Are the changes in the Arctic messing with our weather? Background

Are the changes in the Arctic messing with our weather? Background

(20140115: Revision: This is a revision. In the comments on the original post rlk and ScottLincoln questioned the magnitude of the pressure change in the figure used in the paper. I wrote Kevin Trenberth, the author, and he confirms that the units should be pascals, not hectopascals. I thank rlk and ScottLincoln, and indeed, I should have flagged this as well, rather than noting how large it was and moving along in the original post. The conclusions in the blog are not altered.)

This entry continues with my listing of the big-ticket items in climate change since I last taught in April 2012. In the last entry I wrote about how the technology used to extract oil and natural gas, hydraulic fracturing (fracking), stood as a threat to climate change because it assured the availability of fuels that we preferred and, also, provided desirable jobs. Some would argue that fracking might diminish our use of coal, which is a good thing. This is likely true, but there are several issues that need to be analyzed in that conclusion: selling our coal to other countries, the complete accounting of greenhouse gases associated with fracking, the broader environmental consequences of fracking, the fact that there are no real disincentives for using fossil fuels, and the fact that all burnt fossil fuels have a long-term cumulative effect (My Michigan colleagues integrated assessment of fracking). I don’t want to diminish the importance that the carbon dioxide emissions in the U.S. have, perhaps, been decreasing in recent years; however, it is not a fact that suggests we are on a path to addressing the problems of climate change. I assert we remain in a situation where economic growth, which we require for well-being, is still strongly linked to energy use and carbon emissions. If our economy grows so will our emissions as the preliminary 2013 emissions suggest.

The second big-ticket item that I want to highlight is the work investigating the changes in the Arctic and the possibility that these changes are already influencing the weather in the continental U.S. and, more broadly, in the Northern Hemisphere. I have written about this extensively in my series on the Arctic Oscillation and the hot and cold fluctuations in the U.S. (link to last in series, also see links below). I take some pleasure in noting that back in December I wrote, “The whole Arctic air mass is starting to move east, which means it will get a lot more press.” I did not imagine that it would lead to all of the anxiety about the rogue polar vortex (We the geeks).

I will leave the machinations of polar-vortex mania to my more able colleagues. I want to analyze why this work about the Arctic Oscillation, the polar vortex and wild fluctuations between warm and cold weather is important enough to be on my list of big-ticket items.

There is little controversy that there have been massive changes in the climate of the Arctic. These are most easily noted in the large changes in Arctic sea ice. There is also a whole set of coherent and convergent evidence that documents the changes in the Arctic. The most direct evidence is the increase in temperature, which is much greater in the Arctic than at lower latitudes and in the tropics (Polar or Arctic amplification). Coincident with this warming is a lengthening of the growing season and an increase in activity in the northern forests – the greening of the Arctic (200 blogs ago, Getting Ready for Spring 5). There is controversy about whether these changes in the Arctic are causing changes to the weather at lower latitudes. There is also controversy about if there is a change in the weather at lower latitudes, is it due to the local changes in the Arctic such as loss of sea ice.

I want to start the discussion with Figure 11 from a paper by Kevin Trenberth and John Fasullo entitled, An apparent hiatus in global warming? I will write more about this paper in a future blog. Trenberth and Fasullo provide a self-consistent global analysis that tracks the heating of the planet. Figure 11 of this paper shows the difference in average sea-level pressure between two time periods. An average is taken for 1999-2012 and another for 1979-1998. The difference between the two averages shows an increase in sea-level pressure. This increase is represented by the large red area stretching from the North Atlantic, east of Greenland, to the Arctic Ocean and centered over the North Pole. The maximum magnitude of this increase is about 150 Pa (pascal). To put this in perspective the surface pressure of the Earth is often cited as being 1000 hPa (hectopascal).



Figure 1: Mean annual sea-level pressure differences from ERA-Interim Reanalysis for 1999–2012 and 1979–1998 in Pa (pascal, colors) and for surface wind vectors (arrows) in meter per seconds with the key at top right. (a) Map projection centered on the Pacific and (b) polar stereographic projection of the Northern Hemisphere. (Note the magnitude of pressure is in Pa, not hPa, which is a typo in the original manuscript.)(Figure 11 from An apparent hiatus in global warming?)

This increase in the Arctic sea-level pressure can also be viewed in terms of the strength of the polar vortex, or in terms of wind, the strength of the rotation of the wind. Low pressure is associated with a strong vortex with strong rotation; high pressure is associated with a weak vortex with less rotation (earlier blog on strong and weak vortex). Hence, the observations show that there is a weaker polar vortex. As measured in terms of the Arctic Oscillation, the Arctic Oscillation is more negative. From our narrow U.S. perspective, this is associated with cold and snowy conditions over the eastern half of the U.S. leading to exaggerated political and press attention and excess purchase of toilet paper and bread in supermarkets from Atlanta northwards. It is quite easy to conclude that for the past decade and a half the Arctic Oscillation has been more prominently in its negative phase.

The analysis of Trenberth and Fasullo comes to the conclusion that this change in the Arctic is the consequence of changes in the global distribution of mass of the atmosphere. Specifically, Trenberth and Fasullo trace the changes in the Arctic back to changes in the tropics. Placing the Arctic changes as a part of a global circulation change stands in tension to the conclusions of Jennifer Francis and her collaborators, who are quoted extensively in my blogs. Francis and Vavrus (2012) in Evidence linking Arctic amplification to extreme weather in mid-latitudes correlate the changes in sea-level pressure to changes in the sea ice, the Arctic Oscillation and snow cover. This is a focus on a direct local effect in the Arctic causing changes in the global circulation.

The work that I cite above, in all cases, points to a time in the past 15 years where the Arctic Oscillation is often in its negative phase. There is a difference between the researchers in the determination of cause and effect. The difference in cause and effect leads, perhaps, to different conclusions about the future. The question: in the future will the Arctic Oscillation be more prone to its negative phase? With that question, I introduce another paper, by Elizabeth Barnes and co-authors Revisiting the evidence linking Arctic amplification to extreme weather in midlatitudes. Barnes et al. analyze the simulations used in the most recent Climate Model Intercomparison Project (CMIP-5) and conclude that the models do not support the conclusion that the Arctic Oscillation will become more negative in the future.

In the next blog, I will discuss the arguments offered by these different researchers. Then I will provide my analysis of why I conclude that what is happening in the Arctic makes it to my list of the big-ticket items of the past year.

r


Cold Weather in Denver: Climate Change and Arctic Oscillation (8)

Climate Change and the Arctic Oscillation 2

Climate Change and the Arctic Oscillation 1

Wobbles in the Barriers

Barriers in the Atmosphere

Behavior

Definitions and Some Background

August Arctic Oscillation presentation

CPC Climate Glossary “The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases.”