INTRODUCTION

THE NEED FOR ECOSYSTEM CARBON 

The earth’s climate is crucially dependent on the composition of the atmosphere, and in particular on the concentration in it of greenhouse gases that increase the amount of the sun’s heat that is retained. The two most important of these are carbon dioxide (CO2) and methane (CH4). Both gases are naturally present in the atmosphere as part of the carbon cycle but their concentration has been greatly increased by human activities, particularly since industrialisation. There is more carbon dioxide in the atmosphere now than at any time in the past 650,000 years. In 2006 the global average atmospheric concentration of CO2 was 381 parts per million (ppm), compared with 280 ppm at the start of the industrial revolution in about 1750. The rate at which the concentration is increasing is the highest since the beginning of continuous monitoring in 1959 (Canadell et al. 2007).

The Intergovernmental Panel on Climate Change (IPCC) has stated that limiting global temperature increase to 2–2.4°C and thereby staving off the worst effects of climate change requires greenhouse gas concentrations in the atmosphere to be stabilised at 445–490 ppm CO2 equivalent (see box) or lower (IPCC 2007b). As there is presently about 430 ppm CO2e, this implies limiting future increases to between 15 and 60 ppm (Cowie et al. 2007; Eliasch 2008).

Note on units and quantities

Note on units and quantities1 gigaton of carbon (Gt C) = 109 tonnes of carbon (t C). 

Carbon (C) or carbon dioxide (CO2)? It is when carbon is in the form of carbon dioxide gas in the atmosphere that it has its effect on climate change. However, as it is the carbon that cycles through atmosphere, living organisms, oceans and soil, we express quantities in terms of carbon throughout this report. One tonne of carbon is equivalent to 3.67 tonnes of carbon dioxide. The global carbon cycle (see next page) illustrates how carbon moves and is stored in terrestrial and marine ecosystems and the atmosphere.

CO2 equivalent (CO2e) is a measure of global warming potential that allows all greenhouse gases to be compared with a common standard: that of carbon dioxide. For example, methane is about 25 times more potent a greenhouse gas than carbon dioxide so one tonne of methane can be expressed as 25 tonnes CO2e.           

CARBON IN LIVING SYSTEMS

Living systems play a vital role in the carbon cycle. Photosynthesising organisms – mostly plants on land and various kinds of algae and bacteria in the sea – use either atmospheric carbon dioxide or that dissolved in sea water as the basis for the complex organic carbon compounds that are essential for life. The vast majority of organisms, including photosynthesising ones, produce carbon dioxide during respiration (the breaking down of organic carbon compounds to release energy used by living cells). Burning of carbon compounds also releases carbon dioxide. Methane is produced by some kinds of microbe as a product of respiration in low oxygen environments, such as stagnant marshes and the intestines of ruminants, including cattle, sheep and goats. Methane in the atmosphere is eventually oxidised to produce carbon dioxide and water.

In the biosphere a significant amount of carbon is effectively ‘stored’ in living organisms (conventionally referred to as biomass) and their dead, undecomposed or partially decomposed remains in soil, on the sea floor or in sedimentary rock (fossil fuels are, of course, merely the remains of long dead organisms). 

When the amount of atmospheric carbon fixed through photosynthesis is equivalent to the amount released into the atmosphere by respiring organisms and the burning of organic carbon, then the living or biotic part of the carbon cycle is in balance and concentrations of carbon dioxide and methane in the atmosphere should remain relatively constant (although their concentration will be affected by other parts of the carbon cycle, notably volcanic activity and dissolution and precipitation of inorganic carbon in water). 

Often, however, the system may not be balanced, at least locally. An area may be a carbon sink if carbon is accruing there faster than it is being released. Conversely, an area is a carbon source if the production of atmospheric carbon from that area exceeds the rate at which carbon is being fixed there. In terrestrial ecosystems, whether an area is a sink or a source depends very largely on the balance between the rate of photosynthesis and the combined rate of respiration and burning.

