With a scruffy, desert floor extending as far as the eye can see in all directions – dotted with the Mojave Desert’s signature spiky, gnarled Joshua trees – veteran park ranger Joe Zarki kneels down in the dust.

Pointing to an invasive, red brome grass, which has proliferated here in the past few years, he explains why the rise in temperatures that have helped it thrive could signal the end of Joshua trees within a century. “These exotic grasses dry out, which means that the fires we have are getting larger in size, spread more rapidly, and are harder to put out,” Mr. Zarki says.

Unlike lodgepole pines and other conifers that were replenished in Yellowstone National Park after devastating fires in 1988 in Wyoming, the moisture-sensitive Joshua tree grows in a specific microclimate and cannot easily reproduce itself in the wake of widespread destruction. A recent study showed that Joshua trees are not replenishing after fires that now routinely claim thousands of acres. In the 1970s, a typical fire scorched less than an acre.

The disappearance of the Joshua tree may be just one way in which climate change is affecting our national parks. According to researchers, climate change is contributing to:

• The possible loss of all the glaciers in the Glacier National Park within 20 years.

• Dying coral reefs in Biscayne and Virgin Island National Parks due to increased heat and disease.

• Insect pests thriving and destroying forests ranging from the Great Smoky Mountains to Yellowstone.

• Declining water levels at Lake Mead because of extended droughts.

Global climate change is “the single greatest threat to the health of our national parks,” says Michael Cipra, California desert program manager of the National Parks Conservation Association.

In the case of the Joshua Tree National Park, Mr. Cipra points out that the red brome grasses are also fertilized by nitrogen deposited by greenhouse gases moving east from the Los Angeles Basin. The nitrogen levels are 15 to 30 times higher than in undisturbed ecosystems, nurturing exotics such as the red brome and cheatgrass, which now represent up to 60 percent of the park’s biomass from annual plants.

And when the Joshua tree goes, so may the creatures that depend on it – the Yucca night lizard that nests in its decaying bark and the red-tailed hawks and Scott’s orioles that perch in its branches.

Park officials are already looking to invest more money and effort to research and monitor the changes. The idea, says Jonathan Jarvis, regional director, Pacific West Region of the National Park Service, is that the parks may serve as the proverbial “canaries in the coal mine” – predictors of things to come for other populations and ecosystems.

As a result of climate change, national parks are seeing extended fire seasons, species migration, and increased snow melt, he says.

Researchers and park officers outlined some of their concerns this week to a congressional subcommittee at a special hearing on the “Impacts of Climate Change on America’s National Parks” here in Twentynine Palms, Calif. M. Rebecca Shaw, director of conservation for the Nature Conservancy of California, told the panel, “An explosion of studies in the last five years has documented observed climate impacts on species distributions, wildfire frequency and intensity, pest outbreaks, and sea-level rise.”

“Keeping wildlife populations, rivers, forests, deserts, and our national parks healthy will allow us to support nearly 6.5 million existing jobs and continue to generate $88 billion in state and national tax revenue,” Cipra told the panel, pointing to a 2006 study by the Outdoor Industry Association, which found that fishing, hunting, hiking, and other outdoor pursuits contribute $730 billion annually to the US economy.

The congressional subcommittee also heard from John Coleman, founder of The Weather Channel, who denies that global warming exists. “These scientists know that if they do research and the results are in no way alarming, their research will gather dust on the shelf and their research careers will languish,” said Mr. Coleman.

Nevertheless, committee chairman Rep. Raul M. Grijalva (D) of Arizona said he is proposing substantial funding to help mitigate the effects of global warming on national parks. “You’ve got to hear the other side of this argument, no matter how overwhelming the evidence is to the contrary,” he said. “And then you have to move on and deal with it, anyway.”

National parks can be both a warning system and a laboratory, providing an opportunity to study climate-change effects and communicate them to the public, says Michael Bean, director of the Wildlife Program at Environmental Defense.

For one, parks are relatively unaffected by other threats of major development, making it easier to isolate climate-change effects. “Parks are well studied … so there is a considerable baseline of knowledge on which can be based assessments of change,” he says.

Perhaps most important, “people care about parks and pay attention when something they’ve grown accustomed to is suddenly different. So they serve as a good classroom for information,” adds Mr. Bean.