WASHINGTON — Crazy-sounding ideas for saving the planet are getting a serious look from top scientists, a sign of their fears about global warming and the desire for an insurance policy in case things get worse.

How crazy?

There’s the man-made “volcano” that shoots gigatons of sulfur high into the air. The space “sun shade” made of trillions of little reflectors between Earth and sun, slightly lowering the planet’s temperature. The forest of ugly artificial “trees” that suck carbon dioxide out of the air. And the “Geritol solution” in which iron dust is dumped into the ocean.

“Of course, it’s desperation,” said Stanford University professor Stephen Schneider. “It’s planetary methadone for our planetary heroin addiction. It does come out of the pessimism of any realist that says this planet can’t be trusted to do the right thing.”

NASA is putting the finishing touches on a report summing up some of these ideas and has spent $75,000 to map out rough details of the sun shade concept. One of the premier climate modeling centers in America, the National Center for Atmospheric Research, has spent the last six weeks running computer simulations of the man-made volcano scenario and will soon turn its attention to the space umbrella idea.

And last month, billionaire Richard Branson offered a $25 million prize to the first feasible technology to reduce carbon dioxide levels in the air.

Simon “Pete” Worden, who heads NASA’s Ames Research Center in Moffett Field, Calif., says some of these proposals, which represent a field called geoengineering, have been characterized as anywhere from “great” to “idiotic.” As if to distance NASA from the issue a bit, Mr. Worden said the agency’s report won’t do much more than explain the range of possibilities.

Scientists in the recent past have been reluctant to consider such concepts.

Here is a look at some of the ideas:

• The Geritol solution: A private company is already carrying out this plan. Some scientists call it promising while others worry about the ecological fallout.

Planktos Inc. of Foster City, Calif., last week launched its ship, the Weatherbird II, on a trip to the Pacific Ocean to dump 50 tons of iron dust. The iron should grow plankton, part of an algae bloom that will drink up carbon dioxide from the atmosphere.

• Man-made volcano: When Mount Pinatubo erupted 16 years ago in the Philippines, it cooled the Earth for about a year because the sulfate particles in the upper atmosphere reflected some sunlight.

Using jet engines, cannons, or balloons to get sulfates in the air, humans could reduce the solar heat and only increase current sulfur pollution by a small percentage, said Tom Wigley of the National Center for Atmospheric Research.

• Solar umbrella: For far-out concepts, it’s hard to beat Roger Angel’s. Last fall, the University of Arizona astronomer proposed what he called a “sun shade.” It would be a cloud of small Frisbee-like spaceships that go between Earth and the sun and act as an umbrella, reducing heat from the sun.

These nearly flat discs would each weigh less than an ounce and measure about a yard wide with three tab-like “ears” that are controllers sticking out just a few inches.

About 800,000 of these would be stacked into each rocket launch. It would take 16 trillion of them — that’s million million — so there would be 20 million launches of rockets. All told, Mr. Angel figures 20 million tons of material to make the discs that together form the solar umbrella.

• Artificial trees: Scientifically, it’s known as “air capture.” But the instruments being used have been dubbed “artificial trees” — even though these devices are about as treelike as a radiator on a stick. They are designed to mimic the role of trees in using carbon dioxide, but early renderings show them looking more like the creation of a tinkering engineer with lots of steel.

They would stand tall like cellphone towers on steroids, reaching about 200 feet high with various-sized square filters at the top. Columbia University professor Klaus Lackner envisions perhaps placing 100,000 of them near wind energy turbines. Even if each filter was only the size of a television, it could remove about 25 tons of carbon dioxide a year, which is about how much one American produces annually, Mr. Lackner said. The captured carbon dioxide would be changed into a liquid or gas that can be piped away from the air capture devices.