Climate: Simple = beautiful?
For decades, scientists have thought that the pre-historic Antarctic climate was governed by events at the other end of the planet — in the Arctic. That’s because variations in solar radiation in the northern summer tracked nicely with the temperature record from sediment cores and ice cores taken on or around Antarctica.
Our record of temperatures in the deep south is carried in the ratios of oxygen and hydrogen isotopes (atoms with different masses) contained in ice cores. But Thomas Laepple, of the Alfred Wegener Institute for Polar and Marine Research in Germany, has just published an article maintaining that because the Antarctic ice cores did not accumulate at a steady pace, they have not been interpreted correctly.
Today, more snow (the source of ice), gathers in winter. Because that likely also happened in the past, ice cores from Antarctic tell us more about winter than summer, says Laepple.
And God created winter!
When Laepple and his colleagues factored in this seasonal effect, the level of sunlight in the southern hemisphere suddenly began to explain Antarctic temperatures. Although the orbital cycles still globally affect solar radiation, there was no longer a need to look at the other end of the Earth to explain rhythms in Antarctic temperatures.
The orbital cycles in question are named for Serbian engineer Milutin Milankovitch, who, about a century ago, sought to understand how three slow shifts in Earth’s orbit would affect the amount of sunshine in different regions, different seasons and years.
These orbital variations, which are influenced by gravity of the moon, sun, Jupiter and Saturn, are the basis of the “Milankovitch cycle:”
The Milankovitch cycle
The Milankovitch cycle tries to sum up the interactions of three long-term variations in Earth’s orbit, which affect the amount of solar radiation during different seasons at different places.
The tilt of Earth’s axis (obliquity) changes, in a rhythm of about 41,000 years, between 22° and 24.5° from a line at 90° to the orbital plane.
The axis changes its direction through “precession,” relative to fixed stars over a 21,000-year period, changing the seasonal distribution of sunlight.
The shape, or eccentricity, of the orbital ellipse varies on a complex rhythm that changes our distance to the sun during different seasons.
As scientists examined ocean sediments and then ice cores, the Milankovitch influence on solar radiation in the Northern hemisphere became the accepted explanation for the changing global climate. But when Laepple factored in the seasonal nature of Antarctic snowfall, he found that Milankovitch could explain the climate on the southern continent more directly. “I don’t question that the Milankovitch cycle has an influence on climate,” says Laepple, “but I question that its influence on the Antarctic is coming through the Artic.”
The adjustment was needed because more ice accumulates during winter and does not affect the overall climate record, Laepple says, “but it completely changes the recording of the signal stemming from the precession of the earth axis, which was the evidence for the remote North-South connection.”
How would you explain it?
Climate is never simple, notes Richard Alley, a climate expert and professor of earth science at Penn State, who was not involved in the research. The Milankovitch cycle, he says, “shifts sunshine around on the planet, with more in some places and less in others, changing the length of seasons, the total sunshine during seasons … so it is not surprising that multiple hypotheses can be advanced to explain a given climate record. Ultimately, the mere fact of correlation is not the answer; we seek understanding of the physical linkages. … The new paper provides a clever new idea for a physical linkage, and I anticipate it will get people discussing and studying.”
Any new study concerning climate processes could get sucked into the political vortex enmiring global warming, so we asked if Laepple’s study of Antarctic conditions should lead us to question the widely-accepted theory that burning fossil fuels and changing land use have altered the climate.
Laepple reminded us that he’s studying changes that occur over periods of 10,000 years. He also insists that the new analysis “will not change the basic record of glacial and interglacial periods — the march of ice ages over the past million years. We are focusing on the precession cycle [the 21,000-year cycle affecting the location of Earth’s axis], as this provides the evidence showing where and how the climate is affected by radiation changes, and might hold the key to the mechanism of slow climate change.”
Years ago, scientists began to revise their interpretation of Greenland ice to account for the seasonality in snowfall, Laepple says. “This new study is part of a long discussion. As we interpret the climate record, we have to focus more on how the climate signal gets recorded in the climate record.”
– David Tenenbaum
Terry Devitt, editor; S.V. Medaris, designer/illustrator; Jenny Seifert, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive
- Synchronicity of Antarctic temperatures and local solar insolation on orbital timescales, Thomas Laepple et al, Nature, 3 March 2011 ↩
- Another Antarctic rhythm, Koji Fujita, Nature, 3 March 2011 ↩
- Milankovitch cycles. ↩
- Astronomical theory of climate change. ↩
- Climate of Antarctica. ↩
- Antarctica and the tropical Pacific. ↩
- Climate change and Antarctica. ↩
- The poles and climate change. ↩
- Antarctic climate change fact sheet. ↩
- NASA: paleoclimatology. ↩
- NOAA: paleoclimatology. ↩
- NSF polar news. ↩
- Drilling ice cores. ↩
- Alfred Wegener Institute for Polar and Marine Research. ↩