Anthropogenic climate change and glacial loss explained in a single number: 25!

A new analysis combining climate modeling with glacier dynamics has given us the first estimation of the percent of glacier loss directly due to anthropogenic climate change.  This is strong evidence and important data to understand to educate the general public and refute climate change denialists.  (By the way, speaking of arguments to use against denialists, if you have not seen the documentary Chasing Ice – see it now!!  You would never think you would shed a tear watching a glacier melt).

The central question answered in this paper is how best to attribute the decrease in total glacial mass around the world to anthropogenic causes or internal, interannual variability (or other causes!).  Although we are clear about global temperatures rising drastically in the last several decades compared to the centuries before, glacial loss rates have been generally constant over the past century.  At first glance, this seems a bit strange, and possibly an argument against anthropogenic effects on climate, because we would expect temperature increases to be correlated with glacial melting.  Well, we would expect if we didn’t have an understanding of glacial dynamics…

We can measure glacial loss either by its extent – how far out it stretches – or by its mass balance – its accumulation from sublimation minus its ablation from melting.  Glacial extent actually lags behind mass loss by tens to hundreds of years.  This means that we can understand the long-term effects of climate change by studying glacial extent, since these changes will occur outside of any variability occurring from year to year.  We are seeing glacial extent decrease today, but this actually began about 150 years ago, in the middle of the 19th century.  At that time, anthropogenic climate forcing was inconsequential, so some of that glacial retreat must be due to natural variability.  Specifically, the Little Ice Age ended right around the 1800’s (ice ages are a wondrously complex topic, but current evidence indicates they’re due to fluctuations in Earth’s orbit, known as Milankovitch cycles, that change the intensity of light hitting the northern latitudes).  The Earth then began warming, lead to a ‘natural’ retreat of the glaciers.  But what about this and the last century, when fossil fuel pollution became dominant?  The goal of this recently published analysis is to parse apart the contributions to glacial retreat from the natural variability and from our pollution.

The main results can be summed in a single number- 25 percent!  That’s the percent of total glacial mass loss (melting minus sublimation) that can be attributed to anthropogenic warming.  That may not seem like much, but that’s from the entire period of 1851 to 2010, from the nascent beginnings of the industrial revolution till now.  If we look only from 1991 to 2010, when fossil fuel pollution truly exploded, the analysis finds that 69% of glacial mass loss can be attributed to anthropogenic causes!  This is much worse, and more of a sign of things to come, as we show no signs of drastically reducing emissions.

To come to this conclusion, the authors combined data from general circulation models (CMIP5) to identify the area and volume of each glacier beginning in 1851.  Then, they used model simulations to look at glacial loss due to all possible forcings, i.e., factors influencing mass loss (solar variability, volcanic activity, aerosols, greenhouse gases), and compared this to simulations using only natural forcings (removing the anthropogenic ones).  They found that the model including all possible forcings matched well with actual data of glacial mass loss since 1851, validating their model.  Then, they just compared the amount of glacial loss between the models including all forcings versus those with only natural forcings and calculated the percentage difference that must be due to the anthropogenic causes.  The data below shows the model prediction of mass loss in RED and the experimental data in BLACK (green is a reduced model, not important for now).   The x-axis is time (in years), and the y-axis is mass loss per year.  You can see the decreasing trend in recent decades:

Figure courtesy of [1]

Figure courtesy of [1]

So there it is – quantitative evidence that we are quickening the rate of glacial mass loss – we are responsible for about 2/3 of the loss in the past two decades.  To those who understand the science, this may not be a surprise, but it’s crucial to have this kind of evidence to show policy makers, denialists, and anyone trying to understand the complexities of our climate and our influence on its inner workings.

References

1)

ResearchBlogging.org

Marzeion, B., Cogley, J., Richter, K., & Parkes, D. (2014). Attribution of global glacier mass loss to anthropogenic and natural causes Science DOI: 10.1126/science.1254702

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