Climate change has so much momentum behind it that “either/or” discussions about options are meaningless because it’ll take all we can do just to arrest carbon dioxide at levels double those in preindustrial times, a top climate scientist said Dec. 11.

Daniel Schrag, director of the Harvard University Center for the Environment and professor of earth and planetary sciences, said that most optimistic scenarios of climate change have atmospheric carbon dioxide leveling off at around 500 or 550 parts per million — levels roughly double the 280 parts per million that ice cores show existed in preindustrial air.

More pessimistic “business as usual” scenarios, which factor in continued growth in the developing world, show levels as high as 1,000 parts per million by the turn of the century.

While Schrag said those pessimistic scenarios would almost certainly mean catastrophic change, he cautioned that even the most optimistic scenario represents a considerable increase from today’s level of almost 390 parts per million.

“What people don’t understand is that we will be at 500 parts per million. The question is: Will we go to 1,000?” Schrag said. “Double the preindustrial level might be a catastrophe, but it might also be the best we can do.”

The potential changes in store include droughts, floods, stronger storms, and increased extinctions. Schrag also highlighted the danger of sea level rise. The dramatic ice melt in recent summers has already made much of Russia’s northern coastline and the Northwest Passage from the Atlantic to the Pacific through Canada ice-free for parts of the year. Though the effects of climate change have proven difficult to predict, Schrag said, it’s possible the Arctic Ocean will become ice-free as soon as a decade from now, far faster than recent predictions that it would happen by midcentury.

Melting Arctic ice has ramifications for people living in coastal locations around the world, he said, since it is Arctic ice that helps keep Greenland’s immense glaciers cool. Greenland’s glaciers contain enough water to raise sea level by 7 meters should they melt or slip into the sea. Schrag said scientists don’t know whether that will happen in 1,000 years, 200 years, or even sooner.

Schrag spoke at the Geological Lecture Hall in the third talk in the Harvard Museum of Natural History’s Earth Matters lecture series. The event, “Solving the Climate-Energy Challenge,” featured Schrag and Kelly Gallagher, director of the Energy Technology Innovation Project at the Harvard Kennedy School.

The planetary systems — earth, air, and ocean — that are affected by rising carbon dioxide levels are so complex that scientists still don’t know how the Earth will respond. We are, in effect, conducting a planetwide experiment on the Earth, Schrag said, the outcome of which is uncertain.

“What we don’t know is very likely going to hurt us,” Schrag said.

Another potential problem is that mountain snowpacks are melting. Mountain snow and glaciers, refreshed each winter during cooler times, have been shrinking, threatening areas that rely on snowmelt-fed rivers, such as large parts of the American West. One projection showed melting beginning weeks, even months, earlier, raising the possibility that snowmelt will dry up by the middle of summer, leaving crops to whither.

“If that water runs out by July, agriculture in California is gone,” Schrag said.

Major rivers of Asia on which millions of people depend, such as the Indus, the Ganges, the Mekong, the Yellow, and the Yangtze, all draw some of their flow from melting mountain ice and snow, raising the specter of water shortages for those who rely on these sources.

“This is a really big problem,” Schrag said. “Adaptation [to the reduced flows] is important, but it won’t be sufficient.”

Schrag spent some time going through available energy options, including renewable sources such as wind and solar, nuclear, and coal. He said that carbon sequestration, where the carbon emitted from burning fossil fuels is harvested before reaching the atmosphere and injected underground, will be needed.

Though some are looking for particular technologies to save the day, Schrag said, the global demand for energy is expected to rise rapidly from today’s levels as developing nations such as China modernize and industrialize. As standards of living rise and people around the world demand the amenities common in industrialized nations today, energy demand will shoot up.

Governments are unlikely to deny their citizens that energy, Schrag said, particularly since there remain ample coal supplies in the nations most likely to need it. The question, he said, isn’t whether China will build coal plants — they’re already building them at a rapid rate — it is whether the plants will have clean technology so their emissions don’t add to the climate change problem.

The good news, Schrag said, is that much of the technology to make energy cleaner already exists — carbon sequestration technology is already in use, albeit for another purpose. The oil industry injects carbon dioxide into wells to push oil out, leaving the gas trapped underground. The difficult part is not the technology itself, it is deploying it on a scale large enough to make a difference.

Another option, Schrag said, is to do a direct intervention in the climate — by releasing sunlight-reflecting aerosols into the atmosphere, for example, to help cool the planet while other fixes are done.

Gallagher’s comments focused on the political and policy changes that are needed. Most importantly, she said, it is critical that two nations who have so far been on the sidelines of climate action — the United States and China — get serious about finding solutions and putting them into effect. That is because the two nations account for 46 percent of global carbon dioxide emissions, with China emitting slightly more overall and the United States far more per capita. Without those two countries taking action, she said, it is unlikely that the problem of climate change will be solved.

Gallagher said it is important that the government set a price for carbon emissions soon. A cap-and-trade system — where businesses pay for tradeable permits to emit a set amount of carbon dioxide — or a flat carbon tax are the two most-often-discussed systems. Gallagher said that once a system sets a carbon price, businesses will begin to figure the cost of carbon emissions into their energy plans, which may make technologies not now considered suddenly financially attractive.

Schrag described himself as both an optimist and a realist on climate change. He recognizes the difficulty of what has to be done to limit the coming changes, but said that a lot of progress has been made just over the past few years. Still, he said, though people are paying attention to climate change, there’s a danger that their behavior may not change fast enough to make a difference.

“There’s a lot of momentum in the climate system. By the time people become scared enough about climate change to do something, it may be too late,” Schrag said.