The Natural Fix?

INTRODUCTION

THE NEED FOR ECOSYSTEM CARBON MANAGEMENT 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 (CO 2 ) and methane (CH 4 ). 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 CO 2 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 re- quires greenhouse gas concentrations in the atmosphere to be stabilised at 445–490 ppm CO 2 equivalent (see box) or lower (IPCC 2007b). As there is presently about 430 ppm CO 2 e, this implies limiting future increases to between 15 and 60 ppm (Cowie et al. 2007; Eliasch 2008). CARBON IN LIVING SYSTEMS Living systems play a vital role in the carbon cycle. Photosyn- thesising organisms – mostly plants on land and various kinds of algae and bacteria in the sea – use either atmospheric car- bon 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 car- bon dioxide. Methane is produced by some kinds of microbe as

Note on units and quantities

1 gigaton of carbon (Gt C) = 10 9 tonnes of carbon (t C). Carbon (C) or carbon dioxide (CO 2 )? 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. CO 2 equivalent (CO 2 e) is a measure of global warming po- tential that allows all greenhouse gases to be compared with a common standard: that of carbon dioxide. For exam- ple, methane is about 25 times more potent a greenhouse gas than carbon dioxide so one tonne of methane can be expressed as 25 tonnes CO 2 e.