Summit County Citizens Voice

Beetle-killed areas could have an effect on localized thunderstorm formation and precipitation

Pine beetles killed tens of thousands of acres of trees. Could that affect local weather patterns? Some researchers think it might.

By Bob Berwyn

SUMMIT COUNTY — Researchers who set out to study whether deforestation on the slopes of Kilimanjaro is affecting the mountain’s ice cap concluded that large-scale climate changes have much more of an impact on the glaciers. But they also documented that clear-cutting the mountain’s forests is having a distinct effect on precipitation at the mid-level elevations, where rainfall has been reduced.

The findings could have implications for Colorado, where some forest landscapes are experiencing wholesale changes after the pine beetle epidemic. Both scientists said that, depending on the scale of changes in Colorado and the rest of the region affected by the outbreak, the changes in land cover could have an effect on summer rains by changing the amount of moisture available for convection in the lower atmosphere.

After reading about the Kilimanjaro research, I corresponded with two scientists who have studied how land-use changes can affect mesoscale weather, including Thomas Mölg (University of Innsbruck, Austria), who conducted the study at Kilimanjaro, and Roger Pielke, Sr., of the Cooperative Institute for Research in Environmental Sciences (CIRES).

Since both researchers were kind enough to give detailed written answers to my email questions, I’m posting the Q & A rather than shortening or paraphrasing their quotes.

Q:Why did you set out to study this? What made you think that those vegetation changes might be affecting the glacier?

Thomas Mölg: “Well, it was a hypothesis. Kilimanjaro has a dense forest belt, so we thought that the local evaporation from these forests (and thus its changes when forests change) could have a significant impact on precipitation formation. Our results now suggest that moisture supply by the large-scale climate is indeed more important.”

Q: What is the mechanism that would lead to reduced  rainfall on such a localized mid-elevation scale?

TM: “This is the signature of the local potential for rainfall. The forest belt is the main provider of moisture that can evaporate locally into the atmosphere (and thus contribute to rainfall generation).”

Q: Why do you think reduced rainfall was only observed at the  mid-mountain region? Why not at the higher elevations.

TM: “The problem for high mountains is always that it becomes very difficult for the air flow to transport moisture uphill all the way to the peak (remember: the bulk of moisture in the atmosphere resides within the lowermost 2 km). The higher the mountain, the more difficult. Therefore we emphasized in the paper that the distance to the forest belt is a critical factor.”

Q: I know this is a bit speculative, but is it possible that the widespread deforestation caused by the mountain pine beetle outbreak and associated forest-clearing activities could lead to a similar climate/weather response in the northern Rocky Mountains?

TM: “On a local scale, I can imagine that this could result in a precipitation response. If you need more information on that you should contact Roger Pielke Sr., who has done many studies on land-cover change including mid-latitudes.”

Technically, it’s probably not deforestation yet, because most of  the dead trees are still standing, although based on satellite images I’ve seen, there is a significant change in the albedo both summer and winter because of the loss of dark green needles. Could there be a difference in the response between winter (when most precipitation is snow, driven by frontal or orographic weather systems) and summer (when most of the rainfall is convective)?

TM: “I would expect such a difference, yes; exactly due to the reasons
you mention.”

Q: How could one go about studying whether the changes in the forest landscape are leading to changes in precipitation? How many years of data would you need to to make some supportable conclusions?

TM: “I attach another paper (Pielke is co-author) who looked at this problem in Australia. It is hard to say how many years exactly, as this depends very much on goal/desgin of your study. One common way in terms of methods is running an atmospheric model with different land-cover states. This is the only possibility to make sure that the land-cover change is the only perturbation to the system (all other inputs/boundary conditions are held constant).”

Q: Do the results (of the research at Kilimanjaro) have implications for other glaciated regions?

TM: “In general we believe that LOCAL land-cover changes do not have the potential to distort the glacier response to large-scale climatic changes.”

My questions for Roger Pielke, Sr.:

Can we expect to see, or should we be studying whether the widespread forest cover changes in the southern and central Rockies might have any impact to precipitation on a regional or local scale due to changes in albedo and transpiration (or any other factors).
I’m also wondering about any differences between summer and winter response, and also any other weather/climate feedbacks the beetle epidemic might
cause.

Roger Pielke, Sr: “You have asked an interesting question! The pine beetle tree deaths certainly could effect the microclimate and even mesoscale climate in their vicinity. The loss of any transpiration from the trees in the summer would elevate the daytime temperatures and reduce the humidity some where there are large patches of these dead trees.

“However, I have not seen an analysis of the total area they cover,  including the fraction of an area’s trees that are dead. It needs to cover a large area (e.g. tens of kilometers on a side) before we would expect to see major mesoscale weather effects (such as on thunderstorm development) and this would occur, of course, in the warm season.

“During the winter, when the weather, including snowfall, is so dominated by propagating low pressure systems and the orographic modulation of this snow by the terrain, the dead trees would have relatively little direct weather effect. There could be colder temperatures and more inversions in valleys which have large areas of dead trees, of course, as more snow cover (and hence higher) albedo would result in lower heating rates at the surface during the day.”

Filed under: climate and weather, Colorado, Environment, Forest health, forests, pine beetles Tagged: | bark beetle epidemic, climate, Colorado, mountain pine beetles, Roger Pielke Sr, weather