The evidence outlined above suggests that the terrestrial biosphere is already playing a clear role in slowing the rate of atmospheric and climatic change. This has led to the debate over whether management of the terrestrial biosphere could be employed as a tool to further slow atmospheric change. This management could take the form of enhanced reforestation, reduced forest degradation through logging, slowed down tropical deforestation, and sequestration of carbon in soils through "no-till" agricultural practices [Royal Society (2001)]. These options would also have a number of beneficial environmental side-effects, such as protection of biodiversity, watershed protection, and reduced soil erosion. Another potentially exciting option is the growing of bio-fuels to replace fossil fuels as an energy source.
In the last few years, the potential use of biosphere sinks has moved from academic debate to the forefront of international politics. Since the adoption of the United Nations Framework Convention on Climate Change (UNFCCC) in 1992, world political leaders have moved towards recognising the need to act to prevent dangerous levels of climate change. In 1997, the Kyoto Protocol of the UNFCCC was signed, committing the industrialised countries to reduce their CO2 emissions to various targets below 1990 levels by 2010. Since then, the Kyoto Protocol has faced a number of difficulties, in particular the withdrawal of the USA in 2001, but it finally came into force in 2005. One of the major points of contention has been whether human-induced carbon sinks should be included as a CO2 emissions reduction strategy, and in particular deliberate planned carbon sinks generated by appropriate forest and land management. The debate centres on two points: Whether these sinks really make a long-term contribution to climate change mitigation, and whether these sinks can be reliably measured.
It has been estimated that throughout human history about 190 PgC have been lost from the biosphere to the atmosphere by the clearing of forests for settlement and food production, about 10% of the total carbon content of the biosphere. An intensive and active deforestation reduction and reforestation program could at maximum return 75PgC back to the biosphere by 2050. Such a program could make a significant contribution to reducing net carbon emissions for the next few decades, but over the century becomes increasingly irrelevant compared to the 1400PgC emissions projected by 2100 under the IPCC "business-as-usual" scenario. Some countries (particularly in Europe) and many NGOs have argued that because these sinks can only make a small contribution to the long-term solution, they are a dangerous opt-out from the primary challenge, which is to develop technology and restructure energy supply and energy use to build low carbon emission societies. Other countries (the USA, Canada, Japan and Australia) have argued that the biosphere carbon sinks can be a viable component of an overall CO2 emissions reductions program. The debate still rages, but as of 2001 biosphere carbon sinks are included within the Kyoto Protocol, albeit in a rather arbitrary way.
Whatever the rights and wrongs of the debate, it is increasingly clear that the global carbon cycle will need to be closely monitored for political and economic as well as scientific reasons. The evidence outlined above shows that the scientific community is making progress in understanding the carbon cycle, but we are still somewhat fumbling in the dark. What is lacking is the right observational tool, a "macroscope" that can tell us how much carbon is being emitted from a particular place at a particular time. Over the coming decade, there is the exciting possibility that this macro-scrope will be developed, and that the sharp light of comprehensive global observation will finally be shed on the obscure secrets of the global carbon cycle. That macroscope will be satellite observations of CO2 concentration in high spatial and temporal detail [Houweling et al. (2004)].
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