An unusual extension into the future deals with the likely effects of tripling CO2 concentrations in the atmosphere (1). The base condition was taken as 330 ppm in 1973. A threefold increase to 1000 ppm may well be approached by 2100, assuming a moderate increase of emissions. The GCM study showed climate changes as concentrations rise from 2 x CO2 to 3 x CO2, similar to those expected during the previous rise to 2 x CO2, reinforcing them. In other words, there seems little chance of negative feedbacks developing to curtail global warming. However simple radiative transfer models indicate that a fixed increment of CO2 (e.g. 100 ppm) has less effect in an atmosphere already rich in CO2.
Particularly large increases of annual rainfall are likely over the Pacific Ocean, with minor decreases over most land in the both hemispheres and notably over Australia. Soil would be slightly drier, chiefly because of increased evaporation. The GCM indicated that El Nino events would persist even with 3 x CO2. Clearly droughts and flooding rains will continue to affect farming. Likewise, day-to-day weather is unlikely to be affected.
Another result of the model study concerned the overturning of the ocean current in the north Atlantic, which governs the so-called ‘conveyor belt’, an important longterm control of world climates. The overturning appears to be halved by the CO2 increases.
This modeling study assumed that the concentration of other greenhouse gases and that of aerosols is maintained at the current level. In design the model was more complete than most, incorporating ocean, sea-ice and biosphere as well as atmospheric conditions. Also, by dealing with yearly increments of the gas, the model avoids overestimating the climate effects, as occurs in simply examining equilibrium situations before and after abrupt changes of CO2.
(1) Dix, M.R. & B.G. Hunt, 1995. Climate modelling - doubling of CO2 levels and beyond. Final Report to the Federal Dept of the Environment, Sport & Industries, Australia.