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This Week in Climate Science: A Triassic Warning, Carbon-Absorbing Forests, and Amazon Dams

By David Kroodsma

Welcome to Climate Central’s climate science roundup. This roundup summarizes noteworthy climate science studies published in the previous two weeks, with a special emphasis on articles that might not have been covered by major media outlets.

In this edition:

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Paper Title: A Large and Persistent Carbon Sink in the World’s Forests
Journal: Science
Authors: Yude Pan, Richard A. Birdsey, and 16 others.

 

 

The Gist: Forests around the globe are absorbing and sequestering more carbon dioxide (CO2) than previously reported.

Summary: By combining assessments of forests around the world, these researchers estimated how much carbon the world’s forests have absorbed over the past two decades. They found that between 1990 and 2007, forests absorbed 9.6 (plus or minus 1.6) billion tons of CO2 per year, offsetting about one third of global greenhouse gas emissions released from burning fossil fuels. Most of this sequestration was in temperate countries, where forests have expanded or become denser.

The 9.6 billion ton figure is a sum of how much carbon is being absorbed by forests, minus how much carbon is being released to the atmopshere by deforestation. The world’s forests would be absorbing a whole lot more carbon — about twice as much — if it weren’t for the clearing of forests for agriculture, most of which takes place in the tropics.

The paper contains a detailed table that shows how forest carbon sequestration varies worldwide.


Paper Title: Increased soil emissions of potent greenhouse gases under increased atmospheric CO2
Journal: Nature
Authors: Kees Jan van Groenigen, Craig W. Osenberg, Bruce A. Hungate.
The Gist: Like the previous study on forests, these authors performed a “meta-analysis” — they synthesized other research studies to come to a new conclusion. But if the last paper provided good news (“forests are absorbing more carbon”), this paper tells a different story.

The authors find, based on an analysis of nearly 50 past experiments, that higher levels of CO2 stimulate soils to release more methane and nitrous oxide, both of which are potent greenhouse gases. This is a  positive feedback to the climate system that has not been accounted for, and could increase yearly greenhouse gas emissions by about one billion tons of CO2-equivalent per year. (Most of these extra emissions would come from agricultural soils or wetlands, and not forests.) One billion extra tons per year is a lot, but it is still small compared to other sources of greenhouse gases — humanity puts almost 30 billion tons of CO2 into the atmosphere every year.


Paper Title: Projecting Coral Reef Futures Under Global Warming and Ocean Acidification
Journal: Science
Authors: John M. Pandolfi, Sean R. Connolly, Dustin J. Marshall, Anne L. Cohen.
The Gist: Coral Reefs are in deep trouble due to warming and acidifying oceans, but they may not be in as much danger as previously thought.

Summary: Many research papers on coral reefs and climate change read like obituaries. A combination of warmer temperatures and more acidic waters (caused by the absorbtion of extra CO2) could kill most if not all of the word’s coral reefs.

This study reviews, in detail, the research on how reefs will respond to these stresses, finding that not all may be lost. The study finds that different corals respond differently to temperature and acidity, and some might be able to survive in the projected conditions. Of course, the important word is “some” — most reefs will suffer greatly. The authors also point to periods in the planet's geologic history when rapid climate change, much like we may during see this century, decimated coral reefs. They emphasize that the speed of climate change is important, and that managing other stresses, such as general water pollution and over-fishing, is important to reefs' survival.
 


Paper Title: CO2 emissions from a tropical hydroelectric reservoir (Balbina, Brazil)
Journal: Journal of Geophysical Research
Authors: Alexandre Kemenes, Bruce R. Forsberg, and John M. Melack.
The Gist: In the Amazon basin, reservoirs may emit more greenhouse gases than coal-fired power plants.

Summary: Although hydropower uses almost no fossil fuels to generate electricity, it can still produce greenhouse gases. Reservoirs flood previously productive ecosystems, replacing them with bodies of water where organic matter decomposes into the greenhouse gases carbon dioxide and methane. This problem is worst in warm, shallow dams, such as those found in the Amazon basin.

These researchers studied the Balbina reservoir, which, because it's extremely large (900 square miles), shallow (less than 100 feet deep at its deepest point), and located in the hot Brazilian Amazon, produces large amounts of CO2 and methane. In fact, after analyzing gasses released from the surface of the reservoir and the river just beyond the dam’s turbines, the authors found the dam is responsible for about three million tons of CO2-equivalent per year. That is a lot of greenhouse gases. If a comparable coal-fired power plant were built instead of the reservoir (producing about 115 megawatts of power on average), it would produce one-tenth as much CO2, the study found.

Because of its size, shallowness, and location, the Balbina reservoir produces many times more greenhouse gases than most dams. Nonetheless, it shows that dams are not necessarily greenhouse gas-free sources of electricity.
 


Paper Title: Atmospheric Carbon Injection Linked to End-Triassic Mass Extinction
Journal: Science
Authors: Micha Ruhl, Nina R. Bonis, Gert-Jan Reichart, Jaap S. Sinninghe Damste, Wolfram M. Kurschner.
The Gist: The mass extinction event that occurred about 200 million years ago, between the Triassic and Jurassic periods, may have been caused by a massive release of methane from the ocean floor — an event some scientists fear could happen today.

Summary: By studying carbon isotopes in fossilized plant material, these scientists determined that (most likely), a large volcanic eruption injected enormous amounts of CO2 into the atmosphere about 200 million years ago. This CO2 warmed the planet, which made methane hydrates — a frozen form of methane gas trapped on the ocean floor — become unstable and be released into the atmosphere. Because methane is a potent greenhouse gas, this further warmed the planet. 

Many scientists fear that a similar situation could occur today. If the oceans warm sufficiently through global warming, the methane on the ocean floor may become unstable, instigating a potentially catastrophic positive feedback to global warming (between the Triassic and Jurassic periods, about half of all life on earth went extinct, probably due to this drastic and sudden climate change).

This analysis shows that this worst-case scenario may have happened in the past. Of course, it isn’t clear how likely this event is to happen today. On the one hand, the amount of CO2 released via the Triassic volcanic eruptions was more than 10 times what humanity is expected to put into the atmosphere. On the other, it was probably released over a few thousand years instead of a few hundred, as is the case with humanity’s current experiment. Overall, most scientists still rate the likelihood of methane hydrates being released anytime soon as low.


Paper Title: The role of ocean thermal expansion in Last Interglacial sea level rise
Journal: Geophysical Research Letters
Authors: Nicholas P. McKay, Jonathan T. Overpeck, and Bette L. Otto‐Bliesner.
The Gist: Both the Antarctic and Greenland Ice Sheets may be more sensitive to temperature variability than previously thought.

Summary: During the last interglacial warm period, a little more than 100,000 years ago, global average surface temperatures were about 1°C warmer than today, and sea levels were about fifteen feet higher. The seas were higher due to a combination of the partially melted Greenland and Antarctic ice sheets, and the fact that water expands when it warms.

This study used geological data and computer models to estimate that the vast majority of past sea level rise was due to melting ice and not expanding water. What that means is that a slight warming in the earth’s climate may have melted the ice sheets more than many scientists previously believed.