phone:415/479-5819 · fax:415/492-9325
GRAPEGROWING
|
There is no
convenient "control" planet otherwise
similar to Earth that we can leave
alone while we are changing the composition
of our atmosphere. As a consequence,
scientists use climate models,
complex computer programs that
simulate the three-dimensional movements
and thermodynamics of the
atmosphere and oceans.
Under development for more than
50 years, the modern generation of
models is based on well-understood
physics and tested against a large
array of observations. While the models
are far from perfect, they effectively
represent a wide range of features of
past and current climate.
When these models are run with
only natural contributions driving climate
(blue bands in Figure III), the
projected global and regional temperatures
do not warm over the 20th century,
in contrast to the observations
(black line in Figure III). But when the
models are run with both human and
natural contributions, projected temperatures
(pink bands in Figure III)
and observed temperatures (black
lines in Figure III) agree closely.
HOW CAN WE PREDICT WHAT
WILL HAPPEN FAR IN THE FUTURE?
DIDN'T SCIENTISTS USED TO TALK
ABOUT GLOBAL COOLING? All climate
models substantially agree about
the warming trend currently underway,
and project continued and accelerated
warming in the future.
All of the approximately 20 major
climate models share similar representations
of the main forces driving the
atmosphere and oceans. They differ in
subtle but important details concerning
things like the statistical properties
of clouds, which are important for
determining how much of the sun's
energy is reflected back into space
before the earth has a chance to absorb
it. Most of the ongoing research and
discussion on climate models concerns
these details.
|
Climate model projections of global
cooling from nearly 40 years ago were
also based on a human-caused phenomenon:
very small particles
(aerosols) from smokestacks and farming
causing a cooling effect by reflecting
some sunlight back to space.
15
Climate models have improved in
the last 40 years, but the most important
reason that the global-cooling scenario
did not occur is that emissions of
the cooling pollutants decreased (especially
through deliberate investments
to improve air quality), while the
emissions of greenhouse gases have
continued to increase exponentially.
Aerosols from smokestacks do have a
net cooling effect, which has been acting
to reduce the warming experienced
from greenhouse gases.
The current estimate is that the
direct and indirect cooling effects from
aerosols may offset about 40% of the
warming effect of the main greenhouse
gases related to human activity.
7
WHAT ARE THE PROJECTIONS
FOR THE FUTURE? Economic and
political decisions about energy and
resource use in the coming years will
be critical for determining the amount
of greenhouse gases emitted. This, in
turn, will drive the temperature
changes we experience: more emissions
will result in greater and more
potential damage to the climate
change.
Because of inertia in the climate system,
we are committed to some warming
in the future, even if all greenhouse
pollution was stopped today.
Experts estimate this locked-in warming
from greenhouse gases already in
the atmosphere to be between
0.2°C-1°C (0.35°F-1.8°F) (yellow line in
Figure IV).
21 However, this amount of
locked-in warming is small compared
to the range of warming driven by current
and future human emissions.
The range of emissions considered
by the IPCC entails a warming more
than 66% likely
|
to fall within the range
of +1.1° to 2.8°C (2.0° to 5.0°F) under a
lower greenhouse gas emissions scenario,
and 2.4° to 6.2°C (4.3° to 11.2°F)
under a high emissions scenario. The
group's "best estimate" of likely global
warming is between +1.6° to 4°C (+3.2°
and 7.2°F) by 2100 (Figure IV).
7
In California, the projected annual
average temperature increases for the
state over the next century are
between approximately +1.7° and
3.0°C (3.0° to 5.5°F) for a lower emissions
scenario, and 4.4° to +5.8°C (+7.9°
to 10.4°F) for a higher emissions scenario.
4
This range represents vast differences
in the kind of world we will
live in.
On the low end of the scale, where
the average climate of Monterey today
would be more like Sonoma in the
future, most regions would have a
range of suitable adaptation options,
and conditions would be enhanced in
some regions. By contrast, the higher
end of the range represents a serious to
devastating scenario of the future,
where Los Angeles is warmer than
Death Valley today. Clearly, continuing
grapegrowing in such a vastly
changed climate would be a serious
challenge.
The probability of experiencing
extreme climate events, such as very
hot days or unseasonal freezes, can be
represented by a bell-shaped curve,
with the height of the curve representing
the likelihood of experiencing the
temperatures along the bottom of the
curve (Figure V). Most days are somewhere
in the middle, with very few
days at either extreme.
Projections of climate change leading
to increases in average temperatures
(represented by the temperature
curve shifting to the right in Figure V)
helps to visualize why even small
shifts in average temperature could
mean important changes in impacts of
concern at the extremes.
For example, days over a threshold
such as 100°F where heat stress can
affect vines
|