Terraforming Earth

The Tadart Acacus desert in western Libya--part of the Sahara desert.
Source: Wikipedia

There's a lot of talk about terraforming Mars--raising its temperatures, thickening its atmosphere, reducing atmospheric CO2 from nearly 100% to a more breathable Earth-like 0.4%, and increasing oxygen to 20%.  There are formidable technological and economic hurdles in the way, starting with the fact that we haven't yet even landed a human on the surface of Mars.  But what about terraforming Earth?  For example, could we reverse desertification on the fringes of existing deserts?  Could we even make the deserts green, somehow?

This piece suggests one way--if we covered the Sahara with wind and solar farms, it could change the climate and increase rainfall.

It's the largest hot desert in the world: the Sahara, a blistering landscape of sand, heat, and deadly dryness that swallows 10 nations and is growing bigger all the time.

Because of its searing, sunny conditions, numerous energy projects are already seeking to capitalise on the immense solar potential of the Sahara.

But new research shows an amazing, unprecedented effect of these efforts: solar and wind farms could actually bring rainfall and greenery back to the desert.

"We found that the large-scale installation of solar and wind farms can bring more rainfall and promote vegetation growth in these regions," says one of the researchers, atmospheric scientist Eugenia Kalnay from the University of Maryland.

"The rainfall increase is a consequence of complex land-atmosphere interactions that occur because solar panels and wind turbines create rougher and darker land surfaces."

"Our model results show that large-scale solar and wind farms in the Sahara would more than double the precipitation in the Sahara, and the most substantial increase occurs in the Sahel, where the magnitude of rainfall increase is between ~200 and ~500 mm per year," says first author of the study Yan Li, who began the research at Maryland and is now at the University of Illinois at Urbana-Champaign.

"As a result, vegetation cover fraction increases by about 20 percent."

These effects arise for a couple of reasons. Firstly, wind turbines enhance vertical mixing of heat in the atmosphere, pushing higher, warmer air down to the surface and increasing land surface friction, and ultimately leading to greater likelihood of precipitation.

"This increase in precipitation, in turn, leads to an increase in vegetation cover, creating a positive feedback loop," Li explains.

At the same time, solar panels, which soak up the Sun's rays, reduce what's called surface albedo – the amount of light reflectance at the surface – which also ends up increasing precipitation.

It wouldn't be easy to build this kind of hypothetical infrastructure, of course – we're talking a solar farm roughly the size of China or the United States, punctuated by giant turbines covering about 20 percent of the Sahara.

But if we could pull such an epic feat off, we wouldn't just be kickstarting a gradual greening of the Sahara desert – we'd also completely kick our addiction to fossil fuels, with the complex delivering about 82 terawatts of electrical power annually, the team calculates.

"In 2017, the global energy demand was only 18 terawatts, so this is obviously much more energy than is currently needed worldwide," Li says.

[Read more here]

We're prolly not going to cover the Sahara with wind and solar farms just yet, if only because doing so would provide four times as much electricity as the whole world needs, and anyway, we are still reluctant to build wind and solar farms even when they are profitable.  But it seems very likely that we will need to have a period of negative emissions after 2050, where carbon dioxide is removed from the atmosphere.  And an easy way to do that is to plant a forest. 

It's tempting to think that the Sahara has been ever thus.  But it's not true.  There was a time when the Sahara was wet, and covered with savanna, forests and "huge Saharan lakes".   And although many scientists believe that this was caused by changes in the Earth's axis, some maintain that it was worsened by desertification as a result of the spread of pastoralism.

The Sahara today forms one of the largest and driest expanses of land on Earth. Yet between 5,000 and 10,000 years ago, a period of time commonly referred to as the ‘African Humid Period’, both the climate and ecosystem of the Sahara were dramatically different. Instead of an arid desert landscape, the Sahara was characterized by lush and diverse vegetation, a consequence of monsoons and increased rainfall over the northern Africa landmass.

The ending of this unusually wet period in the Sahara’s history, and transition to modern-day conditions, has long been a target for scientists trying to understand climate and ecological tipping points; until now, most studies have pointed to changes in the Earth’s orbit or natural changes in vegetation as the major driving forces. A new paper in Frontiers in Earth Science by archaeologist Dr. David Wright, from Seoul National University, South Korea, challenges this view and suggests that humans may have also played an active role in driving climate change in this period.

