In 1988 the World Meteorological Organization (WMO) and the United Nations Environmental Programme (UNEP) created the Intergovernmental Panel on Climate Change (IPCC). It provided assessments of the global climate situation with forecasts in 1990 and 1995. Estimates of temperature and sea-level change for 2100 were revised downward somewhat between 1990 and 1995. See also their web page at http://www.unep.ch/ipcc/ipcc-0.html
Their forecasts of annual carbon emissions from fossil fuel burning in 2100 range from 6 GtC per year (6 billion tons of carbon, approximately current levels) to 36 GtC annually. To put that in historic context, between 1860 and 1994 they estimate total emissions to be 360 GtC (or about 2.7 GtC per year), of which 2/3 came from fossil fuel use and 1/3 from deforestation. Currently deforestation and changes in land use add about 1.5 GtC more to the atmosphere. They make forecasts also for emissions of methane and other greenhouse gases.
Resevoirs for carbon in the 1980s were atmosphere (3.3 GtC per year), oceans (2.0 GtC annually), Northern Hemisphere forest regrowth (.5 GtC annually) and an “inferred” sink (1.3 GtC per year).
The 1995 analysis looked at the possibilities for stabilizing global carbon dioxide concentrations which are now about 350 parts per million by volume (ppmv). They can be stabilized at 450, 650, or 1000 ppmv only if CO2 emissions return to 1990 levels within approximately 40, 140, or 240 years respectively.
The mid-range of their temperature forecasts is an increase of 2 degrees Centigrade between 1990 and 2100 (on top of an increase of .3-.6 degrees since the late 19th century). Their forecasts for 2100 range between 1 and 3.5 degrees. They link carbon concentrations of 450, 650, and 1000 ppmv with temperature increases of about 1 (range .5-1.5), 2 (range 1.5-4) and 3.5 (range 2-7) degrees centigrade, respectively. Thermal inertia of the oceans will mean that such temperature increases will be only 50-90% of the ultimate equilibrium temperatures associated with anticipated CO2 increases.
The IPCC projects a sea-level rise of about 50 cm between 1990 and 2100, with a range between 15 cm and 95 cm (nearly 1 meter). This compares with a rise of about 20 cm in the last 100 years. Again, systemic delays mean that sea-level change associated with mid-range scenarios will continue even after atmospheric CO2 stabilizes, with values of perhaps 150 cm by 2500. The IPCC looks at impact by country and region.
They provide forecasts for the affects of these changes on agricultural production that vary widely by geographic region and within them. Affects may be greatly negative or substantially positive.
In comparison with total atmospheric levels of about 800 Gt of carbon, the above-ground biomass holds 600-700 GtC, organic matter in soil contains about 1,500 GtC, and the oceans hold about 40,000 GtC (IPCC, 1995, Volume 2: 63). Methane hydrate deposits below the ocean floor may hold 10,000 GtC (Science News, Nov 9, 1996, p. 299). Thus transfers between the atmosphere and other sinks are critical, especially transfers to or from the ocean. There is great debate about how warming, temperature, and sink concentrations may affect absorption or release. In addition fossil fuel resources contain about 25,000 GtC, with known reserves holding about 1,000 GtC (p. 87).
The International Energy Agency (IEA, 1996: 58) forecasts growth in carbon emissions of 36-50% between 1990 and 2010.