Recently the subject of different trends in different seasons came up. Of course, the astounding heat wave which recently gripped the nation is also on many minds. So I thought I’d look at temperature in the USA (lower 48 states) and see whether or not there are different trends for different seasons. Data are from the National Climate Data Center.
First let’s look at the warming rates for each month of the year, for the entire period of record (1895 to the present):
February shows the most warming, with March not too far behind. All 12 months show a warming trend, and most of them are statistically significant.
If, however, we focus on a very recent time frame, just the data since 1990, the story is quite different:
February shows the strongest cooling trend, albeit not statistically significant. The only statistically significant result is the warming during July. Could it be that in more recent times — the last couple of decades — the USA is warming strongly in summer? What part of the USA shows this?
Let’s compute not monthly, but seasonal trends, defining winter as Dec-Jan-Feb, spring as Mar-Apr-May, summer as Jun-Jul-Aug, and fall as Sep-Oct-Nov. Then we can compute trends for single seasons, for all 50 states. Using the entire period of record (1895 to present), wintertime warming looks like this:
Dots are plotted on a “map” of the USA. Red dots indicate warming trends and blue dots (yes there are a few of them) cooling trends. Large dots indicate larger trends. Most of the country has shown winter warming, strongest in New England and the upper midwest (North Dakota warming fastest). There is one region, however, which has shown overall cooling, namely the southeast, where you’ll find the blue dots. But the blue dots are very small as the cooling is very weak (not even nearly statistically significant). Nonetheless, the southeast hasn’t shown the winter warming exhibited by the rest of the country.
For the spring season we get this:
The story is much the same as for winter, most of the country warming, especially New England and the upper midwest, with the southeast slighly cooling (but not significantly).
Summer shows a different pattern:
Much of the midwest, including the Ohio valley and Mississippi valley, show slight cooling. Warming regions show stronger trend overall than cooling regions, but now the warming area is split between New England in the east and the Rocky Mountains in the west. That same pattern also exists for the fall season:
We therefore see two distinct patterns. For winter and spring, warming is strongly dominant with only the southeast region not participating. During summer and fall the cooling region extends much farther north, and splits the warming part of the country into eastern and western parts.
Turning again to more recent trends, what has happened since 1990? Winter shows a very different story:
All 48 states except Maine and Vermont have cooled, the strongest effect in the midwest and southeast.
But instead of spring mimicking winter, it does a complete turnaround:
Now the midwest is warming strongly, with only the far west showing cooling.
The real cooking, however, has been happening during summer:
47 of 48 states show warming, only New Hampshire indicates cooling (and very weak at that). For the fall season the pattern is much the same:
Over the last two decades, most of the USA has warmed during spring, summer, and fall — but cooled during winter. The eastern part of the country has shown stronger trends (both warming and winter cooling) than the west.
As for the future, regional predictions are very hard to make. If the recent pattern continues, we’ll see even more exceptional heat like we’ve seen this year (both during summer and spring) exactly where we saw it this year (over a very large part of the country, but mainly concentrated in the midwest). No matter how the regional pattern unfolds, over the longer term — the next several decades — we can expect more warming in the USA, and we can surely expect more warming over the globe. Nature has some surprises in store for us … but I don’t think they’ll be the pleasant kind.
We’re now at the point that extreme events like this summer’s heat wave, and even more so the heat wave this March, are just outside the bounds of what we’ve experienced before. Alas, it won’t be long before we see extremes that, frankly, we never dreamed possible. In spite of the fact that they will end the stupid arguments by making global warming simply undeniable, I’m certainly not looking forward to the dreadful days to come. It’s a pity that, through our own failure to act, we will have brought on ourselves the very wrath that will incite public outcry. It’s an even greater pity that it will happen so long after we were warned.
Open Mind 
A very interesting post for me personally, as the ‘recent’ analysis period coincides pretty closely with my term of residence in Georgia. As I recall, the Southeastern long-term cooling trend reflects significant late-19th century warmth here. Of course,the cooler winters of the last couple of years (especially 2009 & 2010) don’t counter that; they seem to reflect the WACC pattern. And there would be curious, very warm interludes as another twist of the jet stream meandered by.
Thanks; this should be very useful for me as well. I am already incorporating the images into a business powerpoint presentation.
