State of the Planet

News from the Columbia Climate School

A World Warmer By Just 2°C Will Be Very Different From Today

nyc water sea level rise
Over hundreds or thousands of years, even the 2 degrees of warming prescribed in the Paris Agreement could cause sea levels to rise by more than 19 feet. That could endanger many coastal cities, including New York City, shown here. Photo: joiseyshowaa on Flickr CC

Even if we limit global warming to within 2°C above preindustrial levels, as the Paris Agreement aims for, the world is in store for some dramatic changes, according to a new study.

Looking at past periods when conditions were as warm or warmer than today, the researchers find that with 2°C of warming, climate zones and ecosystems will shift, rapid polar warming may release additional greenhouse gases, and sea-level will rise by several meters over several thousand years. Further, the researchers find that many current climate models designed to project changes within this century may underestimate longer-term changes.

The assessment is published in Nature Geoscience by an international team of 59 scientists, including three scientists currently at Columbia’s Lamont-Doherty Earth Observatory: Kelsey Dyez, Jackie Austermann, and Julia Gottschalk.

Over the past 3.5 million years, several time intervals were 0.5 to 2°C warmer than the so-called preindustrial temperatures of the 19th century. Although not all of these past warmings were caused by higher CO2 concentrations, they can help to assess the regional effects of warming on a scale comparable to the Paris Agreement goals.

Ecosystems and climate zones will shift

The study finds that 2°C of warming will drive ecosystems and climate zones poleward and to higher altitudes in pursuit of cooler temperatures. Meanwhile, thawing permafrost will release additional carbon dioxide and methane to the atmosphere, driving further warming. The team’s observations indicate that if warming can be limited to well below 2°C, the risk of catastrophic runaway greenhouse gas feedbacks is somewhat lower.

Long-term sea-level rise of more than 19 feet

However, even a warming of 1.5 to 2°C above preindustrial levels will be sufficient to trigger substantial long-term melting of ice in Greenland and Antarctica and sea-level rise of more than 19 feet over hundreds or thousands of years. In addition, rates of sea-level rise will likely increase over those of the last few decades.

“This rise may become unstoppable for millennia,” said Alan Mix of Oregon State University, “impacting much of the world’s population, infrastructure, and economic activity that is located near the shoreline.”

Past warming is stronger than predicted by climate models used for short-term forecasting

Comparing observations of the past with computer simulations suggests that today’s models may underestimate long-term warming and its amplification in polar regions. These observations indicate that the Paris agreement’s proposed limit of 2ºC of warming may be too lenient over longer timescales, says Dyez, who studies past climates. “Beyond the next 80 to 100 years, this guardrail should be held lower, closer to 1 to 1.5ºC of warming to avoid the worst of the climate consequences.”

The information from the past underscores the urgency of reducing carbon emissions quickly.

“The time has come to shift our energy systems away from the ‘business as usual’ burning of fossil fuels,” says Dyez, “especially in consideration of coastal populations.”

This post is adapted from a press release prepared by the University of New South Wales, Oregon State University, and the University of Bern.

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