Researcher helps paint the fullest picture yet of how increases in CO2 helped end the ice age
Harvard scientists are helping to paint the fullest picture yet of how a handful of factors, particularly world-wide increases in atmospheric carbon dioxide, combined to end the last ice age approximately 20,000 to 10,000 years ago.
As described in a paper published April 5 in Nature, researchers compiled ice and sedimentary core samples collected from dozens of locations around the world, and found evidence that while changes in Earth's orbit may have touched off a warming trend, increases in CO2 played a far more important role in pushing the planet out of the ice age.
"Orbital changes are the pacemaker. They're the trigger, but they don't get you too far," lead author Jeremy Shakun, a visiting postdoctoral fellow in Earth and Planetary Science Shakun, said. "Our study shows that CO2 was a much more important factor, and was really driving worldwide warming during the last deglaciation."
Though scientists have known for many years, based on studies of Antarctic ice cores, that deglaciations over the last million years and spikes in CO2 were connected, establishing a clear cause-and-effect relationship between CO2 and global warming from the geologic record has remained difficult, Shakun said. In fact, when studied closely, the ice-core data indicate that CO2 levels rose after temperatures were already on the increase, a finding that has often been used by global warming skeptics to bolster claims that greenhouse gases do not contribute to climate change.
Many climate scientists have addressed the criticism and shown that the lag between temperature and CO2 increases means that greenhouse gases were an amplifier, rather than trigger, of past climate change, but Shakun and his colleagues saw a larger problem – while CO2 measurements taken from air bubbles in the ice cores reflect levels throughout the global atmosphere, temperatures recorded in the ice only reflect local Antarctic conditions.
To get a more accurate picture of the relationship between global temperature and CO2, they synthesized dozens of core samples – 80 in all – collected from around the world.
"We have ice cores from Greenland, people have cored the sea floor all around the world, they've cored lakes on the continents, and they have worked out temperature histories for all these sites," Shakun said. "Putting all of these records together into a reconstruction of global temperature shows a beautiful correlation with rising CO2 at the end of the ice age. Even more interesting, while CO2 trails Antarctic warming, it actually precedes global temperature change, which is what you would expect if CO2 is causing the warming".
"The previous science clearly said that CO2 had something to do with warming," Shakun added. "It has gone up and down in tandem with the ice ages, so it is clearly involved. If it was an amplifier, the question was how big of an amplifier? Does it explain a lot of climate change, or was it a small piece, and other factors were more important? I think this research really points a strong finger at the idea that CO2 was a major player."
Armed with that evidence, Shakun and colleagues were able to sketch out how a series of factors aligned that eventually led to a worldwide warming trend and the end of the ice age.
Most scientists now believe, Shakun said, that the first domino wasn't an increase in greenhouse gases, but a gradual change in Earth's orbit. That orbital change resulted in more sunlight hitting the northern hemisphere. As the ice sheets over North America and Europe melted, millions of gallons of fresh water flooded into the North Atlantic and disruped the cyclical flow of ocean currents.
"Ocean circulation works like a global conveyor belt," Shakun said. "The reason it's important for climate is because it's moving heat around. If you look at it today, the northern hemisphere is on average, a couple degrees warmer than the south, and that's partly because the ocean is pulling heat northward as it flows across the equator in the Atlantic".
"But if you turn the conveyor belt off, it's going to warm the south because you're no longer stealing that heat away. Warming the southern hemisphere, in turn, shifts the winds and melts back sea ice that had formed a cap, trapping carbon in the deep ocean."
As more and more CO2 enters the atmosphere, Shakun said, the global warming trend continues, "and pretty soon you're headed out of an ice age."
While the research strengthens the link between CO2 and the Ice Ages, Shakun believes it also reinforces the importance of addressing CO2-driven climate change in our own time.
"I don't think this tells us anything fundamentally new about global warming," Shakun said. "Most scientists are not in doubt about the human-enhanced greenhouse effect – there are nearly a dozen strong pieces of evidence that it is affecting global climate. This is just one more log on the fire that confirms it."
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Shakun's research was supported by a National Oceanic and Atmospheric Administration Climate and Global Change Fellowship and by the National Science Foundation, and conducted using resources at the Oak Ridge National Laboratory.
http://www.nature.com/nature/journal/v484/n7392/full/nature10915.html
Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation
- Nature
- 484,
- 49–54
- (05 April 2012)
- doi:10.1038/nature10915
The covariation of carbon dioxide (CO2) concentration and temperature in Antarctic ice-core records suggests a close link between CO2 and climate during the Pleistocene ice ages. The role and relative importance of CO2 in producing these climate changes remains unclear, however, in part because the ice-core deuterium record reflects local rather than global temperature. Here we construct a record of global surface temperature from 80 proxy records and show that temperature is correlated with and generally lags CO2 during the last (that is, the most recent) deglaciation. Differences between the respective temperature changes of the Northern Hemisphere and Southern Hemisphere parallel variations in the strength of the Atlantic meridional overturning circulation recorded in marine sediments. These observations, together with transient global climate model simulations, support the conclusion that an antiphased hemispheric temperature response to ocean circulation changes superimposed on globally in-phase warming driven by increasing CO2 concentrations is an explanation for much of the temperature change at the end of the most recent ice age.
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| a, Location map. CBT, cyclization ratio of branched tetraethers; MBT, methylation index of branched tetraethers; TEX86, tetraether index of tetraethers consisting of 86 carbon atoms; , alkenone unsat… |
Es interesante el artículo en cuanto a evidenciar claramente una relación entre las concentraciones de dióxido de carbono y la temperatura media global. Sin embargo, y contrario a lo mencionado en el último parrafo, dudo que este registro sea una pieza más de evidencia a favor del impacto de las emisiones del hombre. Acaso el gráfico no muestra de que nos encontramos ante un ciclo apenas normal de la Tierra? Y si medimos la abundacia relativa de las emisiones humanas, estas acaso no son insignificativas frente a las emisiones naturales? Mientras estas areas de la ciencia no dejen de ser tan cualitativas, la respuesta al impacto global del hombre (partiendo de un impacto local creo que no hay necesidad de analizarlo, pero globalmente los cambios locales no tienen proque afectar un cambio global) seran cuestionables.
ResponderEliminarHola Edward: Agradezco tu comentario. Te respondo que éste es un ciclo anormal en La Tierra, es irregular debido a su temprana aparición. Las emisiones humanas son debido a las actividades humanas. La ganadería, la agricultura desmedida, la contaminación industrial, la quema de petróleo y carbono, los combustibles fósiles, el auge del consumismo y la forma de obtención de los materiales para su construcción, entre otros y la lista aumenta si se tiene en cuenta cada factor del día a día y que los territorios donde hoy se ubican las ciudades no eran baldíos. Es en realidad un ciclo adelantado y progresivo provocado por la humanidad.
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