Jul 08 2008
Sky Islands can Measure Climate Change
This story hit the newsstands almost two weeks ago, but it’s an important one I think. Sky islands are chains of mountains that are isolated in valleys. I’m sure most everyone is aware that as you go higher in elevation, the temperature decreases. Snow-capped mountains are a good example of this. But how much does the temperature change with height?
Wikipedia offers a definition of the dry adiabatic lapse rate. I’ve not seen it written like that before. I only recall seeing it as Γ=g/cp. In either case, the temperature decreases by about 10C for every kilometer you go up in altitude. As an example, I live in Tucson and it’s hot. The forecast for Tucson is 99F for today.
Warm Here, Cool There
However, Summerhaven is located at the top of Mt. Lemmon and is about 6700 feet higher in elevation than Tucson. This is almost exact 2 kilometers. If the temperature cooled at the dry adiabatic lapse rate, Summerhaven would be almost 20C cooler than Tucson. That would be 62F. Ahhh. Nice and cool, and the reason it’s named Summerhaven.
Most of the time, the air doesn’t cool at the dry adiabatic lapse rate. Instead it will cool somewhere between the dry and moist adiabatic lapse rates. The forecast for Summerhaven for today is 75F. This is still nice and cool, and it provides a range of climates as one climbs the mountain. The climate on the top of Mt. Lemmon is similar to that in northern Minnesota.
Sky islands are more important than just a cool place to visit during the summer months. They have distinct vegetation and animal species that live on them. They are called sky islands because they really are like islands. The trees that live at 7500 feet would not be able to survive on the desert floor below. Like on an island, they are trapped from migrating to higher latitudes in the case of climate change. If they average temperatures were rising, we should expect the vegetation to migrate to climates where they are adapted.
Plant Species on Sky Islands
In the journal Science, an international group of scientists found that plants are moving on these sky islands. A Significant Upward Shift in Plant Species Optimum Elevation During the 20th Century. They studied a group of over 100 plant species in the western European mountain ranges, and found that on average they were increasing in altitude by 29 meters per decade.
That’s huge. 29 meters per decade is a change in elevation of 290 meters in 100 years. The researchers looked at the optimal elevation of the plant species in two time periods: 1905-1985 and 1985-2005. When they compared them, there were some species that had moved down the mountains, some that stayed at the same elevation, and some that had moved up the mountins. But on average, they had moved up 66 meters.
Ocean Islands
Let’s go back to the island in the middle of the ocean example. Plants moving up a mountain is synonomous with the island sinking. As it sinks, there is less room for plants and animals to live. Some of them may go extinct at that location. Some mobile animals may try to risk swimming across the unknown ocean in search of a new island. At some point, the island will sink below the ocean surface and all the plants and animals that lived on that island will die.
As the climate warms, plants and animals that live on sky islands are forced higher and higher up the mountain. If the climate warms enough, the mountins will become too warm for a particular species and it will no longer be able to live there. Consider plants that live within the top 300 meters of Mt. Lemmon, in 100 years will they still be able to survive?
The important thing about this study is that it shows that the climate is changing. A rapid change in temperature may be about one hundredth of a degree Celsius per year. That doesn’t sound like a lot, and it’s hard to get people worked up about such a small change when we see changes orders of magnitude larger on daily and yearly cycles.
Small Change or Large Change
Changes in sea level likewise seem small - around 3 millimeters per year. Tides and waves bring changes that are much larger than this. But 3 mm/yr is quite large when you consider that most of our infrastructure is in coastal areas.
But 3 meters per year is something that seems large. The optimum elevation for the vegetation in this study was found to move almost 10 feet every year. Because the elevation changes rapidly in the mountains, so do the climate zones. This allows a larger change to be seen in the effects of climate change.
Review question: If hot air rises, why is it colder at higher altitudes?
References
Lenoir, J., J. C. Gégout, P. A. Marquet, P. de Ruffray, and H. Brisse (27 June 2008). A Significant Upward Shift in Plant Species Optimum Elevation During the 20th Century, Science 320 (5884), 1768. [DOI: 10.1126/science.1156831]
(No Ratings Yet)
Loading ...Related Posts:
10 Responses to “Sky Islands can Measure Climate Change”
To reduce spam, comments are automatically closed 30 days after the last comment. If you would like to comment on any closed thread, please use the contact form at the top of this page.
