PARIS – Levels of carbon dioxide rose hand in hand with warming at the end of the last Ice Age, according to a study that deals a blow to climate skeptics.
French researchers said Thursday that they had answered a riddle that has perplexed scientists. The question arises from bubbles of atmospheric air, trapped in cores of ice drilled from Antarctica that date back to the last deglaciation, which ended some 10,000 years ago.
These tiny bubbles are closely scrutinized, for they contain carbon dioxide, the principal greenhouse gas behind global warming. The higher or lower the carbon dioxide level, according to the conventional benchmark, the greater or lower the atmospheric temperature.
The anomaly is that the carbon dioxide in the bubbles does not correspond to the level of warming indicated by the surrounding snowfall of that time.
Climate skeptics argued that this showed that carbon dioxide rose after Earth’s atmosphere warmed. It would thus imply that global warming today may come, at least in part, from natural means — not from carbon emissions from fossil fuels, as the scientific consensus maintains.
Writing in the U.S. journal Science, a team led by French glaciologist Frederic Parrenin looked at ice from five deep drilling expeditions in Antarctica. By analyzing the isotopic composition of the nitrogen gas in the samples, the researchers said they were able to filter out the confusing signals from the data.
During the last period of deglaciation, the temperature rose by 19 degrees Celsius and levels of carbon dioxide in the atmosphere rose by about 100 parts per million, they said. The discrepancy comes from the physical process by which carbon dioxide bubbles are formed in successive layers of snow.
“The gas bubbles are always more recent than the ice that surrounds them,” France’s National Center for Scientific Research said in a statement.
Further work will be carried out on different ice samples taken from different eras to see whether this result holds. The researchers said the study did not examine the reasons for the rise in temperature that ushered in today’s deglaciation.
There are several natural factors behind global warming, including volcanic eruptions and rock weathering that releases heat-trapping greenhouse gases, as well as modifications in heat from the sun and tiny changes in the Earth’s axis and orbit.
Sahara causes snowfall
One of the driest spots on Earth — the Sahara desert — is increasingly responsible for snow and rain half a world away in the western U.S., a new study released Thursday shows.
Previous studies hinted that these jet-setting particles may retard rainfall in the Sierra Nevada mountains in Northern California by reducing the size of water droplets in clouds. But scientists who flew through storm clouds in an aircraft, measuring rain and snow and analyzing satellite imagery, found the opposite: Far-flung dust and germs can help stimulate precipitation.
During the winter of 2011, a team from the University of California, San Diego and the National Oceanic and Atmospheric Administration traced particles suspended in clouds over the Sierra Nevada range to distant origins — from the skies over the arid Sahara that later mingled with other pollutants in China and Mongolia before crossing the Pacific.
The days with the most particles in the clouds were also “days when we see the most snow on the ground,” said study leader Kimberly Prather, a professor at the University of California, San Diego whose study was published online Thursday in the journal Science.