by Cameron Foster

Created on: February 28, 2009   Last Updated: May 24, 2009

The Earth has warmed since the 1970s, but what is often overlooked in the popular literature today is the effect of human land-use changes. This mainly comes in the form of increasing urban zones, and irrigation of land.

The burgeoning cities of our planet add a tremendous amount of heat to the environment. Dallas-Fort Worth, for example, is consistently several degrees warmer than the surrounding countryside. Other large urban areas from New York City to Chicago show much of the same. This is most noticeable on days when there is not a lot of wind. Windy days mix up the lowest mile of the air more efficiently and help to mask the effect.

Another way in which we have warmed our world, which has nothing to do with methane or carbon dioxide, is with irrigation. The addition of water to the soil affects the boundary-layer air (air near the surface of the earth). Since moist air cools less efficiently than dry air, we would expect overnight lows to be warmer than a similar dry air mass. A study done in California's San Joaquin Valley is quite revealing in this regard.

The study examined seventy years of temperatures down in the valley and also at a site 1,000 feet above in the adjacent hills. Guess what? The valley floor warmed several degrees during the period while the neighboring hillside's temperature changed much less. Is this finding consistent with the theory of greenhouse gas global warming? No. However, it does suggest the great increase in agricultural irrigation during that time frame had a lot to do with that particular warming episode.

Farmers and ranchers claim that the "climate" has changed in places like western Kansas and west Texas. Their observations are that "it has gotten more humid than when I was younger". While we have to be careful to not place too much credence to anecdotal (folksy) evidence, this claim is consistent with a trend towards greater irrigation as well as increasing numbers of man-made reservoirs in these areas. With much more water to evaporate, the average dewpoint, and therefore the relative humidity of the region, will increase compared to the time before irrigation took place.

So, land use changes ARE impacting our climate, and people are directly responsible.

Climate scientists will be the first to tell you (and they are right) that weather and climate are not the same. "Weather" is the given state of the atmosphere at a moment in time: today's "weather" might be cold. "Climate" takes a longer average view of conditions.

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