On Saturday, hundreds of thousands of people will be gathering in almost every nation on the planet to issue a plea to world leaders: It’s time to stop climate change — now.

Warnings have been rolling in from scientists, physicians and economists of the outcome if world leaders continue to drag their feet on reducing greenhouse gases. A key number appearing with greater frequency in those discussions is 350 parts per million as the safe upper threshold for atmospheric concentrations of CO2 — a level we’ve already passed. The effects are showing.

Just this week, U.S. government scientists wrote of dramatic changes in the Arctic, where the sea ice is far thinner than expected and melting in the summers, causing changes in the wind patterns; Greenland’s ice sheet is melting far faster than anticipated; Siberia is seeing greater runoff; and animal species such as walruses, polar bears and caribou are suffering.

Another report, commissioned by the WWF, investigated the speed at which low-carbon industries could transform the economy in time to prevent runaway climate change. Its finding: The transition has to be under way with maximum development of every key low-carbon industry by 2014, and growth must exceed 20 percent a year for decades.

Britain’s Met Office released an interactive doomsday map of what could happen if leaders fail.

People around the world will be loudly voicing those concern on Saturday, when marches, rallies and other actions, from choirs to mountain marathons, spread across more than 4,800 venues in 179 countries for 350.org’s International Day of Climate Action.

In the water just off the coast of New Zealand, representatives of 350 Pacific islands are already making their point. Young and old, they waded in this week to pin up 350 T-shirts, each bearing the name of an island.

The message: We are being hung out to dry by the developed world.

Some of those islands, like Kiribati and parts of the Maldives, are already losing their land to rising sea levels and more severe ocean storms. Last week, the Maldives government held its Cabinet meeting under water to remind the world that this is its people’s future if greenhouse gas emissions aren’t cut quickly.

Ethiopia launched one of the first big rallies of the International Day of Climate Action today, with thousands of schoolchildren and adults marching in Addis Ababa to call for climate action. Africa is on the front lines of climate change as higher temperatures and changing rainfall patterns bring droughts and crop failures. Ethiopians remember the deadly famine of the 1980s, and they are experiencing severe drought again.

“As the countdown begins to the decisive Copenhagen climate talks in December, new voices are making themselves heard,” Nobel Peace Prize recipient Desmond Tutu, former archbishop of South Africa, writes in an opinion article being published today in newspapers around the world.

“The new African assertiveness stems from new science. After Arctic sea ice melted dramatically in 2007, scientists began re-evaluating their predictions. It became clear that basic survival was at stake for many countries.

“The low-lying islands of the Maldives, though poor, have begun saving a portion of each year’s national budget to buy a new homeland if, and when, their current home sinks beneath the waves.

“Kenya’s ongoing drought vividly illustrates what uncontrolled climate change might bring to the African continent.”

Tutu reminds the world that unity defeated apartheid in South Africa. Unity can also bring global action to protect the climate as we know it for future generations.

He echoes scientists in saying that to avoid climate havoc, the nations of the world need to bring the level of atmospheric CO2 back down to 350 ppm. It’s nearing 390 ppm now.

To locate International Day of Climate Action events, check 350.org’s interactive map. Go to the web site to see more photos of actions already under way.

See also:

Nicholas Stern Latest Climate Expert to Endorse 350 ppm Limit

Without Functioning Ecosystem, There Is No Economic Growth

Report: At 385 PPM, Current CO2 Level Already in “Dangerous Zone”

Bad and Getting Worse: Surge in CO2 Emissions Damaging World’s Oceans

James Hansen on Climate Tipping Points and Political Leadership

Religion Emerges as an Influential Force for Climate Action: It’s a Moral Issue

Climate Movement Asks: How Can We Sleep While Our Beds Are Burning?

(Photos, top to bottom: Climate action statements from Ethiopia, the United States, New Zealand and Kiribati, from 350.org / CC BY-NC-SA 2.0)