Does Deforestation Increase or Decrease Radiative Forcing?

[Clifford]

Over the last few centuries we've done massive changes to the surface of the planet.

In some places there has been deforestation, or otherwise removal of trees, and converting these areas to crops (grains, corn, wheat, pasture land, hay, vegetables, etc. Some irrigated, some not. This would include both hardwoods (leafy trees) and softwoods (evergreen trees).

Does this increase or decrease the Radiative Forcing (I.E. The heat being transferred from the sun into the environment)?

[A-L-E-X]

You can add the fact that trees remove CO-2 from the atmosphere so it looks like it's a domino effect.

[Clifford]

You can add the fact that trees remove CO-2 from the atmosphere so it looks like it's a domino effect.

Ahh... yes...

The CO₂ sink.

Of course, grass will also remove CO₂ from the air, but the CO₂ cycle is much shorter, and much of it will be released back into the air within a year or two.

If your trees never burn or rot, then you have a net negative influence on CO₂. And perhaps organic material will eventually be taken out of the carbon cycle.... I.E. Sink to where it it is no longer active.

If you build a house out of wood, it may sequester the carbon for a very long time.

Perhaps we should encourage the Government to keep more archival PAPER DOCUMENTS!!!!

However, maybe one should just consider a tree as delaying the carbon transfer. Grow a tree for a century. Chop it down, and burn to make heat & CO₂. Grow another tree

Many notes indicate that the Amazon is a net Carbon Sink, but it is not clear why it wouldn't be in a steady state.

[WeatherRusty]

Since deforestation is assigned a net warming value on the environment it is tempting to say that radiative forcing is increased. Approximately 20% of recent warming is attributed to deforestation. However, a forested area has a considerably lower albedo than grass land. This means that more direct sunlight is reflected back to space rather than being absorbed by the surface over a deforested area. This would impart a cooling due to radiative forcing alone. So I will say deforestation produces a negative radiative forcing.

Why then the net warming? Forested areas produce a tremendous amount of evapotranspiration which cools the environment considerably. Denuded land can not evaporate water to the atmosphere anywhere near what a forested area can. This process dominates over the radiative properties of the land. Less evaporational cooling results in a warmer surface.

So less radiative forcing is my answer.

[OHSnow]

Since deforestation is assigned a net warming value on the environment it is tempting to say that radiative forcing is increased. Approximately 20% of recent warming is attributed to deforestation. However, a forested area has a considerably lower albedo than grass land. This means that more direct sunlight is reflected back to space rather than being absorbed by the surface over a deforested area. This would impart a cooling due to radiative forcing alone. So I will say deforestation produces a negative radiative forcing.

Why then the net warming? Forested areas produce a tremendous amount of evapotranspiration which cools the environment considerably. Denuded land can not evaporate water to the atmosphere anywhere near what a forested area can. This process dominates over the radiative properties of the land. Less evaporational cooling results in a warmer surface.

So less radiative forcing is my answer.

This is correct. Deforestation increases surface albedo, resulting in a negative radiative forcing. The reduction in latent cooling from evapotranspiration produces intense local warming in tropical areas that have been deforested.

[BethesdaWX]

Ahh... yes...

The CO₂ sink.

Of course, grass will also remove CO₂ from the air, but the CO₂ cycle is much shorter, and much of it will be released back into the air within a year or two.

If your trees never burn or rot, then you have a net negative influence on CO₂. And perhaps organic material will eventually be taken out of the carbon cycle.... I.E. Sink to where it it is no longer active.

If you build a house out of wood, it may sequester the carbon for a very long time.

Perhaps we should encourage the Government to keep more archival PAPER DOCUMENTS!!!!

However, maybe one should just consider a tree as delaying the carbon transfer. Grow a tree for a century. Chop it down, and burn to make heat & CO₂. Grow another tree

Many notes indicate that the Amazon is a net Carbon Sink, but it is not clear why it wouldn't be in a steady state.

ehhhh, I don't think it works that way...trees take in a huge amount of Co2 per year, but the tree doesn't store the Co2 it consumes...it uses it in the photosynthesis process. In 100 years, can you guess how much Co2 one tree takes in?

If the trees store Co2 and don't use it, why would they take it out of the atmosphere?

Co2 may release from decaying wood...but Co2 is released from all biotic decaying matter.

[Clifford]

ehhhh, I don't think it works that way...trees take in a huge amount of Co2 per year, but the tree doesn't store the Co2 it consumes...it uses it in the photosynthesis process. In 100 years, can you guess how much Co2 one tree takes in?

If the trees store Co2 and don't use it, why would they take it out of the atmosphere?

Co2 may release from decaying wood...but Co2 is released from all biotic decaying matter.

CO₂ is "stored"...

Well, at least the Carbon is stored, but not in the form of Carbon Dioxide, but rather in hydrocarbons (wood, cellulose, etc). The Oxygen is released.

If the tree is burnt, it releases energy equivalent to the chemical energy in the hydrocarbons (difference between the hydrocarbon, and the "ground state" as an oxide). And, of course, in burning the CO₂ is reformed and released again. Metabolism is similar, creating heat and CO₂, but at a slower rate.

[skierinvermont]

ehhhh, I don't think it works that way...trees take in a huge amount of Co2 per year, but the tree doesn't store the Co2 it consumes...it uses it in the photosynthesis process. In 100 years, can you guess how much Co2 one tree takes in?

If the trees store Co2 and don't use it, why would they take it out of the atmosphere?

Co2 may release from decaying wood...but Co2 is released from all biotic decaying matter.

