July 27, 2013
High CO2 Reduces Tree Water Loss

Many climate scientists expect a hotter world will be drier in many regions. Tough on plant growth. However, higher atmospheric carbon dioxide reduces the need for trees to open pores to absorb carbon dioxide. This reduces water loss from leaves.

DURHAM, NH, July 10, 2013 - A study by scientists with the U.S. Forest Service, Harvard University and partners suggests that trees are responding to higher atmospheric carbon dioxide levels by becoming more efficient at using water.

Whether this will be a benefit to you personally probably depends on where you live. Trees that release less water into the atmosphere will reduce precipitation downwind.

How efficient trees are in using water has implications for ecosystem function, services and feedbacks to the climate system. These include enhanced timber yields and improved water availability, which could partially offset the effects of future droughts. However, reduced evapotranspiration, or the combination of evaporation and plant transpiration from the land to the atmosphere, resulting from higher water-use efficiency could lead to higher air temperatures, decreased humidity, and decreased recycling of continental precipitation. This could cause increased continental freshwater runoff, along with drought in parts of the world that rely on water transpired in other regions.

Some types of plants can grow more rapidly in high CO2, especially since the high CO2 reduces water loss. But if global warming causes massive droughts then at least in the drought areas the high CO2 won't help any. However, it has been argued that forests are expanding into the Israeli Negev desert as a result of higher CO2. So in some areas higher CO2 will increase biomass density.

If the world really heats up (and I have no crystal ball on climate) then keep in mind that some regions will be winners while others will be losers. Some areas will become too hot to live in during summer. Others will become too dry.

If the costs of energy fall then drought could largely be mitigated in industrialized countries. What we could do: build massive desalinization plants on the coasts and pump the water inland. Imagine thorium nuclear reactors on the Washington state and Oregon coasts pumping desal water inland over the Rockies to Montana and the Dakotas. Hotter and drier plains states could grow crops using desal water flowing into Montana. Part of the water would evaporate but come down as rain.

Since I do not expect humanity to do much intentionally to cut CO2 emissions we are going to find out what higher CO2 emissions will do to global climate

Share |      Randall Parker, 2013 July 27 08:57 PM 

Engineer-Poet said at July 28, 2013 6:41 AM:

Increased growth from CO2 is only possible until the system comes up against the next limiting nutrient, such as phosphorus or potash or nitrogen.

Shane said at July 28, 2013 11:25 AM:

We really don't need any more CO2 in order to get more warming and more plant growth.

From Science:

Investigators employed standard, robust multi-proxy techniques based upon examining sediments recovered from Lake El’gygytgyn in northeast Arctic Russia (100 km north of the Arctic circle in Chukotka, Russia) to determine local temperatures between 3.6 to 3.4 million years ago. Their findings revealed that during this middle Pliocene era period,

…summer temperatures were ~8 degrees C warmer than today, when the partial pressure [i.e., atmospheric concentration of] CO2 was ~ 400 parts per million [ppm]

But if plants do decide to grow more, the limiting factor will be water, rather than phosphorus or potassium.

Phillep Harding said at July 30, 2013 5:49 PM:

I've been eyeing that low area in southern Australia, along with Death Valley. A canal to the edge, then a power station to make use of the water flow into the depression for electricity, and water evaporating for precip down wind.

Ronald Brak said at July 31, 2013 7:53 AM:

Phillep, a canal or pipeline to make lake Kati-thanda Eyre permanent would certainly make things interesting, but the sea level drop is barely enough to make water flow into it and wouldn't be enough to generate power. Evapouration of seawater would leave about 5 centimeters of salt deposit each year which could block the channel. Occasional water flows out to sea in wet years might slow the salt deposition, but water flowing out to sea in Australia is weird. Usually it's more just a standoff between fresh and salt water. Two years ago Australia's largest river actually started flowing into the ocean and we all went down and had a look since it was such a unusual event. It wasn't very impressive.

dscott said at July 31, 2013 2:33 PM:

I'm sure the people of the Sahel and the Sahara are just cursing the rise of CO2 making the desert greener and allowing them to grow more food. Oh, wait, poor people don't care why they aren't starving as much as before... and neither do the plant eating animals. /snark/

Randall Parker said at August 1, 2013 9:15 PM:


Could a large amount of water be moved far enough inland to evaporate and then rain down before heading out to sea again? It is my impression that Australia is a very dry place.


