For decades, scientists have been measuring carbon dioxide and other greenhouse gases to gauge annual increases as well as to better understand how mankind is changing the world's atmosphere.
But scientists have struggled to build an accurate picture of how the gas is continuously shifted around by the atmosphere or precisely how much is soaked up by oceans and plants or emitted by rotting and burning vegetation and other natural processes.
Add to the mix mankind's carbon dioxide emissions from burning fossil fuels, deforestation and agriculture. This complex picture makes independently monitoring a specific region's natural take up of CO2 or a country's carbon emissions currently impossible on a near-real time basis.
HOW MUCH CARBON DOES MANKIND ADD?
The amount of CO2 that moves in and out of natural sources and sinks in the global carbon cycle is much greater than the emissions from mankind's emissions from industry, transport and agriculture. But every year, more and more of mankind's emissions stay in the atmosphere, with plants and oceans unable to absorb all the extra gases.
Mankind's CO2 emissions total about 30 billion tonnes per year. Of this, about half stays in the atmosphere, while about 7.5 billion tonnes is taken up by the oceans and roughly the same amount by land plants.
But exactly how much is taken up by specific regions, such as the Amazon rainforest, remain unclear.
Since 1750, the start of the Industrial Revolution, mankind's emissions have caused the concentration of CO2 in the air to increase by about 100 parts per million (ppm) to nearly 390 ppm. It took until the 1970s to reach the first 50 ppm increase. The next 50ppm increase occurred in the following 30 years, according to the U.N. Climate Panel.
HOW IS CO2 MEASURED?
Long-term records starting in the late 1950s involve daily air sampling in remote locations, such as on Mauna Loa in Hawaii and Cape Grim in Tasmania, Australia.
Since the 1970s, many CO2 measurements have involved collecting air samples from locations around the globe, either on land, ships or from aircraft. The air in trapped in small flasks and then measured by an infrared absorption analyser to determine the concentration of CO2. This process takes about 30 minutes or less but is labour intensive.
A more complicated process is used to measure the radiocarbon content of CO2 to work out how much fossil fuel emissions are contributing to the variations in CO2 in the atmosphere. Fossil fuels such as coal and oil have no carbon-14, a very rare isotope, so studying the various ratios of different types of carbon can help disentangle what is natural CO2 and what comes from mankind.
New devices are being developed to make this process faster and much cheaper.
WHO MEASURES CO2?
Many agencies measure CO2 and other greenhouse gases.
The main ones are:
* Earth Systems Research Laboratory of the National Oceanic and Atmospheric Administration (NOAA). The agency's Global Monitoring Division receives air samples from around the globe to measure concentrations of dozens of gases, including CO2, methane, HFCs and other powerful greenhouse gases.
* Scripps CO2 programme of the Scripps Institution of Oceanography, California, started in 1956. Measurements are taken from sampling stations from the Arctic to Antarctic.
* World Meteorological Agency's Global Atmosphere Watch. Data on myriad gas measurements are collected by the WMO's World Data Centre for Greenhouse Gases at the Japan Meteorological Agency.
* CSIRO Marine and Atmospheric Research Division in Australia
* National Center for Atmospheric Research in Boulder, Colorado. NCAR's Earth Observing Laboratory is running a special programme using a converted business jet to fly a series of pole-to-pole runs to sample the concentration of a variety of greenhouse gases at different heights in the atmosphere.
* Carbon Dioxide Information Analysis Center, of the U.S. Dept of Energy, is a leading data centre for greenhouse gases.
(Editing by Megan Goldin)