TCS Daily

'Not Debatable'

By Willie Soon - August 8, 2003 12:00 AM

"I would stipulate that the earth's climate has changed through the millennia. There's no doubt about that.  I mean I've done enough to know that we've had ice ages and that we've had floods and that we've had volcanoes and we've had lots of naturally occurring events which have affected our climate.  We've had El Niño and his spouse, La Niña; we've had all that. That is not debatable."


-- Senator Hillary Rodham Clinton, July 29, 2003, Senate Environment and Public Work Hearing [see real-audio file at time stamp 1:25:35-1:26:00, available on EPW website.]


Senator Clinton knows that natural climate change includes dramatic swings in temperature, rainfall, snowfall and wind currents. One important task for researchers is to define, quantitatively, the past patterns of climate change, and ultimately to explain them and successfully simulate them in climate models.


Both progress and its opposite in the scientific method -- the many unreported false leads -- continue in climate science at a pace as rapid as New England's changing weather. The case today comes from the coldest region on earth, Antarctica.


A group of Japanese and French scientists reported (in the journal Nature, April 3, 2003) their successful drilling of a 2500 meter-long ice core from an inland location in East Antarctica, Dome Fuji. The core from Dome Fuji holds climatic information extending back 330 thousand years (330 kilo-years or kyr). This remarkable scientific project is the second longest core drilled so far.  The longest -- going back 420 kyr above the ancient buried Lake Vostok -- drilled about 1,500 km away at Vostok, Antarctica.


The intense labor of drilling, protecting and analyzing the core pays big in past climate information. Figure 1 shows the changes in surface temperature deduced from isotopes of hydrogen (deuterium) and oxygen (18O) in the melted ice layers from both Vostok (red curve) and Dome Fuji (green curve) for the past 350 thousand years.


The premise relating the concentration of hydrogen and oxygen isotopes in the melted ice to temperature involves the transport of water vapor, through evaporation and condensation, from the ocean's surface to land-locked ice. The concentration of the isotopes that ends up in the land ice depends sensitively on temperature. So the isotope concentrations are, in effect, temperature sensors mined from the cores.


Figure 1: Surface temperature changes at Dome Fuji (green curve) and Vostok (red curve) ice-core projects.  [Adapted from Figures 1 and 3 of Watanabe et al., 2003, Nature, volume 422, 509-512]


Three points can be seen in Figure 1.


First, the local temperature changes around Vostok and Dome Fuji underwent very large swings -- of about 10ºC -- between the glacial and warm periods (these are called interglacial periods -- an example is the current warm phase, called the Holocene).


Second, although Vostok and Dome Fuji are located in different regions separated by about 1500 km (about the distance between Boston and Atlanta) on the east Antarctic continent, the temperature changes are remarkably correlated. The close agreement of the bumps and wiggles suggest that temperature responses at the two locations are almost synchronous (although Dome Fuji was slightly warmer than Vostok during the interglacial warm periods).


Third is that the Dome Fuji temperature record shows warm peaks -- labeled as stage 9.3 (about 330 thousand years before present), stage 7.5 (about 248 thousand years before present) and stage 5.5 (about 135 thousand years before present) -- were significantly warmer than the current warm peak, the Holocene. The Japanese and French team concluded that "stages 9.3 and 5.5 were much warmer than the most recent 1000 years (~4.5ºC for stage 5.5 and up to 6ºC for stage 9.3)."


These newly-obtained facts present a puzzle.


The air's carbon dioxide concentration in the previous interglacial warm periods (stages 9.3, 7.5, 5.5) seems to have been no more than 300 ppm. But the current carbon dioxide level -- about 370 ppm -- is much higher and yet the Holocene period that we are living in now is still comparatively cooler.


After all, according to the anthropogenic theory of global warming, human-induced emissions of CO2 will lead to a warming of the earth's climate. But in earlier periods, with significantly lower concentrations of CO2, the climate was considerably warmer than today.


What accounts for the discrepancy? Three possible explanations emerge:


(1)   It is well-known that the glacial and interglacial changes in Figure 1 arise mainly from changes in the distribution of seasonal sunlight that the climate system receives as the basic geometry of the earth's orbit around the Sun changes over tens of thousand to one hundred thousand years. In that case, the information in Figure 1 is not very relevant for the issue of predicting global warming from added atmospheric carbon dioxide in the next century or so.


(2)   The air's carbon dioxide content is not the main driver of temperature change. Recent evidence (Caillon et al., March 14, 2003's Science, volume 299, 1728-1731) suggests that during the sharp transition from the glacial to the interglacial period, the temperature found for Vostok started to warm some 600 to 1000 years before the air's carbon dioxide concentration began to rise. Both factors -- rising temperatures and the subsequent rise in the air's carbon dioxide content -- ultimately led to the deglaciation even at the opposite pole of the Earth, in the Northern Hemisphere where areas like Boston, New York City and London were blanketed under one-mile thick sheet of ice.


(3)   Local (Antarctic) surface temperature is not as sensitive to the air's carbon dioxide changes as today's climate simulations suggest. In that regard, most instrumental thermometer records in Antarctica show cooling trends for the past 30 to 50 years -- while the air's global carbon dioxide content rose sharply.


The latest geological ice-core records suggest that large climatic swings in Antarctica are natural phenomena, and that atmospheric carbon dioxide concentration is not the strongest factor in those dramatic temperature changes. If only computer simulations could correctly explain those stipulated and well-documented abrupt shifts in climate.


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