WASHINGTON — In a “sneak peak” disclosing a grim side-effect of future warmer seas, new NASA satellite data show that the vital base of the ocean food-web shrinks when the world’s seas get hotter.

And that discovery has scientists worried about how much food marine life will have as global warming progresses.

The data show a significant link between warmer water — either from the El Niño weather phenomenon or global warming — and reduced production of phytoplankton of the world’s oceans, according to a study in today’s journal Nature. Phytoplankton are the microscopic plant life that zooplankton and other marine animals eat, essentially the grain crop of the world’s oceans.

Study lead author Michael Behrenfeld, a biological oceanographer at Oregon State University, said yesterday that the recent dramatic drop in phytoplankton production in much of the world’s oceans is a “sneak peak of how ocean biology” will respond later in the century with global warming.

“Everything else up the food web is going to be impacted,” oceanographer Scott Doney of the Woods Hole Oceanographic Institute said. He was not involved in the study.

“What’s worrisome is that small changes that happen in the bottom of the food web can have dramatic changes to certain species at higher spots on the food chain,” Mr. Doney said.

This is yet another recent scientific study with real-time data showing the much predicted harmful effects of global warming are not just coming, but in some cases, are already here and can be tallied scientifically, researchers said.

A satellite commissioned by NASA tracked water temperature and the production of phytoplankton from 1997 to 2006, finding that for most of the world’s oceans when one went up the other went down and vice versa, Mr. Behrenfeld said.

As water temperatures increased from 1999 to 2004, the crop of phytoplankton dropped significantly, about 200 million tons a year. On average about 50 billion tons of phytoplankton are produced yearly, Mr. Behrenfeld said.

During that time, some ocean regions, especially around the equator in the Pacific, saw as much as a 50% drop in phytoplankton production, he said.

However, the satellite first started taking measurements in 1997 when water temperatures were at their warmest due to El Niño. That’s the regular cyclical warming of part of the Pacific Ocean that affects climate worldwide. After that year, the ocean significantly cooled until 1999 and the phytoplankton crop soared by 2 billion tons during those two years.

“The results are showing this very tight coupling between production and climate,” Mr. Behrenfeld said. As less food is produced by phytoplankton, the oceans get bluer in color, he noted.

Phytoplankton, which turn sunlight into food, need nutrients such as nitrogen, phosphates, and iron from colder water below, Mr. Behrenfeld said. With warmer surface water, it is harder for the phytoplankton to get those nutrients.

Other oceanographers agree with the El Niño link but said that, with only a decade of data, it is harder to make global warming connections.

“It’s something you certainly can’t ignore, because its potential is quite significant,” James Yoder of the Woods Hole Institute said. “But there are some caveats because of the shortness of the record.”

Mr. Behrenfeld said his study verifies the physical mechanism — how phytoplankton react to warm water — that has been predicted in computer models, thus verifying dire warnings.

Another worry is that with reduced phytoplankton, the world’s oceans will suck up less carbon dioxide , increasing the Earth’s chief global warming gas, NASA ocean biology project manager Paula Bontempi said. That’s because phytoplankton take carbon dioxide out of the atmosphere in making food.

This is at least the third significant peer-reviewed paper in the past six months showing that global warming biological side effects are already happening. Other studies looked at global warming connections to wildfires and die-offs of plant and animal species.