Greenhouse gases

Greenhouse gases

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.

Professor David MacKay FRS talks about cutting greenhouse gases. (3 mins, requires Flash Player).

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.

    Dr John Ashton on the politics of the low carbon transition (2 mins).
  • Listen here.

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.

Spinx Observatory, Jungfraujoch, Switzerland (credit: NOAA ESRL).

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 Carbon 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.

    Sir Crispin Tickell on the equities of measurement
    (3 mins).
  • Listen here.

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.  

National Oceanic and Atmospheric Administration (NOAA) America Samoa Observatory (credit: NOAA ESRL).

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.

[1] C. D. Keeling, 1960, Tellus 12, 200-203

Banner image: Cape Grim laboratory, Australia (credit: Bureau of Meteorology Australia).

Discussion meeting

This article is based on the discussion meeting 'Greenhouse gases in the Earth system' which was held on 22-23 February 2010.

Related policy report

Climate change: A Summary of the Science

Published September 2010.

From the Royal Society's archives

It was the French physicist Edme Mariotte who suggested in the 17th century that, unlike heat from other sources, sunlight can pass through glass. In the 18th century, Horace Bénédict de Saussure FRS, a Swiss natural scientist, built upon Mariotte’s suggestion by building a ‘heliothermometer’: a device that could measure the relative intensity of sunlight. Early analogies to the greenhouse effect represented a big step toward the recognition that air could trap thermal radiation.

The idea that human beings might have an impact on the atmosphere of the entire globe only began to be fully recognised in the later 19th century. British physicist John Tyndall FRS experimented on radiant heat and various gases during the 1850s, recognizing that water vapour and carbon dioxide were capable of absorbing and radiating infra-red far more than other common atmospheric gases. Tyndall was an early Alpinist, with an interest in glaciers, and could make a connection between atmospheric variations of water in particular and possible historical changes in climatic conditions. Then, the Swedish scientist Svante Arrhenius ForMemRS attempted to calculate how levels of carbon dioxide might affect the Earth’s surface temperature, producing a rudimentary “greenhouse law”.

However, without knowing how the Earth’s wider climate history correlates with the composition of its atmosphere over time, it would be difficult to judge the effect of greenhouse gases on global climate. Step in the polar scientists of Antarctica and Greenland, whose ice-cores provided the means of estimating temperatures of the far past and paleoatmospheric composition from trapped gas bubbles.  Although there were earlier attempts at coring, large scale work dates from the International Geophysical Year of 1957-1958, a world-wide collaboration organised in part by the Royal Society. Moreover, atmospheric data from the Society’s Antarctic station revealed that other manmade but non-greenhouse gases, chlorofluorocarbons, caused ozone layer depletion.