The low end of the warming range is assured even if we reduce emissions immediately. The high end results if we continue to increase our CO2 output.
Surface Temperature rise (1.4 to 5.8C increase by 2100)
- Moves the agriculture belts northward
- Fires
- Food supply
- Temperature related extinctions and migration worldwide
- Melts icecaps and glaciers
Ocean temperature increase
- More sea level rise due to expansion of the water in the oceans
- More intense storms hypothesized (more energy available) but other effects could change this idea (e.g. more shear would dissipate storms)
- Changes in the ocean currents making up the thermohaline circulation would change temperatures and weather patterns across the world
- More CO2 in the water means a lower pH thus more acidic waters affecting coral and other sensitive marine life
Atmospheric impacts
- Ozone Layer
- Jet Stream location and velocity
- Prevaling winds (trade winds, etc.)
Runaway Greenhouse risks
- Higher temperatures support more water vapor in the air (water vapor is a much more potent greenhouse gas than CO2)
With these type of impacts, excluding the catastrophic runaway greenhouse, the effects on populations will be asymmetrical. Places like Canada, Siberia, Greenland and even Antarctica may benefit. Europe, South Asia, Central America and Oceania will be devastated. Coastal cities will disappear but inland cities like Atlanta or Denver may prosper. Water supplies will be disrupted in some spots but new rainfall patterns may turn deserts into paradise. We just don't know enough to predict all the outcomes.
In human history vast dislocations always result in conflict. War is the most predictable outcome and it isn't really being discussed.
National Geographic graphic depiction of the impacts
IPCC TAR Reports on Impacts dont expect a clear statement. It is complicated, these guys know it and do not feel compelled to make it easy to absorb. They do say things like this:
"A contrasting future climate for Europe—the result of a rapid, nonlinear response
of the climate system—has been suggested. This involves an abrupt collapse of
the thermohaline circulation in the North Atlantic and consequent cooling in
Europe at least for the first half of the 21st century (e.g., Alcamo et al.,
1994). Although this event has not been ruled out on theoretical grounds (see
TAR WGI Chapter 11), it has not
been simulated by any AOGCM (see TAR WGI Chapter
9) and therefore has not been included in this assessment."
and
"Models predict that land areas in the Arctic will receive substantially increased
snowfall in winter and that the climate will be markedly warmer. Summer could
be much warmer and wetter than present. The climate over the Arctic Ocean does
not change as dramatically, but it will become warmer and wetter by 2080. For
the Antarctic continent, the models tend to predict more snow in winter and
summer. Although temperatures are forecast to increase by 0.5°C, there will
be little impact on melt because they will remain well below freezing, except
in limited coastal localities. The Southern Ocean warms least, especially in
summer. Precipitation increases by as much as 20%, so there will be more freshwater
input to the ocean surface. "
and
"
Projections of components contributing to sea level change from 1990 to 2100
(this period is chosen for consistency with the IPCC Second Assessment Report),
using a range of AOGCMs following the IS92a scenario (including the direct effect
of sulphate aerosol emissions) give:
- thermal expansion of 0.11 to 0.43 m, accelerating through the 21st century;
- a glacier contribution of 0.01 to 0.23 m;
- a Greenland contribution of –0.02 to 0.09 m;
- an Antarctic contribution of –0.17 to 0.02 m.
Including thawing of permafrost, deposition of sediment, and the ongoing contributions
from ice sheets as a result of climate change since the Last Glacial Maximum,
we obtain a range of global-average sea level rise from 0.11 to 0.77 m. This
range reflects systematic uncertainties in modelling."