Extension 17.7: Planting Trees

Planting trees 

How much carbon, where? 

The carbon that is stored in the atmospheric system is huge. But it is not totally clear 

how much carbon is in any of the reservoirs. Planting trees in forests seems the most 

promising way to reduce the atmospheric carbon dioxide, but the uncertainties are great. 

Table E17.7.1 shows the major carbon reservoirs on Earth. 

TABLE E17.7.1 

Carbon Reservoirs (amounts in teratonnes) 

Amount of carbon Amount of carbon 

Reservoir (Reference 686) (Reference 720) 

Atmosphere 0.720 0.750 

Oceans 

Lithosphere 

Inorganic, total 38.400 39.973 

Surface layer 37.400 1.020 

Deep layer 0.670 38.100 

Organic, total 1.000 0.853 

Sedimentary carbonates 60,000 

Kerogens 15,000 

Terrestrial biosphere, total 2.000 2.190 

Living biomass 0.6-1.0 0.610 

Dead biomass 1.200 1.580 

Aquatic biosphere 0.001-0.002 

Fossil fuels 4.130 

Coal 3.510 

Oil 0.230 

Gas 0.140 

Other (peat) 0.250

Energy, Ch. 17, extension 7 Planting trees 2 

Fig. E17.7.1 The carbon cycle. 

(Adapted from Department of Energy data and Refs. 202, 203) 

Another, pictorial, view of the carbon cycle is shown in Fig. E17.7.1. (202,203) The net 

exchange with the oceans is 2.2 Gt and with plants on land is 1.7 Gt. While gross 

exchanges are much larger than human emissions, human emissions are greater than net 

exchanges. 

It is estimated by some that forests contain 1 teratonne of carbon (a bit higher than the 

estimates above). (41) The yearly fluxes are large, too. Carbon exchange between land and 

atmosphere is 120 Pg (120 billion tonnes, or 120 Gt), going both directions; exchanges 

between ocean and atmosphere total 90 Pg (90 Gt). (721) Only about half the CO 2 from


burning of fossil fuels, about 6.3 Pg (6.3 Gt), appears in the atmosphere, which means 

that the other half is going somewhere else. As the amount of carbon in the atmosphere 

increases, though, the amount that is exchanged will have to increase in step. By 2100, the 

exchanges are expected to be larger, and the ocean sink is expected to increase from 1.7 ± 

0.5 Gt/yr (Reference 721) or 2.0 ± 0.6 Gt/yr (Reference 722) to 5 Gt/yr. Land, which is 

thought to have been a net carbon source at mid-twentieth century, is now estimated to be 

a sink (including a source term from deforestation of about 1.6 Gt/yr) of 1.4 ± 0.7 Gt/yr 

(Reference 721) or 1.4 ± 0.8 Gt/yr (Reference 722), and this total is expected to increase 

as well to around 5 Gt/yr. (721,722) The carbon will be there permanently, at least as 

measured in terms of human lifetimes. (85) 

How much carbon is stored or emitted by forests? 

There are several reasons for disagreements and uncertainties (such as indicated above and 

in the next section). However, in the context of the Kyoto Protocol, with possible offsets 

for carbon storage in forests, the situation requires clarification. (723) An experiment that 

combined satellite monitoring with a sophisticated ecosystem model (NASA-CASA) 

suggested that forests lose 1.2 to 1.3 Gt of carbon every year. (724) The highest losses are 

sustained by countries close to the Equator that are losing tropical forest such as Brazil, 

the Congo, and Indonesia. (26-29,31,42) The greatest gains in carbon storage are in the 

Northern Hemisphere, especially in the Unites States and Canada. (724) According to this 

report “the potential for ... carbon gains from forest area regrowth or expansion are 

dwarfed by the continuing losses from deforestation and fires.” (724) 

Work on Pacific Northwest forests that similarly combines satellite mapping and 

modeling showed that the forests used in the pilot study were a source of 1.13 tonnes of 

carbon per hectare per year, rather than acting as a sink. (725) A study at the Harvard


