Is the Earth currently experiencing a warming trend? Yes.
Are human activities, including the burning of fossil fuel and forest conversion, the primary — or even significant — drivers of this current temperature trend? The scientifically appropriate answer — cautious and conforming to the known facts — is: probably not.
"The Earth's climate cycles through 90,000-year Ice Ages interspersed with shorter warm periods."
Indeed, the current warming cycle is not unusual: Evidence from around the world shows that the Earth has experienced numerous climate cycles throughout its history. These cycles include glacial periods (more commonly known as Ice Ages) and interglacial periods, as well as smaller, though significant, fluctuations. During the past 20 years, scientists have been accumulating strong physical evidence that the Earth consistently goes through a climate cycle marked by alternating warmer and cooler periods over 1,500 years (plus or minus 500 years). The evidence indicates that:
- The Earth experienced a Little Ice Age from 1300 to 1850.
- A Modern Warming period began about 1850 and continues to the present.
Figure I tracks the Medieval Warming and Little Ice Age that preceded today’s Modern Warming.
"Within the longer cycle, the climate warms and cools in 1,500 year-cycles (plus or minus 500 years)."
We have long had physical evidence that the Earth has experienced numerous climate cycles throughout its history. The best-known of these is the Ice Age cycle, with 90,000-year Ice Ages interspersed with far shorter interglacial periods. What is new is the evidence of more moderate, persistent climate cycles within these broader cycles.
The message that the 1,500-year climate cycle is real, broad — and sudden — is being dug up from the Earth itself by modern science. The key evidence comes from very long-term proxies for temperature change, especially ice cores, seabed and lake sediments, and fossils of pollen grains and tiny sea creatures that document even small changes in Earth’s temperature over many thousands of years.
In addition, we have a number of shorter-term proxies (cave stalagmites, tree rings from trees both living and buried, boreholes and a wide variety of other temperature proxies) that testify to the global nature of the 1,500-year climate cycles.
A striking example of the effect of this 1,500-year climate cycle can be seen in the temperature-sensitive history of wine-growing in England.
"Evidence from every continent and ocean confirms the 1,500-year cycle."
The Romans grew wine grapes in England when they occupied it from the first through the fourth centuries. Aerial photography, remote sensing and large-scale excavation have recently revealed seven Roman-era vineyards in south central England. One site contains nearly four miles of bedding trenches that could have supported some 4,000 grapevines.1
A thousand years later, during the Medieval Warming of 950-1300, the Britons themselves grew wine grapes in England. The Domesday Book, compiled in the 11th century, recorded 46 places in southern England growing wine grapes. (Richard Tkachuck of the Geosciences Research Institute notes that German vineyards were found as high as 780 meters in elevation during the Medieval Warming, but are found today up to only 560 meters — indicating a temperature difference of 1° to 1.4° C.2) During the Little Ice Age (1300-1850), England was too cold to grow wine grapes. Instead, London often held ice festivals on the frozen Thames River, which hasn’t frozen in the last 150 years.
Now that the Little Ice Age has given way to the Modern Warming, a few hardy Britons have again begun serious efforts to grow good wine grapes in England — but thus far with spotty success. The Web site www.english-wine.com admits that British wine-making is still a very chancy proposition. Only two years in 10 will the wine be very good, and during four of the other years it will be terrible, “largely due to weather....”
British vintners should be hopeful, however. The Modern Warming is still young, and likely to eventually give them several centuries of good wine production. The Earth is apparently having its third natural, moderate — and unstoppable — warming in 2,000 years.
Taken by itself, the cycle of wine-grape growing in England might be seen as an aberration. However, this is just one bit of the emerging body of physical evidence of a natural climate cycle — a cycle too moderate and too long to have been reported in the Viking sagas and earlier oral histories from people without thermometers.
"The changing concentration of oxygen-18 in Greenland ice cores corresponds to the 1,500-year cycle."
None of these pieces of evidence would be convincing in and of themselves. However, in order to dismiss the huge impact of the 1500-year climate cycle, we would have to dismiss not only the human histories from those periods, but also the enormous range and variety of physical evidence presented here.
Importantly, if the current warming trend is, as the evidence suggests, part of an entirely natural climate cycle, actions proposed to prevent further warming would be futile and could, by imposing substantial costs upon the global economy, lessen the ability of people to adapt to the impacts — both positive and negative — of climate change.