Forests cool the world by at least 0.5 C

 From The Guardian

The world’s forests play a far greater and more complex role in tackling climate crisis than previously thought, due to their physical effects on global and local temperatures, according to new research.

The role of forests as carbon sponges is well established. But comprehensive new data suggests that forests deliver climate benefits well beyond just storing carbon, helping to keep air near and far cool and moist due to the way they physically transform energy and water.

The study, which is the first to pinpoint the non-carbon dioxide benefits of different forests, found that the band of tropical rainforests spanning Latin America, central Africa and south-east Asia generate the most local and global benefits.

Researchers from the US and Colombia found that overall forests keep the planet at least half of a degree Celsius cooler when biophysical effects – from chemical compounds to turbulence and the reflection of light – are combined with carbon dioxide.

In the tropics – from Brazil and Guatemala to Chad, Cameroon and Indonesia – the cooling effect is more than one degree. In short, while all forests provide multiple benefits, some are more important than others in keeping the climate stable.

“Despite the mounting evidence that forests deliver myriad climate benefits, trees are still viewed just as sticks of carbon by many policymakers in the climate change arena,” said Louis Verchot, principal scientist at the International Center for Tropical Agriculture (CIAT) and co-author of the study The Unseen Effects of Deforestation: Biophysical Effects on Climate. “Forests are key to mitigation, but also adaptation.”

Deforestation has devastating impacts on biodiversity, food security, and global heating. A recent report by the Intergovernmental Panel on Climate Change (IPCC) warned about catastrophic consequences humanity faces with rising temperatures.

The findings, published in the journal Frontiers in Forests and Global Change, suggest that forests are important to mitigation and adaptation, cooling the air and protecting us from droughts, extreme heat and floods caused by the climate breakdown.

Forest cooling is due to a range of biophysical effects such as the physical aspect of the trees’ wood, leaves and density, as opposed to biochemical factors such as the carbon.

Researchers found that forests emit chemicals called biogenic volatile organic compounds (BVOCs) which create aerosols that reflect incoming energy and form clouds – both are cooling effects. While they also lead to a buildup of two greenhouse gases – ozone and methane – on balance, the cooling outweighs the warming.

Deep roots, efficient water use and so-called canopy roughness also enable forests to mitigate the impact of extreme heat.

These physical qualities allow trees to move heat and moisture away from the Earth’s surface where we live, which directly cools the local area and influences cloud formation and rainfall – which has ramifications far away.

In the tropics, where forest carbon storage and sequestration rates are highest, the biophysical effects of forests amplify the carbon benefits. In other words, tropical deforestation immediately increases extreme heat locally and decreases regional and local rainfall.

“The biophysical factors don’t cool the planet, but they do change the way we experience heat, and that matters,” said Deborah Lawrence, professor at the University of Virginia and the lead author. “The heart of the tropics is at the heart of the planet and these forests are critical for our survival.”

Better protection, expansion and improved management of the world’s forests are considered by many experts as among the most promising nature-based solutions.

Michael Coe, the tropics program director at the Woodwell Climate Research Center and a study co-author, said: “Without the forest cover we have now, the planet would be hotter and the weather more extreme. Forests provide us defense against the worst-case global warming scenarios.”

Forests, such as this one in Indonesia, do lmore than just store carbon. Photograph: Xinhua/Rex/Shutterstock

Rewilding two UK fields

 From ZME Science:

With no special equipment, no fences, and no watering, two abandoned agricultural fields in the UK have been rewilded, in large parts due to the efforts of jays, which virtually “engineered” these new woodlands. Researchers now hope that rewilding projects can take a more natural and hands-off approach — and that jays can shed some of their bad rep.

The two fields, which researchers have dubbed the New Wilderness and the Old Wilderness, had been abandoned in 1996 and 1961 respectively. The former was a barley field, while the latter was grassland — both were adjacent to ancient woodland. Researchers had suspected that the fields would gradually return to wilderness, but it was impressive to see just how quickly this happened, and how much of it was owed to birds.