Forest showed, on the other hand, that the forest there was a net sink of 2.0 ± 0.4 tonnes 

of carbon per hectare per year. (726) This study over nine years used eddy covariance to 

measure the rate of uptake. (726) So from this conflicting evidence, it is inconclusive 

whether forests will be a source or a sink of carbon in the north temperate zone. China’s 

forests are apparently storing carbon at present. (727) 

Tropical forests are seen as a net source of carbon (from deforestation), (724) but 

measurements in mature tropical forests in South America show that tree growth exceeded 

loss in 38 out of 50 sites monitored. (728) Another study in Brazil showed that even with 

deforestation, land use change and deforestation together nearly balance the carbon (the 

Amazon was a source of 200 Mt/yr of carbon over the years of the study). (729) And a 

computer simulation shows that even if carbon dioxide stopped growing, the momentum 

of increased growth in topical forests could continue for a century more. (730) 

There are indications that the warming occurring in the Arctic regions is already leading to 

ecosystem changes there. Permafrost is melting at a rapid rate (in the region of tundra 

studied, 18% had melted in 1959 and only 13% remained unmelted in 2003). (731) By 

2100, central Canada is predicted to have no regions of permafrost left. (a) 

The number of shrubs is growing. (732) In the high north, the boreal forest is expected to be 

a large sink of carbon under warming, but evidence shows that for Alaskan boreal forests, 

droughts coming from changing weather stresses the trees and has not lead to enhanced 

sequestration. (733) A study of many European forests at all latitudes showed that uptake 

increased as latitude decreased, not as latitude increased. (734) Further, it may be that 

afforestation in boreal regions will backfire. The surface albedo changes when the tundra is 

replaced by trees, and this could more than compensate for the carbon sequestration, 

according to Reference 413 and Reference 735. Experimental evidence is mixed; a 1993


study showed that Arctic soils had become drier and become sources of carbon 

dioxide, (736) while a more recent study by the same group found that even though the 

warming has increased, some regions are sinks for carbon part of the time. (737) 

Many people had expected a “carbon dioxide fertilization” effect, in that plants are 

limited in the amount of carbon they can fix by the low concentration of atmospheric 

carbon dioxide in the atmosphere. Experiments are under way to learn about this as well 

as about how plants adapt to a future world with doubled CO 2 concentration, known as 

FACE experiments (FACE stands for Free Air CO 2 Enrichment). In these experiments, 

pipes arranged in a circle (Fig. E17.7.2) emit carbon dioxide at some height above the 

surface (CO 2 is heavier than air and so tends to sink) in a locale and the effects are 

compared with a comparable nearby plot not enriched in CO 2 . FACE studies, which 

originated at Brookhaven National Laboratory, are taking place in seven locations in the 

United States, in Switzerland, and in Panama. (738) 

Fig. E17.7.2 Views of several FACE experimental sites. FACE plots are up to 30 m in diameter and 20 m 

high. 

(Brookhaven National Laboratory) 

A group led by Peter Reich at the University of Minnesota studied how elevated CO 2 

affected grassland in one FACE experiment, 6 native grasses and herbaceous plants in 

various combinations on six plots in Cedar Creek Natural History Area and found that


“enhanced biomass accumulation in response to elevated levels of CO 2 or nitrogen, or 

their combination, is less in species-poor than in species-rich assemblages.” (739) The 

implication of this experiment is that a less diverse biome will be less efficient at carbon 

dioxide absorption than a more diverse biome. 

Another FACE experiment showed that estimates of forest productivity in a CO 2 - 

enhanced world might be too optimistic. Two sites were compared, one moderately 

nutrient rich, the other nutrient poor. Some biomass was added at the nutrient-rich site, 

but none on the nutrient-poor site. The added biomass disappeared after three years. (411) 

This suggests that fertility affects the response to added carbon dioxide, and that it is not 

clear that there will be added carbon sequestration in forests. Two different experiments 

shed some light on this question as well. In one, a group tried and failed to find indications 

of the “carbon dioxide fertilization” effect using inventory data from over 20,000 forest 

plots in five eastern states. (740) So, in this case, we would expect the forest sink 

eventually to disappear. Additionally, Cox et al. used a three-dimensional carbon-cycle 

model to find that “carbon-cycle feedbacks could significantly accelerate climate change.” 

