Scientists Confirm, Again, That Glacier Melting Is Caused by Humans

The Earth has natural ice cycles. Every 100,000 years or so, our planet goes through an Ice Age wherein the Northern Hemisphere is briefly buried under a thick layer of ice. It's also had a Little Ice Age, a period of increased glacial fluctuations lasting a few hundred years.

On a smaller scale, glaciers build up and melt with seasonal and pressure changes. This last cycle serves to regulate the planet's water level, but it isn't completely natural. There are man-made factors that affect the yearly glacial cycle, and unfortunately for us every minor change to glaciers has a big effect on global sea levels. In a new paper published in Science, a team led by Ben Marzeion unpacks just how much our actions are affecting the planet.

When glaciers accumulate less ice mass in winter seasons, it causes a rise in sea level (as less ice means more water). Because the Earth has these ice and water cycles, it's easy to write off these changes as part of the natural self-regulating pattern. But since the 1970s, the shrinking size of glaciers and related rise in sea level suggest that nature isn't acting alone.

Measuring rising sea levels is an interesting way to study the climate because changes take decades to see. Observing glaciers gives scientists an opportunity to track the cycle beyond annual variability; a change from year to year won't trump big picture developments.

The shrinking glaciers scientists are seeing today are in part the aftereffects of the Little Ice Age, which peaked in the mid-19th century. But something else happened since then; the Industrial Revolution brought atmospheric pollution to the hands of men. In their new paper, the researchers wrote that we can therefore reasonably assume that the shrinking glaciers we've seen in the last 150 years are a response to both natural and man-made environmental issues. And they found that the further we get from the Little Ice Age, the more human factors are taking a toll.

Marzeion and his team sought to show just how much human factors are affecting glacier size and rising sea levels by modelling the Earth's ice. They began with the mass balance of all the world's glaciers, which are catalogued in the Randolph Glacier Inventory, taking into account ice dynamics and height changes relative to sea level. Then they reconstructed the area and volume of every glacier in 1851 and modelled each one's evolution going forward in time.

They ran the model with two different data sets: One took into account all known factors that affect glacial size, like solar variability, volcanic eruptions, land-use change, and aerosols dumped into the atmosphere by human activity; the other took into account only natural processes like solar variability and volcanic eruptions.

What the team found shouldn't be surprising. The model with just natural factors was inconsistent with observed glacier mass levels and sea level changes, while the model that took human factors into account was a much closer match to what we see happening.

So not only are human factors taking a visible toll on glaciers and sea level, but the effect is getting steadily greater. Glacial mass lost to human factors has increased from –6 percent during the period from 1851 to 1870 to 69 percent during the period from 1991 to 2010. Natural factors alone would have contributed to about 99 millimeters (give or take 36 mm) to global sea-level rise between 1851 and 2010, but with humans in the mix glacial shrinking contributed to a sea level rise of about 133 millimeters (give or take 30 mm).

That rise in sea level might not sound like a lot, but on an ongoing basis that little bit adds up. A minor decrease in ice buildup when the glaciers freeze corresponds to a rise in sea level that in turn triggers other geologic problems, like unseasonable wet periods, the destabilization of mountain slopes, and the formation of unstable meltwater lakes.

These issues in turn increase the risk of rockslides and floods: A couple millimeters less ice at the Earth's poles can take a serious toll on lives around the globe.