Is desalination used in Australia? Is insufficient water the biggest obstacle to bigger crops?

Engineer-Poet said at August 2, 2013 10:04 AM:

The amount of water required to green up the dry center of Australia is staggering.  Even if humans only had to move that water a few miles inland to evaporation ponds, it would still be an enormous project.

Ronald Brak said at August 2, 2013 6:14 PM:

Similar to the United States, desalination in Australia is a boondogle designed so farmers do not have to pay market prices for water. The cost of desalinating a liter of water is perhaps 20-30 times what farmers currently pay for a liter of water for irrigation. While most farmers would be glad to sell their irrigation water for market prices they currently have no ability to do this. At best they can sell their water allotment at far below the cost of desalinated water. There would need to be massive and sustained increases in food prices to make using desalinated water for irrigation possible. Due to the high capital cost of current desalination technology this would still apply even if the energy used for desalination was somehow magically free.

Randall Parker said at August 3, 2013 10:19 AM:


My interest in desal: I'd like to know the costs of bringing in fresh water if a mega drought hits the US plains states. Desalination might not be the only option. Imagine bringing in fresh water from, say, Alaska rivers via a massive pipeline.

Engineer-Poet said at August 4, 2013 1:18 PM:

Think of the cost of pumping the Mississippi's flow uphill, after desalinating it.

Not gonna happen.

Randall Parker said at August 4, 2013 7:51 PM:

Bismarck North Dakota is at 1686 feet in elevation. Sioux City Iowa is at 1135 elevation. Lake Superior is 600 feet in elevation. Though if the climate heats up I would expect the evaporation rate from Lake Superior to increase.

But Colorado Springs is going to pump water 1600 feet uphill. But agriculture needs much lower water costs than residential.

Engineer-Poet said at August 4, 2013 8:56 PM:

The best way to keep the plains in water would be to prevent it from running down to New Orleans in the first place, no?

The average flow rate of the Mississippi at New Orleans is 600,000 cfps.  At ~28 kg per cubic foot, that's about 17,000 tonnes per second.  Suppose half of that comes from the east, so you only need to move the other half.  Raising 8500 tonnes per second an average of even 1000 feet requires a power of more than 25 GW, not including frictional losses.

Moving water for a single city is small potatoes by comparison.

Randall Parker said at August 6, 2013 8:38 PM:


If we have a long massive drought in the plains states will the Mississippi River even run?

25GW: Sounds like about 15 nukes. Affordable as compared to the costs of depopulating the plains states and giving up on the farm output.

What is not clear: how much of the water that rains on the plains comes down the Mississippi as compared to, say evaporating off of crop plants and coming down away from the Mississippi.

Maybe it'll make sense to cover part of the West in massive glass hot houses that can retain the water.

Engineer-Poet said at August 7, 2013 4:50 AM:

I'll let you calculate the power required to desalinate 8500 tons/sec of seawater.

I'm aware of one technology which might actually make this feasible:  the convection towers proposed in "Defeating the son of Andrew".  They not only yield fresh water, but they capture most of it at much more than 1000 feet MSL and could provide the energy for pumping it by gravity (though electric power transmission is probably cheaper than pipelines gauged for several thousand PSI).  But it's far easier to just nuclearize the economy and remove CO2 from the atmosphere to return rainfall patterns to the cooler, wetter ones of history.

Randall Parker said at August 7, 2013 8:02 PM:


We won't build nukes in time to prevent global warming. If we were going to do that we would already have done it.

Time to start with some numbers:

Desalination plants on average use about 15,000 kilowatt-hours of power for every million gallons of fresh water that’s produced, the Pacific Institute said today in a report.