They mean by this that in their model land became a source of carbon dioxide around 

2050. (741) 

Adding carbon dioxide caused massive (threefold) gains in set seeds in one 

experiment. (742) This occurred in a stand of 19-year-old loblolly pines (Pinus taeda) after 

four years of enriched carbon dioxide. Since most loblolly pine stands are regenerated 

naturally, the spread may become faster than at present. They could take over from other 

species at elevated CO 2 levels. 

At a different site, increased CO 2 levels led to total net primary production gain of 25% 

for pine trees, implying that a lot of carbon could be sequestered. (743) In a study involving


a variety of forest types in Costa Rica, the upper and lower bounds on net primary 

productivity (NPP) varied greatly from site to site. (744) A study over 18 years of records 

from satellites and climate data indicates an increase in NPP. (745) However, additional 

research shows that this is limited to an approximately three-year timespan, after which 

storage decreases significantly. (746) Schlesinger and Lichter find that nearly half of the 

carbon uptake is allocated to short-lived tissues, for example, leaves or needles. When the 

leaves fall to the ground, they do not contribute much carbon to the underlying soil. (746) 

The authors say “Such findings call into question the role of soils as long-term carbon 

sinks, and show the need for a better understanding of carbon cycling in forest soils.” (746) 

Additionally, experiments in fungi indicate that the FACE approach—abrupt alteration in 

the concentration of carbon dioxide, which cannot occur in the “real world”—may not tell 

an accurate story. Fungi living under the condition of gradual increase of CO 2 showed 

little difference from control groups. Fungi exposed to abrupt shifts in concentration 

showed great differences. (b) This may mean that the FACE approach overstates the 

carbon dioxide fertilization effect. 

Lal has emphasized the role of soil carbon storage in agricultural soils. (747) In many 

regions of the world, agricultural soils have been degraded in carbon. Lal estimates the 

total soil carbon loss at 42 to 78 Gt of carbon. Restoring carbon to such degraded soils can 

enhance future crop production: “An increase of 1 ton of soil carbon pool of degraded 

cropland soils may increase crop yield by 20 to 40 kilograms per hectare (kg/ha) for 

wheat, 10 to 20 kg/ha for maize, and 0. 5 to 1 kg/ha for cowpeas.” Lal believes that this 

soil rehabilitation could take place over as little as two decades. (747) 

For carbon storage to take place, soil nitrogen must also be available. Hungate et al. have 

examined the possibilities using GCMs, and conclude that in scenarios with high nitrogen


accumulation, the amount reaching the soils “is less than is required by all CO 2 -only 

simulations and by four of the six CO 2 -climate simulations. Our low estimates of nitrogen 

accumulation yield only 1.2 Pg of nitrogen, insufficient for all simulations.” (748) 

In opposition to intuition, carbon is lost when woody plants invade grassland, not stored. 

Drier sites did gain soil carbon, but wetter sites lost soil carbon. (749) Another study of 

soil carbon found that carbon lost to the soils may have been stored in woody tissue, 

suggesting that there is no long-term carbon storage loss; it would simply be 

transferred. (750) A study of Chinese forests between 1949 and 1998 found that the 

forests had lost on average 22 Gt/yr of carbon between 1949 and 1980, and gained on 

average 21 Gt/yr between 1980 and 1998. (727) A long-term study in the Amazon found 

that the warmer forest ecosystem is changed substantially from the earlier ecosystem. (541) 

Transpiration can increase in a warmer world, and this may dissicate forests in dry 

regions that already struggle to exist. (751) Studies such as these raise the question of 

whether forests have any long-term capacity to store carbon or not. This appears still to 

be an open question. This question is discussed at length in Reference 319 with special 

attention to forests in the context of the situation to come during the twenty-first 

century. 

