May 18, 2017 10:00 am JST

A Japanese lakebed gives scientists a perfect climate record

Sediment samples shed light on conditions 180,000 years in the past

YUTAKA IKEBE, Nikkei staff writer

TOKYO Advances in paleoclimatology are shining a light on the climate conditions of the distant past, with big implications for understanding today's climate -- and what changes may be in store for us.

By analyzing tree rings, ice cores and sediment layers on the bottoms of lakes, scientists have known for some time that the climate has changed drastically through the ages.

A more recent and surprising discovery is that the Earth's climate is not necessarily uniform on a global scale. For example, around 12,000 years ago, at the end of the last ice age, it appears that Europe experienced regional warming while Asia shivered under a blanket of snow.

That finding, reported in March by scientists from the Japan Agency for Marine-Earth Science and Technology, or Jamstec, and Ritsumeikan University, represents the first evidence that the global climate can differ on a regional scale.

LAYERS OF TIME To reach this conclusion, the scientists analyzed silt deposits extracted from the bottom of a very special lake in Japan, where unique topographic conditions have helped preserve a chronological record going back tens of thousands of years.

Lake Suigetsu is one of the five lakes known collectively as the Mikata-goko in Fukui Prefecture. Layers of silt, known as varves, accumulate in the lake each year, like the rings of a tree, each one around 0.7mm thick.

The types of material included in these layers varies with the seasons: Organic matter such as dead plankton settles on the lake bed from spring through fall, minerals like iron precipitate from the lake water at the end of fall, and yellow sand blown over from the continent accumulates in the winter.

Counting the varves is like counting back through the years. Samples extracted from the lake so far provide a historical record stretching back 180,000 years.

The varves of Lake Suigetsu are an extremely precise timescale and were adopted in 2012 as the international standard for dating geological and archeological phenomena up to 50,000 years in the past.

What makes Lake Suigetsu unique is its special combination of topographic features.

Most lakes are subject to an erratic inflow of soil and sediment from rivers, so the layers do not accumulate in an orderly fashion. But in the Mikata-goko drainage basin, all this sediment is deposited in Lake Mikata, which is upstream of Lake Suigetsu. The bottom of Lake Suigetsu, meanwhile, has no oxygen, so there is no life to stir up the bottom and disturb the varves. What's more, the lake is ringed by mountains that block the wind and prevent the formation of waves. Finally, the lake has slowly and continually subsided over time due to the effect of surrounding faults. This means the lake has never become completely filled with sediment, allowing varves to continue piling up.

"This is a rare combination of conditions for a lake anywhere in the world," explained Takeshi Nakagawa, director of the Research Centre for Palaeoclimatology at Ritsumeikan University and leader of the group that collected the varve samples from the lake.

Gordon Schlolaut from Jamstec determined that Lake Suigetsu experienced a frigid climate roughly 12,000 years ago. Schlolaut came to this conclusion by analyzing pollen and other organic matter in the varves and calculating how their ratios changed from layer to layer due to the influence of changes in the climate.

Scientists already knew from records of vegetation and other evidence that Europe was experiencing a temperate climate at that time. Because the cold climate around Lake Suigetsu has been dated with a margin of error of around 50 years, Schlolaut says, it is fair to say that the two climates were happening at basically the same time.

The common assumption is that today's climate change is a globally uniform phenomenon. But the finding about the paleoclimate around Lake Suigetsu shows that this may not always be the case. Scientists conjecture that as Europe warmed at the end of the last ice age, the westerlies pushed northward. This drew warm air up from lower latitudes in East Asia, boosting humidity levels. In Japan and neighboring regions, the result was an accumulation of snow and a cooling of the land.

This regional diversity of climates could have continued for a very long time.

Nakagawa, interested in the way the Earth wobbles while spinning on its axis like a top, discovered a pattern of warming and cooling at Lake Suigetsu with a periodicity of roughly 23,000 years. That pattern broke down around 8,000 years ago, coinciding with when humans first began to practice agriculture. William Ruddiman, a climate scientist at the University of Virginia in the U.S., has proposed that this is evidence of a human-induced global warming trend. Atmospheric levels of two greenhouse gases, carbon dioxide and methane, began increasing in volume around 8,000 years ago and 5,000 years ago, respectively. The study of the varves suggests that human activity might have begun changing the climate far before the industrial revolution.

TREES RING TRUE Tree rings are a useful proxy for studying the climate as far back as 1,000 years ago, since annual tree ring growth is closely related to changes in air temperature and rainfall.

"You can read the tree rings to ascertain the correspondence between human activity and climate change since the Jomon Preriod," explained professor Takeshi Nakatsuka of the Research Institute for Humanity and Nature.

Nakatsuka and his colleagues studied the paleoclimate by analyzing the different isotopes of oxygen in the cellulose that comprises tree rings. They found evidence that temperatures, which had been gradually declining for a thousand years, did an about-face and began rising around the middle of the 19th century -- some 100 years earlier than conventional wisdom says they did.

Ice cores provide another proxy. Japan's National Institute of Polar Research participated in a project that analyzed samples excavated from the ice sheets of Greenland to investigate climate changes going back roughly 4,000 years.

What they discovered was a number of periods when temperatures were as warm as, if not warmer than, the present, despite lower levels of greenhouse gases.

Paleoclimatology shows that abrupt changes in the Earth's climate have been frequent, and that life has survived some rough times. Many of the mechanics behind those changes, however, remain unexplained.

Keyword: Paleoclimatology

Thermometers and rain gauges are just two of the many precision instruments today's scientists have at their disposal to study the climate. But many of these instruments did not exist even 200 years ago, so for paleoclimatology -- the study of the climate of the geologic past -- scientists must extrapolate conditions by analyzing proxies such as tree rings, the gases trapped in the ice sheets at the poles, and the material deposited on the bottoms of lakes and oceans. Ancient texts and diaries also provide clues.

Paleoclimatology has shown that changes to the Earth's climate have occurred repeatedly, in patterns with periodicities ranging from roughly a thousand years to hundreds of thousands of years. Analytical methods have advanced to such a degree that it is now possible to estimate climate conditions with a precision measured in years. Combining this with computer modeling may improve the accuracy of predictions regarding future climate change.

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