Yale Study Shows How Conversion Of Forests to Cropland Affected Climate
![potato-crops-720-browser[1]](https://wattsupwiththat.files.wordpress.com/2014/09/potato-crops-720-browser1.jpg?quality=83)
Writing in the journal Nature Climate Change, Professor Nadine Unger of the Yale School of Forestry & Environmental Studies (F&ES) reports that large-scale forest losses during the last 150 years have reduced global emissions of biogenic volatile organic compounds (BVOCs), which control the atmospheric distribution of many short-lived climate pollutants, such as tropospheric ozone, methane, and aerosol particles.
Using sophisticated climate modeling, Unger calculated that a 30-percent decline in BVOC emissions between 1850 and 2000, largely through the conversion of forests to cropland, produced a net global cooling of about 0.1 degrees Celsius. During the same period, the global climate warmed by about 0.6 degrees Celsius, mostly due to increases in fossil fuel carbon dioxide emissions.
According to her findings, the climate impact of declining BVOC emissions is on the same magnitude as two other consequences of deforestation long known to affect global temperatures, although in opposing ways: carbon storage and the albedo effect. The lost carbon storage capacity caused by forest conversion has exacerbated global warming. Meanwhile, the disappearance of dark-colored forests has also helped offset temperature increases through the so-called albedo effect. (The albedo effect refers to the amount of radiation reflected by the surface of the planet. Light-colored fields, for instance, reflect more light and heat back into space than darker forests.)
Unger says the combined effects of reduced BVOC emissions and increased albedo may have entirely offset the warming caused by the loss of forest-based carbon storage capacity.
“Land cover changes caused by humans since the industrial and agricultural revolutions have removed natural forests and grasslands and replaced them with croplands,” said Unger, an assistant professor of atmospheric chemistry at F&ES. “And croplands are not strong emitters of these BVOCs — often they don’t emit any BVOCs.”
“Without doing an earth-system model simulation that includes these factors, we can’t really know the net effect on the global climate. Because changes in these emissions affect both warming and cooling pollutants,” she noted.
Unger said the findings do not suggest that increased forest loss provides climate change benefits, but rather underscore the complexity of climate change and the importance of better assessing which parts of the world would benefit from greater forest conservation.
Since the mid-19th century, the percentage of the planet covered by cropland has more than doubled, from 14 percent to 37 percent. Since forests are far greater contributors of BVOC emissions than crops and grasslands, this shift in land use has removed about 30 percent of Earth’s BVOC sources, Unger said.
Not all of these compounds affect atmospheric chemistry in the same way. Aerosols, for instance, contribute to global “cooling” since they generally reflect solar radiation back into space. Therefore, a 50 percent reduction in forest aerosols has actually spurred greater warming since the pre-industrial era.
However, reductions in the potent greenhouse gases methane and ozone — which contribute to global warming — have helped deliver a net cooling effect.
These emissions are often ignored in climate modeling because they are perceived as a “natural” part of the earth system, explained Unger. “So they don’t get as much attention as human-generated emissions, such as fossil fuel VOCs,” she said. “But if we change how much forest cover exists, then there is a human influence on these emissions.”
These impacts have also been ignored in previous climate modeling, she said, because scientists believed that BVOC emissions had barely changed between the pre-industrial era and today. But a study published last year by Unger showed that emissions of these volatile compounds have indeed decreased. Studies by European scientists have produced similar results.
The impact of changes to ozone and organic aerosols are particularly strong in temperate zones, she said, while methane impacts are more globally distributed.
The sensitivity of the global climate system to BVOC emissions suggests the importance of establishing a global-scale long-term monitoring program for BVOC emissions, Unger noted.
I termed this cooling effect as “rural-cold-island effect” similar to “urban-heat-island effect”. Here the important point is: met network is dense in urban areas but it is sparse in rural areas. Thus the urban-heat-island effect is over emphasized in the global temperature averaging and under emphasized the rural-cool island effect. In fact satellite measurements takes this in to account. Because of this the satellite data based temperature pattern is far lower than the ground based observations. There is a need to look in to this angle and derive the correct picture of global average temperature trend — rhythmic part is already there.
Dr. S. Jeevananda Reddy
Since the mid-19th century, the percentage of the planet covered by cropland has more than doubled, from 14 percent to 37 percent.
Say what? Do you mean 37% of land area? Even that sounds like way too much.
Since the mid-19th century, the percentage of the planet covered by cropland has more than doubled, from 14 percent to 37 percent.