In terms of need, the fuelwood crisis in developing countries is leading to deforestation, so 

afforestation projects are an important counter to these losses. However, most near-term 

forest carbon storage projects will take place in the developed world. (752) 

The U.S. land carbon “sink” has been growing for reasons that had eluded scientists, but a 

suggestion that might work is that it is due to the increased rain falling on the continental 

United States. (753,754) Nemani et al. found that increases in humidity and precipitation 

were correlated with increased growth and accounted for two-thirds of it. A mechanism


exists that can explain this correlation. In wetter times, the stomata (pores) in the leaves 

open more, taking in more CO 2 . More water reaches the roots, too, helping stimulate 

growth. The possible problem with this explanation is the uncertainty over what led to 

the increased rainfall in the first place. This study suggests that if that pattern of 

increasing humidity and rainfall changes, the increased growth of the “sink” for carbon in 

the plants will, too. (754) 

It may now appear questionable whether we know anything at all. We are very lucky to 

have learned a substantial amount in recent years about the carbon cycle. Details of the 

effect of CO 2 on forests remains somewhat cloudy, but we now have a better set of 

questions to ask about how forests store carbon (and how the terrestrial biosphere as a 

whole stores carbon). (755) Most investigators are still convinced that forests can absorb 

significant amounts of carbon. In fact, there is mounting evidence that forests can act as 

sinks of carbon until advanced tree ages. (756) The carbon in an old forest is found in tree 

trunks and limbs, but it is also found deep in the soil in roots and carbon-rich zones 

surrounding the roots, where it can remain for centuries. (757) Additionally, as Marland has 

pointed out, use of trees for fuel would seem to indicate a smaller storage capacity, while 

it replaced a much larger source of CO 2 , so that a proper accounting should include these 

beneficial alternative uses. (758) 

Several estimates have been made of the actual area needed to absorb all the carbon 

currently being produced.(Reforestation will only delay the effects of the greenhouse 

warming that occurs as long as fossil fuels continue to be used.) Marland (759) estimates, 

on the basis of measurements of carbon fixation by American sycamore trees on a 

plantation in Georgia, that one hectare absorbs 7.5 t/yr, so an area of some 7 million km 2 

would be needed to fix all CO 2 

being released. For comparison’s sake, 7 million km 2 is 

about the area of Australia, one-third the area of North America, or one-fourth the area of


Africa. F. Myers (760) of the World Wildlife Fund instead used tropical pines and 

eucalyptus to estimate a net fixation of 10 tonnes of carbon per hectare per year. Thus, 

foresting of 465 to 670 Mha, compared to a current forest area of 3.4 Gha, would be 

required to sequester the 5 Gt/yr released by fossil fuel burning. (40) Marland and 

coworkers at the New Zealand Forest Research Institute found that Pinus radiata that is 

harvested and replanted socked away 112 tonnes of carbon per hectare over the course of 

the lifetime of the trees. (761) 

A recent estimate of the one-time cost of planting enough trees to absorb 2 to 3 Gt of 

carbon per year is $186 to $372 billion. (762) It would cost $230 to $1,000 per hectare for 

planting plus land cost of $400-$1,000 per hectare. (762) The estimated cost is $2.2/t of 

carbon removed. (40) This cost should be compared to the cost of doing nothing. A sea 

level rise could cause $300 million in land loss alone. A recent estimate by the EPA of the 

total cost of protecting against a 1 m rise in sea level would be $73 to $110 billion. (496) Of 

course, not all the CO 2 

would have to be compensated. It would be possible, for example, 

to forest 300 Mha for $120 billion and achieve a 26% CO 2 

reduction. (763) 

One study of vegetative storage asserts that “[t]emporary carbon stores would only be 

beneficial for climate change impacts related to the cumulative impact of CO 2 

, but it could 

even worsen impacts mediated via the instantaneous effect of temperature or those related 

to the rate of change. Hence, the planting of trees is only beneficial in reducing climatechange 

impacts if the most serious impacts are those related to the cumulative effect of 

increased temperature.” (764) 

Costs are expected to range between $10 and $150 per tonne and worldwide up to 2 

Gt/yr could be stored at this price, but secondary benefits occur as well that can reduce 

the final cost—if these are high enough, forest storage of CO 2 

may be a “no regrets”


storage option. (765) One problem in making comparisons among projects is that the 

phrase “dollar per tonne” may mean different things, depending on the author. In the 

review article (Reference 784), it was found that after correcting for this disparity, many 

projects were feasible at estimated costs of under $50/t. The authors write “when 

compared to source control of fossil fuel emissions ... [various estimates] ... for the world 

suggest that one-third to one-half of a sizable carbon reduction program could be cost 

effectively accomplished through carbon sequestration.” 

