T rees are vital to life on Earth. They providefuel, fibre, food and shelter for humans, andhabitats for a vast range of animals. One of

the most important functions of trees is their ability touse atmospheric carbon dioxide (CO2), a greenhousegas, in photosynthesis:

6CO2 + 6H2Olight→

chlorophyllC6H12O6 + 6O2

The biomass produced stores large amounts ofcarbon, and oxygen is released.

Carbon dioxide levelsReconstructions of Earth’s past climate, and theobserved impact of vegetation on global atmos-pheric carbon dioxide levels, have changed the waywe perceive the Earth’s system. Figure 1 shows pastatmospheric carbon dioxide and temperaturevariations measured from tiny air bubbles trappedup to 3 km deep in Antarctic ice. Although there aremajor variations over glacial cycles of around100 000 years, atmospheric temperature and carbondioxide follow each other — we say that there is aclose correlation between temperature and carbondioxide levels. Notably, carbon dioxide remainedbelow 300 parts per million (ppm) until thetwentieth century.

The inset in Figure 1 shows atmospheric carbondioxide levels from 1958 to the present, measured atMauna Loa in Hawaii. These measurements highlightthe link between vegetation and climate, as well ashuman influence. They show the annual rise and fall

12 Catalyst

Forests, carbon and climate

MathiasDisney

550

500

450

400

350

300

250

200

20

15

10

5

0

–5

–10

–15

VostokEPICATemperature difference

CO2 from Antarcticice cores

Mauna Loa

600 000 500 000 400 000 300 000 200 000 100 000 0

1960

360

320

1980 2000

Car

bon

diox

ide

(par

tspe

rm

illio

n)

Mea

nte

mpe

ratu

redi

ffere

nce

from

the

pres

ent

day

(°C

)

Years before present

Figure 1 Atmospheric carbon dioxide concentrations over the last 650 000 yearsfrom Antarctic ice cores. The inset shows atmospheric carbon dioxide in moredetail from 1958 to the present measured in Mauna Loa, Hawaii. The green lineshows historical temperature variations relative to the present day (right handaxis). (Sources: all data publicly available from NOAA — see Box 2)

Scientists are only beginning to understandjust how complex and fascinating therelationship between trees, the carbon cycleand climate really is. Here, Mathias Disneyshows how aspects of your GCSE sciencecourse relate to his current research.

GCSE key wordsPhotosynthesis

RespirationCarbon cycle

Climate changeGreenhouse gas

Sim

onFr

aser

/SPL

Tropical rainforestin Hawaii

of carbon dioxide caused by the vegetation in thenorthern hemisphere growing up during summer anddying back during winter; the whole Earth is‘breathing’. There is much less land mass supportingvegetation in the southern hemisphere. Figure 1 alsoshows there has been an increase of nearly 20% incarbon dioxide levels since 1958. This rapid increase ismainly due to burning fossil fuels (gas, oil, coal).There is strong evidence that this is having ameasurable impact on climate.

Trees and climateSo how do trees fit into the picture? Figure 2 showsthe stores of carbon and the fluxes (flows) that makeup the global carbon cycle. The soil/plant system actsas a huge store for carbon. It contains three times asmuch carbon as the atmosphere. The land and oceaneach absorb approximately a quarter of all fossil fuelemissions; the other half remains in the atmosphere.As atmospheric carbon dioxide concentration rises,trees grow more rapidly (so-called carbon dioxidefertilisation) and hence absorb more carbon dioxide.At a certain concentration of carbon dioxide,however, this increase will level off as other factorsbecome limiting.

Figure 2 also shows that soils are a vital part of thecarbon cycle. Microbes in the soil decompose deadorganic plant matter, storing huge quantities ofcarbon. But they respire while doing so, releasingcarbon dioxide. As global temperatures rise, thesemicrobes will become more active, releasing morecarbon dioxide and further increasing warming.

