vegetation and climate - אוניברסיטת ת"אcolin/courses/pses/ch10_vegetation and...

Vegetation and Climate The Biosphere is the part of the Earth where living organisms live.

Biomass generally refers to all vegetation on the planet, in the oceans and

on the continents, live or dead. Although biomass has a large influence

on the Earth’s climate, termites, cows, sheep also play a role in the climate

system.

Jan 2004

Jul 2004

The strong connection between surface vegetation and climate was first demostrated in 1947 by Holdridge who showed that potential distribution of global vegetation types depends primarily on two parameters: temperature and precipitation.

The vegetated surface of the Earth is part of the lower boundary of

the atmosphere where important exchanges of heat, radiation,

momentum, moisture, trace gases and aerosols occur.

The wavelength dependence of surface reflectance is also significant.

The chlorophyll in vegetation absorbs strongly at visible wavelengths;

consequently, leaves may reflect only about 0.05 of the solar radiation

shorter than 0.7m, but approximately 0.25 of longer wavelengths.

Soils also increase their reflectance with increasing wavelength, although

more gradually.

Cooling of the boundary

layer is achieved by

evapotranspiration. Under

normal conditions, plant

leaves take in carbon dioxide

from the air and

release moisture as part of

the photosynthesis process.

The release of

moisture, through

evapotranspiration, causes a

cooling of the air, since

energy normally used to heat

the surface is now used to

evaporate water.

The removal of vegetation

results in the warming of the

surface.

The rates of the fluxes of gases are modulated by the ability of roots to extract

water from the soil and by the resistances imposed by the stomatal controls

of leaves. Leaf surfaces are equipped with small openings or pores called

stomata which allow CO2 to enter the leaf and O2 to escape during

photosynthesis. In addition, water is lost through the stomata during

transpiration. It is estimated that 99% of the water absorbed by the roots of

the plant is lost by the leaves via the stomata.

In times of warm climate, when the atmosphere is rich in CO2, plants

need relatively few pores to take in all the CO2 they need, so their leaves

develop comparatively few stomata. In times of cold climate, more

stomata are needed because less CO2 is readily available. The stomata

are also important in conserving the plants moisture in times of dry

conditions. Warmer conditions result in the stomata closing to prevent

the loss of water.

Storage of Carbon:

The absorbed CO2 from the atmosphere is used partially for the

production, within plants, of a wide variety of organic materials. Some,

such as leaves and fine roots, are generally short-lived and become

available to decomposers within a few years of production. Therefore

the carbon is returned to the atmosphere fairly quickly. However, woody

branches and stems of trees can live for decades to centuries, storing the

Carbon in the biosphere instead of in the atmosphere.

Photosynthesis in

C4 plants is 6 times

faster than in C3

plants

Will increasing atmospheric CO2

green the planet?

The biosphere is a major sink for atmospheric CO2. There is a clear annual

cycle in the global CO2 atmospheric concentrations resulting from the

withdrawal and production of CO2 by the terrestrial and oceanic biosphere.

Human use of the landscape generally reduces carbon storage. The

harvesting of forests (deforestation) generates CO2 both from the decay

and burning of wood produces. Conversion from forests or grasslands to

agriculture releases CO2 both through the loss of vegetation and litter

inputs. However, regrowth of forests and the reverting of agricultural land

to native vegetation results in a net sink of CO2 in the atmosphere.

Historical trends

There is an indication that over the last

century the contribution of the terrestrial

biosphere to atmospheric CO2 has been

increasing. However, the main source of

this increase is from Latin America and

tropical Africa resulting from land use

conversion, as well as deforestation. The

contribution from Europe and North

America is actually decreasing over time.

It is estimated that 20-40% of the increasing

trend in CO2 results from changes in the

biosphere. It is also believed that only within

the last 60 years has the input from the

burning of fossil fuels been greater than the

input from terrestrial ecosystems. The

largest single component of the Carbon flux

from the biosphere is presently due to tropical

deforestation.

Land Use change

Increasing atmospheric CO2 trend

The land biosphere may be responsible for the uptake of about 1/3 of all

the CO2 that is released into the atmosphere. On an annual basis, land

vegetation removes from the atmosphere about 100 Gt of carbon, compared

to the 6 Gt released by the burning of fossil fuels. However, about the

same flux of Carbon returns to the atmosphere from the respiration of

live plants and the decay of dead ones. The gross annual flux of Carbon

through the biosphere is very similar to that of the world’s oceans.

