A 0.8 Ma history of changing greenhouse gases

Polar ice cores have presented us with the most exquisite records of how high-latitude climate has changed in the recent past from indirect clues presented by variations in stable isotopes of oxygen and deuterium (temperature change), dust and sulfate content (aridity and volcanicity respectively) in layers of ice. That proxy record extends back to 800 ka in the Dome C core from Antarctica, showing in great detail the course of the last nine glacial-interglacial cycles, both the astronomical effect of a changeover from a 40 ka pacing to one of around 100 ka and many intricacies on a millennial time scale. The most tangible archive of information resides in the air bubbles trapped by the original snow that eventually turned into ice. That reveals how the intricate pacing of climate change has been almost perfectly tracked by the global carbon cycle as shown by changes in the concentrations of carbon dioxide and methane. This was first demonstrated by cores through the Greenland ice cap, which penetrate just the last glacial episode and the warmth before and after.

After several years of painstaking bubble analyses at many collaborating labs, the full 800 ka greenhouse-gas records from Antarctica have now appeared (Luthi, D. and 10 others 2008. High resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature, v.  453, p. 379-382. Lulergue, L. and 9 others 2008. Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years. Nature, v.  453, p. 383-386). These long records demonstrate the close connection between climate and greenhouse gases that must be maintained by complex (and not fully understood) feedback mechanisms. Different Earth processes affect the two principal gases, methane probably being controlled by effects of varying temperature and rainfall on peat-rich swamps in the tropics, whereas carbon dioxide’s main driver is capture and release of carbon by the oceans. The central feature remains that of astronomical forces, with perhaps some sign of a signal from the 413 ka component of orbital eccentricity from a shift in the range of temperatures and greenhouse gases in 100 ka cycles around 450 ka ago, and a broad change in methane concentrations. Yet, despite being a pole away from high northern latitudes where comparison of the Greenland ice record with North Atlantic sea-floor sediment data revealed a northern cause for dramatic short term shifts, much the same millennial cycles characterise the whole Antarctic record. It could be that these rapid changes are proxies for the course of northern climate vagaries – there are about 75 of them in the methane Antarctic record. So stunning are the new data that they are sure to spur attempts to go back even further by more drilling in Antarctica, probably in the eastern ice cap where current air temperature and snow fall are extremely low and a greater length of time may be preserved in a smaller thickness of ice. That is because the faster snow and ice accumulate the more rapidly flow removes the record: the reason why the thick Greenland ice, although capable of yielding time resolution of as little as individual years, cannot retain records much beyond 200 ka.

See also: Brook, E. 2008. Windows on the greenhouse. Nature, v.  453, p. 291-292.


The yellowing of the Sahara

As Earth emerged steadily from the last glacial maximum, around 14.8 ka when temperatures were close to those of the Holocene yet sea level still had a way to rise before reaching its current level, the Sahara became a land of wetlands, lakes and grassland. Many caves within its modern arid confines contain superb artwork depicting its fauna and the forager-hunters that preyed on it. Around the time of the earliest Pharaonic civilisation on the Nile floodplain (~3000 BCE) the humid episode ended, forcing inhabitants of the Sahara either to the Nile valley of the Mediterranean coast. Having spanned the millennium-long climatic upheaval of the Younger Dryas and the relative stability and warmth of the early Holocene, why it ended is something of a mystery. A small, amazingly beautiful lake in northern Chad seems to hold the key, as it has existed and gathered sediment for at least 6 thousand years (Kröpelin, S and 14 others 2008. Climate-driven ecosystem succession in the Sahara: the past 6000 years. Science, v. 320, p. 765-768), Lake Yoa is one of several permanent lakes fed by ancient groundwater from the vast Nubian Sandstone aquifer, yet receives negligible rainfall. The uppermost lake sediments are laminated in an annual fashion so that each layer and its contents of aquatic organisms, pollen and dust can be precisely dated.

Between 4200 and 3900 years ago the lake changed from a freshwater habitat to a salt lake when evaporation overcame recharge by rain. However, the environment as a whole did not change suddenly, but progressively. The sudden change in salinity resulted from Lake Yoa losing any outflow, which previously had removed salts accumulated by evaporation of the inflowing groundwater. The lake would then no longer have had any use for humans and their livestock, but conditions did not drive people out of the Sahara suddenly.

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