Article: Zennaro, P., N. Kehrwald, J. Marlon, W. F. Ruddiman, T. Brücher, C. Agostinelli, D. Dahl-Jensen, R. Zangrando, A. Gambaro, and C. Barbante (2015), “Europe on fire three thousand years ago: Arson or climate?” Geophys. Res. Lett., 42, 5023–2033. DOI: 10.1002/2015GL064259.
Humans and Climate
When did human impacts start to influence climate? This statement is heavily debated by many paleontologists, politicians, and the general public.
By now, we are all aware that the Industrial Revolution started to convert previously stored fossil fuels into the carbon dioxide in our present day atmosphere. This fossil carbon is driving many of the current and projected future climate changes.
However, many scientists and anthropologists argue that human impacts on climate began much earlier that the 1800’s. The primary reason: fire.
Wildfires are a natural part of the Earth’s system. Sediment records of charcoal tell us that wildfire activity naturally increases during warm and dry climate periods and decreases during cool climate periods. But let us take this one step further. Climate and wildfires are integrally connected. When the climate is dry, lightning strikes can cause uncontrollable wildfires to break out. These fires, in turn, release dark particulates and carbon dioxide into the atmosphere, trapping more heat energy.
Enter humans! Fire is pivotal to our early ancestors….and still is today! We use fire to cook our food, stay warm, and (especially to our early ancestors) to clear vegetation. One of the most common agricultural practices in early human history applied a slash and burn technique to convert forests into croplands and villages. Those sedimentary charcoal records suggest that humans have been using slash and burn practices in Europe as early as 3,000 years before the present (ybp). In fact, fire activity between 2,000 to 3,000 ybp increased worldwide (everywhere except for Australia), even though the climate was stable.
Thus, have we under-estimated early human impacts on climate and our ecosystem? Zennaro et al., aimed to supplement those sedimentary records with ice cores to investigate this question further.
The approach: Ice and Biomarkers
Just like sediments, glacial ice can be used as a record for historical events, such as volcanic eruptions and wildfires. Glaciers accumulate new layers of ice every year, effectively trapping atmospheric gases (as air bubbles) and airborne particles. When something like a massive wildfire occurs, tons of soot and charcoal get ejected into the air and can travel great distances before they fall out of the air (usually by a process called wet deposition, commonly known as precipitation!). These particles can be added to those annual layers of glacial ice, preserving that moment in time for scientists!
But instead of charcoal, Zennaro et al. used a biomarker called levoglucosan to reconstruct the fire history over the last 15,000 years. Levoglucosan (see Figure 3) is a molecule that is created when woody materials, like trees, are burned. So the more levoglucosan in the ice core, the more fire activity during that time period.
A ~1500 meter ice core from the Northern Greenland Eemian glacier was drilled from 2008-2012. Ice pieces between 5 to 10 centimeters long were used to measure the flux of levoglucosan using liquid chromatography and mass spectrometry. Levoglucosan flux, rather than concentration, was used since the accumulation of snow was not constant. In other words (purely hypothetical), imagine that the same amount of levoglucosan deposited in 1900 and 1910, but there was 2X more snow accumulation in 1900; thus, the levoglucosan concentration in 1900 would be half that of 1910 but the flux would be the same.
The age of this ice core ranged from 1 to 14,962 ybp.
What did they find?
The levoglucosan ice core record showed that there was fire activity throughout the Holocene and it peaked around 2,500 ybp (Figure 4). The lowest concentrations of this fire biomarker was at the beginning of the record between 15,000 to 12,000 ybp. Levoglucosan concentrations increased at the transition between the glacial and interglacial periods. The researchers hypothesized that the change to an interglacial period would have decreased ice cover and expanded the extent of boreal forests, likely resulting in an increase in fire activity (simply, there is just more area available for burning).
Around 4,000 ybp, there was an abrupt increase in levoglucosan with the study-wide peak between 2,000 and 3,000 ybp. This levoglucosan reconstruction of fire activity agreed with the Global Charcoal Database which uses pollen and charcoal fluxes from sediment cores to reconstruct human influence and fire activity.
Fire activity (frequency, duration, and intensity of fires) is a combination of natural drivers (solar insolation and temperature) and human activity (i.e. slash and burn). The ice core, using δ18O (a stable but heavy isotope of oxygen that is used to estimate paleo-temperatures), suggests that the warmest temperatures would have occurred around 7,000 ybp, then cooled until the industrial era. Likewise, solar insolation has decreased in the Northern Hemisphere over this period, which should cause a shorter fire season.
Thus, if temperature was the primary driver of fire activity at this time, we would expect fire activity to also decrease. Instead, levoglucosan and charcoal records suggest that fire activity increased around 4,000 ybp and peaked ~2,500 ybp. Thus, the major natural climate drivers do not support this peak in fire activity.
So, the best explanation for this peak in fire activity? You guessed it! Humans.
Deposits on the Greenland glaciers are mostly from Eurasia and North America. A combination of pollen records and literature on early European settlement suggest that massive forest clearings started around 4,000 ybp and forests were reduced to near-modern levels in places like England by 2,500 years before present. As a side note, climate drivers do support any change in fire activity in North America.
After this maxima, fire activity in Europe began to decrease, which makes perfect anthropological sense. As population increases and cities are established, wild fires become less frequent since there is less biomass (forests already cut down), and the region becomes fragmented making fires harder to spread.
It is very likely that humans have been influencing our climate, by releasing carbon dioxide into the atmosphere, even before the Industrial Revolution. The slash and burn agricultural practice to clear away forests likely caused the European peak in fire activity 2,500 years ago, demonstrating that humans have been changing our climate and ecosystem as early as 4,000 years before the present.
The slash and burn practice allowed humans to create communities and croplands for food, but harmed forest ecosystems and affected our climate. What are your thoughts on early human activities?