An article written by Professor Euan Nisbet, Professor Peter Liss FRS, Dr Andrew Manning and Professor Ralph Keeling.
Half a century ago, Charles David Keeling began his measurements of atmospheric greenhouse gases at the South Pole and Mauna Loa. Keeling's measurements led him to remarkable conclusions1 - the Earth breathes, in and out – plants take in carbon dioxide as leaves are put on in spring and summer, and the process reverses as leaves fall and decompose in autumn and winter. This breathing can be observed in the atmosphere far from where the action is taking place, such as at the top of Mauna Loa volcano in the middle of the Pacific Ocean. The overwhelming influence of the northern hemisphere in the cycle showed Keeling that land plants, not ocean plankton, dominate the seasonal cycle of carbon uptake and release. The timing of the CO2 rise and fall between Hawaii and the South Pole showed him that air mixes from pole to pole on an approximately annual time scale.
Keeling found that each year there is more CO2 in the air than the year before, and our use of fossil fuels is the cause of this growth. Today we are all too aware that the burden of greenhouse gases in the air is increasing due to human activity, and that this is changing the climate of our planet.
Today, many stations world-wide collect long-term records of atmospheric CO2. Some also monitor other greenhouse gases and related tracers (e.g. methane, oxygen and stable isotopes). In the oceans, there are large-scale collaborative measurement programmes. On land, ecosystem studies monitor carbon fluxes around the world. The US ‘CarbonTracker’ programme is providing valuable regional insight, while the forthcoming EU Integrated Carbon Observing System (ICOS) promises much. Modelling tools have been developed, some of which use observational data, while others aim to mimic the Earth System to provide predictive capacity. Aircraft sampling and satellite work offer powerful partners to ground-based studies.
There are problems. The spatial gaps in measurement are huge, especially in the tropics. The heart of the biosphere is missed. Southern Asia, immensely important to both biosphere and economy, is barely measured, nor most of Africa and South America. Some important nations, including the UK whose remote islands (e.g. Ascension in the Atlantic and Chagos in the Indian Ocean) are in key locations, barely contribute to the UN’s Global Atmosphere Watch for CO2. Isotopic records of carbon gases, potent in identifying sources, are few. The measurement network for oxygen, the biological inverse of CO2, is in its infancy. Release of nitrous oxide (N2O), an easy target for greenhouse reduction, is poorly understood. The oceans are being acidified by uptake of CO2 and are losing oxygen from breakdown of organic material, but our observational network, and our understanding, is sparse. Few aircraft sample in the middle troposphere. As for satellites, the European SCIAMACHY instrument is aging, and the US Orbiting Carbo
n Observatory (OCO) crashed on launch, though the Japanese GOSAT is producing excellent results. Better measurement, at all vertical levels from the Earth’s surface to space, should be high on the agenda for the period to 2030.
Under the Kyoto Protocol of the UN Framework Convention on Climate Change (UNFCCC), Annex I nations (developed nations and economies in transition) are required to declare their greenhouse gas emissions. These emissions are calculated ‘bottom-up’ (for example, by working out how much oil and coal are burned by a country, and how much methane is emitted by its cows, landfills and gas leaks) and are published in very precise terms. However, evidence from atmospheric measurements show emissions of many industrial greenhouse gases tend to be greater than reported – disagreeing with reported ‘bottom-up’ emission measurements by factors of two or more. The implications are significant for activities such as carbon trading, and generally challenge the validity of promises to reduce emissions.
There is some hope here. Verification of emissions by atmospheric measurement is necessary. It is already becoming possible and may play a major role in future international agreements on climate change. Using the data on greenhouse gases so carefully collected at monitoring stations, new methodologies are being developed that promise to step from coarse global scale understanding to regional and country scale emissions. The European Commissioner for Research has recognised the need for better top-down verification of emissions, while the US National Research Council’s Committee on Methods for Estimating Greenhouse Gas Emissions published similar recommendations.
We have learnt much about the biosphere. We are able to track the bulk carbon flows across the planet. The work of understanding how the biosphere breathes, and our impact on it, is ongoing. In the air, on land, and in the oceans, scientists are studying the ways gases move through the Earth System.
Big changes are occurring, which will likely have large impacts on climate, but they are not well understood. Will ocean acidification and de-oxygenation cause large swathes of tropical oceans to become ‘marine deserts’? Will plant growth increase as CO2 rises? All these questions are on the agenda: in the answers we will learn much about our home, its robustness to change, and its fragility.
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