November 25, 2013
Putting A Cap On CO2 Emissions Won’t End Global Warming: Study
redOrbit Staff & Wire Reports - Your Universe Online
The carbon dioxide content already present in the Earth’s atmosphere would continue to cause warming for hundreds of years, even if the emissions were to suddenly stop today, according to new research published in Sunday’s edition of the journal Nature Climate Change.
The study, which was led by scientists at Princeton University and funded by the Swiss National Science Foundation (SNSF), suggests that it could take far less CO2 than previously believed to reach global temperatures experts have deemed unsafe, the Ivy League school said in a statement.
As part of their research, the investigators created a simulation of Earth upon which carbon dioxide emissions suddenly ceased after 1,800 billion tons of the greenhouse gas had entered the atmosphere. Within a millennium of this sudden CO2 shutoff, the carbon itself began to fade. Within 20 years, 40 percent of it had been absorbed by the planet’s oceans and landmasses, and 80 percent of it had been soaked up after 1,000 years.
“By itself, such a decrease of atmospheric carbon dioxide should lead to cooling. But the heat trapped by the carbon dioxide took a divergent track,” the university said. “After a century of cooling, the planet warmed by 0.37 degrees Celsius (0.66 Fahrenheit) during the next 400 years as the ocean absorbed less and less heat.”
“While the resulting temperature spike seems slight, a little heat goes a long way here. Earth has warmed by only 0.85 degrees Celsius (1.5 degrees Fahrenheit) since pre-industrial times,” it added. “The Intergovernmental Panel on Climate Change estimates that global temperatures a mere 2 degrees Celsius (3.6 degrees Fahrenheit) higher than pre-industrial levels would dangerously interfere with the climate system.”
In order to prevent the planet from reaching that point, humans would have to limit cumulative CO2 emissions to less than 1,000 billion tons of carbon, approximately half of which have already entered the Earth’s atmosphere. The lingering warming effect that the Princeton researchers found, however, means that the two-degree plateau could be reached with far fewer carbon emissions.
“If our results are correct, the total carbon emissions required to stay below 2 degrees of warming would have to be three-quarters of previous estimates, only 750 billion tons instead of 1,000 billion tons of carbon,” said first author Thomas Frölicher, who conducted the work as a postdoctoral researcher in Princeton’s Program in Atmospheric and Oceanic Sciences (AOS).
“Thus, limiting the warming to 2 degrees would require keeping future cumulative carbon emissions below 250 billion tons, only half of the already emitted amount of 500 billion tons,” added Frölicher, who now works as a researcher at the Swiss Federal Institute of Technology in Zurich (ETH Zurich).
The results of their study contradict a widely-accepted scientific consensus that the planet’s temperature would remain the same or decline if emissions suddenly stopped. However, those previous studies did not take into account the oceans’ decreasing ability to absorb atmospheric heat (especially the polar oceans).
While CO2 steadily dissipates, Frölicher’s team was able to determine that the oceans that remove heat from the Earth’s atmosphere gradually begin absorbing less and less. Ultimately, the residual heat offsets the cooling that takes place as a result of reduced amounts of atmospheric carbon dioxide.
The researchers demonstrated that the change in ocean heat uptake in the polar regions has a more pronounced effect on global mean temperature than changes in low-latitude oceans. This phenomenon is known as “ocean-heat uptake efficacy,” and according to Frölicher, it plays a “central role” in climate change and has been underrepresented in previous studies.
“Scientists have thought that the temperature stays constant or declines once emissions stop, but now we show that the possibility of a temperature increase can not be excluded,” Frölicher said. “This is illustrative of how difficult it may be to reverse climate change – we stop the emissions, but still get an increase in the global mean temperature.”