Ancient trees tell different climate story
Professor Liu Yu says tree rings are key to understanding and predicting climate change. For more than a decade, the deputy director of Chinese Academy of Sciences' Institute of Earth Environment in Xian has ventured into some of the wildest spots in the mid-eastern Tibetan Plateau region to collect samples and take measurements. Liu has run simulations on computers to determine annual temperatures in the region over the past 2,485 years. His research has yielded a particularly interesting, perhaps even shocking, finding: global warming may have stopped.
Where did you find trees more than 2,000 years old in a country with a long history of systematic logging?
I need luck to find a tree more than 100 years old in the lowlands. But on some mountains of the Tibetan Plateau, where the altitude reaches up to 4,000 metres, I have run into forest after forest of Qilian junipers that have remained undisturbed for thousands of years.
How does a tree survive in that kind of harsh environment for so long?
The Qilian juniper is one of the oldest surviving tree species on earth. In the high altitude of the eastern Tibetan Plateau, where poor soil, little rainfall and low temperatures make it impossible for other trees to survive, the juniper has perfectly adapted to the harsh environment by growing very slowly. We recently found one that is close to 2,000 years old, but less than 8 metres tall. In the study of tree rings, slow-growing trees provide information on variations in climate over a long period. Qilian junipers grow only in China.
Is the study of tree rings popular in China?
Chinese researchers have studied tree rings for more than seven decades, and some scientists have produced original studies containing a trove of important data on the climate. But due to historical reasons, their research methods did not quite fit with international mainstream thinking, and therefore not widely recognised. Since the 1990s, as climate change has become a political and diplomatic issue, the study of tree rings has received increased government funding, allowing Chinese researchers to use the best tools and methods and produce well-received results.
Are the field trips fun?
To collect samples, we sometimes need to dance with death. At altitudes of more than 3,500 metres above sea level, my research team has to combat low air pressure, a lack of oxygen, headaches and sometimes life-threatening illnesses. The best Qilian junipers for our purposes often stand alone on a cliff, where they can be fully exposed to the elements. We have to watch our step when approaching such trees. Some researchers from overseas have slipped and died in less-perilous situations. Most virgin forests of Qilian junipers are in areas inaccessible by road, and are barely, if ever, visited by humans. The remoteness turned each of my more than 20 data-collection trips over the past 10 years into unforgettable adventures. Standing so high up, with an ancient tree, one has the opportunity to enjoy some of the most breathtaking landscapes on earth.
Do you need to cut down a tree to take samples?
No, we do not cut down trees. Once a tree is selected, we use a long, fine tube similar to a chopstick to bore through the trunk and reach the core. It's standard practice used by scientists for a long time and does not cause the tree any serious damage. In the laboratory, all the samples must be polished with sandpaper. The work is time-consuming because the samples cannot be used for measurement until they are smooth enough that, when put under a microscope, we can see a perfect outline of their cells.
What do you do with the data?
Using a set of scientific techniques, we measure the width of the rings and convert the variations into changes in annual temperatures. We have published two papers in the English versions of Science in China - Series D: Earth Sciences and in the Chinese Science Bulletin.
What have tree rings told us about climate change over the last two millennia?
Popular belief is that industrialisation has led to the fastest rate of warming witnessed by humans; that we are at the warmest time of the modern era; and that we are causing global warming by emitting carbon dioxide into the atmosphere. None of that fits the records in tree rings.
In northern China, the warmest period occurred from AD401-413, which had an annual mean temperature 0.16 degrees Celsius higher than today's. Other periods, including 604-609, 864-882 and 965-994 had temperatures higher than in recent decades. Our results are supported by historical documents from the period. Archaeological records in Loulan, Xinjiang, show that pomegranate, a fruit rich in vitamin C, was used as currency during the Eastern Jin dynasty (AD317-420). The fruit could not possibly have appeared in northern China without a climate much warmer than today's.
And we are not experiencing the most dramatic climate change in recent history, either. Over the past 2,485 years, the biggest climate change took place during the Eastern Jin dynasty. The period had two stages, with the temperature plummeting first and then soaring. In the warming period, the mean temperature [in the Tibetan Plateau region] increased suddenly from 1.66 degrees to 2.67 degrees in 30 years. In the cooling period, the mean temperature dropped to below that of the Little Ice Age [an abnormally cold period that lasted from about 1550 to 1850]. The coldest years, with a mean temperature of 1.38 degrees, occurred from 362-369, and the temperature was about 1.5 degrees lower than the mean temperature of the late 20th century.
So what causes climate change?
We believe that the sun and atmospheric circulations play a vital, if not decisive, role in this. The millennial cycle of solar activity determines the long-term trends of temperature variations. Almost all sunspot minimums [periods of sometimes several decades when sunspots become rare] correspond with low-temperature intervals. Meanwhile, atmospheric circulations affect temperature changes from decade to decade. To quote Professor Zhu Kezhen, the father of climate change studies in China: 'The big changes in the earth's climate have been controlled by solar radiation, but the small changes by atmospheric circulation.'
Can tree-ring records tell us anything about the future?
Our results show that the temperature continued to increase until 2006, and will now decrease until about 2068. After 2068, the temperature will increase again until 2088.
Do you think your research will help Beijing gain ground in climate negotiations?
I am a scientist, and I know nothing about politics. But the climate- change debate, in my opinion, has more political significance than scientific. Diplomats can sit at negotiating tables talking about carbon caps while scientists have not reached an agreement on the role of carbon dioxide in global warming. But political decisions must be based on sound scientific foundation, or they will be useless, if not dangerous.