But on time scales of decades or more, the CO2 concentrations are

controlled mainly by the exchange with the oceans. Deforestation and

other conversions and uses of land surface are thought to contribute a

net flux of 1-2 Gt of carbon per year to the atmosphere, 20-40% of the

contribution due to the combustion of fossil fuels.

The biosphere is also a source region for two other important greenhouse

gases: methane (CH4) and nitrous oxide (N2O). Both methane and nitrous

oxide are increasing in the atmosphere. The major anthropogenic sources

of N2O are from fertilization of agriculture.

The production of CH4 in terrestrial ecosystems is limited to highly

reduced conditions, as are found in wetland soils and sediments, and the

digestive system of animals. On a global scale methane production is

dominated by fluxes from natural wetlands and bog areas, and agricultural

wetlands such as rice paddies. Anthropogenic sources of CH4 contribute

345 Gt CH4 to the atmosphere annually, compared to natural sources of

180 Gt. Rice paddies are

the most important

anthropogenic source of

methane, followed by

biomass burning

(especially in the tropics),

gas and coal drilling, and

landfills.

Methane concentrations are increasing in the atmosphere at a rate of nearly

1% per year. The rate has slowed in recent years. There is a risk that large

regions of permanently frozen soil (permafrost) in high latitude boreal and

tundra regions may become a major additional source of CH4 if

temperatures warm in these regions. In many areas, deep layers of peat

exist below these frozen soils. Higher soil temperatures and longer frost-

free seasons could increase the CH4 emissions significantly.

Aerosols

The biosphere is also a major source of aerosol particles that can be used

as cloud condensation nuclei, and ice nuclei in the production of

precipitation. In addition, these particles can interfere with the direct

incoming solar radiation to reduce the radiation at the surface (cooling

effect). The oceanic biosphere plays a major role in the production of

sulfate aerosols. Through the production of Dimethyl Sulfide (DMS),

which are the dominant aerosols in the atmosphere. Release of gases such

as isoprene from vegetation can also result in the formation of organic

aerosols. Finally, burning of biomass results in large amounts of aerosols

pumped into the atmosphere on seasonal time scales.

Possible consequences of future climate change on vegetation:

Long term climate change (over thousands of years) result in the migration

of species to new climatic regions favourable for the type of vegetation.

Short term rapid climate change (hundreds of years), as is being observed

today, may be too rapid for natural migration of species, resulting in many

species becoming extinct. Natural migration of species occurs at a rate of

tens of kilometers per century, while future climate warming in mid- to

high latitudes would require the species to migrate at rates of hundreds of

kilometers per century.

In addition, there are also close

associations between the

distribution of major vegetation

types and the distribution of

major soil types which may hinder

the free movement of species

boundaries.

Today

2xCO2

Vegetation – Climate Feedbacks

SW-LW

SW-LW

HL

HL

HS

HS

soil

soil

Ts P

Climate effects of

deforestation

Positive feedback due to vegetation changes

BIOTIC RESPONSE TO RECENT CLIMATE CHANGE

Biota

Location

Change

Climate Link

Treeline

Europe, New Zealand

Upward shift

Warming

Arctic Shrub Tundra

Alaska

Spread

Warming

Alpine Plants

Alps

Upward shift

Warming

Biota

Antarctica

Spread

Melting

Zooplankton

Calif. & N. Atlantic

Population increase

Warming

Butterflies (39 spp)

N Amer. & Europe

Northward shift, 200 km

Warming

Birds, lowland

Costa Rica

Expansion upward

Dry-season mists

Birds, migratory

England

Northward shift , km20 Winter Warming

Foxes, red, white

Canada

Northward boundary shift

Warming

Gaia Hypothesis:

James Lovelock suggested that the biosphere always acts in a way to

regulate the Earth’s environment and climate so that it remains in some

type of equilibrium. The adjustments by the biosphere to any perturbation

of the system is done by the process of natural selection (e.g. DMS-CNN-

cloud albedo cycle). However, Gaia does not explain why the biosphere

did not regulate the climate during the ice ages.

Homework

Groenigen et al., 2011: Increased soil emissions of potent greenhouse gases under increased atmospheric CO2, Nature, 475, 214-218.