Maybe, maybe not. There is a new paper that looks at what climate scientists call “synoptic midlatitude temperature variability” and the rest of us call “cold snaps” and “heat waves.” The term “synoptic” simply means over a reasonably large area like you might expect a cold snap or heat wave to be. Specifically, the paper (Physics of Changes in Synoptic Midlatitude Temperature Variability, by Tapio Schneider, Tobias Bischoff and Hanna Plotka, published in Journal of Climate) concludes that as human-caused greenhouse gas pollution increases, the frequency of cold snaps in the northern hemisphere will go down. Naturally, as temperatures warm up we would expect the highs to get higher, the averages to be higher, and the lows to be higher as well (and thus fewer cold spells). But the new research actually argues that the cold spells (the cold extremes at synoptic spacial scales) will become even less common. This is potentially controversial and conflicts with other recently published research.
The paper is rather technical so, I’ll give you the abstract so you can go take a class in climate science, then come back and read it:
This paper examines the physical processes controlling how synoptic midlatitude temperature variability near the surface changes with climate. Because synoptic temperature variability is primarily generated by advection, it can be related to mean potential temperature gradients and mixing lengths near the surface. Scaling arguments show that the reduction of meridional potential temperature gradients that accompanies polar amplification of global warming leads to a reduction of the synoptic temperature variance near the surface. This is confirmed in simulations of a wide range of climates with an idealized GCM. In comprehensive climate simulations (CMIP5), Arctic amplification of global warming similarly entails a large-scale reduction of the near-surface temperature variance in Northern Hemisphere midlatitudes, especially in winter. The probability density functions of synoptic near-surface temperature variations in midlatitudes are statistically indistinguishable from Gaussian, both in reanalysis data and in a range of climates simulated with idealized and comprehensive GCMs. This indicates that changes in mean values and variances suffice to account for changes even in extreme synoptic temperature variations. Taken together, the results indicate that Arctic amplification of global warming leads to even less frequent cold outbreaks in Northern Hemisphere winter than a shift toward a warmer mean climate implies by itself.
Why is this controversial? Because we have seen research in recent years indicating that with Arctic Amplification (the Arctic getting relatively warmer than the rest of the planet as global warming commences) the manner in which warm air is redistributed from sun-facing Equatorial regions towards the poles changes, which in turn changes the behavior of the Polar jet stream. Rather than being relatively straight as it rushes around the globe, separating temperate and sub-polar regions (and defining the boundaries of trade winds, and moving along storms) it is thought that the jet stream has become more often very curvy, forming what are called Rossby waves. These waves, recent research has suggested, can become stationary and the wind within the waves moves relatively slowly. A curvy jet stream forms meteorological features such as the “ridiculously resilient ridge” which has brought California nearly continuous dry conditions for at least two years now, resulting in an unprecedented drought. A curvy jet stream also forms meteorological features called “troughs” such as the excursion known last year (incorrectly) as the Polar Vortex, which also returned in less severe form this year; a bend in the jet stream that brings polar air farther south than usual, causing a synoptic cold spell of extensive duration. These changes in the jet stream also seem to have brought some unusual winter weather to the American Southeast last year, and have been implicated in steering Super Storm Sandy into the US Northeast a few years ago. And that flood in Boulder, and the flood in Calgary, and the June Of All Rain here in Minnesota last year, and so on. This is the main global warming caused change in weather systems responsible for what has been termed “Weather Whiplash” and may rank up there with increased sea surface temperatures as factors underlying the observable, day to day effects of human caused climate disruption.
I’ve talked about jet streams, Rossby waves, and such in a few places:
- Linking Weather Extremes to Global Warming
- Global Warming and Extreme Weather
Even more recently was a paper by Dim Coumou, Jascha Lehmann, and Johanna Beckmann, “The weakening summer circulation in the Northern Hemisphere mid-latitudes” that argued:
Rapid warming in the Arctic could influence mid-latitude circulation by reducing the poleward temperature gradient. The largest changes are generally expected in autumn or winter but whether significant changes have occurred is debated. Here we report significant weakening of summer circulation detected in three key dynamical quantities: (i) the zonal-mean zonal wind, (ii) the eddy kinetic energy (EKE) and (iii) the amplitude of fast-moving Rossby waves. Weakening of the zonal wind is explained by a reduction in poleward temperature gradient. Changes in Rossby waves and EKE are consistent with regression analyses of climate model projections and changes over the seasonal cycle. Monthly heat extremes are associated with low EKE and thus the observed weakening might have contributed to more persistent heat waves in recent summers.
Coumou notes that “when the great air streams in the sky above us get disturbed by climate change, this can have severe effects on the ground. While you might expect reduced storm activity to be something good, it turns out that this reduction leads to a greater persistence of weather systems in the Northern hemisphere mid-latitudes. In summer, storms transport moist and cool air from the oceans to the continents bringing relief after periods of oppressive heat. Slack periods, in contrast, make warm weather conditions endure, resulting in the buildup of heat and drought.” Co-author Jascha Lehmann adds, “Unabated climate change will probably further weaken summer circulation patterns which could thus aggravate the risk of heat waves. Remarkably, climate simulations for the next decades, the CMIP5, show the same link that we found in observations. So the warm temperature extremes we’ve experienced in recent years might be just a beginning.”
These seem to be conflicting views.
So, how do the scientists who have published the recent paper that stands in stark contrast with these other recent findings explain the difference? I asked lead author Tapio Schneider to comment.
