What have you done lately about climate change?

In the last two weeks, about 700 Americans -- with more to come -- have been arrested in front of the White House, calling on President Obama to block the construction of the $7 billion, 1700-mile Keystone pipeline project that will bring Canadian tar sands oil to the largest refineries in the United States.

They include Bill McKibben, the writer, activist and founder of 350.org, who led the protests; James Hansen, NASA's leading climate scientist; Gus Speth, who lead the Council on Environmental Quality under President Carter and went on to become dean of the Yale School of Forestry; Greenpeace executive director Phil Radford; actresses Daryl Hannah and Margot Kidder; and my friend and rabbi, Fred Scherlinder Dobb of Adat Shalom Reconstructionist Congregation in Bethesda.

Standing behind them are the nation's leading environmental groups -- Greenpeace, the Sierra Club, the Natural Resources Defense Council, the Environmental Defense Funds, Friends of the Earth, the National Wildlife Federation and others. In a letter to Obama, they described the Keystone pipeline ruling as “perhaps the biggest climate test you face between now and the election.”

"There is not an inch of daylight between our policy position on the Keystone XL pipeline, and those of the very civil protesters being arrested daily outside the White House,” the groups said.

There are strong arguments for and against the pipeline, which we'll get to in a moment, but first, a few words about McKibben and the protestors. I went to the White House to talk with them because I share their belief that climate change is not just another issue, but the defining issue of our time. To be sure, it's not an issue that mobilizes people, at least not yet, for a number of reasons: You can't see carbon dioxide pollution, climate science is complex (but clear in its basics), global warming is a slow moving threat and the troubled U.S. economy has crowded virtually every other concern off stage since the summer of 2008.

But ... greenhouse gas emissions are rising steadily, as are atmospheric concentrations of CO2. That means we are edging closer to catastrophe every day.

That's why hundreds of people stepped up to get arrest while many of us enjoyed the last couple of weeks of summer.

Said Speth: "It's time to step outside the system, and do some things we haven't done before."

On the Climate Progress blog, Peter Anderson, a Wisconsin recycling consultant, wrote:

For me -- like for most of us -- the precipitate that galvanized my newfound resolve in the face of a corporate choke hold on Congress was the simple, elemental, drive to protect my children: my three girls, now grown up, and my 14 year old boy who is still a child. Theirs is the generation that, in place of an inheritance, will be left to inhabit an overheated world that my cohort is callously leaving behind as, in a blissful state of denial, we party the night away.