Using aerial data, the researchers monitored the two sites. After just 24 years, the New Wilderness had grown into a young, healthy wood with 132 live trees per hectare, over half of which (57%) were oaks. Meanwhile, the Old Wilderness resembled a mature woodland after 39 years, with 390 trees per hectare.

“This native woodland restoration was approaching the structure (but not the species composition) of long-established woodlands within six decades,” the researchers explain in the study.

Part of this reforestation was done by wind, and researchers suspect that previous ground disturbance may have aided the woodland establishment — which is good news, as it would suggest that agricultural areas may be reforested faster than anticipated. However, animals — and in particular Eurasian jays, thrushes, wood mice, and squirrels — were also essential in helping the forests take shape. This handful of species provided much of the natural regeneration needed for the forest to develop. Jays, in particular, seem to have done a lot of heavy lifting.

Eurasian Jay (Garrulus glandarius). Image credits: Luc Viatour.

But just because land can rewild itself in time doesn't mean that a bit of help wouldn't make the process faster.  In my own experience, planting a copse on bare land where the topsoil had been removed,  spreading grass clippings on the subsoil transformed the soil, with grass (which stops the soil drying out and washing away) covering the bare earth in 2 seasons.

We will need to plant billions of trees to soak up CO2.  This story suggests that that nature will help us if she is given a chance.  Plus....hooray for jays.

Zero carbon by 2050

If we want to stop catastrophic climate change and global heating, we need to cut emissions of CO2 and methane to zero by 2050.  Let's split those 30 years up into decades, aiming to cut emissions by 1/3rd of the 2019 level each decade.

2020-2030

This will be the decade where we have to close down as many coal power stations as we can.  The good news is that in most countries, wind or solar or both are now cheaper than (new) coal.  In developed countries, most coal power stations are old, and will soon have to be retired.  When they are, they will be replaced by wind and solar.  Even with 10 hours of storage, wind and solar are the cheapest power source in the USA, except for existing coal power stations which have been fully depreciated and have had their debt paid off.  But of course, they are precisely the power stations which will need to be retired over the next decade.

Even in China, where coal is cheap, large-scale solar will this year reach grid parity, meaning it can compete with the wholesale price of electricity, which is determined by China's massive coal fleet.  China produces 35% or world CO2 emissions, and is the largest consumer of coal.   A change here will be very important for world emissions and the global climate.

So the target is that by 2030, the number of coal power stations still operating will be small.  They'll simply be too costly to keep going.  This is much faster then even the relatively optimistic BNEF forecasts (they forecast just 25% from renewables by 2030).  Nevertheless, the cost curves as well as the increasing global panic about catastrophic climate change suggest this will be likely.

During this decade, we should also try to switch heating from gas/oil to electric, and we will start the switch to electric transport.  Of which more below.   Electricity and heat production contributes 25% of global CO2 emissions, so we'll need to find more areas to cut emissions by 1/3rd by 2030.

2030-2040

This will be the decade where we electrify transport.  Battery costs are falling by 20% compound per annum.  This means that we should cross the $100/kWh battery pack cost line by 2023, which will mean that the "sticker price" of EVs will be comparable to ICEVs.  Already, in China and India (where it is very important that the growth in demand for personal transport isn't satisfied by petrol cars) small, cheap EVs are available.   Once again, the twin pincers of public anxiety about climate change and the plunging cost of EVs will rapidly squeeze fossil fuels out of the market. Assuming EVs reach 100% of new car sales by 2030, then by 2040, almost all the emissions from road transport will have stopped, assuming a 10 year vehicle life, which is lower than what it is now, but government will likely want to accelerate the transition by banning polluting cars and lorries from town centres as well as buying back aging fossil fuel clunkers.

In developed countries, these emissions are about 1/3rd of total emissions.  In developing countries, they make up a smaller proportion on average, though the percentages vary widely.  But demand for cars is growing fast in developing countries, so a transition to EVs will prevent big rises in emissions from this sector.

It will also be the decade when we make cement production and iron & steel carbon-neutral.  We have the technologies to do this now, but these processes are still more expensive than making them the old way.  Expect carbon taxes or regulations, to force a shift.

