Could The Sahara Be Made Green

Some models that involve both vegetation and climate have suggested the hidden potential for far more extreme changes in the climate of the Sahara than we have witnessed over the past century. These models have concentrated so far on just the western half of the Sahara desert. They tend to find that if you were to blanket the whole of the western Sahara desert in a leafy cover of grass or bushes, the climate of the region would be transformed. The low albedo, the greater roughness, the capture of rainfall and its evaporation from leaves would result in monsoon rains that normally stay to the south of the desert coming farther north. According to these models, the rain made by all this vegetation would actually be enough to sustain the vegetation cover itself and the Sahara (at least, the western Sahara, and perhaps the whole Sahara) would vanish! It would be replaced by the sort of open cover of small bushes and scattered patches of grass that we see just along the northern and southern edges of the Sahara at present, something more like "semi-desert" than the desert of the present. The higher rainfall zones to the south would also move farther north, bringing much moister climates to areas that are nowadays scrub and semi-desert. According to these models, then, the imaginary "green Sahara'' and the present-day "brown Sahara'' are both equally probable, equally stable in the present-day world and it is just by chance that we have one rather than the other. Some people have suggested that, given this possibility, we should set out to create a greener and more useful Sahara region for ourselves by progressively planting trees and other vegetation inwards from the edges of the desert.

However, climate modeling is a complex business and different groups' models often come up with different conclusions from one another. Some models (e.g., one put together by Hans Renssen and colleagues) set up in slightly different ways suggest that the "green" Sahara is not actually a possibility in the present-day global climate system: that even if we blanketed the whole desert in vegetation the feedbacks it set up would not manage to bring in the rains needed to keep the vegetation going. Given the uncertainties, any large-scale exercise in climate engineering that sets out to transform the Sahara through planting vegetation would risk becoming an expensive failure.

Box 5.2 Simulating climate: GCMs and mesoscale models

What changes can we expect for the Earth's climate over the coming decades, as greenhouse gases increase? Because of the importance of knowing the answer to this question, a lot of effort is going in to understanding and forecasting climate change. The world's most powerful computers (known as supercomputers) are used to calculate the effects of a given rise in greenhouse gases on global climate, using a "model": a simplified world inside the computer, complete with oceans, continents, mountain ranges and an atmosphere.

Such models are also being used to investigate how vegetation creates its own climate, and what will happen to global climate if the vegetation cover is altered. As well as looking into the future, models can be made to look backwards in time, to understand how climates in the past worked, including, for example, the effects of past vegetation changes feeding back on climate.

To get a broad global perspective, climate scientists try to simulate the circulation system of the whole planet, with what is known as a general circulation model (or GCM) (Figure 5.6). To model the entire global climate system is of course no easy task, and one which has taken a long time to get more or less right. Basically, the world in the computer is divided up into a grid covering its surface, and each grid cell is labeled as "ocean" or "land". If it is land, that surface grid cell is assigned an altitude, and also some attributes that relate to vegetation cover such as albedo and roughness. Up above the surface of each grid square, the atmosphere is represented as a stack of cubes. Each cube has its own composition and density of gases, and it exchanges energy with the cubes next to, above and below it, or (if it is at the bottom of the atmosphere) with the surface below it. Air is also exchanged sideways, and upwards and downwards from each grid cell, simulating the wind and also the process of convection. In the newest models, the ocean is also divided into stacks of cubes, much like the atmosphere except that these are under the surface and the fluid that fills them is not air but water. Heat and water move between these ocean boxes, simulating surface currents plus the sinking or

Simulates circulation and temperature distribution of the whole planet

Amesoscale model...

Slots into the broader climate picture of the GCM, to simulate in detail climate of one particular region

Figure 5.6. How {top) a GCM works and (bottom) how a mesoscale model slots into it.

upwelling of water. Winds and ocean currents push against one another, churning endlessly across the surface of the planet.

It is remarkable how many details of the climate system these GCMs can simulate. When a GCM is set up to run with the present-day atmospheric composition, the major wind belts and ocean surface currents can all be simulated quite accurately. Air masses form and move across the surface, colliding to give weather fronts. The patterns of average temperature and rainfall are closely similar to what we observe in the present world, and they go through their correct seasonal cycles. Furthermore, from year to year the global climate also goes through internally generated climate fluctuations that mimic those on the real earth.

When climate modelers are satisfied that their model works well for the present world, they can begin to tweak certain aspects of it to see how these will change the climate. For instance, they can add more greenhouse gases and observe the heat balance, rainfall and circulation systems changing in response. They can also change the vegetation cover and see how climate responds to this alteration in albedo, roughness and evapotranspiration. Some of the broader scale studies of vegetation-climate feedbacks use this sort of approach to reach their conclusions about the importance of vegetation cover in making climate.

Climate modeling has come a long way in the past couple of decades, as the quantity of data that computers are able to handle per unit time has increased enormously. But, there is always still room for improvement in models. A major problem in simulating the climate is still the coarseness of models—it is not possible to include every small bump or valley in the landscape, and yet such little microclimatic differences might add up to significant broad-scale effects. Many processes—such as the formation of thunderclouds—occur at a scale smaller than a single grid cell so they must be assumed to occur rather than simulated directly. Decreasing the size of the grid cells in the model increases its accuracy, but doing so magnifies the computing task enormously. Modelers have to strike a balance between the time taken to run a model, and the accuracy that it can produce. As computing power has increased, the size of the grid cells in models has decreased from 5 x 5° in the 1980s, to 0.5 x 0.5° in the latest models. At the time of writing, the world's most powerful computer system—located in Japan—was built especially for the task of running the most sophisticated climate models.

One way to get more fine-scale accuracy, without running up against enormous computing problems, is to focus on simulating one area of interest in detail and leaving the rest of the world outside it at a lower resolution. For this purpose, climate modelers use a special add-on model that works at a regional scale, known as a mesoscale model. A mesoscale model works in many ways like a GCM except that its grid squares and boxes are smaller and cover a more restricted area—the region simulated by such a model will be at most a few hundred or a thousand kilometers across. A mesoscale model partly creates its own climate from the sunlight that falls on it, but it slots into a broader GCM which supplies heat and water vapor in at the edges, and takes these away from the edges too when the wind blows out from the area. A mesoscale model is ideal for exploring how detailed changes in vegetation will affect a regional climate. Many of the vegetation-climate feedback studies mentioned in this book were carried out with mesoscale models coupled to broader GCMs.

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Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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