Dr. Ricky Rood's Climate Change Blog

Just Temperature Redux: What about the Cherries and Apples?

By: RickyRood, 1:23 AM GMT on May 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?

r



Figure 2: Larger image Ripe by Jennifer Bruce from Absolute Michigan


Climate Impacts and Risks Climate Change

Updated: 2:34 AM GMT on May 21, 2012

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When Students Listen: Atlantic Multi-decadal Oscillation

By: RickyRood, 9:27 PM GMT on May 07, 2012

When Students Listen: Atlantic Multi-decadal Oscillation -

This week a student who took my class a couple of years ago and also helped me with my class this past term, Kevin Reed, wrote me a message that he remembered my commenting in class that the Atlantic Multi-decadal Oscillation might not be real. He made reference to a paper in Nature entitled Aerosols implicated as a prime driver of twentieth-century North Atlantic climate variability. A good thing about students is that they get to read all sorts of interesting things, send them back to me, and help me appear smarter than I am.

The term “Atlantic Multi-decadal Oscillation” (aka AMO) has been used to define the variation of sea surface temperature in the North Atlantic Ocean. What did I mean in class when I said “it might not be real?” There is no doubt that the temperature of the Atlantic Ocean varies, and as we take and accumulate measurements we identify extended times when the ocean is warmer or cooler than average. When these data are plotted, we see these warmer and cooler time spans persist for a few tens of years; hence, a multi-decadal oscillation. The plot below is taken from a good article in Wikipedia, and the plot was made from data that is available at the Earth Systems Research Laboratory.



Figure 1: An area index that measures how much warmer or cooler the North Atlantic Ocean is from a long-term average (from Wikipedia). (The indices for the Atlantic Multi-decadal Oscillation are not especially well documented in the web resources that even a reasonably informed practitioner can find. The indices tend to be averages of the Atlantic surface temperatures from somewhere in the deep tropics to Greenland. They are then subtracted from long-term means. The 20th century mean is used in some papers. This example demonstrates some of foibles of data, data documentation, and data presentation on the web.)

The Atlantic Oceanographic and Meteorological Laboratory has a nice set of Frequently Asked Questions about the Atlantic Multi-decadal Oscillation.

I want to revisit this figure that I use as a template to think about the natural science of the Earth’s climate.




Figure 2: A summary figure I use to organize the basics of climate science and global warming.

A focus on the Atlantic Multi-decadal Oscillation is most naturally categorized in Figure 2 as “internal variability.” When we talk about global warming in the past 100 years and the next 100 years, internal variability usually refers to states of the atmosphere and ocean that are persistent for some amount of time – weeks, months, years, decades. For example, in El Nino the temperature of the ocean in tropical eastern Pacific is warm and stays warm for a few months or more. Then in La NIna the eastern Pacific is cool and stays cool for a few months or more. There is an easy feeling of the Earth oscillating back and forth between the warm and cool times in the eastern Pacific Ocean. For El Nino and La Nina, there are many related changes in atmospheric circulation (the trade winds change) and precipitation (tropical convention moves east and west). All of these related changes fit together, and they describe the atmospheric and ocean behaving as a coherent system. This coherent behavior allows us to understand cause and effect; it allows the possibility for prediction.

On a scale of a million years, the cycles between the ice ages and temperate times might be internal variability. This would be related to, for example, carbon dioxide coming into and out of the ocean due to changes in temperature and biology. So far, I have been diligent not to call internal variability “natural variability.” El Nino and La Nina are “natural,” but that does not mean that their behavior will remain the same as the climate warms. To call internal variability “natural” suggests this idea of a “natural” and a “manmade” climate that are two different things, and this idea is clearly not the case. We have our climate, there is internal variability, there is manmade warming, and they all occur together, and they will change together.

The Atlantic Multi-decadal Oscillation is internal variability. When I stand in front of class and say the Atlantic Multi-decadal Oscillation might not be real, I mean several things. At the foundation of my statement is that we don’t have this story of coherent behavior like we have in El Nino and La Nina; we don’t have a construction of the atmosphere and ocean behaving as a connected, dynamic system. In fact, I would argue that the issues I raise in the caption of Figure 1, for example mushy definitions of indices, indicate the challenges of the Atlantic Multi-decadal Oscillation. We know there is a signal, but it is even hard to describe that signal very well. When we link back to cause and effect, one of the leading ideas is that it is related to subtle changes of global scale ocean circulation, which we neither model nor observe very well. So I don’t say that the signal of the temperature change is unreal, but I suggest that the Atlantic Multi-decadal Oscillation is not a coherent sloshing back and forth between warm and cold.

