Let’s get global warming and long-cycle natural climate change put in a little historical perspective for us today. Yes, the Medieval Warm Period and whether Greenland was actually green will be examined by two different climate scientists.

Got a question about climate science? These next few weeks at Rooted we’re running the Ask a climate scientist series. Keep the questions coming as well, by either leaving one as a comment, or — preferably — emailing me directly. Some of you have asked similar questions, so keep an eye out for them, and let me know if that didn’t answer exactly what you were wondering.

These answers are coming from American Geophysical Union’s Climate Science Q&A service, where more than 700 volunteer scientists provide factual and peer-reviewed climate science information to journalists. The AGU only comments on science, not climate policy.

Crikey reader Peter asked:

While there is lots of evidence and also personal experience which strongly suggests to me that climate change is real, the place for long-cycle natural climate change never seems to be referred to. For example, there is good evidence that Greenland was indeed a green land in about 900 AD and that the Thames would experience some freezing in the late 1700’s. Could someone put this apparent long-cycle activity into a perspective for us.

Professor Jeffrey Park, from Yale University, responds:

It is easy to overinterpret terminology. The phrases “Medieval Warm Period,” “Little Ice Age,” and even “Greenland” have some history behind them. Near the year 1000AD the Norse Vikings established farming settlements in Greenland. Subsistence farming would have been impossible in Greenland in 1900AD, so this is clear evidence of unusual warmth in the North Atlantic region at that time. Similarly, anecdotal evidence of unusually cold winters in the early-modern period in Northern Europe (roughly during the Protestant Reformation) and of temporary advances of glaciers in the European Alps suggested a period of cooler climate there. How large were these climate excursions? How long did they last? How widespread were they? Because we don’t have eyewitnesses at all places in all centuries, scientists have tried to answer these questions with climate-proxy data.

Climate proxies are biological, chemical or geological processes that are sensitive to temperature. The variation of climate-proxy observations can be calibrated in the last 150 years against actual temperature measurements, and only then can be applied to the centuries before 1850AD. Calibration is important because many systems are sensitive to multiple factors. For instance, tree-ring widths vary with both temperature and precipitation, because both influence tree growth. Ice cores drilled from glaciers offer yearly ice-thickness variations, and also clues to temperature in the mixture of different isotopes of hydrogen and oxygen in the ice. Ocean corals are also temperature sensitive, and have yearly growth cycles. Having a variety of climate proxies allows scientists to “spread their bets” when developing a robust temperature proxy, and also allows for the study of temperature changes in polar latitudes, temperate latitudes and tropical latitudes, and on different continents.

The proxy data sets are still growing as scientists collect data from more locations, but the overall pattern strongly supports some general statements. First, neither the Medieval Warm Period, nor the Little Ice Age, were uniformly warm or uniformly cool. If these climate anomalies had been based originally on historical records from the tropical Pacific, the Medieval period would have been called a “cooling” and the Little Ice Age might have been called a “warm period.” Historical documents from the tropical Pacific don’t exist, so we are stuck with nicknames that describe the climates of Northern Europe at roughly 1000AD and 1500AD.

Second, the proxy data tell us that the “Medieval Warm Period” is accurate terminology, because the global and Northern-Hemisphere temperatures from proxy data during roughly 950-1250AD are warmer than all centuries before the 20th century, just not uniformly so. The geographical variations of proxy-data suggest a climate process that resembles a very slow El Nino, with temperatures in the tropical Pacific trending one way, temperatures in the northern continents trending the opposite way.

Third, the “Little Ice Age” is an exaggeration, because the Little Ice Age compares with the most-recent real Ice Age (100000-20000 years ago) as a snowflake compares with a blizzard. In the Little Ice Age (roughly 1450-1750AD) we have reports of Dutchmen ice-skating on frozen canals, but in the real Ice Age all of Sweden was buried beneath a kilometer-thick sheet of ice. A careful examination of the historical data in both Europe and the Ottoman Empire suggests that the climate extremes that characterize the Little Ice Age rarely persisted for more than a decade, suggesting an amplification of one of the natural cycles of atmosphere-ocean heat exchange that can be detected with careful statistical analysis of 20th-century temperature data.

The fourth general observation from proxy data is that the global-average temperature variations experienced in the Medieval Warm Period and the Little Ice Age were smaller deviations from the norm than we are experiencing in the 21st century. The 21st-century pattern of global warming does not look like a natural climate cycle, because there is scarcely anywhere on the globe where a cooling trend counter-balances the warming. Also, typical “natural climate variability” involves a heat exchange between the atmosphere and ocean: the atmosphere warms by extracting heat from the ocean, or cools when the ocean extracts heat from the atmosphere. “El Nino” and “La Nina” represent a cyclic exchange of heat between atmosphere and ocean in the tropics, for instance. In the second half of the 20th century, historical measurements tell us that both the ocean and atmosphere experienced warming. Heat is still being exchanges between atmosphere and ocean (we still have El Nino events!), but these are wiggles on an upward trend.

The fifth general observation we get from the proxy data is that natural temperature variations that were smaller that those we are experiencing in the 21st century affected human civilization greatly. The expansion of the Vikings would have been impossible without the Medieval Warm Period. The climate extremes of the Little Ice Age caused famine in Europe and nearly caused the Ottoman Empire to collapse. For the Ottomans, see the webpage of the historian Sam White of Oberlin College.

Also, the name of “Greenland” was essentially a real-estate scam, similar to calling a housing subdivision in the Australian outback “Verdant Groves.” Even the Vikings were guilty of false advertising! Joking aside, around 1000AD when the Viking explorers visited and settled in Greenland, the climate along certain portions of the coast was warm enough to support a marginal agriculture, similar to the lifestyle of a Norwegian farmer, based on grains and cattle. Almost all of “Greenland” was covered by a kilometer-thick (or more) ice sheet, even when the Vikings first arrived. Drill cores from the ice sheet have yearly layers going back hundreds of thousands of years, so the green part of Greenland has not reached far inland during the last few 100 kyr. The Vikings would not have been frightened away from establishing farms within sight of the ice sheets, because that situation was common in Norway, though the glaciers in Norway are/were not as extensive.

A good reference for the history of the Greenland Ice Sheet (and Antarctica) is The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future by Richard B. Alley.

The climate in the Northern Atlantic was relatively warm at the turn of the previous millenium (1000AD), but cooled gradually in the centuries afterward. It took until 1400AD or so before agriculture failed and the Norse Greenlanders starved. The paradox is that Eskimo villages were founded in the neighborhood of the declining Norse settlements, with an economy based on fishing. Archeologists argue over why the Norse failed to copy the eskimos and survive.

Dr Stephen Mulkey from the University of Idaho reviewed this answer.