منتديات المُنى والأرب - Expansion of deciduous trees could make Arctic warm faster

Expansion of deciduous trees could make Arctic warm faster

A new simulation of deciduous tree expansion into the Arctic region shows that the trees would block the highly reflective snow and increase cloud cover, causing the area to warm even faster than it already is
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Warming in the Arctic has been more pronounced than elsewhere.

Although temperatures have risen throughout the globe, they've gone up most dramatically in the Arctic. Past warm periods indicate that deciduous tree expansion into the Arctic is a common occurrence when the region warms up, so a new study has looked into the impact trees could have on the regional climate. As expected, the increase of the leafy trees would result in less reflective ground, but the study suggests they could also induce more cloud cover and an increasingly warm surface and ocean that have more turbulent weather patterns.
Studies of climate change suggest that deciduous trees will eventually expand into the Arctic region, a notion that is supported by paleontological studies of the region's past warm periods. The currently forested parts don't change from evergreen to deciduous—rather, deciduous trees expand into areas that currently lack other vegetation. The authors of the new paper estimate that the Arctic region could be warm enough to support a substantial population of deciduous trees within the next 10 years.
Although the trees may invade as a result of climate change, it's also expected that they'll influence the local climate, since the foliage will alter the reflective properties and water cycle of the land it will be growing on. The authors used a climate model to study how the albedo (diffusive light reflection) and transpiration of the deciduous trees affected the overall temperature, as well as amount of water vapor in the air.
The evergreens that already populate the Arctic region have a very low albedo and absorb a lot of light due to their dark color; however, their warming effect isn't terribly dramatic because of a relatively low population. Evergreens also have a very low evapo-transpiration rate, meaning they don't add much moisture to the atmosphere.
Deciduous trees have twice the albedo of evergreen trees, meaning they don't absorb the light as much. However, the simple fact that deciduous trees would begin growing in the region would reduce the surface area that is currently dominated by reflective snow; even when their leaves are gone, the sheer number of dark branches would provide an overall decrease in albedo and an increase in the heat held by the surface.
Equally importantly, the authors found that the relatively broad leaves of deciduous trees have much higher transpiration rates, adding moisture to the air. That can both increase cloud cover (decreasing the light that reaches the ground), which should have a cooling effect, and increase warming due to the greenhouse effect of water vapor.
The simulation indicated that the expansion of leafy trees, by their ability to increase water vapor and absorb light, would result in an increase of the surface temperature in the Arctic region of about a Kelvin over the next 20 years.
One key process the authors identified is that the transpired water vapor would further mix throughout the air and cause a warming over the ocean as well, leading to the melting of highly reflective sea-ice. That would create a darker ocean, which would also warm faster. The end result is an Arctic region that continues to warm, allowing more leafy trees to grow (which warm the atmosphere further), creating a system that may self-perpetuate until it runs into some limit on tree growth.
This isn't the only paper that looked into how changes in water vapor can influence regional climates; a paper in EOS looks into the effects of dams on weather patterns. The authors review data that suggest that dams can result in a dramatic increase in the water vapor in their immediate regions, destabilizing the precipitation rate and increasing the probability of thunderstorms and flooding.