Arctic Amplification: Aerosols and Greenhouse Gases
Has the far north always warmed faster than the rest of the Earth?
One of the received truths that I have always passed on to students in my classes is that warming is occurring more rapidly in the Arctic than in the temperate or tropical zones. This truth has a long history. Earlier essays in this series have reached back to Svante Arrhenius and his massive calculations, published first in 1896, on the impact of carbon dioxide on global temperatures. As something of a footnote to that paper, Arrhenius had calculated differential warming south to north in response due to “nebulosity” or differences in clouds and aerosols south to north.
The Assessment documents of the Intergovernmental Panel on Climate Change (IPCC) have become the standard for synthesis of our understanding of climate science. In the second report (1995) a greater temperature increase at high northern latitudes was asserted and predicted as a response to greenhouse gases. Also described was a self-reinforcing feedback with loss of sea ice in the Arctic Ocean. Decreased reflection of sunlight by the disappearing ice and increased absorbance into the revealed seawater increases heat content of the Arctic Ocean, leading to further increases in melting, and further warming. Loss of sea ice in the Arctic is one of the best documented and most dramatically visible changes in our warming world.
Mentioned in the 1995 report, but not well known at the time, was the quantitative role of aerosols in modifying the impact of greenhouse gases on temperature in the Arctic.
By the time of the 5th Assessment in 2013, preferential Arctic warming was well-established and the role of aerosols had become clearer, both in terms of their direct effect in reflecting sunlight and their role in cloud formation (more on that in a minute). But the report also concluded that aerosols were one of the largest sources of uncertainty in estimates of radiative forcing, or global warming impact.
In 2018, the IPCC issued an updated report on the role of aerosols. Two conclusions stand out relative to this essay. The first is that aerosol production varies widely in different regions, and this, along with a relatively short residence time in the atmosphere, means that concentrations in the atmosphere also vary widely among regions. The second is that aerosols still remain one of the greatest sources of uncertainty in climate prediction models.
This essay was prompted by a recent article by Mark England and colleagues (title and citation below) that provides an update on changes in Arctic temperatures since 1940, and also discusses the possible impact of aerosols. We’ll explore their results in a minute, but first some background on aerosols and preferential heating in the Arctic.
What are aerosols?
Aerosols are minute particles (~a millionth of a meter, or one ten thousands of an inch) that in the atmosphere can reflect and/or absorb sunlight, depending on their properties. Let’s simplify this and start with two categories of particles, “dust” and “soot.”
Dust would include generally light colored particles thrown into the atmosphere for example by volcanoes or everyday actions like plowing fields or driving on unpaved roads. In general, these are light colored and reflect sunlight. They have a cooling effect.
Soot would include the dirty smoke you see in the exhaust from the diesel truck or a poorly managed fireplace, or any other smoke-generating process. In the scientific parlance, this is sometimes called “black carbon” as it is often the residue of incomplete combustion of organic materials like wood or gasoline. Black carbon absorbs sunlight, warming the atmosphere, and can also accumulate on snow and ice, especially in the far north, turning a highly reflective surface into a highly absorbing surface, again increasing temperatures.
But there is more. We need to add one small molecule, sulfur dioxide, to the mix (literally). This compound is a major product of the combustion of coal or any other organic material with a high sulfur content. Sulfur dioxide acts as a highly reflective aerosol, reducing sunlight reaching the surface of the Earth, but also has a second crucial property: It is effective in “nucleating” water droplets. What this means is that in a humid atmosphere, sulfur dioxide will increase cloud formation and possibly alter precipitation as well. Those clouds can then have complex effects on the temperature of the atmosphere depending the type of clouds formed and where they are in the atmosphere. All of this adds to the uncertainty that aerosols bring to climate predictions.
Arctic Amplification
Preferential heating of the Arctic, driven by the combined effects of greenhouse gases and the feedback to loss of sea ice, as well as other processes driven by these two primary forces, is called Arctic Amplification. Through all the iterations of the IPCC reports, and similar documents by agencies around the world, it has only become more certain that the far north is warming faster than regions to the south. Images like this one can be found on many, many sites.
These images are produced using measured values extrapolated by the suite of climate models that have been constructed to make projections about the future. Long-term measurements of temperature on the ground in the Arctic have been relatively sparse.
New Information
The title of the paper by England and colleagues is The Recent Emergence of Arctic Amplification (full reference below). Their research has added clarity on the time course of temperature change in the Arctic, and the role of aerosols in that change. It also makes an interesting contribution to a story about the multiple impacts of a single pollutant.
Arctic Amplification is charted as the ratio of temperature change in the far north compared with global averages, and for me, the surprising word in the title is “Recent.” I have been teaching that this has been a well-established and long-term trend!
The article begins by integrating several recent compilations of observed temperatures in the Arctic (citations are from 2014-2020); detailed data not available to the IPCC in 1995, and certainly not vailable to Arrhenius!
This figure from that paper documents a 40 year period of cooling in the arctic, from about 1940 to 1980. This is then followed by the expected pattern of rapid increase far above the global average.
How does this mesh with the knowledge that greenhouse gases have been increasing consistently for the period covered by these graphs? Is there something else at work here?
There is, and given how this essay began, you will not be surprised to learn that aerosols are at the heart of the story. England et al. propose that changes in the production of aerosols and their concentration in the Arctic atmosphere “masked” the impact of greenhouse gases and the ice loss/warming feedback in the Arctic for that 40 year period.
