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Planetary Watch: Climate Is on the March

by Bill Chameides | January 26th, 2009
posted by Erica Rowell (Editor)

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When we think of variety related to Earth, seasons do not immediately spring to mind. But scientists have just discovered that over about the past half century, the seasons have shifted by roughly 2 days. How much should we care? Plenty.

As students from Duke’s Nicholas School of the Environment visit the Papahanaumokuakea (pronounced Pa-pa-hah-now-mo-koo-ah-keh-ah) Marine National Monument, two new papers suggest that subtle but profound changes in the Earth’s climate are underway that could threaten that very ecosystem.

‘And the Seasons They Go Round and Round’

Winter, spring, summer, fall. They come and go each year with unchanging regularity. Right? Not quite. A new paper in Nature documents that a subtle but profound shift in the seasons is underway. The culprit? Very likely global warming.

The seasons are a direct effect of the tilt of the Earth on its axis. As the Earth orbits the sun, first the Northern Hemisphere points toward it, bringing about our summer and the Southern Hemisphere’s winter; then the Southern Hemisphere points toward it and the seasons flip.

Summer solstice (~ June 22) marks the day when the Sun is in the sky’s most northern point, and winter solstice (~ December 22) marks the day when the Sun is in its most southern point. But even though summer solstice occurs when the Northern Hemisphere receives its maximum annual dosage of solar heat, it is not the Northern Hemisphere’s warmest day of the year on average. There is a seasonal lag between the solstice and the warmest day, which typically arrives weeks later. How much lag varies, with low-latitude oceans having the largest lag (of about 65 days) and polar regions experiencing the shortest lag (of about 15 to 20 days). (More on lag.) Similarly, the year’s coldest day lags behind the winter solstice. That is perhaps why urbanites from Paris to New York tend to flee their cities for the country in August and why snowbirds head to the south in January and February.

The reason for the lag? Thermal inertia. Similar to the way that a pot of water will continue to heat up over an electric stove after the switch is turned off, the Earth’s surface temperature continues to heat for awhile even though the amount of heat from the sun has begun to decline. (Same is true for cold temps.)

We now learn from a report published in Nature by Alexander Stine and Inez Fung at the University of California, Berkeley, and Peter Huybers from Harvard University that the timing of the summer and winter peaks over land (outside the tropics) is changing. Over a 54-year period from 1954 to 2007, these peaks have shifted earlier by about 1.7 days.

Interesting, but should we care about a 2-day shift in the seasons? Here are three reasons why I think we should:

  1. Stine and his co-authors were able to show that the shift is historically anomalous – significantly larger than any measured shift since 1850. Something unusual is going on.
  2. The shift to earlier arrivals of the peak cold and warm days over land suggests that the land’s thermal inertia is decreasing. Since the thermal inertia of land surfaces is largely determined by its moisture content, this suggests that extratropical lands are drying out – making them more susceptible to droughts.
  3. This shift in the seasons is not predicted by the climate models. While some argue that the failure of climate models to predict some global warming effects robustly (such as hurricane intensity or soil moisture) is reason to discount all predictions of dire consequences of global warming, I don’t agree. If the models get it wrong, don’t assume they overestimate impact, they could underestimate it. In this case models don’t predict seasonal shift – so they are missing something that could be important.

‘Climb Every Mountain’

When the climate changes, flora and fauna can have a problem. They can’t put on a sweater or go indoors and turn on the air conditioner. What they can and often do do instead is move with the climate. Researcher I-Ching Chen of the University of York in England and colleagues have documented such a migration in the Proceedings of the National Academy of Sciences. On the basis of identical censuses of 6 moth communities living on Mount Kilabalu in Borneo conducted in 1965 and 2007, the authors conclude that over the period the moths have increased the average altitude of their habitat by about 70 yards in search of cooler temperatures.

A 70-yard climb of Borneo moths may not seem like much of a problem, but it’s important to keep in mind that if moths are feeling the stress of climate change and moving, other species are likely feeling the stress as well. These moths and other species that similarly move upward to get away from warming temperatures may have found an effective strategy for now, but there is only so high they can go. Species living in isolated or unique ecosystems might reach a point beyond which they cannot go and thus would eventually have few options.

And that brings us back to the Papahanaumokuakea Marine National Monument in the Northwestern Hawaiian Islands, where our Nicholas School students are studying this week. The species there inhabit a unique habitat. If climate change makes the islands uninhabitable for many of those species, it is not clear where they will go.

filed under: animals, climate change, faculty, global warming, Planetary Watch, science
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