This is complete nonsense!
https://www.cia.gov/library/publications/the-world-factbook/fields/2097.html
World arable land: 10.43%
permanent crops: 1.15%
other: 88.42% (2011)
This entry contains the percentage shares of total land area for three different types of land use: arable land – land cultivated for crops like wheat, maize, and rice that are replanted after each harvest; permanent crops – land cultivated for crops like citrus, coffee, and rubber that are not replanted after each harvest; includes land under flowering shrubs, fruit trees, nut trees, and vines, but excludes land under trees grown for wood or timber; other – any land not arable or under permanent crops; includes permanent meadows and pastures, forests and woodlands, built-on areas, roads, barren land, etc.
All around the world, crop yields have rapidly increased since 1980 according to World Bank’s crop yield data:
http://data.worldbank.org/indicator/AG.YLD.CREL.KG
USA’s crop yields alone have increased 70% since the early 80’s (from 3,984kg/hectar to 6,744/hectar– last 4-yr avg) and this is true for almost all countries around the globe.
Farmers are growing much more on less land; not exactly in agreement to the narrative portrayed by CAGW warmunists and to the assumptions of this paper.
Ironically, this incredible increase in crop yields is partially due to increased CO2 levels/CO2 fertilization and also to: cheap gas, cheap petrochemical fertilizers/insecticides and GMO advances; all of which are anathema to the CAGW warmunists/enviro-wackos.
I swear the warmunists are out to destroy mankind, not save it….
I would like to see figures comparing effects on atmospheric water vapour of farmland vs forest. For years now I have felt that this farmland-forest is probably by far the biggest anthropogenic input and have been disappointed that up to now have never read anything about it in the climate wars. The carbon business is a scientific sideshow albeit it gets nearly all the screaming headline ink.
PS. I am thinking about the forest-to-farmland change in the past few thousand years. I don’t think since the industrial revolution is the huge shift. In China, India and Europe the big changes were from around 1000 to 1500 AD. Europe used to be one huge forest, for example. Now there aren’t hardly any. Same in England. I suspect similar story in Africa albeit probably they deforested more like 1000 BC. Probably no way of calculating the effect climate-wise but still I wonder if there isn’t quite a significant one.
I wonder if the Black Death (with the resultant reafforestation) resulted in a warming of the climate?
Off course not, it caused the little ice age because since there were less people there must be less global warming, since we all know it is only man that can cause global warming!
Did not the skeptic science kidz write an article claiming that a) The Black Plague killed off a lot of people, and therefore b) a lot of farmland went back to forests, thereby C) soaking up CO2, and D) causing the LIA?
Isn’t this the exact opposite?
“The conversion of forests into cropland worldwide has triggered an atmospheric change that, while seldom considered in climate models,”
Wait a moment, doesn’t that mean that the iterative climate models accumulate a small error with each time step, always in the same direction… meaning that after a week or two of simulated time they’re completely off the tracks…
…and that the climate scientists would then introduce an arbitrary unphysical fudge factor counteracting that error; call it a parameter and NEVER TALK ABOUT IT…
…and that therefore climate modeling is advanced fudgeology…
Hey that sounds like easy money count me in!
Is it possible that Nadine Unger could be so short sighted as to not take into account that forest vegetation stays put but farm vegetation (crops) goes someplace ~else~; I.E. that it MOVES to another location?
Am I supposed to focus on the rotting leaves that fall to the forest floor but ignore the methane generated by cattle eating harvested feed stock? Are we supposed to now also ignore the CO2 emitted from burning bio-fuel to claim it is “saving the planet”?
Being that his paper is pay-walled I have no way to check … but if he is not accounting for the subsequent release of GHG’s from the harvested crops in all the places where that happens, including GHG’s gases emitted from sewerage treatment facilities which came from farms for example – then this “study” is a complete crock.
Surely albedo in the Arctic and Antarctic where the suns radiated heat reflects back off ice and snow leaving the air temperature below zero (cold) is different to a field full of crops where the radiated heat is absorbed first by the field,crops etc and the resulting warm air rises as thermals (hot) and gives glider pilots and soaring birds of prey the fun they have come to know and love.Do modelmakers ever go outside,or am I just being to simplistic here.
The great American white pine forests were chopped down en masse 100-150 years ago so it follows this must have introduced a step-cooling effect, rather than a “spread out” cooling effect from 1850 to 2000, right? I recall recent articles that pine forests emit lots and lots of volatile organic compounds. The while pine logging occurred on an industrial scale and removed multiple state-sized swaths of pine trees in a small amount of time, making the Tunguska event, which knocked down 830 square miles of Siberian forest in 1908, not even worth mentioning (sorry, I just did). This particular step-cooling effect happened right about when the warmists like to start their Great Century of Warming charts. Maybe the number adjusters should consider adjusting the long ago temperature charts upwards rather than the usual downwards?
