Geoscience Reference
In-Depth Information
better represented (e.g., those relating to
clouds).
G
POSTSCRIPT
Our ability to understand and project climate
change has increased considerably since the
first IPCC Report appeared in 1990, but many
problems and uncertainties remain. Key needs
include (not in order of importance):
•
Greater understanding and modeling of ocean
processes and atmosphere-ocean coupling that
bear on the heat flux at the ocean surface,
the ability of oceans to absorb CO
2
, especially
by biological processes, and their role in heat
uptake that delays the climate system's
response to radiative forcing.
•
The development of more refined forcing
scenarios through a better understanding of
impacts of economic growth, forest clearances,
land-use changes, sulfate aerosols, carbona-
ceous aerosols generated by biomass burning,
and radiative trace gases other than CO
2
(e.g.,
methane and ozone). Refined estimates of
forcing by indirect aerosol effects merit
particular attention.
•
An improved ability to distinguish between
anthropogenic climate change and natural
variability, especially through the use of
ensemble simulations.
•
An improved understanding of threshold
behaviors (sometimes termed 'tipping points')
through which a warming climate may pre-
condition key systems, such as ice sheets,
sea ice and permafrost, to exhibit rapid
decay.
•
Incorporation of a realistic carbon cycle and ice
sheet dynamics in AOGCMs.
•
Better understanding of feedback processes,
notably those involving clouds, water vapor, sea
ice and the carbon cycle. Feedbacks involving
polar cloud cover and carbon release due to
thawing permafrost merit particular attention.
•
Continued, systematic collection of instru-
mental, proxy and space-based observations
of climate variables. This requires a commit-
ment by national governments to maintain
surface observation networks and satellite
remote sensing systems.
•
Further increases in climate model resolution
so that small-scale physical processes can be
The most fundamental measure of the earth's climate state is the global averaged surface air
temperature. It is influenced by a variety of climate forcing factors operating on a suite of timescales.
Climate variations over timescales of millions of years can be linked to plate tectonics. The great
Ice Ages and interglacials that have characterized the past two million years can be linked to
periodicities in the earth's orbit around the sun, influencing the seasonal distribution of solar
radiation over different parts of the surface. The observed increase in global mean surface air
temperature over the past 100 years may be attributed primarily to human-induced increases in
atmospheric carbon dioxide and other greenhouse gases, partly compensated by the cooling
effects of aerosol loading. These are known as radiative forcings in that they alter the globally
averaged radiation budget at the top of the atmosphere. Solar variability, another radiative forcing,
has played a minor role since the mid-twentieth century. The general rise in global mean surface
temperature over the past 100 years contains inter-annual to multi-decadal variations. These reflect
natural internal variability in the coupled atmosphere-ocean-land system as well as transient
radiative forcings such as large volcanic eruptions (e.g., Mt. Pinatubo).
