Consider: Meteor Crater, the Arizona landmark many people visualize when they think of an asteroid impact, was the result of a relative interplanetary pebble. That chasm, nearly a mile across and more than 500 feet deep, was the result of impact by a space rock only about 80 feet in diameter.

But the Apollo-class asteroids - those whose solar orbits, like that of our most recent unwelcome guest, take them across the orbit of Earth - include far larger threats. Like the three-mile rock that blasted the 62-mile-wide Manicouagan impact structure in Quebec more than 200 million years ago - one of the largest impact craters still visible on the planet's surface. Or the six-mile space bomb believed to have eradicated the dinosaurs 100 million years later.

Scientists tell us that such truly catastrophic hits come every 100 million years or so, which makes us roughly due, the way such things are measured. But not to worry, they say; as far as they can observe and predict, nothing really nasty is headed our way. Of course, the length of that view is the cosmic equivalent of an eye's wink. We know of some 150 Apollo objects up to five miles across. It's likely that there are thousands we haven't tagged yet.

When the one with our name on it comes, they say, we'll know it - for a moment, at least. A land hit would obliterate thousands of square miles, followed by 13-magnitude earthquakes, volcanic eruptions and dust clouds that would blot out the sun for months. An ocean hit would produce mega-tsunamis up to three miles high, not only eradicating coastal life, but flooding the interiors of continents as well. The atmospheric steam cloud would alternatively turn Earth into an equatorial hothouse, then an icebox.

No, we would not survive as a civilization, most likely not as a species. That's why those who watch for and plot the orbits of these doomsday rocks are doing such important work. And why those who dream of thwarting the cataclysm dream such important dreams.

"It's just been an absolute time bomb that's gone off both in the scientific community and, ultimately, in our public policymaking," said Rep. Jay Inslee (D-Wash.), who received a two-hour briefing on the subject in May with five other House members. "It's another example of when you put gigatons of carbon dioxide into the atmosphere, you have these results none of us would have predicted."

Thomas E. Lovejoy, president of the H. John Heinz III Center for Science, Economics and the Environment, has just rewritten the paperback edition of "Climate Change and Biodiversity," his latest book, to highlight the threat of ocean acidification. "It's the single most profound environmental change I've learned about in my entire career," he said last week.

A coalition of federal and university scientists is to issue a report today describing how carbon dioxide emissions are, in the words of a press release from the National Center for Atmospheric Research and the National Oceanic and Atmospheric Administration, "dramatically altering ocean chemistry and threatening corals and other marine organisms that secrete skeletal structures."

For decades, scientists have viewed the oceans' absorption of carbon dioxide as an environmental plus, because it mitigates the effects of global warming. But by taking up one-third of the atmosphere's carbon dioxide -- much of which stems from exhaust from automobiles, power plants and other industrial sources -- oceans are transforming their pH level.

The pH level, measured in "units," is a calculation of the balance of a liquid's acidity and its alkalinity. The lower a liquid's pH number, the higher its acidity; the higher the number, the more alkaline it is. The ph level for the world's oceans was stable between 1000 and 1800, but has dropped one-tenth of a unit since the Industrial Revolution, according to Christopher Langdon, a University of Miami marine biology professor.

Scientists expect ocean pH levels to drop by another 0.3 units by 2100, which could seriously damage marine creatures that need calcium carbonate to build their shells and skeletons. Once absorbed in seawater, carbon dioxide forms carbonic acid and lowers ocean pH, making it harder for corals, plankton and tiny marine snails (called pteropods) to form their body parts.

Ken Caldeira, a chemical oceanographer at Stanford University who briefed lawmakers along with NCAR marine ecologist Joan Kleypas, said oceans are more acidic than they have has been for "many millions of years."

"What we're doing in the next decade will affect our oceans for millions of years," Caldeira said. "CO2 levels are going up extremely rapidly, and it's overwhelming our marine systems."