I also looked at the ranking of summers in terms of hottest. I found the Southeast states had their hottest summer (in the NCDC database starting in 1895) in 2010. Summer of 2011 came in either 2nd or 3rd in most of these Southeastern states. I suspect other Eastern states also have top 10 hot summers recently.
If this summer sets yet another record, or near record, for hottest summer in the database, that would make three years in a row. The odds of getting three consecutive top 3 or top 5 records in a random 118 year database are likely pretty long.
And next year’s summer could be an El Nino year…
The cost of these events in terms of crop losses are huge, and then there are the Americans who die due to the persistently hot weather. I am afraid the cost of climate change is coming home to roost. But this information is useful, and persuasive.
Paul Klemencic
It’d be interesting to see how the change in continental US winter temperatures over time change compared with changes in Arctic winter temperatures over time.
And then there’s George Will…http://www.slate.com/blogs/trending/2012/07/09/george_will_record_heat_waves_and_drought_are_just_summer_get_over_it_.html Words fail me….
When Will spouts off on something like that, I would love Gavin Schmidt or Michael Mann to suddenly pop up out of the wings and say, “No, George, that’s not so…” As much as I like E.J. Dionne, it’s hard for a nonscientist to quickly come up with rebuttals (of course, there are skilled science communicators out there who are not practicing scientists).
I don’t get it. All he has to do is say that global warming hasn’t stopped since kids entering high school this year were born, since young men and women graduating college entered the 2nd grade and since Celine Dion’s My Heart Will Go On topped the charts. Or, in other words, since 1998. The guy’s got a teflon reputation; he caught zero flack for treating us to this BS analysis a few years ago and it’s just as not-exactly-true as it was then so why not stick with it instead of inventing new BS?
There was a good article about this in Media Matters. Interesting analogy.
Will is simply a victim of motivated reasoning. He takes a lawyer’s approach to science and sees his own case but not the balance of the evidence.
Deech: Will is simply a victim of motivated reasoning.
BPL: He’s not a victim. He’s a perpetrator.
I sometimes wonder whether Will’s problem might not be innumeracy–specifically an inability to think inductively or to understand probability. It reminds me of a friend who posited that if there is such a thing as an idiot savant, there must also be such a thing as a genius stupide–smart in all ways except one, where he is utterly incompetent.
I pretty certain that GFW is just stupide. Incompetent in all things, including baseball.
Horatio also wonders whether Will’s problem might not be innumeracy
As Ron Marquart noted about George Will’s opinion piece “It’s not about the Environment” (which now seems to be MIA on the web)
It appears that Will knows enough about probability to appreciate that it’s highly improbable (>99%?) that most members of the general public will investigate what USGS actually said.
No need to posit – I personally know a few of those…
A loosly related question came up in a discussion at Paul Hudson’s blog about the Central England Temperature data set. This seems to show significantly different trends in different seasons but more interestingly, I think, there also seem to be significant differences in the year-to-year variation in average monthly temperatures, with much greater variations in winter months than summer ones. (Graphs have been provided here: http://somethingtododownthepub.com/2012/07/07/40/)
A number of questions come to mind, such as:
1) Is there in fact a significant difference in the variance and if so what causes it? (My speculation is that it’s something to do with the jet stream tending to be over the UK in the winter, which means small changes in its path would have big changes in the weather, whilst in the summer, with the jet stream well to the north, things are more stable – this year being a notable exception!)
2) Is there a similar pattern elsewhere, with greater variation in temperatures in the winter than the summer, or is it specific to the UK, or indeed to this one data set?
3) What implications are there for the detection and attribution of extreme temperature events?
It would be useful to see how the dustbowl period looked in a similar set of graphs. Without them, I can see the deniers saying that this is just another dustbowl period. With them showing (as I suspect they will) a completely different pattern, the uniqueness of the last 22 years can be more solidly established.
Very interesting. Two questions:
1) You recently showed phase shifts in (I think) sea ice data that caused odd behavior in monthly anomaly data. Is there any similar sort of phase shift in annual temperature minima/maxima that contribute to these seasonal changes to some extent? It seems like the changes are quite large, so I doubt this would account for much.
2) Your USA48 1990-2012 plot shows slight (but probably not statistically significant) trend. Since warming is accelerating, it is expected that, without any actual dependency on the time of year, changes in rates between Januaries would be less than changes in rates between Decembers in the same calendar year. Again, this is a very small effect, but it’s one that becomes more severe the smaller the time window you use.
