The earth's climate appears to have been more variable over the past 7,000 years than often thought. This is the conclusion of a new study forthcoming online this week in the U.S. scientific journal "Proceedings of the National Academy of Sciences" (PNAS). In the study, scientists from the Potsdam-based Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, and Harvard University show that sea surface temperatures reconstructed from climate archives vary to a much greater extent on long time scales than simulated by climate models. The consequence: either the analysed climate archives supply inaccurate temperature signals, or the tested models underestimate the regional climate fluctuations in the Earth's recent history.
In order to reconstruct climate history, it is necessary to study natural climate archives since, in terms of the Earth's history, humankind has only very recently begun measuring the planet. There have been instrumental measurements of ocean temperatures for only around 150 years now. For periods prior to that, scientists have to rely on "proxies", i.e. indicators enabling indirect conclusions to be drawn about climate data from earlier times. Such climate archives generally refer to spatially limited areas and differ in their temporal resolution. They may also include significant "background noise".
"In our study we weren't interested in how warm the climate might have been at time X in a specific region. We wanted to retrospectively analyse how much the regional climate temporally varies over decades to millennia," explains Dr. Thomas Laepple from the Alfred Wegener Institute. "One of our biggest challenges was to make it possible to compare various measured data and climate archives from a wide variety of regions and filter out the natural noise that can greatly distort the signal of climate archives."
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