Human Effect On Climate The Grasslands Of The Great Plains In The

There may be some places in the world where "climate-engineering" by humans altering vegetation cover has already occurred, albeit unintentionally. In the 1800s the grasslands of the central USA were transformed at a pace and on a scale unmatched in any other region in history. Settlers poured westwards in their millions, ploughing up the deep prairie soils to plant wheat and corn fields stretching for hundreds of miles.

Did this affect the climate? The debate about it goes back a long way, to the time when the land was still being ploughed up. In the Plains climate, a drier than average year could prove disastrous for crops, so there was plenty of interest in ensuring that the rainfall was as reliable and abundant as possible. In the 1880s, Samuel Aughey, a professor in the young state ofNebraska, suggested that ploughing a prairie soil helps it to retain water better because—with the mat of vegetation on the surface broken— water soaks in rather than running straight off into rivers. This store of water held in the crumbly soil will then evaporate, being recycled as rain which falls to earth again, instead of being lost to rivers and the sea. This idea became encapsulated by the plains farmer's adage: "Rain follows the plough." Although the idea got a lot of attention, it is now thought that ploughing actually does not have such an effect on rainfall.

Others at the time suggested that the best thing to do to ensure steady rainfall was to plant more trees. In the 1860s a US government official named Joseph Wilson pointed out that since deforestation seemed to have decreased rainfall in other parts of the world (see Chapter 6), planting trees in the Great Plains would surely increase the rainfall there. He advocated covering a third of the Great Plains in trees to ensure an adequate supply of rain. Congress was impressed enough by his arguments to pass an act that offered free land parcels to farmers who planted a certain percentage of their land with trees. However, the farmers were not motivated by these incentives— few trees were planted, and the act was eventually repealed.

More recently, aided by modern climatological knowledge and computers, scientists have been able to take a more informed look at the effects of converting the prairies to grain fields. Some modeling studies by Eastman and colleagues suggest that replacing the grasslands of the central Plains with crops caused the peak temperature reached during the afternoon to increase by between 1 and 6°C, depending on the location and time of year. The warming in the model strengthens during the growing season, and decreases as the crops are harvested. The most important factor in causing this warming is that the crops have fewer leaves per unit area than the grasslands. With fewer leaves there is less transpiration of water, and less uptake of energy in latent heat; hence, the air can get warmer over the crops.

Settlers may have affected the climate across the Great Plains even before they had managed to plough up most of the land for crops. Up until the mid-1800s, the Plains supported vast herds of bison, numbering in the tens of millions. The mass slaughter of these animals during the early phases of settlement would have greatly reduced the grazing of prairie grasses. With more leaf area accumulating uneaten, there would have been more evaporation of water from the leaves. Climate models suggest that this could have cooled summer temperatures by 0.4-0.8°C, due to extra latent heat uptake by the evaporating water. This would then have been followed by the main phase when the farmers ploughed the landscape and planted crops, which reduced leaf area to below what it had been in the grazed prairie and caused a raising of temperature as explained above.

In the modern Great Plains, particularly towards the western edge, farmers irrigate their crops with water from underground aquifers. What does all this extra water on the fields do to the climate? Modeling studies suggest that the uptake of heat into evaporation from irrigated crops (compared with non-irrigated crops or prairie) will cool the air and create a sort of "sea breeze" blowing outwards to nearby hotter, non-irrigated areas. Measurements comparing irrigated and non-irrigated areas of northeastern Colorado show that, as the models predict, temperatures are several degrees C cooler where there is irrigation, due to latent heat uptake, altered wind patterns and cloudiness (Figure 5.7). As irrigation in the area has expanded over the

Figure 5.7. Temperature map for a warm day in northeastern Colorado. Irrigated areas such as suburbs and agricultural land (shaded) have cooler temperatures than non-irrigated areas. Surface temperature at 13: 00, 1 August to 15 August 1986. Contour from 38 to 28 by 2. After Bonan.

last 45 years, there has also been a cooling trend in climate, as would be expected. The models also predict an increase in rainfall over irrigated areas as a result of both the extra water evaporated, and the movement of air that results from the temperature contrasts between irrigated and non-irrigated land. Observations from northern Texas show that extensively irrigated areas have more rainfall than otherwise similar areas that do not get much irrigation.