“During the African Humid Period the Sahara had a completely different vegetation regime” explains Wright. “All of the plants that are found in the Sahara today were there, but you also had plants that are found in the Sahel, the semi-arid zone to the south of the Sahara, and even types of plants that are found in the Congo rainforest”. This so-called ‘Green Sahara’ was also capable of supporting large animals – rock paintings made in northern Africa dated to this time period depict crocodiles, elephants and giraffes, animals that could not be sustained in the Sahara today.

The wet conditions also had an important influence on human sustainability and cultural development, allowing humans to thrive in foraging and fishing communities. “Unlike a lot of other places, people in the Sahara became very sedentary, there was really no need for agriculture”, says Wright. “One of the dietary staples of people living in that period was Nile perch, an enormous 150 kg fish, and this was only possible due to the huge Saharan lakes which could support abundant fish and fishing populations”.

But such favorable conditions didn’t last. Although the exact timing and spatial distribution is still under debate, there is consistent agreement in geological and archaeological records that beginning approximately 8,200 years ago, the Sahara began a trend towards more and more arid conditions. Over the course of the next 3500 years, the landscape of northern Africa shifted from a diverse, wet ecosystem to conditions similar to those found today.

The underlying causes of this drying and desertification has previously been attributed to subtle changes in the Earth’s orbit, which in turn influenced atmospheric weather patterns and led to a reduction of the amount of rainfall in northern Africa. But Wright, whose scientific research has led him to exploring Neolithic-age archaeological sites all over the world, suggests that this is not the full picture. “In East Asia there are long established theories of how Neolithic populations changed the landscape so profoundly that monsoons stopped penetrating so far inland”, explains Wright, also noting in his paper that evidence of human-driven ecological and climatic change has been documented in Europe, North America and New Zealand. Wright believed that similar scenarios could also apply to the Sahara.

[Read more here]

If you think that this sounds far-fetched, consider that forest clearing in the Amazon is reducing rainfall, exactly as Dr Wright  postulates happened 8000 years ago in the Amazon. I read somewhere once that a tropical rain forest actually evaporates/transpires more water into the air than the same area of tropical sea, but I couldn't find the reference.  However,  a large oak tree transpires 400 litres (110 gallons) of water every day, when it is in leaf.  This is similar to the rate at which warm seas evaporate water into the air.  So it seems plausible.

Now, deserts from in the mid latitudes are caused by Hadley cells.  Essentially, what happens is that hot air at the equator rises, flows polewards until it reaches roughly latitude 30, and then descends, heating and drying as it descends.  This produces the bands of deserts around the world between 20 and 30 degrees latitude.  However, the humidity and consequent atmospheric instability on the east coast of continents stops this effect.  Compare the climate of Savannah, Georgia (latitude 32 degrees N) on the east coast and the climate of LA (latitude 34 degrees N) on the west coast. This difference is caused by ocean currents: on west coasts the ocean currents flow from cool to warmer regions, on east coast from warm to cooler, producing atmospheric instability and humidity, which leads to rain, not just over the immediate coast but deeper into the adjacent landmass. 

The atmospheric instability over a rain forest is likely to be as great as over a warm ocean.  Thus if we could establish a rain forest over the Sahara artificially, it might conceivably be self-sustaining. 

Most of the water released by evapotranspiration to the atmosphere as water vapor will be returned to the forest as rain, so rainforests provide their own rainfall. Although forests account for only about 15%-20% of global water evaporation, approximately 65% of the rainfall over land is due to them. Lowered levels of atmospheric water vapor reduce cloud cover and rainfall, so if forest is removed, rainfall in that region will be substantially reduced. 

[Read more here]

The only area where this won't work is the western Sahara where cool ocean currents produce a surface air temperature inversion which makes rising thermals and therefore rain impossible, except for the trailing edge of sub-polar frontal cyclones.

We may need to afforest the Sahara (and Arabian) deserts to remove carbon from the atmosphere as well as reducing the world's temperatures (rainforests are much cooler than deserts), and we will be able to do it using cheap electricity from solar and wind to produce desalinated water and cheap pumping.  To me, this seems at least as desirable as terraforming Mars, and far cheaper.  And we may have to take desperate measures over the next few decades to prevent runaway global warming.