Great stuff, as usual. Thanks Tamino! From an ‘anthropogenic aerosol-forcing’ point of view, it might be worth consulting Leibensperger et al. ACP 2012. Looking at their Fig. 5 (and considering the region most affected as apparent in Fig. 4), your 1895-2012 time series makes quite some sense to me. The most pronounced negative aerosol forcing is simulated in fall (a bit less in summer), which corresponds surprisingly well from a spatial point of view with the trend of USA 48 fall (and summer) for 1895-2012. The fact that the forcing was strongest in the 1970-1990 period (which is within the latter half of the entire record) could explain why it is so easy to identify in the NCDC trend maps.
Things become less obvious in case of the last 22 years, the phase during which the negative aerosol forcing has decreased considerably (past the 1990s). Although we do see a strong warming trend with regard to the annual average over the entire USA 48 region, one would expect to find a distinct peak over the mid-Atlantic US particularly in fall … if it weren’t for the natural variability aka noise, which prevents us from drawing unambiguous conclusions. While it seems a straight forward solution to look at the NAO-AO regime (may it be driven by ENSO, Arctic sea ice retreat, the solar-stratosphere-link, or simply random fluctuations) in order to identify the ‘controlling’ factor for winter temperature trends, this method won’t provide too much of an insight for the rest of the year. Therefore I leave it with Fig. 3 in Cohen et al. GRL 2012, which shows seasonal CRUTEM3 NH trends from 1987-2010. Leaving winter aside, the strongest trend evolves in fall, consistent with what we would expect in an aerosol brightening phase (which simultaneously took place over the US and Europe). However, one should bear the high noise level in mind, as 22 years certainly are too short a period of time to obtain statistically significant trends.
Looks like shifts in the jet stream, but increasingly it looks like those are being driven by Arctic melting, which is a temperature effect.
And they ask why climate change and not global warming.
Zach, some of your questions may be answered by considering to some recent reports by meteorologists, that can be accessed at some of the links in this comment I just submitted to the previous thread to this site.
(hopefully the comment was accepted, even with a lot of embedded links)
And there is one lone soldier over there on WUWT, fighting on with incredible forbearance, who summarized the current meteorological theory pretty well:
Glenn Tamblyn says:
July 8, 2012 at 3:00 am
Marian
“It is caused by a persistent blocking high pressure pattern”
Here we go again….
Yep, pay attention to descriptions of weather events that include that word ‘blocking’. It is the description that tags what looks like a possible Global Warming phenomenon. And one that till recently hadn’t been appreciated that well. Possibly a disconnect between the Meteorologists and the Climatologists.
A big factor in weather patterns are the Polar Jet Streams. Relatively high speed and high altitude eastward travelling air flows, they act as barriers, dividing climate zones because weather systems can’t easily cross them. So in the northern hemisphere, this tends to divide the colder polar weather patterns from the warmer mid latitude weather. But the jet streams don’t just travel across at one latitude. They meander north & south. When it meanders south, colder air from the arctic can penetrate southwards – colder weather. When it meanders north, warmer air from lower latitudes can penetrate north – warmer weather.
And if the meandering of the jet-stream happens to freeze in place for a period, which it does from time to time, then whatever type of weather is happening nearby tends to intensify – warm gets warmer or cold gets colder. This is what happened in western Russia in 2010. Warmer air from the Mediterranean was able to keep moving northwards because the Jet Stream had ‘locked up’ for a time. Our weather, no matter where in the world you live, is best when it cycles frequently between cooling and warming influences. Too much of any factor is bad.
So what is happening to the Jet-Stream? Preliminary research (this is still an emerging field of study) is suggesting that the North Polar Jet Stream is slowing. It is meandering further north & south. And it is becoming more prone to ‘locking up’ for periods. All of which isn’t good news; that will definitely lead to more extreme weather events in the regions influenced by it. More snowstorms and rain if you are north of it, more heatwaves, drought and fires if you are south of it.
The recent intense thunderstorm activity in a band across from Chicago to Washington DC, a phenomenon called a Derecho, may have occured because of this. A large warm air mass moves north from the regions like Colorado where the fires are happening. This comes up against the Jet Stream which forms a barrier to it’s further northward movement. All that energy then starts to spill out sideways, following the normal pattern of air movement eastwards. And thunderstorms start plowing eastwards towards Washington DC.