“Meanwhile, the disappearance of dark-colored forests has also helped offset temperature increases through the so-called albedo effect.”
That’s a cart of equine fecal matter by the simple fact that the steady state for forests is COOLER than grasslands and farm fields. On Unger’s planet the darker looking suburbs with all the trees are always warmer than the city.
Forests are almost always COOLER! And cooler not just because of transpiration – because of photosynthesis, forests convert more UV into chemical energy.
http://www.forestthreats.org/products/publications/Comparison_of_cropland_and_forest_temperatures.pdf
(Riitters et al., 2002; Wickham et al., 2008b).
Based on our results, reforestation in the continental United States can add promotion of cooler surface temperatures to the numerous other ecological benefits that forests provide.
There is, just up the road from me, a place called Kershope Forest. It butts up against a much larger stand of trees going by the name of Kielder Forest. Sometimes I go there.
Even well before you can even see any trees and in a car, you can feel ‘something’, a chill in the air. Get closer still and you see what looks like and actually is, low fog. Still can’t see any trees at this point.
When you do see the trees, you do indeed see a low cloud hanging over the forest on what would be an otherwise bright day. Those tress have a MAJOR effect on their local environment.
Then work out what’s going on water-wise. Does a mature tree get through 100 gallons of water per day?
If so, lets use UK gallons= 454 litres. To evaporate that much water needs about 1,000,000,000 joules of energy, or, over the course of one day, a power of over 11KW.
Spread that over the area covered by the tree, say 100 metres square and the cooling power is????
Is someone remind me what the warming power, watts per square metre of our CO2 emissions are supposed to be…….
“Study: Conversion Of Forests to Cropland Cooled the Climate”.
Sounds plausible to me.
It may be warmer in a field than in a forest in the DAYTIME, but remember there is also nighttime. I see that overall, fields would be cooler than forests. Compare the Sahara Desert with Florida. Although the Sahara Desert gets much hotter in daytime, it gets much colder at night, so overall Florida is warmer than the Sahara Desert at a similar latitude. I suspect that the same effect works for forest versus field, but to a lesser extent.
Note that in addition to chopping down trees, there are also plenty of drainage ditches built to keep fields from becoming too soggy. That would also have an effect on the water cycle and water vapor in the air. Perhaps it was the settling of North America that helped bring about the little ice age.
Is’nt the Sahara cooler at night due to lack of cloud cover and amount of inland desert area.
Using sophisticated climate modeling ha!
“During the same period, the global climate warmed by about 0.6 degrees Celsius, mostly due to increases in fossil fuel carbon dioxide emissions.”
The state of climate “science” today: one insight that may advance the science is more than destroyed by coupling them with facts that remain to be proved especially in light of contrary theories such as that advanced by Murry Salby:
Better link: (but the audio is poor)
“The conversion of forests into cropland worldwide has triggered an atmospheric change that, while seldom considered in climate models, has had a net cooling effect on global temperatures, according to a new Yale study.”
This is said with absolute certainty, which is staggering because this ‘conclusion’ (net cooling) is based on assumptions and a crude application of the Albedo ‘effect’ without even checking that this is what happens. It is not what happens. It is more than likely that any warming since the end of the Mini Ice Age has been driven by land clearance, including draining and urbanisation, otherwise its natural.
This is an interesting twist that I knew nothing about, but to acknowledge the complexity of the system and then assert that this tidbit is the final piece to the puzzle is just plain stupid. Transpiration from grasslands can be almost an order of magnitude higher than woodlands, cycling water (the real greenhouse gas) much faster, thereby maintaining more water molecules in the atmosphere.
But all the above contradicts BOTH the “usual assumption” (by the CAGW religion) that the snows and glaciers on Mt Kilimanjaro were melting “because of global warming” AND it’s usual counter-argument (that the snow and ice was lost on Mt Kilimanjaro “because the forest down low were being rapidly cut down for farmland and as firewood” to feed the local peoples…..
One thing that needs consideration is winter. Albedo on cropland, especially covered w/snow, is high. Forests, especially w/evergreens, not so high. And conifers spreading into treeless, usually snow-covered sub-Arctic tundra would presumably warm the area alittle in winter/spring.
