Climate change: The long reach

Earth is warming. Sea levels are rising. There’s more carbon in the air, and Arctic ice is melting faster than at any time in recorded history. Scientists who study the environment to better gauge Earth’s future climate now argue that these changes may not reverse for a very long time. Think millennia.

People burn fossil fuels like coal and oil for energy. That burning releases carbon dioxide, a colorless gas. In the air, this gas traps heat at Earth’s surface. And the more carbon dioxide released, the more the planet warms. If current consumption of fossil fuels doesn’t slow, the long-term climate impacts could last thousands of years — and be more severe than scientists had been expecting. Climatologist Richard Zeebe of the University of Hawaii at Manoa offers this conclusion in a new paper. It appeared August 5 in the Proceedings of the National Academy of Sciences.

Most climate-change studies look at what’s going to happen in the next century or so. During that time, changes in the planet’s environment could nudge global warming even higher. For example: Snow and ice reflect sunlight back into space. But as these melt, sunlight can now reach — and warm — the exposed ground. This extra heat raises the air temperature even more, causing even more snow to melt. This type of rapid exaggeration of impacts is called a “fast feedback.”

Zeebe says it’s important to look at fast feedbacks. However, he adds, they’re limited. From a climate change perspective, “This century is the most important time for the next few generations,” he told Science News. “But the world is not ending in 2100.”

For his new study, Zeebe now focuses on “slow feedbacks.” While fast feedback events unfold over decades or centuries, slow feedbacks can take thousands of years. Melting of continental ice sheets and the migration of plant life — as they relocate to more comfortable areas — are two examples of slow feedbacks.

Zeebe gathered information from previously published studies investigating how such processes played out over thousands of years during past dramatic changes in climate. Then he came up with a forecast for the future that accounts for both slow and fast feedback processes.

Climate forecasts that use only fast feedbacks predict a 4.5 degree Celsius (8.1 degree Fahrenheit) change by the year 3000. But slow feedbacks added another 1.5° C — for a 6° total increase, Zeebe reports. He also found that slow feedback events will cause global warming to persist for thousands of years after people run out of fossil fuels to burn.

“This study uses our understanding of how the climate works to build an idea of what might happen in the future,” Ana Christina Ravelo told Science News. Ravelo is a climate scientist at the University of California, Santa Cruz. She pointed out that Zeebe’s study also is conservative — which means it might greatly underestimate the true boost in Earth’s temperature.

Power Words

carbon dioxide A gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. This colorless, odorless gas also is released when organic matter (including fossil fuels like oil or gas) is burned. Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

climate The weather conditions prevailing in an area in general or over a long period.

climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

feedback (as in climate) Processes that propel or exaggerate a change in some direction. For instance, as the cover of Arctic ice disappears with global warming, less of the sun’s warming energy will be reflected back into space. This will serve to increase the rate of Earth’s warming.

ice sheet The broad blanket of ice, most of it kilometers deep, that today covers most of Antarctica. An ice sheet also blankets most of Greenland.