|
 For the Northern Hemisphere temperature,
recent decades appear to be the warmest since at least about
1000AD, and the warming since the late 19th
century is
unprecedented over the last 1000 years. Older data are
insufficient to provide reliable hemispheric temperature
estimates. Ice core data suggest that the 20th century has been
warm in many parts of the globe, but also that the significance
of the warming varies geographically, when viewed in the context
of climate variations of the last millennium.
Large and rapid climatic changes affecting
the atmospheric and oceanic circulation and temperature, and the
hydrological cycle, occurred during the last ice age and during
the transition towards the present Holocene period (which began
about 10,000 years ago). Based on the incomplete evidence
available, the projected change of 3 to 7°F (1.5 - 4°C) over the
next century would be unprecedented in comparison with the best
available records from the last several thousand years.
Is the sea level rising?
Global mean sea level has been rising at
an average rate of 1.7 mm/year (plus or minus 0.5mm) over the
past 100 years, which is significantly larger than the rate
averaged over the last several thousand years. Depending on
which greenhouse gas increase scenario is used (high or low)
projected sea-level rise is projected to be anywhere from 0.18
(low greenhouse gas increase) to 0.59 meters for the highest
greenhouse gas increase scenario. However, this increase is due
mainly to thermal expansion and contributions from melting
alpine glaciers, and does not include any potential
contributions from melting ice sheets in Greenland or
Antarctica. Larger increases cannot be excluded but our current
understanding of ice sheet dynamics renders uncertainties too
large to be able to assess the likelihood of large-scale melting
of these ice sheets.
Can the
observed changes be explained by natural variability, including
changes in solar output?
Since our entire climate system is
fundamentally driven by energy from the sun, it stands to reason
that if the sun's energy output were to change, then so would
the climate. Since the advent of space-borne measurements in the
late 1970s, solar output has indeed been shown to vary. With now
28 years of reliable satellite observations there is
confirmation of earlier suggestions of an 11 (and 22) year cycle
of irradiance related to sunspots but no longer term trend in
these data. Based on paleoclimatic (proxy) reconstructions of
solar irradiance there is suggestion of a trend of about +0.12
W/m2 since 1750 which is about half of the estimate given in the
last IPCC report in 2001. There is though, a great deal of
uncertainty in estimates of solar irradiance beyond what can be
measured by satellites, and still the contribution of direct
solar irradiance forcing is small compared to the greenhouse gas
component. However, our understanding of the indirect effects of
changes in solar output and feedbacks in the climate system is
minimal. There is much need to refine our understanding of key
natural forcing mechanisms of the climate, including solar
irradiance changes, in order to reduce uncertainty in our
projections of future climate change.
In addition to changes in energy from the
sun itself, the Earth's position and orientation relative to the
sun (our orbit) also varies slightly, thereby bringing us closer
and further away from the sun in predictable cycles (called
Milankovitch cycles). Variations in these cycles are believed to
be the cause of Earth's ice-ages (glacials). Particularly
important for the development of glacials is the radiation
receipt at high northern latitudes. Diminishing radiation at
these latitudes during the summer months would have enabled
winter snow and ice cover to persist throughout the year,
eventually leading to a permanent snow- or icepack. While
Milankovitch cycles have tremendous value as a theory to explain
ice-ages and long-term changes in the climate, they are unlikely
to have very much impact on the decade-century timescale. Over
several centuries, it may be possible to observe the effect of
these orbital parameters, however for the prediction of climate
change in the 21st century, these changes will be far less
important than radiative forcing from greenhouse gases.
What About the Future?
Due to the enormous complexity of the
atmosphere, the most useful tools for gauging future changes are
'climate models'. These are computer-based mathematical models
which simulate, in three dimensions, the climate's behavior, its
components and their interactions. Climate models are constantly
improving based on both our understanding and the increase in
computer power, though by definition, a computer model is a
simplification and simulation of reality, meaning that it is an
approximation of the climate system. The first step in any
modeled projection of climate change is to first simulate the
present climate and compare it to observations. If the model is
considered to do a good job at representing modern climate, then
certain parameters can be changed, such as the concentration of
greenhouse gases, which helps us understand how the climate
would change in response. Projections of future climate change
therefore depend on how well the computer climate model
simulates the climate and on our understanding of how forcing
functions will change in the future.
The IPCC Special Report on Emission
Scenarios determines the range of future possible greenhouse gas
concentrations (and other forcings) based on considerations such
as population growth, economic growth, energy efficiency and a
host of other factors. This leads a wide range of possible
forcing scenarios, and consequently a wide range of possible
future climates.
According to the range of possible forcing
scenarios, and taking into account uncertainty in climate model
performance, the IPCC projects a best estimate of global
temperature increase of 1.8 - 4.0°C with a possible range of 1.1
- 6.4°C by 2100, depending on which emissions scenario is used.
However, this global average will integrate widely varying
regional responses, such as the likelihood that land areas will
warm much faster than ocean temperatures, particularly those
land areas in northern high latitudes (and mostly in the cold
season). Additionally, it is very likely that heat waves and
other hot extremes will increase.
Precipitation is also expected to increase
over the 21st century, particularly at northern mid-high
latitudes, though the trends may be more variable in the
tropics, with much of the increase coming in more frequent heavy
rainfall events. However, over mid-continental areas
summer-drying is expected due to increased evaporation with
increased temperatures, resulting in an increased tendency for
drought in those regions.
Snow extent and sea-ice are also projected
to decrease further in the northern hemisphere, and glaciers and
ice-caps are expected to continue to retreat.
Source: National Oceanic and Atmospheric
Administration
http://www.ncdc.noaa.gov/oa/climate/globalwarming.html
|
Search
Weather
Weather News
|