Battery technology may well have advanced far enough that we will be able to fly long distance without using jetfuel.  Or we will have shifted to carbon-friendly jetfuel.  Or we'll be flying long distance by SpaceX's suborbital shuttle, fuelled by green methane, and short distance by electric planes.  Once again, carbon taxes will help shift air travel towards zero-carbon alternatives. 

Emissions from transport and industry (iron & steel, cement, chemicals, mostly) make up another third of global emissions.  By 2040, these will have stopped.  They'll have to.  Together with what will have been done in the 2020s, total emissions will have fallen by roughly 2/3rds, a compound rate of decline of 5.5% per annum.

2015.  Source: EPA

2040-2050

By 2040, emissions from electricity generation, transport, and industry will have fallen dramatically.  But there will remain some emissions, by far the most important being agriculture, land-use, land-clearing, etc.  There's no particular reason to wait until 2040 to deal with these.  We could start transitioning now.  After all, we have alternatives to meat.  And perhaps by 2030 or so, most ppl will be terrified enough of climate change to change their personal lifestyles.  But change here will be hard.  With electricity generation, the future is already happening now.  Renewables are simply cheaper.   With EVs that will soon be the case.  But with meat, we're asking people to change life-long habits.  It'll have to be done, it's just that politicians will postpone action as long as they can get away with it.  Once again, a carbon tax would help the shift.   If you think that the outrage generated by trying to get our economy to switch to green electricity was over the top, wait till you tell people they must eat less meat.  Yet, I have hope.  Synthetic meats are taking off.  Vegetarianism and veganism are rising trends.   And if meat substitutes taste just like the real thing but don't inflict dreadful cruelty on animals and have a huge negative effect on the environment, then why not?

2020-2050

In each decade, the necessary year-on-year percentage decline will increase, even though as a percent of the starting point, the decadal declines will be roughly the same.   If we cut emissions 1/3rd by 2030, then we have to cut emissions by 1/2 from 2030 to 2040.  And from 2040 to 2050 by 100%.  These seem to be large percentages, but they will only look like that because of previous successes.

Many of the shifts will begin before the decade I've selected for each of them, though I expect my selected decade will be when they reach their culmination.  If the transitions are sped up, maybe we can reach near-zero emissions by 2040, if we move in all sectors.  And if we start massive re-afforestation we might achieve negative emissions, and will for the first time in the last 200 years see falling atmospheric concentrations of greenhouse gases.  We must surely hope so.

Brazil's forest destruction off the scale

From ZME Science:

Brazil’s National Institute for Space Research (INPE)'s report explains that between January and November of this year — which were the first 11 months in office for Jair Bolsonaro, a far-right leader who has eased restrictions on exploiting the Amazon — a total of 8,973.3 square kilometers (3464.6 sq mi) of the forest have been cut down.

That is almost double the total recorded over the first 11 months of 2018 (4,878.7 sq km).

The data was recorded by the DETER (Detecção de Desmatamento em Tempo Real), a satellite-based real-time deforestation detection system employed by INPE. The system uses data from the MODIS sensor aboard the Terra and Aqua NASA satellites. The system is mostly used as an indicator of the rate of deforestation but does not represent the whole area cut down, which is measured by the PRODES project.

According to PRODES readings — the system is more reliable but slower to compile data than DETER — between August 2018 and August 2019, the total deforested area in the Brazilian Amazon exceeded the 10,000 square kilometer threshold for the first time since 2008. It would represent a 43% increase over the preceding 12 month period (when the total was 7,033 sq km).

Areas of the Amazon that see indigenous habitation have experienced some of the fastest-rising rates of deforestation (74.5%) over the preceding period, INPE adds.

Ricardo Galvao, INPE’s former president, was sacked by the Bolsonaro government in early August under accusations of exaggerating the report on deforestation. On Friday, Galvao was named one of the 10 most important scientists of the year by the journal Nature.

How long before they stop even producing the statistics?

Carbon budget nearly used up

From Open Mind:

The “carbon budget” is an estimate of how much CO2 we can still emit, but still have a good chance to keep global warming from going over the 1.5°C limit into “dangerous” territory. The budget has recently been revised (upward, thank goodness) to about 420 GtCO2 (420 billion tons of carbon dioxide).