He told me that yes, there is a tension between the other work (the Comou et al paper) and his work, but there is also overlap and similarity. “Coumou et al. state that amplified warming of the Arctic should lead to reduced zonal jet speeds at fixed levels in the troposphere. This is an uncontroversial and well known consequence of thermal wind balance. Then they say that the reduced zonal jet speeds may lead to reductions in eddy kinetic energy (EKE), which is a measure of Rossby wave amplitude. That this can happen is likewise well documented. What affects eddy kinetic energies is a quantity known as the mean available potential energy (MAPE), which depends on temperature gradients (which also affect jet speeds) and other quantities, such as the vertical temperature stratification. Coumou et al. focus only on one factor influencing the EKE, the temperature gradient.”
The tension, he told me, is in what the other researchers (Coumou et al) draw from their results. “They show that warm summer months usually are associated with low EKE in the current climate, consistent with common knowledge: unusually warm conditions are associated with relatively stagnant air. They use this correlation in the current climate to suggest that reduced EKE in a future climate may also imply more (monthly) heat waves. While intuitive, this is not necessarily so. They say their suggestion is not in contradiction with our results because we considered temperature variability on shorter timescales (up to about two weeks), while their suggestion for more heat waves is made for monthly timescales. However, why the longer timescales should behave so differently is not made clear. “
As an onlooker, I take the following from this. First, there may be differences in time (and maybe space) scales of the analyses that might make them less comparable than ideal. Second, Schneider and Bischoff seem to be emphasizing synoptic cold outbreaks specifically. Schneider told me that they did look at temperature variability over longer time scales, but that did not make it into the paper. He said, “Even on monthly timescales, midlatitude temperature variance generally decreases as the climate warms, with a few regional exceptions (e.g., over Europe).”
Also, note that Schneider, Bischoff and Plotka, in this paper, do not address the specific problem of stationary Rossby waves, which probably has more to do with rainfall (lacking or heavy) than temperature, but is an important part of current changes in weather.
There has been some additional criticism of Schneider’s work on social media, etc. and perhaps the most significant one is this: Schneider, Bischoff and Plotka may have oversimplified the conditions in at least one of their models by leaving out continents. Also, Schneider et al has been picked up by a few of the usual suspects as saying that climate change will result in milder winters or less severe storms. This is not actually what the paper says. When people think “milder winter” they usually mean fewer severe storms, but various lines of evidence suggest that the notheastern US will experience more storms. For, example, see “Changes in U.S. East Coast Cyclone Dynamics with Climate Change” and “Global Warming Changing Weather in the US Northeast.”
UPDATE: I’ve received a comment from Dim Coumou pertaining to the differences between Schneider Et Al and Comou Et Al:
I see mostly overlap between the two studies, whereby ours really is an observational study analyzing data over the last 35 years (focusing on summers), and theirs is a theoretical and modeling study (focusing on winters). Interestingly both studies report similar dynamical changes but indeed come to somewhat different conclusions as to what this means for surface weather extremes.
We show that circulation (and notably EKE) has weakened in summer, that this has made weather more persistent and therefore favored the occurrence of prolonged heat waves in recent years. (We´re really focusing on present day climate and only show future projections for comparison with observations). As also discussed in our paper a drop in EKE leads to a reduction in weather variability on short timescales (less than a week) so this is consistent with the findings by Schneider et al. So indeed the issue of timescales is very important, and in my opinion prolonged extremes lasting several weeks are more important from an impact point of view.
Schneider, Bischoff and Plotka are well respected scientists and they are using methods that are generally accepted within climate science, yet have come to a conclusion different from what some of their colleagues have proposed. This is, in my opinion, a very good thing, and, certainly, interesting. I would worry if every climate scientist came up with the same result every time they tried something slightly different. The patterning (or changes in patterning) of air and sea currents under global warming has been the subject of a great deal of recent research, and there is strong evidence that changes are happening (such as in sea currents in the North Atlantic, and the jet stream effects discussed here) that have not been directly observed before. Because of the high level of internal (natural) variability, climate science works best when chunks of time 20 or 30 years long are considered. If we are seeing changes now that have really started to take off only five or ten years ago, and that are still dynamically reorganizing, how can the more ponderous, long term and large scale, thinking of climate science adjust and address those rapid changes? Well, we are seeing that process now in the climate change literature, and this paper is one example of it. I look forward to an honest, fair, and vigorous discussion in the peer reviewed literature.
Caption for the figure at the top of the post: FIG. 6. CMIP5 multimodel median values of 850-hPa potential temperature statistics for (left) DJF and (right) JJA. (a) Synoptic potential temperature variance u02 for the years 1980–99 of the historical simulations. (b) Per- centage change of the synoptic potential temperature variance u02 in the years 2080–99 of the RCP8.5 simulations relative to the years 1980–99 of the historical simulations shown in (a). (c) Percentage change of the squared meridional potential temperature gradient (›yu)2 in the years 2080–99 of the RCP8.5 simulations relative to the years 1980–99 of the historical simulations. (To calculate the gradients, mean potential temperatures were smoothed with a spherical harmonics filter that damped spherical wavenumbers greater than 6 and completely fil- tered out wavenumbers greater than 10.) (d) Percentage change of the squared mixing length L0 2 5 u0 2 /(›y u)2 implied by the variance and meridional potential temperature gradient, in the years 2080–99 of the RCP8.5 simulations relative to the years 1980–99 of the historical simulations. Synoptic potential temperature variations are bandpass filtered to 3–15 days. In the dark gray regions, topography extends above the mean 850-hPa isobar. The light gray bar blocks out the equatorial region, where potential temperature gradients are weak and their percentage changes become large.