A new global warming report issued on April 5 by the United Nations paints a near-apocalyptic vision of Earth’s future: hundreds of millions of people short of water, extreme food shortages in Africa, billions of people in Asia at risk from flooding; millions of species sentenced to extinction; rampant disease.

Despite its grim vision, the report was quickly criticized by many scientists surveyed by the Los Angeles Times, who said its findings were watered down by governments seeking to deflect calls for immediate action. Even in diluted form, the report paints a bleak picture, noting that the early signs of warming are already apparent.

The report is the second of four scheduled to be issued this year by the UN, which assembled more than 2,500 scientists worldwide to give their best predictions of the consequences of a few degrees’ increase in global temperature. The 1,572-page document was endorsed by officials from more than 120 countries, including the United States. The first report, released in February, said global warming was irreversible but could be moderated by large-scale societal changes.

On Saturday, April 14, at more than 1,300 simultaneous events coast to coast, Americans of different hues and views will call for such large-scale changes by imploring Congress to enact immediate cuts in carbon emissions and pledge an 80 percent reduction by 2050.

The true expression of a viral grassroots movement, organized online through word of mouth, email outreach and the Internet community, Step it Up! is the largest day of citizen action focusing on global warming in our nation’s history and the largest environmental protest of any kind since Earth Day 1970.

Conceived by writer Bill McKibben and six recent graduates of Middlebury College, the initiative has been embraced by environmental organizations, religious networks, campus groups and individuals from virtually all walks of life. The Sierra Club, the National Resources Defense Council and the National Wildlife Federation have all committed real efforts to organizing Step It Up! rallies. Student groups have been particularly enthusiastic, led by Energy Action, the PIRGs and the Campus Climate Challenge campaign, which has thrown its organizational weight and energy behind Step It Up!, as well as the evangelical student movement, which has also embraced the cause.

As McKibben writes in an open letter on the Step It Up! website, “The enormous participation in today’s movement is a wake-up call to legislators from across the country. Their constituents are urgently demanding that America get on the path towards reducing carbon emissions before it is too late.”

Along with lots of marches, rallies and concerts, some of the activities this Saturday will creatively highlight the dangers and losses of a rapidly warming earth. There’ll be ski mountaineers in Wyoming descending the shrinking Dinwoody Glacier; hikers ascending Oregon’s threatened Mt. Hood; scuba divers underwater photographing the endangered coral reefs off Key West; rock climbers hanging banners from Seneca Rocks in West Virginia; gardeners planting native trees, shrubs, grasses and flowers in the Shartel median at 33rd Street and Shartel Avenue in Oklahoma City; demonstrators painting a blue line through downtown Seattle to illustrate how far the rising seas could penetrate; activists on the levees in New Orleans’ Ninth Ward and Vermonters hauling sap from a maple sugar tree that is producing much earlier than it ever has before.

Join your voice to this growing chorus of people determined to save their planet. Click here to find an April 14 event near you, help spread the word or sign up to organize an event yourself. And watch the Step it Up! YouTube video below for tips on how to make your action as successful as possible.