On the other side of the world, parts of another arid region may have been affected by climate feedbacks that result from land use change. In southern Israel over the last 50 years, intensification of farming (including increased irrigation), reduced grazing and tree-planting has resulted in lower albedo and more evapotranspiration from vegetation. Since the early 1960s there has been a dramatic increase in autumn rainfall, by as much as 200-300% depending on the location. It seems plausible that the climate change has been a result of the progressive change in land use in this area. The increased upwards movement in the atmosphere above these lands seems to suck in moist air off the Mediterranean, which gives much of the rainfall.

The Sinai desert of Egypt has cooler daytime temperatures than the adjacent Negev desert of Israel, by 3.5-5°C in the early afternoon. It seems that the key factor that makes the Sinai cooler is its lack of vegetation, due to a lot more goat and sheep-grazing and cutting of firewood. With more high-albedo soil exposed, the Sinai reflects back more sunlight and cannot heat up as much. But, doesn't this contradict what I said at the beginning of the chapter—that dark vegetated areas tend to be cooler because they evaporate more moisture? In fact, it is the exception that proves the rule that, without evaporation, dark vegetated areas would always be hotter. Conditions in the Sinai and Negev are so dry that there is no soil moisture to evaporate much of the year. So, the dark vegetation cover in the Sinai (although it is fairly sparse) merely absorbs the sun's rays but does not suck heat away into transpiration.

In slightly moister—but still arid—areas such as the Sonoran Desert in the southwest USA and Mexico, adding a bit more vegetation makes things cooler not hotter. The heavily grazed Mexican side of the border is several degrees hotter during the day than the lightly grazed US side. This is because in this case there is enough moisture in the soil for the extra leaves on the US side to have a cooling effect by transpiring more water, and this dominates over the warming caused by the darker vegetated surface.

Box 5.3 Interactive vegetation schemes in climate modeling

To simulate vegetation-climate feedbacks, it is necessary to pass back and forth between a climate model and the vegetation cover. Initially, a particular climate and a vegetation distribution are set up together. The vegetation distribution can be whatever the modeler is interested to try out, and does not need to be anything that corresponds to the actual present-day vegetation, or anything that is in balance with the climate. The purpose of the exercise is to see how the two of them—vegetation and climate—get along together. The vegetation is allowed to modify the climate (using such feedbacks as albedo, roughness and evaporation), and the modified climate is allowed to modify the vegetation (using the sort of bioclimatic relationships mentioned in Chapter 2). The two are allowed to interact, until they eventually settle down into some sort of steady state. The state that is arrived at can then be compared with what happens with a different starting point for vegetation—for example, more desert or less desert. Or it can be compared with a world in which vegetation only responds passively to climate and does not feed back to change the climate. Making such comparisons allows us to find how important vegetation is in making climate.

In the early days of modeling vegetation-climate feedbacks, this back-and-forth interaction was worked out as many separate steps. The first run of the computer would give a particular climate, and a particular vegetation distribution would now be added in. Adding vegetation would modify the climate. Then, the simulation would be stopped, and the vegetation distribution would now be changed to something which corresponded to this altered climate. The simulation would be started again exactly where it left off except with a new modified vegetation, which now had a chance to modify the climate further. The process would be repeated again and again, until eventually vegetation and climate reached a balance with one another.

Now, this rather clumsy process of stopping and re-starting the model has been replaced by interactive models. The climate and vegetation respond to one another smoothly and continuously. The key to this is to have a vegetation scheme that in effect has the plants dynamically growing or dying off as the climate around them changes. One example of such a scheme is CLIMBER, which seems an appropriate acronym because the vegetation pulls itself along in its interaction with climate.

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