One reason we are interested in the Atlantic Multi-decadal Oscillation is that we know that there are strong relationships between the temperature of the ocean in the tropical North Atlantic and drought and flood in North America and Africa. We know that warm Atlantic sea surface temperature is very highly linked to hurricanes in the United States. One of the scientists most quoted as a skeptic of the science of global warming, Bill Gray, bases much of his climate change argument on the role of Atlantic Multi-decadal Oscillation as a proxy for global climate. (For those who are interested, go back to Forms of Argument, and look at the focus on isolated information and reliance in this case on the reality of a process that is both hard to model and observe. How does this stand up in the face of all that we can observe?)

Back to the paper in Nature referenced in the first pargraph, Aerosols implicated as a prime driver of twentieth-century North Atlantic climate variability. This paper is a set of model simulations of the past century and a half. The simulations are associated with the Coupled Model Intercomparison Project Phase 5 (CMIP5). CMIP5 represents a coordinated set of simulations run by scientists around the world with the most recent production-ready climate models. I expect a set of interesting new results to be reported from these simulations especially with regard to the role of aerosols and land use in the climate. Aerosols, particulates in the atmosphere, have strong regional climate impacts, and are closely related to air quality – two of the other items listed in my Figure 2 template.

The point of Aerosols implicated as a prime driver of twentieth-century North Atlantic climate variability by Ben Booth and coauthors is that temperature changes associated with different amounts of aerosols at different times can explain the majority of the variability observed in the temperature of the North Atlantic Ocean. Natural sources of aerosols include volcanoes, which cool the Earth’s surface. Other natural sources are salt and soil dust. Manmade aerosols include pollutants, soot, and soil dust. (old Rood blogs - Volcanoes and Long Cycles, and Black Carbon) What aerosols do is to change the absorption and reflection of solar radiation; the absorption and reflection of clouds; and how efficiently heat is held near the Earth’s surface. In the simulations by Booth and others, the predominant impact of aerosols is related to effects on solar radiation – both directly by reflection (volcanoes) and indirectly by changes to clouds. Earlier studies have investigated the effect of volcanoes, and this study brings to the forefront the importance of other sources of aerosols, many of them manmade, in modulating global climate with strong regional influences.

The numerical experiments in Booth et al. (2012) are well designed. But they are complex, and, well, numerical experiments. I hold such numerical experimentation as an important part of scientific methodology of the 21st century. They help us think in a field where our ability to execute controlled experiments is limited. To me, these experiments suggest a strong, well-based explanation of the variability of North Atlantic temperatures. However, scientific method requires more scrutiny, more use of observations, and independent verification of the results. But as it stands right now, we have at hand a plausible explanation of cause and effect that explains the majority of the observed variability.

To finish another long article – The work of Booth et al. (2012) extends back to 1860. The Atlantic Multi-decadal Oscillation extends back, well seemingly, at least 8000 years. In Nature Communications there is an article Tracking the Atlantic Multidecadal Oscillation through the last 8,000 years by Mads Faurschou Knudsen and others (2011). This paper concludes that for most of the time since the last ice age ended, there has been a 50 – 70 year oscillation, which they attribute to atmospheric-oceanic coupled behavior modulated by variations in the orbit of the Earth. As I understand this paper, the authors tested whether or not variations in the Sun could explain their observed variability. Since solar variability did not explain their observations, they accepted the hypothesis that atmospheric-oceanic coupled behavior provided the explanation. They did not investigate the impact of aerosols.

As it stands in my mind today, the variability in the North Atlantic temperature behavior is strongly influenced by aerosols in the atmosphere and a trend due to increasing carbon dioxide. If there is oscillatory behavior in the temperature, it is due to increases and decreases in atmospheric aerosols, perhaps on top of a smaller atmospheric-oceanic dynamic variation that still requires explanation. A good step forward, I think.


r

Climate Models Climate Change

Updated: 8:02 PM GMT on May 21, 2012

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About RickyRood

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.

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