Humans have been burning fossil fuels on a large scale for only about 150 years, yet that has started a cascade of profound changes that at their current pace will still be felt 10,000 years from now, a new study shows.
Coastal areas, in particular, will experience the long-term effects as rising seas slowly redraw the world map as we know it and continue to rise long after emissions are brought down. Even in a scenario in which global temperatures warm to only about 2° Celsius above pre-industrial times, the analysis shows that several of the world’s coastal megacities will eventually be submerged.
That long-term view is important. Policymakers today often discuss climate impacts only through the end of this century. The study provides a new perspective for considering the compounding effects of today’s carbon emissions and what their impact will mean for future generations. It also feeds into international discussions underway this year on whether humans have pushed the planet into a new geologic epoch that many are calling the Anthropocene.
“It is becoming increasingly evident that humans have become a geological force of nature,” said Anders Levermann, a co-author of the study and a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory and the Potsdam Institute for Climate Impact Research in Germany. “The emission of greenhouse gases is already changing our climate. If continued, sea level will continue to rise and consume the coastlines and our cultural heritage there for centuries to come.”
In the new study, Levermann and his co-authors analyzed the long-term impact that continuing to burn fossil fuels will have over the next 10,000 years. They created four scenarios and gave the world a carbon budget for each—a total amount of carbon that could be released into the atmosphere from coal, oil and gas and other sources, with those emissions declining to zero over the next 300 years.
The results reveal the powerful, long-term impact that human choices made today have when they contribute to already high carbon concentrations in the atmosphere and oceans:
- In the atmosphere, 20-50 percent of airborne carbon emissions from human activities over the next 100 years will still be present 1,000 years from now.
- 60-70 percent of the change in ocean surface temperatures—an important driver of extreme weather and droughts—will still be evident in 10,000 years.
- Sea level will slowly rise, and will remain at or near its peak for at least 10,000 years, long after emissions have declined.
The climate changes already underway are happening at rates not seen before. Warming that ended the last ice age proceeded over nearly 8,000 years. Today, we are on pace for a similar temperature rise over the span of a few centuries.
“This long-term view shows that the next few decades offer a brief window of opportunity to minimize large-scale and potentially catastrophic climate change that will extend longer than the entire history of human civilization thus far,” the authors write.
How Much Carbon?
So far in our history, humans have burned through about 570 petagrams of carbon (PgC), equivalent to 570 billion metric tons, the study notes.
Of the four scenarios in the study, the one with the smallest impact would contribute 1,280 PgC between the years 2000 and 2300. The result would push temperatures more than 2° C above pre-industrial times—a level of warming that world leaders have agreed not to exceed but that our current trajectory would surpass. Under the most destructive scenario, the world would burn through 5,120 PgC and warm by 7.5° C.
The scientists analyzed the impact of the four scenarios using carbon cycle, climate, and land-ice models. All four, they found, produced a climate state not previously experienced by human civilization.
Sea Level Rise
Sea level rise is slow and varies location to location, but all four scenarios suggest widespread damage to coastal cities.
Even in the scenario with the smallest impact, several megacities—including Shanghai and New York—would be almost completely submerged over time. In 122 countries, at least 10 percent of their current population-weighted area would eventually be under water; in 25 countries, that rises to 50 percent.
With current annual emissions rates at about 10 PgC per year, we would reach that scenario in about 120 years. Sea levels would take longer to rise, but the effects would be locked in unless the resulting emissions can eventually be removed from the atmosphere.
“If we want to keep cities like New York, Tokyo, Calcutta and London, we cannot burn all the coal that we have found, we need to leave a significant portion of it in the ground,” Levermann said. “Cleaner energy sources exist, and they need to be used intensively if we want to pass on Miami and New York to future generations.”
The scientists also explored each major source of sea level rise and how it would respond in each scenario.
Looking at the scenario with the smallest impact, the scientists found that thermal expansion of the oceans would drive sea level rise first, pushing average global sea level up as much as 1.1 meters over 2,300 years, then slowly falling. As warmer ocean water breaks up the ice shelves holding back Antarctica’s glaciers, the ice released into the ocean would raise sea level as much as 24 meters over time. The Greenland ice sheet would partially melt, contributing up to 4 meters more over 10,000 years.
Under the rest of the scenarios, the Greenland ice sheet melts completely, leaving the island ice-free and causing about 7 meters of sea level rise. Under the most destructive scenario, that would happen within 2,500 years, and ice loss from Antarctica would contribute as much as 45 meters more over 10,000 years.
“It takes sea level rise a very long time to react—on the order of centuries,” said Peter Clark, the study’s lead author and a climate scientist at Oregon State University. “It’s like heating a pot of water on the stove; it doesn’t boil for quite a while after the heat is turned on—but then it will continue to boil as long as the heat persists. Once carbon is in the atmosphere, it will stay there for tens or hundreds of thousands of years, and the warming, as well as the higher seas, will remain.”
The scenarios provide a broad view of the future if current trends continue. Many questions remain that will matter to global adaptation efforts, Levermann noted. The big one is the rate at which Greenland and Antarctica are losing ice, which determines how quickly sea level will rise. It’s a question scientists at Lamont are working on.
The paper appears in the journal Nature Climate Change. Clark and Levermann were joined by 20 co-authors from across Europe, the United States and Canada.