A squeeze on funding for satellites to look down on the Earth’s environment at a time of growing need for research into the effects of climate change is creating alarm among scientists and on Capitol Hill.

The National Aeronautics and Space Administration, renowned for its pioneering role in science, is seeing its science budget shrink and its satellite Earth observation capacity endangered even as the agency’s overall mission grows.

Three and a half years ago, President Bush announced “a new plan to explore space and extend a human presence across our solar system,” including a return to the moon by 2020, a step toward Mars and beyond.

The ambitious program vastly expanded NASA’s mission at a time when its near Earth science programs — arguably more relevant to humankind’s needs — were in decline.

Since Bush announced his Vision for Space Exploration, the administration has reduced future-year funding for NASA’s Science Mission Directorate by a total of $4 billion, according to the House Science and Technology Committee’s space and aeronautics panel.

A two-year study released last January by the National Academy of Sciences’ National Research Council found that NASA’s Earth science budget had declined 30 percent since 2000 and was threatened to fall even further.

“There is not adequate funding for Earth science in NASA to accomplish the mission that it has been assigned — to use the global vantage point in space to provide information on the immediate future of Earth, on which we can base sound policy decisions to protect our future,” University of Michigan Professor Lennard A. Fisk told the House panel last month.

Richard A. Anthes, president of the University Corporation for Atmospheric Research in Boulder, Colo., and co-chairman of the committee that wrote the National Research Council report, told senators in February that “at a time when the need has never been greater, we are faced with an Earth observation program that will dramatically diminish in capability over 10-15 years.”

Anthes said that between 2006 and the end of the decade, the number of operating missions would decrease dramatically and the number of operating sensors and instruments on NASA spacecraft would decrease by about 35 percent. By 2015 the decrease would be 50 percent.

Substantial loss of capability is likely over the next several years due to a combination of decreased budgets and aging satellites already well past their design lifetimes, he said.

He cited the elimination of the requirements for climate research-related measurements on the National Polar-orbiting Operational Environmental Satellite System (NPOESS) as an example of the nation’s failure to sustain critical measurements.

Anthes said environmental challenges posed by climate change call for study of ice sheets and sea level change, shifts in precipitation and water availability, transcontinental air pollution, shifts in ecosystem structure, impacts on human health and occurrence of extreme events such as hurricanes.

Anthes was testifying to the Senate Commerce Committee shortly after the Intergovernmental Panel on Climate Change (IPCC) released its first report of the year.

He is to testify this week to the House space and aeronautics panel, chaired by Rep. Mark Udall, D-Colo.

Udall and Rep. Bart Gordon, D-Tenn., chairman of the full committee, wrote to Bush in April expressing concern at the “mismatch” between NASA funding and the tasks it was being asked to undertake.

“Stresses from that mismatch can be seen in all of the agency’s programs,” they said.

NASA’s grant-based research programs as well as missions such as Explorer, Discovery and Earth System Pathfinder that played key roles in scientific advances and training the next generation of scientists and engineers had suffered significant cutbacks in recent years, the letter said.

Udall and Gordon asked for a meeting with Bush to discuss the issue but were turned down.

The administration’s fiscal 2008 budget request for NASA’s Earth science and applications program is $1.5 billion, an increase of 2 percent over the current-year request.

An aide said that at the Thursday subcommittee hearing members are likely to question witnesses about the importance of NASA Earth science missions to U.S. and global climate research efforts.

Members may also ask what role other countries should play in NASA’s future Earth science system and what the agency is doing to better use Earth science research data to address society’s needs.