When global warming kicks into overdrive, how will we know?

Global warming is bringing abrupt change to some aspects of the climate system as well as to ecosystems and societies, according to a panel of scientists looking at the issue for federal science and national-intelligence agencies.

The panel, convened by the National Research Council, recommends the establishment of an early-warning system for abrupt change, noting that even slow warming trends can reach levels that threaten to drive plant and animal species extinct, disrupt agriculture, and trigger political instability in the space of a few years to a few decades – well within the span of a human lifetime.

Such a monitoring system, whose specific definition the panel acknowledges it has left to others, is crucial to help decisionmakers anticipate abrupt changes as much as possible and begin to adapt ahead of time.

Several areas in particular are well shy of the monitoring effort needed to better understand how they respond to global warming and to spot changes that could herald a change to a new “normal.” Among them: sensors monitoring the release of methane from permafrost on land and on Arctic Ocean continental shelves, and sensors that can monitor outlet glaciers on Greenland and the West Antarctic Ice Sheet, and ocean conditions that can affect the pace at which the glaciers slide toward the sea.

“There are cameras everywhere, but they watch us,” said James White, a University of Colorado researcher who studies abrupt climate change in Earth's geological past and chaired the National Research Council panel. “Remarkably, very few watching devices are pointed at the environment, which is our most economically important and precious asset.”

Typically, the notion of abrupt change has been applied to the climate itself, the panel notes. Indeed, sudden dramatic changes in climate have occurred naturally, Dr. White noted at a briefing on Tuesday. Even during ice ages, temperatures in the Northern Hemisphere rose by 10 to 15 degrees C (18 to 27 degrees F. ) “in far less than a human lifetime,” he said, only to plunge again.

Such changes show that the climate has thresholds, although the panel doesn't anticipate anything as large as the swing White cited. Keys to a monitoring system are an understanding of what triggers abrupt change and the tools needed to keep an eye out for the appearance of those triggers at as early a stage as possible.

The panel actually saw a reduced threat for sudden change in two areas that scientists have been worried about, noted Richard Alley, a glaciologist at Pennsylvania State University in University Park, Pa., and another member of the National Research Council panel.

In the North Atlantic, where surface and deep-ocean currents form a conveyor belt that drives ocean circulation far beyond the Atlantic basin, “we now have good confidence that while we are pushing in a direction that will change the North Atlantic” in ways that affect people and fisheries, “it's very unlikely that it's going to happen rapidly.” Dr. Alley said at the briefing.

The other abrupt-change bright spot, he continued, involves the Arctic's reservoir of carbon, trapped in soils on land and as methane ices under the surface of the continental shelves. Initially, concerns centered on the sudden release of this potent greenhouse gas in a relatively short period, accelerating global warming.

“There were worries about the possibility of giant methane belches changing the world almost instantly,” he said. “Now, we have fairly high confidence that there are safety valves. Giant methane belches are not the big worry.”

Instead, the carbon is more likely to be released slowly as the climate continues to warm.

Researchers are still uneasy about the future of the West Antarctic Ice Sheet, one of two icecaps atop the continent, Alley added. The West Antarctic Ice Sheet has been losing mass under the influence of warmer surface temperatures, especially on the Antarctic Peninsula, and under the influence of warm seawater eroding the underside of outlet glaciers whose bottoms are grounded on the seafloor.

That latter mechanism has the largest impact on the pace of ice loss. Greenland's glaciers, as well as those in East Antarctica, generally have natural brakes to the rate at which they flow into the sea. That is not the case with big portions of the West Antarctic Ice Sheet.

“There are really bad-dream scenarios for West Antarctica that could still cause very rapid sea-level rise that we just don't have well quantified yet,” Alley said.

Meanwhile, rapid change has come to other aspects of the climate system.

One prime example, the panel notes, is the Arctic Ocean's sea ice. It has been been declining in extent and thickness during the summer melt season at least since satellites first began to keep track in 1979, and probably much longer.

This doesn't affect sea level, but it does increase the ocean's ability to absorb heat in the summer and release it later in the fall, reducing the length of the seasonal freeze-up and altering ecosystems by affecting a wide range of organisms, from algae to certain populations of polar bears.

In addition, researchers are exploring links between changes to the duration and extent of snow and ice cover at the top of the world to atmospheric circulation patterns that affect storm intensity and duration at lower latitudes in the Northern Hemisphere.

The report comes at a time when global greenhouse-gas emissions have risen to record levels and when one research team warns that a target of holding global warming to 2 degrees C above pre-industrial levels by 2100 is insufficient to ward off the worst effects of global warming.

A month ago the Global Carbon Project, in international effort to monitor greenhouse-gas emissions in the atmosphere, noted that in 2012, CO2 emissions from fossil fuels and cement-making reached the highest level in human history and were 58 percent higher than in 1990, the base year against which emissions limits were defined in the 1997 Kyoto Protocol. Atmospheric concentrations of CO2 reached 392.5 parts per million in 2012, the highest level the atmosphere has held in more than 800,000 years, when the climate was warmer than today.

Concentrations have gone over the 350-ppm mark that some scientists say is needed to hold global average temperatures to a level that avoids reaching tipping points in the climate system.

Meanwhile, at global climate talks in Warsaw last month, the road to emissions reductions took an unexpected turn. Japan announced it was backing out of plans to cut its greenhouse-gas emissions by 25 percent by 2020 – in no small part because of the reactivation of fossil-fuel power plants in the wake of the nuclear disaster at the Fukushima Daiichi nuclear station following a major earthquake and tsunami in March 2011. Some observers estimate that Japan's emissions will rise by 3.1 percent instead.

The largest increases in CO2 emissions last year came from Asia, with China leading the way, followed by India and Japan. China's CO2 emissions rose by 71 percent over the preceding year, according to the Global Carbon Project analysis.