Staying within the 1.5°C limit doesn’t make us “safe” — there are still consequences of climate change, dangerous and costly, and we’re already paying the price despite not having hit 1.5°C yet. But going above 1.5°C takes us into what is best described as: nobody wants to go there.

If all of the “budget” amount of 420 GtCO2 went into the atmosphere, it would raise CO2 concentration by 50 ppm (parts per million). But when we emit CO2, only about half of it remains in the air. The other half is absorbed, mainly by the oceans and by plant life. So, our 420 GtCO2 budget translates to an increase of about 25 ppm in atmospheric concentration.

The air right now has about 410 ppm CO2. Adding another 25 will bring that number up to 435 ppm. In my opinion, that’s the number we should be looking at. Instead of a 420 GtCO2 emissions budget, we should be talking about a 435 ppm CO2 concentration limit.

That’s the kind of limit we can actually keep track of, with precision and accuracy.

[Read more here]

This chart shows atmospheric CO2, as an annual average, measured at Mauna Loa (Hawaii) and is called the Keeling Curve.

The second chart shows the Keeling Curve with the scale adjusted to show the carbon budget limit as ppm.

The final chart shows the projected Keeling Curve if we continue with BaU (business as usual), i.e., don't cut CO2 emissions.  And we cross the 1.5 degree carbon budget line in 2029, i.e., in 10 years.

It's obvious that we are not going to keep our emissions low enough to prevent 1.5 degrees C of warming.  Let's look at what's happening. 

First, the good news.  Because wind and solar are now so much cheaper than coal, the build out of new coal plants has fallen sharply, and old ones are being shuttered and being replaced by gas/renewables.  Emissions from electricity generation make up roughly 25% of total emissions.  Let's say it takes 20 years for all coal powered generation to stop.  That means (back of the envelope calculation) that this will reduce emissions by 1.25% per annum, ceteris paribus. 

Transportation is  roughly 15% (globally; it's 30% in the USA.)  This will fall only slowly (if at all) in early years because EVs will make up only a small proportion of the global vehicle fleet even when they reach 100% of new sales.    The average life of a car in the USA is about 12 years; globally, it's closer to 20.  When EVs are cheaper than ICEVs, the chances are that vehicle replacement will speed up, because EVs are already cheaper to run than ICEVs.  So let's be generous and assume an average life of twelve years.  That will mean that once EVs make up 100% of sales, emissions will fall by 8% per year, or 1.2% per annum of total emissions.  Put together, that means, ceteris paribus, that emissions will start to fall by 2.5% per annum.  However EVs won't reach 100% of vehicle sales until 2025 or later. And that glosses over the fact that we haven't yet got electric planes or electric freighters. 

There's still industry (20%): fossil fuels used for smelting ores, making steel and cement and chemicals, and so on.   Could we cut emissions in industry by 3 or 5% a year?  Yes, with a mixture of regulation and a carbon price.   There are low carbon replacements for cement; we can reduce iron ore to get iron using hydrogen (made with renewable electricity of course); we can make synthetic natural gas and gradually replace  natural gas with methane generated by the Sabatier process.  So let's assume that we can cut industry's emissions by 3% per year.  That adds another 0.6% a year off total emissions, making in round numbers a potential cut in emissions of 3% per year.

Agriculture (cow farts and burps; land clearing and forest burning; diesel for tractors, etc)  make up  25% of global emissions.   We could stop land clearing and forest burning now, if we had a firm conversation with Australia, Indonesia, Brazil, Borneo and a couple of others. And we could also plant new forests or replace destroyed forests.  This would actually soak up CO2 from the atmosphere, giving us negative emissions.

