The Bulletin of the American Meteorological Society's annual State of the Climate report was released earlier this month, and just like last year, it concluded that the earth continues its overall warming trend. Also, similar to last year, one might notice that the eastern United States may be warming, but is warming at a slower pace than most other regions of the world. On the opposite end of the spectrum, the western part of the country is consistently warmer than normal and setting records. Why is that?
The answer seems to depend on whom you ask.
If you ask some climatologists and forestry officials, you might be interested to learn that the reforestation of former plantation land in the Southeast is a contributing factor. Areas that had been cleared for farming in the 19th century have been allowed to return to a more natural state or have even had forests actively planted in them.
The additional woodland aids in counteracting carbon dioxide increases since trees absorb carbon dioxide and emit oxygen. However, this explanation only works with trees that have not yet reached their stability point at which they absorb through photosynthesis as much CO2 as they release in the form of leaf litter, respiration, and decay. Young forests are more productive in this sense.
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In the west, there has been a trend of deforestation as woodland is cleared for development and agriculture.
Other scientists will tell you that the polar jet stream is to blame, at least in the winter. We've seen examples a couple years in a row as the polar jet breaks our country in half with a persistent ridge of high pressure in the west and a persistent trough in the east. The western U.S. suffers from warmer than normal temperatures and drought, while the eastern U.S. experiences cooler than normal temperatures and more than average precipitation.
Still others may point to an increase in Arctic sea ice, which may be affecting the jet stream, or the fact that we haven't had a strong El Nino or La Nina in recent years that might change the pattern.
The real key is to understand that the atmosphere is fluid and dynamic and always changing. These possible causes do not stand alone. They are all interconnected, and a trend in one area - whether it be changes in land usage or the sea surface temperature in the eastern Pacific Ocean - will eventually affect the others, in at least some small way. It's rarely ever as simple as pointing to one thing and crying, "That's it!" Almost always, it is a number of things and I would bet that often some of those things aren't even being considered yet.