Vegetative growth over North America has increased over the last several decades. The 

About half the tree cover originally on Earth before the beginnings of agriculture has been 

cut. (719) Planting some five to six times the number of trees currently being cut should 

lead to absorption of the 5 Gt of carbon per year needed to absorb the human-generated 

carbon. The timescale is several decades before large plantations in the tropics would 

seriously affect the CO 2 

level in the atmosphere. (766) We already know that deforestation 

of some 7 to 20 million hectares per year (mainly in the tropics) is pumping about 1 Gt of 

carbon per year into the atmosphere. Conversion of old-growth forests to young forests 

will not decrease CO 2 

. It is better to leave old-growth trees standing. On-site carbon 

storage does not approach old-growth storage for 200 years. (767) 

An additional question is whether, as most models (including the IPCC models) assert, 

that increased temperatures lead to increased respiration, which leads to a situation in 

which the forests’ decaying organic matter becomes a source of carbon dioxide. However, 

some experiments have found that the decomposition rates in soils do not vary with 

temperature. (768) A study of plots in grassland showed that soil respiration acclimatized 

to warming, in basic agreement with Reference 768. (769) An additional piece of evidence is 

that a study of artificially warmed tundra showed almost no difference in carbon emission


from the control plot. (770) So perhaps the forests will continue to be able to store carbon, 

even at heightened temperatures. 

Controversy over the size of the United States sink 

As there are no carbon meters available directly, the transfers of carbon must be found in 

indirect ways. Several lines of reasoning indicate that the Northern Hemisphere acts as a 

carbon sink. The gradient of CO 2 north-south is smaller than expected, and the oceanic 

partial pressure of CO 2 also indicates carbon uptake in the north. (85) One method, known 

as “inversion modeling” uses the available data and partitions it using models of oceanic 

uptake and release, and models for the global distribution of the released CO 2 through the 

action of the winds. With this method, Fan and coworkers found that most landmasses 

were poorly constrained in terms of their ability to store of produce carbon, but that 

North America was well-constrained. (771) Reference 771 claims a storage for North 

America of 1.7 ± 0.5 Gt/yr, most south of 51° N (that is, the United States). Believing 

this is fraught with difficulty, because this is about the amount of carbon emitted by 

North America through burning fossil fuels! 

A different method was used by Houghton, Hackler, and Lawrence, who adopted a model 

such as the Harvey model to look from the bottom up at carbon uptake and emission. (772) 

Their analysis was carried out for the period from 1700 to the present. They found that 

about one-quarter of carbon flux is attributable to land-use changes. They also found that 

the United States was a sink for carbon, with the greatest storage occurring in the 1950s 

and 1960s. In the 1980s, carbon accumulated at 0.15 to 0.35 Gt/yr. (772) They interpret 

the storage as being a result of land management practices in recent times (about half the 

total storage). (674,718,772) This is both good news and bad. Carbon is accumulating in 

North America. But, since the changes have been made some time ago, they are soon due


to saturate (that is, the storage will go away as the ecosystems reach equilibrium), and 

this is not admissible under the Kyoto Treaty as a credit. (772) Other investigators also 

found that changes in land use have large effects, using a computer model of vegetative 

ecosystems. (674,718,773) It is well-known that a lot of U.S. farmland has returned to forest 

over the last century (it is estimated that 9 to 11 Mha of former agricultural land is now 

forested). 