Now, 3% per year cut in emissions would mean a 65% cut over the next 20 years, and 70% over the next 40---assuming it began now, which it won't.  That would be quite respectable if we hadn't left things so late.  Remember, it's not enough for emissions to fall for the Keeling Curve to stop rising.  They have to fall to zero for the accumulated atmospheric CO2 to stop rising.   Global temperatures have risen about 1 degree C from the 1880s.  They are now rising by 0.2 degrees C per decade.  Even if CO2 emissions peak this year (thanks in part to a recession) they won't reach zero for 40 years. So global temperatures will go on rising for at least the next 40 years, and likely by at least 0.2 degrees C per decade.  That will take us up by another 0.8 degrees from here.  Unless we accelerate the de-carbonisation process. 

Can we do that? Of course we can, if we really wanted to.  We could accelerate the retirement of coal power stations, from 20 years, to, say, 15.  We could encourage the take-up of EVs and the retirement of old ICEVs with tax tweaks.  A carbon tax would help shift the market away from carbon-intensive processes to carbon-free ones, for example, from making iron and steel with coal to making it with hydrogen. If the funds raised via the carbon tax were distributed as a "carbon dividend" to the people, the opposition to such a tax would be reduced. 

If we stopped land clearing and burning and actually started planting forests instead, net agricultural emissions would plunge.  Would the public stand for a red meat tax?  No.  But there is vat-produced meat just starting to be produced.  As we become more and more worried abut heatwaves, droughts, floods and rising sea levels, the politics could shift very rapidly   

To get to zero emissions by 2050, we need to cut emissions by 14% per year, starting this year.  But even 10% per year would cut emissions by 97% by 2050, which is pretty close to zero.  A 7% per annum cut would reduce emissions by 90% by 2050.  Not perfect, but, hell, that's still pretty good.  It could be done.  But it'll take willpower, willpower to resist denialists and fossil fuel fightback and ignorance and stupidity.  I suspect we won't get real action until we get panic.  When people start dropping like flies from the heat in the US.  When Miami is flooded every day.  When the summer heat in China starts killing children.  When 'once in a hundred year' floods start happening every second year.  Then, we'll get global co-operation.  Then, something serious will be done.  And until then, progress will be slow. 

We won't limit increases to 1.5 degrees C.   We might just manage 2. 

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.

Natural climate solutions

Farmland near Leongatha, Victoria

The key to reducing greenhouse gas emissions is to switch all electricity generation to renewable sources, because, in principle, most activities can be electrified.  For example, we can heat our homes with electricity, not gas; we can electrify transport (and where we can't, we can create carbon-neutral fuels); we may even be able to electrify some industrial processes, such as making iron and steel.  But that will still leave agriculture and land clearing, which are responsible for 20% plus of emissions.   

The good news is that there are inexpensive changes we can make which could reduce net agricultural emissions to zero.

Conserving and restoring American forest, farm and natural lands could cut a substantial chunk of the country's emissions, helping meet greenhouse gas reduction goals without relying on undeveloped technologies, a new report finds.

A team of 38 researchers spent more than two years looking at "natural climate solutions"—a range of strategies that includes planting trees in cities, preventing the conversion of natural grassland to farmland and shifting to fertilizers that produce less greenhouse gas emissions.  

In a study published Wednesday in Science Advances, they report that these solutions, if deployed across agricultural lands, forests, grasslands and wetlands, could mitigate 21 percent of the country's net annual greenhouse gas emissions, getting the U.S. closer to meetings its goals under the Paris climate agreement.

The researchers found that reforestation had the single largest maximum potential to store carbon or take it up from the atmosphere—nearly 307 million metric tons. Most of the potential lies in forests in the Northeast and south-central regions of the country. "Natural forest management" strategies, which include things like extending harvest cycles or reduced-impact logging, could mitigate an additional 267 million metric tons. (The researchers calculated the overall net emissions of the U.S. as 5.8 billion metric tons, factoring in existing carbon sinks.)

The researchers looked at a number of solutions in agriculture, including avoiding the conversion of grassland to cropland, using cover crops planted in the off-season that add carbon to the soil, and using fertilizer more judiciously.  The solutions also included "biochar"—a form of charcoal made from a number of sources, including agricultural residue, that can be used to build healthier soil—and the practice of "alley cropping," or planting trees between crops.

Altogether these agricultural practices have the potential to mitigate nearly 440 million metric tons of carbon dioxide a year, the researchers found.

[Read more here]