I'm a professor at U Michigan and lead a course on climate change problem solving. These articles often come from and contribute to the course.
By: Dr. Ricky Rood , 5:45 AM GMT en Junio 02, 2012
Heads and Tails: Still thinking about Spring 2012
In March 2012 I planted potatoes in Colorado and it was 85 degrees F. A couple of weeks ago I put out some basil, and it frosted. As I said in the last blog, following the March heat wave I watched with interest the caster that has weather events and earthquakes on the WU homepage. There was a period of time when there were record highs and, a couple of hundred miles away, record lows.
In my blog Just Temperature, I highlighted several measures and displays of temperature information that made a consistent picture of a warming planet. If I count correctly, we are now at the 327th consecutive month that has been above the 20th century average. That average includes the 1930s, a notoriously warm decade, and it includes all of those warm months since 1985. So that average is by its definition, a high number compared with say, the 1800s. This march of warmer that average months is, by itself, pretty compelling.
In that blog, I also revisited the nice plot adapted from a 2009 paper by Jerry Meehl and a host of other authors. (Original Paper, Paper Discussion from NCAR ) It is reproduced here. This figure shows, for the U.S., the number of new record highs divided by the number of record lows – the ratio of highs to lows. In a simplistic, intuitive way, if the average temperature where staying the same, then one would expect the number of new record highs and the number of new record lows to be about the same. What is seen in the figure is as we go from the 1980s to the 1990s to the 2000s, there is trend of record highs out numbering record lows by a factor of 2 to 1.
Figure 1: Adapted from Meehl et al. (2009) the ratio of U.S. record highs and record lows by decade.
So let’s return to that WU caster information. I wondered about what sort of message I was getting from these little nuggets of information. If I picked a few days from the caster, I counted about as many highs and lows. The folks at Climate Central have developed and published a record temperature tracker. It packs in a lot of information. If you look at the daily maps, then you see the waves of warm records and cold records moving across the continent. I can see the hot days in March when I planted potatoes and the cold days in May when I planted basil. But if you take May as a whole, there were 3,188 daily high records compared with 421 daily lows. If I calculate my ratio, that is more than 7 times as many highs as lows. And that was for May, a month when my impression from the WU caster information and my basil it was relatively cool.
So what about March, when everyone knew it was hot? There were 7,755 records highs and 287 record lows, a ratio of more than 27. The temperature tracker also pulls together information about warm nights and cool days. For a variety of reasons warm nights are of special interest. From a climate scientist’s point of view, warm nights are often associated with the greenhouse effect, primarily due to water vapor and clouds. It doesn’t take a very thick cirrus cloud to maintain warm nighttime temperatures. Or, if it is simply high humidity, then it stays warm. So if the amount of water vapor in the atmosphere is increasing because it is getting warmer, then the nighttime temperature should remain high. Therefore, we might expect a trend in increasing nighttime temperatures to be a robust measure of warming. This gets confusing, because it is the daily lows getting higher. If we look at the number of warmest nighttime low records in March 2012, the number was 7,517. There were only 603 records set for coolest daily high. (What does the extra water do about the daytime highs?)
We see here a very warm spring. It’s also been very dry, but I will leave that until a later. (I know I should write shorter, more frequent articles to maintain the excitement amongst my readers.)
The Climate Central record temperature tracker is based on data at the National Climatic Data Center. They keep a nice records table, which also has easy comparisons to last year. If you look at the ratio of January through May of records maximum to record minimums for 2011 and 2012, it shows what an extraordinary year we have had so far. This year the ratio of highs to lows is nearly 12 compared with 1.7 in 2011. The 2011 number is far more similar than 2012 to the information in Figure 1 - still a pretty strong imbalance between highs and lows.
So I want to end this blog with a party trick. We have had 327 months in a row above the average temperature of the 20th century. If we played the game that there was a 50% chance of each month being above (heads) or below (tails) average, we have now rolled heads 327 times in a row. How likely is that? I think that is one half raised to 327th power, which is about 1 chance in a number that is 1 with 98 zeros after it. That makes buying a mega lotto ticket look like a solid investment. We live in a extraordinary spring in an extraordinary times. After a rocky start, my potatoes look pretty good.
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