You may recall that aerosol pollution affects climate as well as people’s health. Wetlands 

emit 115 to 237 Mt/yr. (774) In a similar way, sulfur pollution appears to affect the health 

of bogs (in a favorable way) as well as the health of people. Wetlands emit methane, and 

warmer wetlands emit greater amounts of methane (a global warming feedback), but 

apparently increasing sulfuric acid deposition reduces emissions of methane from 

peatlands, and Gauci et al. apply this knowledge to wetlands in general. (775) The 

researchers estimate that current levels of sulfur deposition on wetlands reduce the 

amount of methane emitted by 15 Mt/yr, decreasing emissions by 8%. If current trends 

continue, by 2030 the resuction will be 26 MT/yr, or 15% of total wetland 

emissions. (775) From this research, it is possible to infer “that the observed increase in 

atmospheric CH 4 concentration throughout the 20th century is the result of factors other 

than growth in the contribution from wetlands.” (775) 

Bonan found that changing forest to cropland caused cooling to occur, rather than the 

reverse. (776) The computer study showed a correlation between land use and cooling over 

the course of the twentieth century. (776) Rice is an important crop for billions of people 

worldwide, and methane emissions from rice cultivation are an important source of 

greenhouse gases. (153) The emissions from rice cultivation can be greatly reduced. (777) 

Urban land also behaves differently from forest or grassland, storing less carbon and


emitting more nitrous oxide. (778) Aye et al. consider these effects to be arising at different 

urban scales, (778) 

[O]ur results support three hypothetical links between human decisions and 

biogeochemistry at multiple scales. At the regional scale, our data suggest that 

bio-geochemical fluxes depend on which land-cover types are converted to 

urban use. The conversion of grasslands to urban use (increasing N 2 O, 

decreasing CH 4 ) will have a much different effect than conversion of corn to 

urban use (moderate decrease in N 2 O, no change in CH 4 ). At the landscape or 

neighborhood scale, we hypothesize that effects of urbanization depend on 

landscape architecture, especially the proportion of the landscape that is 

lawns vs. impervious surfaces. At the household scale, we hypothesize that 

individual management decisions (irrigation and fertilization levels) are 

important determinants of biogeochemical fluxes. 

Additionally, Nemitz et al. find that urban traffic, particularly at rush hour, releases CO 2 

and heat directly into the city and affects local climate. (779) Similarly, Grimmond et al. 

find their “observations at a single suburban site in Chicago show significant diurnal 

variations in CO 2 concentrations. The CO 2 concentrations are elevated relative to global 

background values” at ground level, but not as much at higher elevations. (780) Grimmond 

et al. also find that carbon dioxide is being removed from the air in leafy neighborhoods. 

As a result of all this research, it is clear that land-use change is an important cause of 

regional—and global—climate change. 

This agrees with the IPCC and other studies. However, it seems to be the direct opposite 

of what has been found by others. Computer models of forest growth on the Great 

Planins shows that the opposite occurs. (781) Climate change with increased temperatures 

leads to changes in composition, but little change in diversity. As research has shown the 

effect of warming is not in increased daytime temperatures, but in increases in the 

nighttime temperatures. (322) The so-called diurnal temperature range has gotten smaller.


Interestingly, the decrease in the range in the last few decades may have set into motion a 

process that makes the range even smaller. The range decreased 3° to 5° C between 1950 

and 1993. (412,782) The study found that satellite observations over Canada could not be 

explained by the totality of non-biospheric parameters. Only if the effects of vegetation 

are included could the observations be explained. The vegetation seem to have caused an 

additional warming. (412) 

A study by Pacala et al. (783) constituted of an extensive land-based survey of the lower 

48 states and correlations with results from 27 different atmospheric studies. They find 

that trees and other vegetation took up 0.3 to 0.7 Gt/yr of carbon between 1980 and 

1989. (783) 

According to a review of the literature, “the terrestrial biosphere was largely neutral with 

respect to net carbon exchange during the 1980s, but became a net carbon sink in the 

1990s.” They attribute the change to northern temperate regions. They note that tropical 

land areas were approximately in balance, (784) as others have found. (728,729) There is 

possibly a local carbon sink that offsets the emissions from tropical deforestation. Clark 

finds the opposite, however, from her studies in Costa Rica. (785) 