So it’s complicated & may be quite dependent on location, precip, latitude, forest-type, etc.
“…..Conversion Of Forests to Cropland Cooled the Climate……”
Really ??
Pray tell, what caused the ice ages???
Did some aliens from a far away solar system denude the earth’s forests which then resulted in an ice age. And these aliens did this several times !!! Wow.
More BS from the climate frauds. They cannot explain the historical climate changes; ice age to warming to ice ages to warming to……. you get the picture. Yet, they presume to isolate one factor that MAY affect climate (take your pick; forests , crops, CO2, cow farts, etc.), and claim this one factor will increase/decrease global temperatures, and these “scientists” have NO IDEA how the many myriad factors that can affect climate interact !!!
They can’t even predict the weather two months hence….oops, sorry, weather is not climate, unless of course the weather “affirms” the AGW thesis.
Has anyone noticed that the AGW folks are all anti-capitalists or socialists?
Well, Fukuoka’s method of bringing rain to desert areas was to plant trees near deep aquifers to bring up the water. Trees attracted rain somehow and he could change microclimates that way in previously dried-out areas. Shortly put: trees attract rain = create more cloud cover = cooling. Therefore, to my mind, it is quite possible that widespread deforestation has a general warming effect. Of course, there could be other effects which go into Earth’s complex system which obviate such cooling. The point is, where is the hard core analysis on this in climate science? You would think this topic would have been researched and argued about to death in the past two decades (if not earlier), but I don’t recall ever reading anything substantive about it. I always assumes the global warming thesis was reasonable until a few skeptics stimulated me to look into it a little and the more I looked the more amazed I was at just how haphazard the so-called ‘scientific method’ is, moreover how brazenly they come to sweeping conclusions based on tiny amounts of data points. It’s the same in so many science-related fields. At this point, I trust engineers to use physical sciences to come up with nifty technological advances, but the more theoretical approaches including evolution, climate etc.) I regard as little better the religulous masturbation. The debates go on forever because the scientific method is not designed to deal with such issues and shouldn’t be used that way. In any case, unless and until they include things like deforestion / reforestation in their models, they will remain worthless as they have thus far proven.
I wonder what the net effect would be, then, if we could somehow pull a massive stunt off such as completely “regreening” the Sahara Desert with grasslands (as it once was, as is my understanding) OR (less likely, but intriguing idea) turn it into something as lush as the Amazon rainforest. Of course there are many variables present here. The biggies that come to mind right away: On the one hand, a lot more carbon uptake than the current sand and rock, naturally, but no doubt counteracting any slight cooling effect due to removal of CO2 from the atmosphere would be the fact that the Sahara from space looks rather bright. So there’s the loss of reflectivity, or its albedo effect, right? Yes, either project would require massive infusions of fresh water and a pricey distribution system, and would be an engineering marvel for all time to behold. So also add to the mix massive redistribution of water from one point to another, even if such volumes could be easily found nearby. I’m sure the list goes on and on…
Still, it would be interesting to see what would happen. Imagine for example the possible food production factor alone. In any case, are we to assume then, based on the study above, that a massive grassland or cropland would cool the planet slightly more than an Amazonian type vegetative zone?
Does this count as another explanation of the hiatus?
The paper by Unger is primarily about the effects of deforestation on BVOC emissions. However in her introduction she mentions that “…the disappearance of dark-colored forests has also helped offset temperature increases through the so-called albedo effect.” In other words deforestation is thought to have a local cooling effect on account of the increased reflectance of visible light by seasonally bare earth as compared with perennially vegetated areas. However visible light is not the only part of the electromagnetic spectrum that has to be taken into account when assessing the effect of forest vegetation on climate. About 50% of near-infrared radiation between 0.7 and 1.3 μ (microns) will be reflected by leaves, the other 50% being transmitted. Without the leaves, 100% of that part of the spectrum is transmitted, and, if it strikes bare earth, will be absorbed, thus warming the earth’s surface (as compared with a forested area) and counteracting part of any albedo cooling effect from deforestation.
Ref: Knipling (1970): Physical and Physiological Basis for the Reflectance of Visible and Near-Infrared Radiation from Vegetation. Remote Sensing of Environment, 1, 155-159.
This paper is accessible at http://naldc.nal.usda.gov/download/37948/PDF
I get the impression that that the role of near-infrared radiation in surface climate has been neglected by climatologists in recent decades. Might it be inconvenient for supporters of the CO2-driven global warming paradigm to have to take into account other anthropogenic factors than CO2 emissions which may have contributed to surface warming in the 20th century?