A new study in Nature says the Earth, previously headed for an Ice Age before the Industrial Revolution, is likely to maintain its current warm phase in the glacial cycle for an unprecedented amount of time.
The researchers―Andrey Ganopolski, Ricarda Winkelmann, and Hans Joachim Schellnhuber of the Potsdam Institute for Climate Impact Research―first examined the effect of the Earth’s orbital characteristics on the glacial cycle, but found that increased carbon dioxide (CO2) played a more important role. Additionally, they found a critical relationship between CO2 and solar radiation that could aid in predicting the beginning of the next glacial period.
“This illustrates very clearly that we have long entered a new era, and that in the Anthropocene humanity itself has become a geological force. In fact, an epoch could be ushered in which might be dubbed the Deglacial,” co-author Hans Joachim Schellnhuber said in a press release for the study.
Interglacial periods are the phases in Earth’s history with generally low amounts of global ice, and glacial periods have the most ice. The study uses the commonly accepted theory that glacial periods occur when Northern Hemisphere summer solar radiation (the amount of solar energy reaching the Earth’s surface) is at its lowest.
If summer solar radiation is low in the Northern Hemisphere, where there is more land, snow does not melt as readily. This build up of snow leads to more reflectivity―albedo―at the surface. As global albedo increases, even less snow melts and this process continues enhancing itself; this positive feedback loop could potentially trigger a glacial period.
This concept was used to support the study’s claim that our planet was headed for a glacial period prior to the Industrial Revolution since the solar exposure was, and still is, close to its minimum. The authors argue that the level of CO2 and low amount of solar radiation seen prior to industrialization should have led to a rapid buildup of ice sheets.
The team also considered the effect that the Earth’s orbital shape might have on climate. The eccentricity of the Earth is currently in a low phase―the Earth’s orbit fluctuates over thousands of years between having a more pronounced elliptical shape in its high phase and a more circular one in its low phase. In its current orbital pattern, the Earth does not get far enough from the sun during the Northern Hemisphere summer to achieve the solar radiation minima that typically spur the buildup of ice. They believed that the current near-circular orbital pattern may have countered the effects of the cooling that would be expected from the lower solar radiation.
In other words, the team thought the shape of the Earth’s orbit could explain why we have not entered an ice age.
In order to test this theory, the researchers used paleoclimate data (data derived from studying natural indicators of the conditions found in previous geologic times) from two similar glacial periods to see if there are any important similarities to the period we are in today, known as the Holocene.
The theory that the orbit had caused the delayed ice age was challenged by the fact that similar orbital patterns have led to glacial periods in the past. It was found that neither period matched the Holocene’s characteristics well enough, again showing the unprecedented behavior of the glacial cycle.
Though not a providing a perfect replication of current circumstances, this paleo data provided the closest geological approximation of similar global conditions and was incorporated into their simulations to try to get the most accurate representation of when the next glacial period should begin. The team used a highly sensitive model which had accurately modeled the last eight glacial cycles to examine the effect CO2 and orbit patterns had on this cycle.
Ultimately, they concluded that carbon dioxide, not the Earth’s orbital shape, was the more important factor.
The team surmised that even accounting for the planet’s current near circular orbit “…the Earth system would already be well on the way towards a new glacial state if the pre-industrial CO2 level had been merely 40 ppm [parts per million] lower than it was during the late Holocene…” This clearly shows the importance of the increased CO2 levels.
Using their data, a more accurate threshold of CO2 levels was determined in order to predict the onset of the next glacial period; with this threshold the team was able to find the “glacial inception” point for various levels of solar radiation.
Based on all but one of the scenarios put forth by the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, the team concluded that the Earth will not see another glacial period for upwards of 100,000 years or more ― which is remarkable for glacial cycles.
The researchers argue that even without further human influence, the system would still have an exceedingly long time between glacial periods.
“[O]ur study also shows that relatively moderate additional anthropogenic CO2-emissions from burning oil, coal, and gas are already sufficient to postpone the next ice age for another 50.000 years,” the lead author said in a press release for the study.
“The bottom line is that we are basically skipping a whole glacial cycle, which is unprecedented. It is mind-boggling that humankind is able to interfere with a mechanism that shaped the world as we know it.”