So assuming 10 cents per kwh (close enough) I read that as $1500 per million gallons. Next I assume: 269 gallons per ton. But maybe you are using a different kind of ton? Assuming 269 is correct (and that you do not mean tonne of water): 269*8500=2286500 gallons per second. Is that right? 2.3 million gallons. So then 2286500/1000000=2.2865*$1500=$3429.75 per second. So now we have a price per second. Feel free to fix it.

So what about a cost per day? Times 60*60*24=$29.6 million per day. That's low. Have I done something wrong? Times 365 days = $108160586000 per year. That's only about $108 billion per year. Seems like chump change.

I figure at high volume we could achieve much lower costs of power. But that cost above is just for power. Everything else is the other half of But, again, in volume with large fleets of nukes costs would be much lower. Plus, even convert nuke power to electricity? Why not just boil water with the heat?

Randall Parker said at August 7, 2013 8:10 PM:

Here is another cost source for desal water:

The San Diego County Water Authority has agreed to buy at least 48,000 acre-feet of water from the plant each year for about $2,000 an acre-foot. An acre-foot equals about 326,000 gallons, roughly enough for two families of four for a year.

So 1 million divided by 326,000 gallons and then times $2000 = $6134 per million gallons, about 4 times the cost I quote above. I would guess in a much larger scale operation costs would be half what San Diego is going to pay. So say $200 billion. Granted, additional costs for moving the water over the Rockies. Though the thought occurs to me: Why not just irrigate deserts and turn them into massive farm areas?

Another idea: Just run pumps in the ocean near the Western shore of southern Alaska and British Colombia to increase the amount of moisture headed toward the great plains? Alberta and the Dakotas would get more rain.

Engineer-Poet said at August 8, 2013 4:20 PM:

I get $296 million per day (2.963304×10⁸, if that renders correctly) or $108 billion/year.  And that's only the energy cost of operation.

Just run pumps in the ocean near the Western shore of southern Alaska and British Colombia to increase the amount of moisture headed toward the great plains?

Hmmm.  A concept I saw years ago was for downdraft evaporation towers in California to cool the hot inland air using seawater.  This would also add humidity, which would be carried inland.  Rainfall could only increase.  However, unless the Midwest's rainfall comes from air which passes over California, that wouldn't help.

Randall Parker said at August 10, 2013 12:04 PM:

$100 billion: Another idea: Pump the water inland from Oregon to the Utah Great Salt Lake and let it evaporate from there. So the lake accumulate more salt. Not seeing that as a problem.

To be clear on the pumps: I mean spray ocean water into the air to cause more to travel inland.

Where the plains and midwest rain comes from: That's why I said BC and Alaska. There's got to be a correct place to do this.

Engineer-Poet said at August 11, 2013 10:54 AM:

During wet years, Utahns have had to set up evaporation ponds uphill of the Great Salt Lake to keep it from flooding SLC.  I don't think you're going to get any support for a scheme to use their real estate to increase rainfall elsewhere.

Ronald Brak said at August 14, 2013 8:14 PM:

When the interior of the US dries up, and it seems to be happening now:


I don't think there will be grand schemes to maintain crop production on currently arable land. I instead think that once rainfall in an area drops too low in an area farmland there will be abandonned or at least switched to dryland agriculture something like in the sheep/wheat belt of Australia. There are a couple of reasons why I think this. Firstly, there is a coordination problem. If people can't coordinate enough to prevent global warming drying out parts of the United States how will they coordinate enough to raise taxes and pay for grand schemes to allow agriculture to continue in drought stricken regions? And secondly it should be cheaper to increase food production closer to where the water is rather than to attempt to move water to where food is currently grown. The cost of pumping fresh water into the US's dead heart could instead pay for an awful lot of high intensity agriculture where water is available. I suppose the good news is that agricultural robots might make good use of hilly land that is currently not profitable for large scale mechanised farming. (Not sure how the Amish will feel about this, however.)

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