Is there a way to test whether aerosols are the explanation? It is a cliché at this point to say that we are running a complex experiment on the atmosphere right now, but there is no way experimentally to, say, remove all the aerosols over the Arctic and see what happens (we will conclude with another approach to the manipulation of aerosols at the end of this essay).
But there is another approach. One of the advantages of the complex climate models available to the research community is that simulated experiments can be run for situations that could never be accomplished by the usual method of controlled comparisons.
What England and colleagues have done, then, in addition to compiling data on Arctic temperatures, is to analyze a set of climate model runs that performed this non-aerosol experiment virtually (model data are from Deser et al. (2020). Running a model multiple times with slightly different initial conditions is called an ensemble approach, and allows some understanding of uncertainty in the predicted response – in this case to the change in aerosol concentrations. It’s similar to taking multiple measurements of something in the field or lab. In this figure, the dots represent the results of a single run, and the solid line is the average of all the runs.
The difference between the measured temperatures in the first figure and what the model predicts would have happened if aerosol concentrations had been constant since 1920, is captured in this figure (Thanks to Dr. Mark England for producing this figure which was not in the original paper in this form). Negative numbers from 1940 through 1980 show that atmospheric aerosols depressed temperatures globally, but to an even greater extent in the Arctic (the blue dots and line). This effect peaked in 1970 and upward trends in this difference between modeled and measured temperatures after 1970 reflect reductions in aerosols and their impact on temperature. After 1980, measured temperatures are above what the models predict temperatures would be if aerosol concentrations had remained constant at 1920 levels.
If aerosols reduce Arctic warming, and average Arctic temperatures declined from 1940 to 1970, and only began the expected increase from 1970 on, this would suggest that aerosol concentrations have declined since 1970, at least in the far north.
How and why might this have happened?
A switch in topics now to acid rain. One of the primary environmental issues in the 1980s, acid rain was driven by emissions of sulfur and nitrogen oxides from electric generating stations, especially those burning coal. Acid rain is a story for another time, but the upshot of a couple of decades of research was the enactment of policies, in both North America and Europe that have effectively reduced sulfur dioxide emissions by close to 90% (Figure here is for U.S. emissions).
At the hemispheric level, this dramatic change, really a fine environmental success story, is also recorded in the Greenland ice cap as a sudden decrease in sulfur concentrations in ice formed after 1990. Note that the figure here captures only the decline in emissions, which would have been near zero before 1850, increasing until reaching a peak around 1970.
Note also that these dramatic changes in emissions have happened in the industrialized parts of two continents regionally adjacent to the Arctic.
Such significant reductions in the human-caused emissions of sulfur appear to have altered the radiative balance of the atmosphere, removing some of the cooling effect of aerosols. In a sense, solving the acid rain problem has exacerbated the climate problem, as noted by others (see references below).
Our discussion of aerosols might bring another thought to mind. If these particles, especially sulfur dioxide, are so effective at reflecting sunlight, why not purposefully emit them into the layer of the atmosphere where they would be most effective in increasing reflectance of the sun’s energy back to space? The term applied to this concept is solar geoengineering, and you might imagine that it’s a hot topic. Do we want to “solve” the climate problem by adding back to the upper atmosphere a pollutant that we have worked so hard to eliminate in the lower atmosphere? Do we know enough to do that without harm?
Solar geoengineering is much too big and too controversial a topic to address here, but it is likely to be part of the climate mitigation conversation for some time (see reference below to a recent report out of the U.S. National Academies). Given that the role of aerosols constitutes one of our biggest uncertainties in the climate system, it would seem prudent to err on the side of caution here. We have a lot more to learn.
For now, England and colleagues have sharpened our understanding of Arctic Amplification and the role of aerosols in this process.
Sources
The England, et al. paper is:
England, M.R., I. Eisenman, N.J. Lutsko and T.J.W. Wagner. 2021. The Recent Emergence of Arctic Amplification. Geophysical Research Letters, in press.
https://doi.org/10.1029/2021GL094086
That earlier essay on this Substack site about Arrhenius and his climate change calculations is here:
https://lessheatmorelight.substack.com/p/climate-change-in-four-easy-steps
The image of global temperature changes is from NASA and can be found here:
https://data.giss.nasa.gov/gistemp/maps/index_v4.html
The second figures in this essay is from the paper by England et. al. The third has been redrawn by Mark England for use in this essay. Thank you.
The model data reported by England et al and in this essay are from
Deser, C., Lehner, F., Rodgers, K., Ault, T., Delworth, T., DiNezio, P., et al. (2020). Insights from Earth system model initial-condition large ensembles and future prospects. Nature Climate Change, 10, 277–286. https://doi.org/10.1038/s41558-020-0731-2
The figure on sulfur dioxide emissions in the U.S. uses data from the U.S. EPA national tier 1 data base
Information on reductions in sulfur deposition to the Greenland ice cap is here:
https://www.ipcc.ch/site/assets/uploads/2018/03/TAR-05.pdf
The IPCC assessment reports are at the IPCC site:
https://www.ipcc.ch/
Two secondary sources that discuss the role of aerosol reduction in increasing warming are:
https://www.sciencedaily.com/releases/2020/11/201117085924.htm
https://www.scientificamerican.com/article/cleaning-up-air-pollution-may-strengthen-global-warming/
The National Academy report on Solar Geoengineering is:
Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance (2021)
https://www.nap.edu/download/25762
A recent review paper relevant to this essay is here:
Schmale, J, P. Zieger and A.M. Ekman. 2021. Aerosols in Current and Future Arctic Climate. Nature Climate Change