The amount of carbon stored, the form that it is stored in and the rate of turnover – that is the rate at which carbon is organically fixed or released as carbon dioxide or methane – vary greatly from place to place. These are dependent on a variety of conditions of which climate (chiefly temperature and, on land, precipitation) and nutrient availability are the most important. Changing climate will itself have an impact on the natural distribution of biomes and ecosystems and on the carbon cycle both globally and locally. 

HUMAN IMPACTS ON THE CARBON CYCLE

Humans are affecting the carbon cycle in a number of ways. The burning of large amounts of fossil fuels releases long-stored organic carbon into the atmosphere. Production of cement produces atmospheric carbon through the burning of calcium carbonate. Many land-use changes also tend to increase the amount of atmospheric carbon: conversion of natural ecosystems to areas of human use (agriculture, pasture, building land and so forth) typically involves a transition from an area of relatively high carbon storage (often forest or woodland) to one of lower carbon storage. The excess carbon is often released through burning. From the point of view of climate regulation, increasing livestock production, notably of ruminants, has a particularly marked effect as it increases the production of the highly potent greenhouse gas, methane.

Historically, it is estimated that since 1850 just under 500 Gt of carbon may have been released into the atmosphere in total as a result of human actions, around three quarters through fossil fuel use and most of the remainder because of land-use change, with around 5% attributed to cement production. Of the total around 150 Gt is believed to have been absorbed by the oceans, between 120 and 130 Gt by terrestrial systems and the remainder to have stayed in the atmosphere (Houghton 2007).

The most recent estimates indicate that human activities are currently responsible for annual global carbon emissions of around 10 Gt, of which around 1.5 Gt is a result of land use change and the remainder comes from fossil fuel use and cement production (Canadell et al. 2007). This has led to an average annual rate of increase of carbon dioxide concentrations in the atmosphere of just under 2 ppm for the years 1995–2005 compared with around 1.25 ppm for the years 1960–1995 (IPCC 2007b). 

STABILISING OR REDUCING THE AMOUNT OF ATMOSPHERIC CARBON

Stabilising or reducing the amount of atmospheric carbon can be achieved in essentially two ways: by reducing the rate of emission, or by increasing the rate of absorption. Any successful strategy is almost certain to need both approaches, and will require contributions from all sectors (Cowie et al. 2007; Eliasch 2008).

Reduction in emissions can be achieved through a reduction in fossil fuel use, in cement production or in adverse (that is carbon-releasing) land-use change, or a combination of these. 

Removal of carbon dioxide from the atmosphere can be achieved either mechanically or through biological means. Mechanical removal, referred to as carbon capture and storage (CCS), entails the collection of CO2 emissions from fossil fuel at concentrated sources such as power stations and cement plants and their storage in geological formations such as spent oil fields (IPCC 2005). Biological mechanisms exploit the ability described above of photosynthesising organisms to capture CO2 and store it as biomass or as organic matter in sediments of various kinds. 

The biological management of carbon in tackling climate change has therefore essentially two components: the reduction in emissions from biological systems and the increase in their storage of carbon. These can be achieved in three ways: existing stores could be protected and the current high rate of loss reduced; historically depleted stores could be replenished by restoring ecosystems and soils; and, potentially, new stores could be created by encouraging greater carbon storage in areas that currently have little, for example through afforestation. In this report, we consider the roles that natural and human-dominated ecosystems can play in reducing emissions and in removing carbon from the atmosphere and we refer to the latter as ‘biosequestration’.

If well designed, a biological approach to carbon management can offer other benefits. Natural ecosystems, especially forests, are often rich in biodiversity as well as carbon; protecting one may serve to look after both (UNEP-WCMC 2008; Miles and Kapos 2008); they may also offer a range of other ecosystem services such as soil stabilisation, local climate amelioration and recycling of waste products. Good management of these ecosystems, and of agricultural systems, can pay dividends in terms of water and nutrient availability and reversal of land degradation, having positive impacts on livelihoods and helping in poverty reduction (Lal 2007; Smith et al. 2007a).

That is not to say ecosystem carbon management is straightforward. There are serious technical, social and economic challenges and some risks of unintended consequences. This report examines the state of knowledge about both its potential and challenges.

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