So, what is Climate Change doing to this pattern? The strength of the Jet Stream is driven by how large the temperature difference is between the equatorial latitudes and the poles. This temperature gradient drives an energy flow towards the poles. And one of the major places this energy ends up is in the JetStream.
So as Global Warming progreses, the northen polar region is warming faster than the tropics. So the temperature difference between these two regions is dropping. With the result that the strength of the Jet Stream is dropping. More meandering. More frequent ‘lock-ups’. And so more frequent occurances of extreme weather events.
AGW predicts more extreme weather. And this looks like a significant mechanism that drives that. The temperature difference between the Arctic and the Tropics declines. The Jet Stream slows and meanders more. And the weather systems bounded by the Jet Stream become more intense.
So how can the US be suffering heatwaves while the UK is seeing a very wet summer? They are on opposite sides of a slower, lazier Jet Stream
Why aren’t we seeing this down here in the southern hemisphere? The Southern Polar Jet Stream typically runs further south of us here in Australia, New Zealand, Southern Africa. The Southern Hemisphere has far more ocean than land that tends to moderate weather extremes. The Antarctic isn’t yet seeing the major temperature changes that the Arctic is. So the basic driver of this isn’t there. And if it was happening, it would be happening down in the Southern Ocean where we don’t pay much attention.
Interesting data, Tamino. I remember when I first started looking into climate change about ten years ago, that the data seemed to indicate winter was warming much more than summer. However, in recent years, I’ve noticed that trend has flipped. Winters (besides the most recent) have tended to be colder and snowier, while summers have become hotter. This will probably end up being the third consecutive top ten hottest summer, based on the warm June and the scorching start to July. I remember last year reading a study from Stanford that predicted that, by 2050, every summer will be hotter than the then-hottest summer. The current hottest summer is still back in 1936, last year was only a tenth of a degree lower though.
Given the trifecta of recent hot summers, I’m wondering if we haven’t already entered a period of permanent hot summers. Given the fact that the jet stream is typically near the Canadian border during summer and has been gradually migrating northward with time, it’s not hard to see why the summers of the future may be permanently hot. Most of America lies between 30 and 45 North. Moscow, Russia is at 55N, and in 2010 from late June to mid August, it experienced temperatures on par with the even the hottest readings most northern American cities have ever experienced. Temperatures there reached 90+ nearly every day during that period with temperatures near or above 100 on several occasions. If it can happen there, it can most certainly happen here.
The decrease in wintertime US temperatures, I think may be a product of declining sea ice affecting the predominant state of certain oceanic oscillations or the jet stream configuration. More study is probably necessary to see whether these trends continue or if all seasons beginning warming uniformly.
Now look at the maps Tamino made above. You can see where the jet locks in place for awhile in each season. In the Winter, the jet stream is meandering much more than previously as the lower latitudes cool somewhat with normal seasonal cooling, but the Arctic tropospheric pressure is higher than before loss of the Arctic ice pack (Figure S4 in the Cohen paper). The Rossby waves have larger amplitudes that extends further into north into Canada. Longitudinally, the jet stream seems to get locked in place for time with a flow in a loop in the Rossby wave cutting south over east side of the Rockies, leading to cold temps in the East. As an example, this happened early in the winters of 2010 and 2011, and again a second time in the winter of 2010.
Then in the spring the Rossby wave tends to get locked up for a time with a loop in the jet stream flowing north just east of the Rockies. This pulls warm GOM air up into southern Canada, and the east, and especially the northern tier of US states east of the Rockies heat up early. This pattern occurred in March of 2012, leading to very high temps, and happened in again in late June for the big heat wave just experienced. The Rossby waves tend to persist in one place longer in the winter and spring, because the jet stream has slowed since the increase in polar tropospheric pressures.
In the summer, the Rossby waves can lock up anywhere across the US, and we see the full impact of AGW. As just mentioned the jet stream looped north in the Rockies, and gave us the heat wave.
In the fall, this pattern of the jet stream flowing north over the Rockies persists, but with a higher amplitude to the Rossby wave extendIng up into Canada, and the northern tier of states warm disproportionately. We saw this happen last October, 2011.
Some guesswork on this, but more or less, this generally describes what meteorologists seem to be thinking and working on. Many aren’t talking because of a rush to analyze and publish work on this.
Climate crock Episode 2 Weird Weather talks about this some, but didn’t have the plots Tamino made here to discuss.