In some quarters of the media, global warming is presented as a natural rebound from an epoch known as the Little Ice Age. Is it possible the rise in global temperatures represents a natural recovery from a prior colder era? The best way to answer that is to understand what the Little Ice Age was and determine if natural forcings alone can explain the recent rise in global temperatures.
The Little Ice Age refers to the period from 1300-1850 when very cold winters and damp, rainy summers were frequent across the Northern Europe and North America. That era was preceded by the Medieval Warm Period from 950-1250 featuring generally warmer temperatures across Europe. Before we get into the temperature data, lets take a look at the physical and cultural evidence for the Little Ice Age.
You can see the retreat of the glaciers in the Alps at the end of the Little Ice Age to the current day. In the Chamonix Valley of the French Alps, advancing glaciers during the Little Ice Age destroyed several villages. In 1645, the Bishop of Geneva performed an exorcism at the base of the glacier to prevent its relentless advance. It didn’t work. Only the end of the Little Ice Age halted the glacier’s advance in the 19th century.
The River Thames Frost Fairs
The River Thames in London froze over 23 times during the Little Ice Age and five times, the ice was thick enough for fairs to be held on the river. When the ice stopped shipping on the river, the fairs were held to supplement incomes for people who relied on river shipping for a living. These events happened in 1684, 1716, 1740, 1789, and 1814. Since then, the river has not frozen solid enough in the city to have such an activity occur. An image of the final frost fair is below:
The Year Without a Summer
The already cold climate of the era was exacerbated by the eruption of Mt. Tambora on April 10, 1815. If volcanic dust reaches the stratosphere, it can remain there for a period of 2-3 years, cooling global temperatures. The eruption of Mt. Tambora was the most powerful in 500,000 years. Its impact was felt across Europe and North America during the summer of 1816. From June 6-8 of that year, snow fell across New England and as far south as the Catskill Mountains. Accumulations reached 12-18 inches in Vermont. In Switzerland, a group of writers, stuck inside during the cold summer at Lake Geneva, decided to have a contest on who could write the most frightening story. One of the authors was Mary Shelley and her effort that summer is below:
Let’s take a look at what the hard data says about the Little Ice Age. Below is a composite of several temperature reconstructions from the past 1,000 years in the Northern Hemisphere:
The range of uncertainty is wider as we go back in time as we are using proxies such as tree rings and ice cores rather than direct temperature measurements. However, even with the wider range of uncertainty it can be seen that temperatures in the Northern Hemisphere were about 0.50 C cooler than the baseline 1961-90 period. Was the Little Ice Age global in nature or was it restricted to the Northern Hemisphere?
Recent research indicates that the hemispheres are not historically in sync when it comes to temperature trends. One key difference is that the Southern Hemisphere is more dominated by oceans than the Northern Hemisphere. The Southern Hemisphere did not experience warming during the northern Medieval Warm Period. The Southern Hemisphere did experience overall cooling between 1571 and 1722. More dramatically, the Southern Hemisphere is in sync with the Northern Hemisphere since the warming trend began in 1850. This indicates the recent global warming trend is fundamentally different than prior climate changes.
Keep in mind that we are dealing with global averages. Like a baseball team that hits .270, but may have players hitting anywhere between .230 and .330, certain areas of the globe will be hotter or cooler than the overall average. During the 1600’s, Europe was colder than North America, and the reverse was the case during the 1800’s. At it’s worst, the regional drops in temperature during the Little Ice Age were on the order of 1 – 2 C (1.8 to 3.6 F). At first glance, that might not seem like much. We tend to think in terms of day-to-day weather and there is not much difference between 0 and 2 C (32 and 35 F). But yearly averages are different than daily temperatures.
We’ll take New York City as an example. The hottest year on record is 2012 at 57.3 F. The average annual temperature is 55.1 F. If temperatures were to climb by 3 F, the average year in New York City would become hotter than the hottest year on record. Again, using the baseball example, a player’s game average fluctuates more so than a career batting average. You can think of daily weather like a game box score, and climate as a career average. It’s much more difficult to raise a career batting average. In the case of climate, it takes a pretty good run of hotter than normal years to raise the average 2-3 F.
Lets go back to the climate history. Global temperatures dipped about 0.5 C over a period of several centuries during the Little Ice Age. Since 1800, global temperatures have risen 1.0 C. This sharp increase gives the temperature graph the hockey stick look. The latest warming trend is more than just a return to norm from the Little Ice Age. There are two other factors to consider as well. One is the increasing acidity of the oceans, the other is the cooling of the upper atmosphere.
Carbon dioxide reacts with seawater to form carbonic acid. Since 1800, the acidity of the oceans have increased by 30%. A rise in global temperatures alone does not explain this, but an increase in atmospheric carbon dioxide delivered to the oceans via the carbon cycle does. As carbon dioxide in the atmosphere increases, it traps more heat near the surface. This allows less heat to escape into the upper atmosphere. The result is the lower atmosphere gets warmer and the upper atmosphere gets cooler. The stratosphere has cooled 1 C since 1800. A natural rebound in global temperatures would warm both the lower and upper atmosphere, observations do not match this. However, increased carbon dioxide in the atmosphere does explain this.
The Little Ice Age looms large historically in that the colder climate played a role in many events leading to modern day Europe and America. What caused the Little Ice Age? That is still a matter of debate. The Maunder Minimum, a sustained period of low solar activity from 1645 to 1715, is often cited as the culprit. However, solar output does not vary enough with solar activity to cause the entire dip in global temperatures during the Little Ice Age. As the old saying goes, correlation is not causation. That’s were the science gets tough. You need to build a model based on the laws of physics explaining causation. While the cause of the Little Ice Age is still undetermined, the origin of modern global warming is not. To deny that trend is caused by human carbon emissions, you have to explain not only the warming of the lower atmosphere, but the cooling of the upper atmosphere and increase in ocean acidity.
To date, no one has accomplished that.
*Image atop post is Hendrick Avercamp’s 1608 painting, Winter Landscape with Ice Skaters. Credit: Wiki Commons.