News•December 3, 2013

Gradual Climate Changes Could Cause Sudden Impacts

Abrupt shifts in the climate have already begun, with more possible by the end of this century according to a report released Tuesday by the National Research Council. At the same time, even gradual changes to the climate could lead to more unforeseen, sudden impacts. The report recommends creating an early warning system, instead of simply reacting to the changes.

Greenhouse gas emissions from human activities have already altered the climate. Most changes have been gradual, but the possibility of abrupt shifts concerns the scientific community. Anthony Barnosky, a paleoecologist at University of California, Berkeley, likened gradual changes to being able to see the road while driving. With unexpected changes, the “road drops out from under you,” he said at a press conference.

A map showing temperature changes in 2100 compared to the 20th century average under a very high greenhouse gas emissions scenario.
Credit: NOAA View

Tipping points from abrupt climate change became a research priority in the early 2000s. At that time, there were growing concerns that melting ice in the northern Atlantic could cause ocean circulation to come to a halt.

“Since then, knowledge has gotten better,” Richard Alley, a geoscientist at Penn State and one of the report’s authors, said in an interview. “Now we are less worried about an Atlantic shutdown.”

The new report highlights shifts that are of growing concern. The precipitous decline of summer Arctic sea ice since 1979, but particularly over the past decade, is one of the most notable sudden changes already occurring. That trend is likely to continue and have a cascade effect on ecosystems in the region as well as impacting shipping, oil and natural gas exploration, and national security.

While changes in the Arctic might be most visible, it’s changes in the globe’s lower latitudes and oceans that are of equal or greater concern, Alley said. Those areas are where the majority of people and animals live and most of the world’s food is produced.

The current rate of climate change is one most likely not seen in 65 million years. The rate of warming is likely to increase in the coming century, and that means some species, particularly those in mountainous regions, might not be able to adapt fast enough or they’ll simply run out of room to migrate.

The top map shows velocity of climate change estimates based on the new Science study. The bottom map shows velocity of climate change based on previous estimates. Note the two maps use different scales.
Credit: Diffenbaugh and Field, 2013

Beyond changes, Alley said the report also focuses on impacts including, “more strongly emphasizing that even gradual climate change can have abrupt impacts on people and ecosystems.”

Agriculture was one area in particular where Alley thought gradual shifts in temperatures could pose problems for crops and the farmers that grow them.

“By end of the century, average summers are supposed to be hotter than the hottest we’re already seeing,” he said. “Our food is already heat stressed. If we move to unprecedented levels (of warmth), what does that do for eating?”

Research indicates that by 2050, global corn yields could fall 15 percent if greenhouse gas emissions continue on their current trajectory. Most U.S farmers have the means to adapt to or offset such decreases by planting alternative crops or investing in drought-resistant strains of corn or increased irrigation.

But for farmers in developing countries, the options are much more limited. Yields of wheat, other cereal crops, and rice are also likely to decrease in areas where they’re most needed, particularly in Africa and India, compounding the problem.

Sea level rise is also a gradual shift that could have a big consequences for coastal communities. In the U.S., coastal communities contributed $6.6 trillion to the national GDP in 2011 and population density along the coast is four times greater than inland. Globally, sea level has risen about 9 inches since 1880, but the rate of change varies based on local conditions.

Slow and steady changes in the ocean’s height can cause major damage to coastal infrastructure. Alley used New York’s subway system during Superstorm Sandy as an example.

“Either the water got into it the subway or it didn’t. The difference can be billions and billions of dollars,” he said. “A pretty small sea level change can be sufficient to push you over that threshold and make a really big difference.”

Damage on Staten Island in the immediate aftermath of Sandy. Despite scenes like this and calls for a new conversation around climate, the storm has had no lasting impact on the national discourse.
Credit: Somayya Ali/NASA GISS

Both worries about sudden climate shifts and the potential of gradual ones to create jarring impacts led the authors to recommend developing an early warning system for unanticipated climate shifts and impacts.

“It would include a lot of elements that we already use to keep track of the climate,” Alley said.

Satellites, weather stations, and greenhouse gas monitoring efforts would all likely be key components. Alley said that some of them, notably satellites, have been threatened by funding cuts in the U.S., which could hamper the development of a warning system.

Understanding little-known elements of the climate system is another important part of assessing threats. The West Antarctic Ice Sheet contains enough ice to raise sea levels 10-13 feet. Sediment records where the ice sheet meets the ocean indicate that it’s capable of rapidly flowing into the ocean and melting, but what it takes to push it over the edge is unclear. Reducing that level of uncertainty could provide coastal cities critical information on how to adapt.

Understanding who and what are most vulnerable to these threats is key, too. Some of those efforts are underway, particularly at the local level. Alley cited New York’s work in the wake of Superstorm Sandy to identify vulnerable infrastructure and populations across the city as well as U.S. Forest Service efforts to use bark beetle outbreaks to predict where large wildfires would occur as two key examples.

Putting these pieces together will ultimately require a concerted effort that brings together physical and social scientists along with decision makers. The Famine Early Warning Systems Network, which was created in the wake of the East Africa famine in the mid-1980s, and a drought warning system in the U.S. both provide lessons of how an abrupt climate change early warning system could function.

By following these and other examples, the authors envision a tool that would give decision makers a timeframe to take proactive measures for what they call “inevitable surprises” rather than reacting to them.