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 , 01:23 AM GMT on mai 21, 2012
Just Temperature Redux: What about the Cherries and Apples?
March and April were very warm in the United States, and especially in March when it was 86 degrees F in Detroit, there was a lot of press attention to the heat (my blog at the time). Following the March heat wave I watched with interest the caster that has weather events and earthquakes on the homepage. There was a period of time when there were record highs and, a couple of hundred miles away, record lows. There were these waves moving (very) warm air north and (very) cool air south (another old Rood blog Warm, Cold, Warm, Cold). This is what weather does, moves heat from the tropics to the poles; it tries to smooth out the distribution of temperature, heat, energy. The climate of the Earth is strongly linked to the Equator to Pole temperature contrast. (I note that, at this writing, a May 20 record high in Holland, MI, of 92 F. In fact, May 20 is pretty much coast-to-coast high.)
So I am watching these highs and lows, expecting someone to write to me and tell me how cold it was in Tennessee, and what do you say to that you alarmist?
The past few months provide us a nice example of climate, and a useful framing for thinking about the future. Scientists are always explaining that just because the globe is, on average, warming, that does not mean that it no longer gets cold. When I have written about this in the past, I always start with the Sun still goes away at the winter pole; it gets cold; the pole is relatively isolated, so there are cold pockets of air up north. (Yes, I am presuming a Northern Hemisphere bias.) So it’s cold up north, and down south it’s hot. If you think about the Earth, the seasons, the distribution of land and ocean, an increase in average global temperature suggests an increase in the average temperature between, say, 30 degrees latitude south and north. Half of the Earth’s area lies in those bounds, and, well, the Sun is always there.
Next if we think about weather and climate, the contrast between the temperature at the equator and the pole is a measure of the amount of mixing that the atmosphere and ocean need to do to work towards a balance. If someplace up north is still getting about as cold as it used to get, because the Sun is down and it is a bit isolated, and there is more and more build up of heat in the tropics, then something has to give. Using climate and weather models as a guide, we see large mixing events in the late winter, perhaps more characteristic of events of, historically, early spring.
Figure 1: From an old, but good, blog: Warm, Cold, Warm, Cold. A schematic picture that represents a wave in temperature. There are hot and cold parts of the wave.
So there are bursts of warm air north in late winter or earlier in the spring. But there are still pockets of cold air and these get pushed south. The variability, hot and cold contrast in this case, actually increases. The bursts of warm air appear as the onset of spring, leaves and flowers come out. And there they sit waiting for the return of the cold air. This year’s warm spring did great damage to the sour cherry crop (Michigan, Wisconsin, New York) and the apple crop all across the upper Midwest. (Iowa, Michigan).
This scenario of a warm period followed by a frost that kills fruit blossoms is not new. I grew up in the South, and just about every year there was some strip of peach-growing land that was damaged by the onset of spring, followed by a frost. What this current case study lets us think about is what does a warming climate bring to table? Earlier warm spells extending farther north. Increased vulnerability as larger areas of land are impacted by the mixing of the increasing temperature contrasts. Increased crop risk as new weather threats encroach on new regions. There are adaptation strategies for these risks, but they come at a cost.
So I want to finish this blog with something of a change of gears. It relies on a paper brought to my attention by Chris Burt. It is a paper in Nature entitled Warming experiments underpredict plant phenological responses to climate change by E. M. Wolkovich (2012) and many others. There are a couple of points I want to make about this paper.
First, the paper is a nice exposition about how biological scientists think about the intersection of their field with climate change. Advancing onset of leafing and flowering is one of the most sensitive indicators of the onset of spring. Though many factors influence when plants start to leaf out and flower, temperature is the predominate factor. The variable that is used as a proxy for climate is mean annual temperature, and variability of the mean annual temperature represents the variability in the onset of spring.
The second point I want to make about the paper is a clarification – perhaps a translation between different scientific fields. As pointed out in Wolkovich et al. (2012), there is substantial observational evidence that spring is coming earlier. This move to earlier times is especially evident in the northern hemisphere and more evident at higher latitudes, say, in Michigan or Canada. When Wolkovich et al. (2012) talk about “warming experiments” they are not talking about experiments with climate models. They are talking about experiments that artificially warm plant communities to investigate their sensitivity to increased temperatures. In this paper, they find that such experiments do not explain the observations of the onset of spring in natural plant communities.
Returning to climate change - Wolkovich et al. (2012) estimate that for each degree C that mean annual temperature increases the onset of leafing and flowering will move forward by 5-6 days. Given temperature trends for the past forty years, this translates to 1.1 to 3.3 days per decade. And returning to the cherries and apples, these types of trees are especially vulnerable to bloom followed by a frost, especially in high latitudes. So if you are an orchard fruit grower, how do you use this information? Do you treat this year as a simple fluke of weather, or do you look to start a replacement program with different types of fruit or different hybrids as the orchard is refurbished? Or do you look to ways to manage the temperature in the orchard, and perhaps a market advantage with earlier fruit?
Figure 2: Larger image Ripe by Jennifer Bruce from Absolute Michigan
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