TABLE E17.7.2 

Carbon Stocks in World Biomass and Soils 

Area Global Carbon Stocks (Gt C) 

Biome (10 9 ha) Vegetation Soil Total 

Tropical forests 1.76 212 216 428 

Temperate forests 1.04 59 100 159 

Boreal forests 1.37 88 471 559 

Tropical savannas 2.25 66 264 330 

Temperate grasslands 1.25 9 295 304 

Deserts and semideserts 4.55 8 191 199


Tundra 0.95 6 121 127 

Wetlands 0.35 15 225 240 

Croplands 1.60 3 128 131 

Total 15.12 466 2011 2477 

Source: Reference 73, Table 1 

The IPCC has certainly a role to play in this question, and they have. (72) Table E17.7 2 

shows in detail the estimated amounts of carbon in the near-surface and above-surface 

biomass. According to the IPCC, about 270 Gt of carbon has been emitted between 1850 

and 2000 due to burning of fossil fuels and 136 Gt from land-use change. (73) After 

accounting for the amount in the atmosphere, which changed by about 176 Gt in that 

time, they find that about 230 Gt must have been taken up by the planet by some 

mechanism (they assume the carbon was divided roughly equally between oceanic and 

terrestrial uptake). (73) 

The net planetary uptake was about 0.2 ± 1.0 Gt/yr in the 1980s and 0.7 ± 1.0 Gt/yr in 

the 1990s. (73,784) As is usual with IPCC, there are several different scenarios used, which 

give slight differences among the apportionment of carbon storage. An IPCC Definitional 

Scenario was chosen in order to report everything equally clearly. In this scenario, several 

estimates of the effects of deforestation and afforestation were given. Between 2008 and 

2012, the report expects a carbon sequestration of 197 to 584 Mt/yr of carbon from 

afforestation and reforestation. The expected deforestation is estimated to be about 1.8 

Gt/yr. This means a release of between 1.2 and 1.6 Gt/yr of carbon. The bright side, as 

the study points out, is that it is feasible to expect a 20% increase in reforestation and 

afforestation, which would remove a further 11 to 45 Mt/yr of carbon (but unfortunately, 

this is still small in comparison to the emissions).


Some critics doubt that trees can do the job. The Royal Society says that such measures 

can take care of just one-quarter of the sequestration needed by 2050. (786) 

The IPCC report finds that improved forest management, improved cropland 

management, improved grazing land management, conversion of cropland to grassland, and 

improved agroforestry could provide about 270 Mt/yr of carbon storage for the Annex I 

countries (about 260 Mt/yr from better management and 30 Mt/yr from land-use 

change). (73) This implies that the Annex I countries could virtually wipe out the effects of 

their carbon emissions by adopting improved management practices and converting some 

cropland to grass. This, in turn, means that achieving the Kyoto emissions goals will be 

less costly than had been thought, especially by U.S. politicians who have argued against 

the adoption of the Kyoto treaty because of its projected huge costs (see the extended 

discussion of costs and responsibilities of carbon emission in Extension 17.8, Why we 

should support the Kyoto Protocol). 

For the world as a whole, the situation looks different, according to the IPCC, as the landuse 

change contribution totals 335 Mt/yr and better land-use management 570 Mt/yr in 

carbon storage. (73) This assumes, of course, that land management changes from cropland 

to grassland or forest will result in greater carbon storage. 

References in addition to those listed for this chapter are shown in red in the text, and 

listed below: 

a. P. Camill, “Permafrost thaw accelerates in boreal peatlands during late-20th century 

climate warming,” Clim. Change 68, 135 (2005). 

b. J. N. Klironomos, M. F. Allen, M. C. Rillig, J. Piotrowski, S. Makvandi-Nejad, B. E. 

Wolfe, and J. R. Powell, “Abrupt rise in atmospheric CO 2 overestimates community 

response in a model plant–soil system,” Nature 433, 621 (2005).

Extension 17.7: Planting Trees

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