This is just one of many feedbacks between theforest system and climate, some positive (reinforcingwarming) and some negative (slowing downwarming). One of the most fascinating and importantchallenges facing scientists is how to measure thecarbon balance of forest systems over many life cyclesand predict how this may alter with changing climate.

Measuring how forests ‘breathe’Each year a large amount of carbon cycles through theEarth’s vegetation and soil. Because forests are notclosed systems, measuring the carbon balance of aforested region is difficult. We need to know howmuch carbon flows in and out over a given timeperiod. To do this, scientists draw an imaginary boxaround the forest system and, using instrumentsmounted on towers above the forest canopy (seepages 18–19), measure the flux of carbon dioxide,resulting from flow, into the system (due to photosyn-thesis) and out of it (due to plant and soil respiration).

l Can you manage the global carbon cycleand climate? Have a go using this simpleonline model:www.sei.se/forests/breathingforests.htm

13April 2007

Box 1 The history of the greenhousegas concept1827 Fourier developed the greenhouse gasconcept in which carbon dioxide raises theatmosphere’s temperature by about 15°C.1860 Tyndall found that water vapour makes thelargest contribution to greenhouse warming.1896 Arrhenius calculated that doublingatmospheric carbon dioxide would bring about a5°C temperature increase.

Figure 2 The globalcarbon cycle showingthe reservoirs (white)of carbon in billiontonnes (gigatonnes,Gt) of carbon. Fluxesare in GtC/year.Geological changesare measured inmillions of years (myr)

Respiration/photosynthesis

Runoff/erosion0.8

0.6

1.9

3.3

1.9

0.2

~200myr

Marine biotaphotosynthesis/respiration*

* values for marine photosynthesis and respiration are uncertain

Atmospheric:730 + 3.3/yr(380 ppm)

Atm

ospheric carbon cycle

Sediments:20 000

Rock carbonates

Marine carbon cycle

Fire 5

Volcano0.1

Photosynthesis120

Respiration6055

~200myr

Vegetation: 500

Soil: 1,500

Fossil fuel(known reserves)Gas 550Oil 750Coal 3700

Terre

strial carbon cycle

Energy use5.3

Cement0.1

Fire/land usechange

1.7

Human impact

90

120

+7.1/yr

Although scientistsrefer to carbon in theenvironment, it isusually in a compound:carbon dioxide in the atmosphere;hydrocarbons in fossilfuel; carbohydrates,proteins and lipids inplants and animals; andcarbonates in rocks.

EPICA is the EuropeanProject for Ice Coring in Antarctica. The ice core went to 3190 metres covering720 000 years. TheVostok ice core wentback 420 000 years.

A typical forestFigure 3 shows a forest breathing (a localised versionof the global breathing in Figure 1). From bottom totop it shows 5 years of seasonal variation in the netcarbon flux; horizontally it shows variations over eachday. Negative values (green to red) indicate where theforest is a sink of carbon dioxide, when more carbondioxide is absorbed through photosynthesis than isreleased by respiration (during the day and insummer). Positive values (blue) indicate where theforest is a source of carbon dioxide (during the nightand in winter). By adding up all the positive andnegative values we can calculate the forest carbonbalance over time.

Soil respirationInstruments placed on the soil enable us to measurehow much of the total carbon dioxide flux comesfrom soil respiration. This is vital to understandinghow soil respiration will change with climate. Suchmeasurements show that forest systems can be very

different from each other, and that the carbonbalance of a forest varies over its life cycle.

Young and old forestsA young forest may be a net source of carbon dioxideto the atmosphere because the soil disturbed duringplanting may emit more carbon dioxide than is takenup by the vegetation. It may take 15 or 20 years tobecome a net carbon dioxide sink. As a forest getsold, the size of the sink may decrease as the trees nearthe end of their lives. Crucially, after death, treebiomass decomposes, releasing most of its storedcarbon back into the atmosphere. So the long-termstore lies in the soil.