Trees do store a substantial amount of CO2.. they are made of carbon and as they grow they store more of it. They also use and burn a large amount of carbon in cellular respiration. But on net, a growing tree is a net sink of carbon. Only once it is burnt or decomposes does the tree return all of the CO2 to the atmosphere.

This is why young forests which are growing rapidly are net carbon sinks, while old forests are carbon neutral because the amount of growth is equal to the amount of decomposition.

However, many old forests are becoming carbon sources because as they warm the decomposition process is rapidly accelerated, while the growth process is not.

However, maybe one should just consider a tree as delaying the carbon transfer. Grow a tree for a century. Chop it down, and burn to make heat & CO₂. Grow another tree

This is exactly the idea behind biomass burning. The trees remove the CO2 from the atmosphere, and when you burn them it is net neutral. However, it is done with much faster growing species like willow. This is unlike fossil fuels which take carbon that has been stored for millions of years and put it in the atmosphere.

[Clifford]

6 votes?

2 people followed the IPCC reasoning

4 people didn't.

The IPCC equates albedo (reflectivity) to Radiative Forcing.

Chapter 2, Section 2.5, Page 180-184

http://www.ipcc.ch/p...g1-chapter2.pdf

Trees are typically absorb more sunlight, and reflect less than other crops, at least in theory.

The argument breaks down when looking at deserts (which is recognized by the IPCC).

Myhre&Myhre(2003)

http://folk.uio.no/g...hre_jclim03.pdf

In arid regions it is very uncertain how a

conversion to pasture influences the surface albedo, but

likely the changes are modest. In our simulations the

difference in surface albedo values between pasture and

barren is substantial. This results in a large decrease in

the surface albedo in the arid regions with conversion

to pasture, as a result of an unrealistically low surface

albedo of pasture in these areas. Satellite observations

indicate that pasture in such regions have not decreased

the surface albedo as shown in Figs. 3e,f.

While not forests, this indicates some of the complex relationships between reflectance and energy.

Wikipedia has a good image of Albedo.

http://en.wikipedia.org/wiki/Albedo

When you look at the "Clear Sky Albedo", the Amazon and the Congo show up as Dark Blotches.

However, they do not show up as dark blotches under the Total Sky Albedo.

The Sahara shows up as one of the lightest spots under clear sky albedo, but comparatively less so under total sky albedo.

So, what is the difference between the Sahara and the Amazon?

In the Sahara, light is either reflected, or absorbed as the equivalent of heat, and thus albedo equates to radiative forcing.

In the Amazon, some of the light gets absorbed as heat. But, a large portion is converted into chemical energy.

In a steady-state, the chemical energy eventually gets released, or converted to other forms of energy. However, it is a complex chain of events with some of the energy being stored long term as chemical energy (conversion of CO₂ into hydrocarbons). Some of it pulls water and moisture from sub-soil to the leaves where it evaporates, eventually forming rain and clouds, and a negative feedback for the albedo.

It also creates an energy elevator... where the vaporization of water at the surface cools at the surface. Then, as the vapor rises and recondenses in the upper atmosphere, it releases the energy in the upper troposphere, with the water falling back to earth as relatively cool droplets.

Trees actually have complex feedback mechanisms to lower their temperatures in hot weather, and to raise their temperatures, or otherwise prevent deep freezing in cold weather.

The IPCC also compares the albedo of trees to snow, and conclude that trees penetrating the snow decreases the albedo of the area, and thus leads to warming. They fail to consider that the snow is on the ground during the times with the least sunlight, and snow may have as much of an insulating effect as reflective effect in the winter. So trees may actually release energy in the winter, perhaps acting as a summer --> winter energy buffer.

The question here was about croplands.

I know that walking under a dense forest canopy can be much cooler than walking across a meadow, or a wheat field. However, it is hard for me to quantify the heat above the forest canopy.

Modern irrigation may create some of the same evaporative cycles that are naturally maintained by the forest.

The IPCC attributes a negative Radiative Forcing to conversion to cropland, primarily based on changes to clear sky albedo (reflectivity). The overall energy balance is probably closer to zero when changing to irrigated cropland (or cropland with adequate summer rain). It may be a slight positive when converting to non-irrigated rangeland.

The other issue is the bolus of CO₂ released at the time of conversion from forestland to cropland.

The IPCC has quantified this as the equivalent to a 12-35ppm increase of CO2, resulting in a calculated +0.2 to +0.57 w/m² RF.

Overall, they conclude a net negative RF (cooling) of about -0.25 w/m² for the change, although it is unclear if that includes the contribution of the CO₂ bolus.

[skierinvermont]

I can't answer. It has negative radiative forcing but it causes surface warming.

[Clifford]

I can't answer. It has negative radiative forcing but it causes surface warming.

If you're saying negative forcing with cutting the trees down.

Positive forcing from growing trees.

Then you have to find the heat somewhere.

Dig down 6-10 feet or so and you can get an average crust temperature which can be compared between the rain forest and the wheat field.

Or... you can compare heat above the canopy.

Or... averaged IR & Light emissions to space?

If a forest can be carbon negative, it can store solar energy.

However, it is possible it could move energy to higher levels of the troposphere, or perhaps buffer the annual intake/release of energy differently than an inorganic system might.

[Clifford]

I bumped into this page on the Ryan Maue web page:

http://www.coaps.fsu...nt/lh_flux.html

Latent Heat Flux.

As I understand it, it is the cycle of evaporation at ground level, and transport of heat up to the upper troposphere where the water condenses and releases the heat.

Looking at the Amazon, and the Congo (daytime), according to this about half of the sun's energy is going into this water respiration cycle.

That just seems like too much energy being transported upwards using the water cycle.