35 years, with 16 of the 17 warmest years on record occurring since 2001.
Together with a warming climate, we see changes in local weather patterns: increased rainfall in some areas, drought in others, and storms that are more destructive. Warmer oceans contain more energy, leading to the powerful hurricanes we've seen in recent weeks. Climate change may not cause extreme weather but apparently intensifies it.
Many sources contribute to climate change, some natural and some manmade. But in the scientific community, the consensus is that burning fossil fuels has caused the bulk of the warming. Burning fossil fuels produces carbon dioxide and other greenhouse gas emissions, which trap heat in the Earth's atmosphere.
The world's big economies -- China, the United States, the European Union, India and Russia -- are responsible for much of the world's carbon emissions. But developing nations are heavily dependent on hydrocarbon fuels and are becoming more so.
The United Nations and other groups have put forward a number of international attempts to counter climate change. Most notably, the 2016 Paris climate agreement -- signed by the EU and nearly 200 nations, including the U.S. -- called on all countries to contribute to reducing greenhouse gases and to mitigate the effects of climate change.
In the U.S., former President Obama's Clean Power Plan aimed to reduce emissions from electrical generating facilities by over 30 percent by 2030. But the Trump administration is expected to eliminate the Clean Power Plan and withdraw from the Paris agreement.
It is unclear how our climate policy will unfold, given the current political divide. Democrats, in general, are committed to climate treaties and far-reaching reductions in emissions. Republicans -- with some notable exceptions, such as former Secretary of State George Shultz and former Environmental Protection Agency administrators Bill Ruckelshaus and Bill Reilly -- want to back away from those commitments.
President Trump has argued that climate change was a hoax invented by China to limit America's competitiveness. His view strikes a chord with voters who are suspicious of agreements that limit American sovereignty.
Many voices call for more study. While study is important, it is not sufficient. The topic is complex, involving statistical simulations, scientific judgments and a heavy dose of politics. It's important for us to learn all we can about what's happening and how human behavior influences climate change. But while we study, we should try to mitigate the problems.
Currently, however, the U.S. lacks a coherent plan to deal with climate change.
What we do about climate change will have an impact on how we build our homes, plant our crops, construct our roads and bridges, and locate schools and enterprises. It comes into play in many different activities.
We have to pay attention to what climate policy means for jobs, economic growth, burdens on business and American competitiveness. Surveys show that Americans want to limit the damage caused by climate change, but they are wary of a growing web of regulation.
But two developments can't be ignored when we talk about climate change.
First, the CIA, the Defense Department and the State Department classify climate change as a current and ongoing national security concern.
National-security officials see climate change as a threat to the safety of American citizens and institutions. They point out that flooding, drought, crop failures and other consequences of climate change put pressure on governments and increase the risk of instability and conflict. For example, Nigeria, a state of strategic importance, is challenged by desertification of the north and rising sea levels that threaten Lagos, a city of 20 million people.
They also point out that climate change and shifting weather patterns are exacerbating the spread of diseases like Ebola and malaria, threatening food production and increasing the flow of refugees.
They are also concerned that reductions in the Arctic ice cover will lead to conflict over tourism, shipping, and resource exploration and extraction. When disasters strike, our military is often called to respond. And as one Defense Department official put it, referring to rising sea levels, "You can't dock a ship if the pier's underwater."
The other development is that American states and local governments are acting on their own to combat climate change. At least 11 states -- including two of the most populous, California and New York -- have vowed to defy Trump and abide by the Paris agreement. And 381 mayors from cities with combined populations of 67.9 million have pledged to meet the Paris commitments by investing in renewable fuels and energy efficiency.
While our nation dithers over policy, our national security and many state and local officials understand that we ignore the question of climate change at our peril. My view is that the threat is unforgiving and now is the time to act. Our generation is going to be judged in no small part on how we respond to this existential threat. Our actions today will determine what kind of planet we leave the generations that follow us.