Scientists worldwide are tracking changes in our natural environment to better understand issues like global warming. Researchers at the Natural History Museum of Los Angeles say some changes are natural and others are caused by humans, and the challenge is to distinguish between the two.

The scientists are part of a global effort to track changes in the atmosphere and oceans by monitoring plant and animal populations.

Leslie Harris is the Los Angeles Natural History Museum's collections manager for polychaetes, which are segmented worms equipped with bristles. Polychaetes come in many sizes and are common in the ocean, and Ms. Harris says they serve as a barometer of ocean health.

"For example, if we go out to the Hyperion sewage plant, they have an ocean discharge of the sewage from the city of Los Angeles," she said. "They pump out millions and millions of gallons of sewage every day. Well, if you take samples of the sea bottom and then identify and count the animals that are in the sediments, you'll find that polychaetes often dominate. And by looking at the types of polychaetes and where they live, we see how much of an effect there is from the outfall and how far it reaches."

Some of the bristled sea worms actually thrive on processed wastewater, but the creatures may also be harmed by escaping contaminants.

While some scientists study living creatures, others turn to the fossil record for clues about the earth's past climate. Temperatures have shifted dramatically over the ages, and paleontologists see evidence of that. Luis Chiappe, curator of Vertebrate Paleontology at the museum, first studied dinosaurs in his native Argentina. In 1997, he co-discovered a large collection of fossilized dinosaur eggs in the Patagonia region. Today he is studying the remains of an ancient Tyrannosaurus Rex from the eastern part of the U.S. state of Montana.

He says fossilized plants and animals from the Mesozoic era reveal a very different climate from today's. Now, eastern Montana, where this dinosaur was found, has hot summers and cold winters, but 65 million years ago, conditions were very different.

"It was a lot warmer than today, and essentially tropical forests formed the shores of a seaway, because at the time, there was a seaway that connected the Gulf of Mexico with the Arctic," he said. "And T-Rex and other dinosaurs, triceratops and so on, they all lived on the western side of the coastal plains of the seaway."

Over time, the earth has experienced ice ages alternating with periods of global warming. Paleontologist Ken Johnson says the shells of ancient mollusks found near Los Angeles illustrate that history.

"And we study them because they act as ancient thermometers," he said. "They show us what the ocean temperatures were like in the past."

During the ice ages, mollusks now found in Alaska lived near Los Angeles. When temperatures were higher, the Los Angeles coast was home to mollusks that now live off the coast of Central America.

Researcher Sam McLeod points out the fossilized skull of a gray whale that lived off the coast of California 100,000 years ago. It was a time of global warming when the polar ice was melting and sea levels were high. He says the changes seen in these fossils show that such cycles were natural.

"Now we don't understand all the details about how and why global warming occurs, or why you have the reverse, which is ice ages, where you have a lowering of the sea levels because the water gets tied up in the glaciers and the polar ice," Mr. McLeod said. "But it is a natural phenomenon that does occur. It can have severe impacts. But it's something that has occurred many times in the past, and we know is going to happen in the future, and apparently we're in the middle of at least a slight global warming period right now."

He adds that humans are contributing to the warming process by releasing so-called greenhouse gases into the atmosphere. While scientists debate the precise extent of the human impact, he says that global warming has the potential of creating major changes in the earth's environment.

Researcher Regina Wetzer studies sea life to monitor the biodiversity of the local waters. She says new species are easy to find on any California beach because so many species remain unclassified. She points to some tiny sea creatures in a petri dish.

"It's a common myth that we know everything that's out there," she said. "It's very easy to go to the shore and look at organisms this size, a few millimeters or even a few centimeters across, and many of them are new and have not been previously described. So in order to be able to look at anything like global change down the line, we need to know who's there first and describe them. And that's where the role of museums come in."

She says that, unfortunately, many species are being lost before they can be described, sometimes through natural processes and sometimes as a result of human activity.

Scientists need not look far to find species that have become extinct. In the neighboring city of Pasadena, a species of freshwater shrimp, called the Pasadena shrimp, appears to be gone. The shrimp were once abundant in local rivers, but the rivers were paved over with drainage channels and the last specimens of the Pasadena shrimp were collected in 1933.

Scientists say there is a trade-off. Humans need a place to live, and in creating it, they may displace other species. But George Davis, crustacea manager for the Natural History Museum, says it is not a matter to be taken lightly, and that crustaceans like this small shrimp are also important.

"Crustaceans are at the base of the food chain, right there with plankton and what have you," he said. "And if you wipe if out, you wipe out an awful lot of the marine environment, which eventually is going to affect man as well. So it's a good idea to know what we've got before we get rid of it, and in this case, I don't think we'd want to get rid of it."

Mr. Davis adds that another threatened crustacean could well have met its end at human hands, depriving medicine of an important diagnostic tool. For years, horseshoe crabs were harvested from beaches and ground up for fertilizer. But scientists discovered that the crab's blood contains a substance that reacts to endotoxins, dangerous chemicals produced by certain bacteria. The pharmaceutical industry uses blood drawn from the creature for a diagnostic serum, then the crab is returned unharmed to its natural habitat.

Humans are not the only creatures that change an ecosystem. Invasive plant or animal species can have an unexpected effect on the local ecology. Changes in the weather have an even greater impact. These scientists say that the balance of nature is delicate, and their challenge is to monitor these shifts and understand the extent to which humans are causing them.