Greenhouse Gases Overturn 2,000 Years of Natural Cooling
04.09.2009 - Atmosphere & Space, Ice & Snow, Flora & Fauna, Human Dimension, Other, Arctic
A new study, led by Northern Arizona University and the National Centerfor Atmospheric Research (NCAR), shows Arctic temperatures in the1990's to have reached their warmest level in at least 2,000 years. The study, to be published in Science, shows that if it hadn't been foranthropogenic greenhouse gas emissions, the Arctic would otherwise be gettinggradually colder.
"Scientists have known for a while that the current period of warming was preceded by a long-term cooling trend," says Darrel Kaufman of Northern Arizona University. "But our reconstruction quantifies the cooling with greater certainty than before."
The reconstruction, which is based on evidence from Arctic lakes sediments combined with previously published data from glacial ice and tree rings, were compared to temperatures obtained through simulations run with an NCAR computer model of global climate called the Community Climate System Model. The results, consequent with the analysis of lake sediments and glacial ice and tree rings, made scientists optimistic as to the quality of future computer projections for temperatures in the Arctic.
Furthermore, this study is the first of its kind to ever quantify the natural cooling of the Arctic due to a change in the Earth's tilt. This change in the orbital cycle of the Earth and consequent lengthening of the distance to the sun had for centuries caused summer temperatures in the Arctic to drop by as much as 0.2°C every thousand years until the trend bottomed out during the Little Ice Age, which occurred between the 16th and 19th centuries. However in spite of this orbital trend continuing, 20th century carbon emissions began to have a stronger influence and the Arctic began to warm instead. Because of "Arctic amplification", a phenomenon exchanging the sunlight-reflecting surface of the ice for sunlight-absorbing surfaces such as the land or ice-free sea, the region has been warming up to three times as fast as any other part of our planet.