The species heading downhill, they just can’t stand the heat any longer and try commiting vegetational suicide?
Climate Change is most talked about thing these days. But the sadest part is that most of us are even not aware about that.
Is it 29 meters greater than normal? Meaning, have they always been moving at 29 meters per decade, or were they always at the same place since the last glacial?
[Reply: They measured the elevation during the two time period mentioned above, and then compared them. They didn't talk about what was 'normal'. Although, if the climate (temperature, rainfall, etc.) were not changing, I would not expect the vegetation to change altitude.]
“[Reply: They measured the elevation during the two time period mentioned above, and then compared them. (...)]”
Well, no. From presence-absence data of surveys from the past and present they chose an undisturbed subset and estimated something they considered as ‘optimum elevation’ for each species by using a model that’s made for disregarding every aspect but mean temperature and they compared shifts of that optimum elevation estimate over time.
So - other way round - they claim that given date of global warming and elevation they can predict mean species composition of survey plots at this elevation. Just kidding, it’s not what they say they can do. It’s what I say they should succeed in if they were right.
There is one tiny problem with your discussion of the adiabatic lapse rate… it applies to parcels of are that are, for one reason or another, moved from one elevation to another. It does NOT mean that the thermal profile of the troposphere normally follows that rate.
In fact, the usual state of the atmosphere is relatively stable… that is, the actual lapse rate is less than adiabatic (about 6.5C per 1000 meters). That is, when a parcel of “warm” air rises through natural convection, it cools at the adiabatic rate and eventually ceases to rise when it reaches an elevation where the the surrounding air is warmer than the parcel being buoyed aloft.
Lapse rates greater than dry adiabatic are quite unstable. Lapse rates between dry and moist adiabatic are potentially unstable (if the air parcel can be buoyed to an elevation where cloud formation can begin), and lapse rates less than moist adiabatic are always considered to be stable.
Your temperature example of the difference between the mountaintop and the city is just about spot-on for a standard atmospheric lapse rate (23.4C for 2km).
Also, your claim that the mountaintop climate is similar to northern Minnesota, is simply wrong. Climate (including microclimate) is much, much more than just temperature. The reason for the upward expansion of a species range may be due to many factors other than a change of ambient temperature. For example, deposition of critical micronutrients through precipitation, or even dust. Or, perhaps, decreases in air pollution (notably sulfates, but there are certainly other candidates) are placing less stress on these species, allowing their range to expand.
Yes, it might be warming… but it might be a myriad of other things, too. It’s best not to jump to conclusions until you actually do some additional research to discover the “why.”
Just my $.02
DaveK
Oops… tired old eyes and no reading glasses to proof my post…
That should have been 23.4F over 2 km (13C)
Thanks
DaveK
DaveK,
I stated in the post that the temperature does not cool at the dry adiabat, but somewhere between the dry and moist adiabat. You must have missed that part.
I’ve lived in Minnesota most of my life. I now live a few miles from Mt. Lemmon and have been there many times. From experience, the climate on Mt. Lemmon is similar to northern Minnesota.
Sorry, I guess I must have missed that. It’s a nice coincidence that the temperature differences you cite correspond very closely to the Standard Atmosphere lapse rate.
Nonetheless… I’d still bet dollars to donuts that the “climate” on Mt. Lemmon is quite a bit different from the “climate” in Northern Minnesota. Now, this is just conjecture on my part, not having been to either location. But I’d guess that for example:
Diurnal temperature swings are greatly different in the two locations.
Rainfall/snowfall patterns are quite different (e.g. it probably rains a lot more in Northern Minnesota during the summer).
Humidity patterns are quite different.
Typical cloud cover patterns are quite different.
Etc…
Yes, the temperatures may be similar, but likely not the “climate.” And I’ll bet both are really great places to visit in the summertime!
DaveK
Hard to say, summer is “monsoon season” in Arizona, and thunderstorms and rain in the afternoon are quite common in normal years.
So I presume you’re telling us that summer thunderstorms and rain are uncommon in northern Minnesota???
Like I said… I haven’t been to either place.
But I’d think that one would have a high-desert type of climate, and the other a more warm-summer continental climate type.
DaveK