Satellite observationsScientists complement their detailed forest measure-ments with observations from satellites whichmonitor forest productivity on a global scale. Figure 4shows satellite estimates of global vegetation produc-tivity over the globe for 1999.

14 Catalyst

Figure 3 Daily variations in carbon dioxide fluxes(horizontal axis) measured at Aberfeldy in Scotland,over a 5-year period. We see the forest breathing incarbon dioxide (green to red, negative values) due tophotosynthesis during the day and during the summer;and out (blue, positive) due to respiration at night andduring the winter. (Source: J. Moncrieff)

Rob

ertB

rook

/SPL

Dav

idC

.Cle

gg/S

PL

It may take a young forest 15 or 20 years to become anet carbon dioxide sink

Deforestation reduces the amount of carbon dioxideabsorbed from the atmosphere

6

3

0

−3

−6

−9

−12

−15

−18

−21

−24

2002

2001

2000

1999

1998

19970 4 9 12 16 20 24

Carbon dioxide flux (mmol m−2s−1)

Hour

Dat

e

Biomass is the total drymass of an organismsuch as a tree.

Developing modelsAll these measurements help us to develop models ofhow the system works and make predictions aboutwhat will happen as climate changes. We can also usesuch models to make predictions about the impact ofactions such as deforestation, increased fossil fuelconsumption and planting trees to offset carbonemissions.

Other feedbacksOne of the difficulties in considering how forests andclimate interact is that there are many other factorsinvolved, in addition to carbon dioxide. Some of thesefactors have positive effects, some negative.

Recent research suggests that forests may be asource of methane (CH4), another importantgreenhouse gas. Forests, being large and dark,absorb more sunlight energy than light-colouredareas, making the surface warmer. Trees can alsoencourage the production of clouds both by tran-spiring water into the atmosphere and throughproduction of compounds which encourage cloudformation. Clouds can reduce heat loss from theground and atmosphere beneath (clear nights arecolder than cloudy ones), but also reflect light fromthe sun before it reaches the ground. The net impactof forests on climate is a combination of all these

(and other) factors, whose complexity scientists arestill trying to unravel.

ConclusionAt the start of the twenty-first century, it is clear thatmanaging the Earth’s carbon cycle is crucial to theplanet’s wellbeing. Scientists are revealing how muchwe have left to learn about the subtle interplaybetween trees, carbon storage and climate. Thisscience is fascinating, because we don’t know theanswers. It is also urgent if we are to predict the fullconsequences of humanity’s global climate experimentfrom burning fossil fuel.

Mathias Disney wrote this article with input from ShaunQuegan, John Grace and Andreas Heinemeyer. All are membersof the Centre for Terrestrial Carbon Dynamics research team.Find out about the team’s work at http://ctcd.group.shef.ac.uk/ctcd.html. Click on People to read their biographies.

A limiting factor is onethat controls the rate ofa process.

15April 2007

Box 2 Useful websitesl This site shows the way many nations arecollaborating in research on the carbon cycle:www.globalcarbonproject.orgl The website of the Intergovernmental Panel onClimate Change is at: www.ipcc.chl A great place to learn more about the carboncycle is at: http://earthobservatory.nasa.gov/Library/CarbonCyclel You can find ice core data at:www.ncdc.noaa.gov/paleo/icecore/antarctica/vostok/vostok_data.htmll You can find data on carbon dioxideconcentrations at: www.cmdl.noaa.gov/projects/web/trends/co2_mm_mlo.dat

Figure 4 Satellite-derived map of gross primaryproduction, the total amount of carbon absorbed due to photosynthesis, in grams of carbon per squaremetre during 1999. Values range from 0 (dark blue) to4 kg C/m2 (brown). The heavily productive regions canbe clearly seen across the tropical forests in theAmazon and Central Africa (Source: T. Quaife)

0 Gross primary production gC/m2 4000