Some have questioned global-warming predictions based on computer models, but ocean acidification is less controversial because it involves basic chemistry. "You can duplicate this phenomenon by blowing into a straw in a glass of water and changing the water's pH level," Lovejoy said. "It's basically undeniable."

Hugo A. Lo?iciga, a geography professor at the University of California at Santa Barbara, is one of the few academics to question the phenomenon. A groundwater hydrologist, Lo?iciga published a paper in the May edition of the American Geophysical Union's journal that suggested the oceans may not become so acidic, because enough carbonate material will help restore equilibrium to them.

Lo?iciga wrote that although seawater in certain regions may become more acidic over time, "on a global scale and over the time scales considered (hundreds of years), there would not be accentuated changes in either seawater salinity or acidity from the rising concentration of atmospheric CO2."

Two dozen scientists have written a response questioning this assumption, since it would take thousands of years for such material to reach the oceans from land.

"The paper by Lo?iciga ignores decades of scholarship, presents inappropriate calculations and draws erroneous conclusions that simply do not apply to real ocean," they wrote. They added that, unless carbon dioxide levels in the atmosphere stabilize soon, the seas will soon exceed the Environmental Protection Agency's recommended acidity limits.

Scientists have conducted a few ocean acidification experiments in recent years. All have shown that adding carbon dioxide to the water slows corals' growth rate and can dissolve pteropods' shells.

Langdon, who conducted an experiment between 1996 and 2003 in Columbia University's Biosphere 2 lab in Tucson, concluded that corals grew half as fast in aquariums when exposed to the level of carbon dioxide projected to exist by 2050. Coupled with the warmer sea temperatures that climate change produces, Langdon said, corals may not survive by the end of the century.

"It's going to be on a global scale and it's also chronic," Langdon said of ocean acidification. "Twenty-four/seven, it's going to be stressing these organisms. . . . These organisms probably don't have the adaptive ability to respond to this new onslaught."

Stanford University marine biologist Robert B. Dunbar has studied the effect of increased carbon dioxide on coral reefs in Israel and Australia's Great Barrier Reef. "What we found in Israel was the community is dissolving," Dunbar said.

Caldeira has mapped out where corals exist today and the pH levels of the water in which they thrive; by the end of the century, no seawater will be as alkaline as where they live now. If carbon dioxide emissions continue at their current levels, he said, "It's say goodbye' to coral reefs."

Although the fate of plankton and marine snails may not seem as compelling as vibrantly colored coral reefs, they are critical to sustaining marine species such as salmon, redfish, mackerel and baleen whales.

"These are groups everyone depends on, and if their numbers go down there are going to be reverberations throughout the food chain," said John Guinotte, a marine biologist at the Marine Conservation Biology Institute. "When I see marine snails' shells dissolving while they're alive, that's spooky to me."

Rep. Rush D. Holt (D-N.J.), a scientist by training, attended the congressional briefing on ocean acidification. He said these developments are "new to me, which was surprising because I usually keep up with things."

"The changes in our climate are severe and urgent even if it weren't for this, but this just adds impact and urgency to the situation," Holt said.

Metropolitan trains in the teeming city of 17 million were running late as rain water submerged rail tracks.

India's monsoon flooding started on Tuesday in the western city and has killed at least 24 people in eastern India.

A year ago, two days of very heavy rain showed up the pathetic infrastructure and dismal emergency response in Mumbai, India's richest city.

The floods had killed hundreds of people then and closed down the city for nearly a week.

On Wednesday, traffic remained disrupted on several major roads and many flights to and from the city were running late.

Scores of people holding colorful umbrellas waded carefully through muddy water as emergency workers used dozens of water pumps overnight to reduce the flooding.

Schools and colleges were closed and waterlogging had affected life in crowded residential areas like Borivali, Santacruz, Andheri and Kurla.

In the past 24 hours, Mumbai, home to Bollywood, received 17.5 cm (7 inches) average rainfall.